p i
J 7
'
COLLOID CHEMISTRY
AN INTRODUCTION, WITH SOME
PRACTICAL APPLICATIONS
BY
JEROME ALEXANDER, M.Sc.,
Chairman, Special Committee on Colloids, Division of Chemistry
and Chemical Technology, National Research Council
Member Amer. Institute Chemical Engineers
ILLUSTRATED
NEW YORK
D. VAN NOSTRAND COMPANY
25 PARK PLACE
1919
J>>
COPYRIGHT, .1919,
BY
D, VAN NOSTRAND COMPANY
Stanbope
F. H. GILSON COMPANY
BOSTON, U.S.A.
PREFACE
THIS little book is the result of an attempt
to compress within a very limited space, the
most important general properties of colloids,
and some of the practical applications of col-
loid chemistry. Its object will be accomplished
if it is helpful in extending the sphere of in-
terest in this fascinating twilight zone between
physics and chemistry.
J. A.
NEW YORK,
Nov. 1, 1918.
415477
TABLE OF CONTENTS
CHAPTEB PAGE
I. INTRODUCTION 1
II. CLASSIFICATION OF COLLOIDS 10
III. CONSEQUENCES OF SUBDIVISION 14
IV. THE ULTRAMICBOSCOPE 17
V. GENERAL PROPERTIES OF COLLOIDS 24
VI. PRACTICAL APPLICATIONS OF COLLOID CHEMISTRY . 36
BIBLIOGRAPHY 85
AUTHOR INDEX 87
SUBJECT INDEX 89
COLLOID CHEMISTEY
CHAPTER I
Introduction
Although many facts and principles con-
cerning colloids have from time immemorial
been known and utilized empirically, the
scientific foundation of modern colloid chemis-
try was laid by an Englishman, Thomas Gra-
ham, F.R.S., Master of the Mint. In two
basic papers on this subject, the first entitled
"Liquid Diffusion Applied to Analysis," read
before the Royal Society of London, June 13,
1861, the second entitled "On the Properties
of Colloidal Silicic Acid and other Analogous
Colloidal Substances," published in the Pro-
ceedings of the Royal Society, June 16, 1864,
Graham pointed out the essential facts regard-
ing colloids and the colloidal condition, and
established much of the nomenclature in use
at the present day. In the first of these
i
2 \ COLLOID CHEMISTRY
papers Graham says: "The property of vola-
tility, possessed in various degrees by so many
substances, affords invaluable means of separa-
tion, as is seen in the ever-recurring processes
of evaporation and distillation. So similar in
character to volatility is the diffusive power
possessed by all liquid substances, that we
may fairly reckon upon a class of analogous
analytical resources to arise from it. The
range also in the degree of diffusive mobility
exhibited by different substances appears to be
as wide as the scale of vapor tensions. Thus
hydrate of potash may be said to possess double
the velocity of diffusion of sulphate of potash,
and sulphate jti potash again double the
velocity of sugar, alcohol and sulphate of
magnesia. But the substances named belong
all, as regards diffusion, to the more "vola-
tile " class. The comparatively "fixed " class,
as regards diffusion, is represented by a differ-
ent order of chemical substances, marked out
by the absence of the power to crystallize,
which are slow in the extreme. Among the
latter are hydrated silicic acid, hydrated alu-
mina and other metallic peroxids of the
aluminous class, when they exist in the soluble
INTRODUCTION 3
form; with starch, dextrin and the gums,
caramel, tannin, albumen, gelatin, vegetable
and animal extractive matters. Low diffusi-
bility is noj^the only property which the bodies
last enumerated possess in common. They are
distinguished by the gelatinous character of
their hydrates. Although often largely soluble
in water, they are held in solution by a most
feeble force. They appear singularly inert in
the capacity of acids and bases, and in all the
ordinary chemical relations. But, on the other
hand, their peculiar physical aggregation with
the chemical indifference referred to appears
to be required in substances that can intervene
in the organic processes of life. The plastic
elements of the animal body are found in this
class. As gelatin appears to be its type, it is
proposed to designate -substances of this class
as colloids, and to speak of their peculiar form
of aggregation as the colloidal condition of
matter. Opposed to the colloidal is the crys-
talline condition. Substances affecting the
latter form will be classed as crystalloids. The
distinction is no doubt one of intimate molec-
ular constitution.
" Although chemically inert in the ordinary
4 COLLOID CHEMISTRY
sense, colloids possess a compensating activity
of their own, arising out of their physical
properties. While the rigidity of the crystal-
line structure shuts out external impressions,
the softness of the gelatinous colloid partakes
of fluidity, and enables the colloid to become
a medium for liquid diffusion, like water itself.
The same penetrability appears to take the
form of cementation in such colloids as can
exist at high temperature. Hence a wide
sensibility on the part of colloids to external
agents. Another and eminently character-
istic quality of colloids is their mutability.
Their existence is a continued metastasis. A
colloid may be compared in this respect to
water, while existing liquid at a temperature
under its usual freezing-point, or to a super-
saturated saline solution. Fluid colloids ap-
pear to have always a pectous modification;
and they often pass under the slightest
influences from the first to the second condi-
tion. The solution of hydrated silicic acid,
for instance, is easily obtained in a state of
purity, but it cannot be preserved. It may
remain fluid for days or weeks in a sealed tube,
but is sure to gelatinize and become insoluble
INTRODUCTION 5
at last. Nor does the change of this colloid
appear to stop at that point. For the mineral
forms of silicic acid deposited from water, such
as flint, are often found to have passed, during
the geological ages of their existence, from the
vitreous or colloidal into the crystalline con-
dition. (H. Rose.) The colloidal is, in fact,
a dynamical state of matter, the crystalloidal
being the statical condition. The colloid
possesses Energia. It may be looked upon
as the probable primary source of the force
appearing in the phenomena of vitality. To
the gradual manner in which colloidal changes
take place (for they always demand time as an
element) may the characteristic protraction of
chemico-organic changes also be referred. . . .
"It may perhaps be allowed to me to apply
the convenient term dialysis to the method of
separation by diffusion through a septum of
gelatinous matter. The most suitable of all
substances for the dialytic septum appears to
be the commercial material known as vegetable
parchment, or parchment-paper. . . ."
At the beginning of the second paper above
referred to, Graham states: "The prevalent
notions respecting solubility have been de-
6 COLLOID CHEMISTRY
rived chiefly from observations on crystalline
salts, and are very imperfectly applicable to
the class of colloidal solutions." From this
it may be seen that Graham appreciated the
fact that all the laws of crystalloidal solutions
could not be applied to colloidal solutions.
In the case of crystalloidal solutions the dis-
solved substance is present in a state of molec-
ular subdivision, and, according to the ioniza-
tion theory, is in many cases dissociated into
ions. With colloidal solutions, on the other
hand, we have a lesser degree of subdivision,
and the particles in solution are larger and
more cumbersome. As Graham remarked,
"The inquiry suggests itself whether the
colloid molecule may not be constituted by the
grouping together of a number of smaller
crystalloid molecules, and whether the basis
of colloidality may not really be this composite
character of the molecule." This is to-day
the idea generally accepted.
COLLOID CHEMISTRY DEFINED
Colloid chemistry deals with the behavior
and properties of matter in the colloidal con-
dition, which, as we now know, means a certain
INTRODUCTION 7
very fine state of subdivision. While there
are no sharp limitations to the size particles in
colloidal solutions, it may in a general way be
stated that their sphere begins with dimensions
somewhat smaller than a wave length of light,
and extends downward well into dimensions
which theory ascribes to the molecules of
crystalloids. (See Table II, p. 12.)
SUSPENSION vs. SOLUTION
With the aid of the ultramicroscope, which
renders visible particles approaching in mi-
nuteness molecular dimensions, Zsigmondy has
shown that there is no sharp line of demarca-
tion between suspensions and solutions, but
that with increasing fineness in the subdivision
of the dissolved substance, there is a progres-
sive change in the properties of the resulting
fluids, the influence of gravity gradually
yielding to that of the electric charge of
particles, of surface tension and of other forms
of energy. Thus in the case of metallic gold,
subdivisions whose particles are 1 M and over
act as real suspensions and deposit their gold,
whereas much finer subdivisions (60 AM and
under) exhibit all the properties of metal
8 COLLOID CHEMISTRY
hydrosols or colloidal solutions. In the ultra-
microscope the coarser subdivisions show the
well-known Brownian movement, which
greatly increases as the particles become-
smaller, until at the present limit of ultra-
microscopic visibility (about 5 MM) it becomes
enormous both in speed and amplitude.
On the other hand, there is no sharp dis-
tinction between colloids and crystalloids, but
as the particles in solution become smaller and
smaller, the optical heterogeneity decreases
correspondingly, finally vanishing as molec-
ular dimensions are approached.* That even
crystalloid solutions are not in a strict sense
homogeneous, is indicated by an experiment
of van Calcar and Lobry de Bruyn (Rec. Trav.
* In an article entitled "Pedetic Motion in Relation to
Colloidal Solutions " published in Chemical News, 1892, Vol.
65, p. 90, William Ramsay, Ph.D., F.R.S, (afterward Sir
William Ramsay), clearly expressed this view in the fol-
lowing words: "I am disposed to conclude that solution
is nothing but subdivision and admixture, owing to attrac-
tions between solvent and dissolved substance accompanied
by pedetic motion; that the true osmotic pressure has,
probably, never been measured; and that a continuous pas-
sage can be traced between visible particles in suspension
and matter in solution; that, in the words of the old adage,
Natura nihil Jti per sattum."
INTRODUCTION 9
chim. Pays-Bas, 1904, 23, 218), who caused
the crystallization of a considerable part of
saturated crystalloid solutions at the periph-
ery of a rapidly rotating centrifuge.
CHAPTER II
Classification of Colloids
The broadest classification of colloids is that
of Wolfgang Ostwald (Koll. Zeitschr., Vol. 1,
page 291), who grouped them according to the
physical state (gaseous, liquid or solid) of the
subdivided substance (dispersed phase), and
of the medium in which the particles of the
subdivided substance are distributed (disper-
sion medium).* Table I (page 11) shows the
nine resulting groups and gives some instances
of each.
Ostwald's classification, however, is more
theoretical than practical, for the properties of
colloids are dependent mainly upon the specific
nature of the dispersed substance and its degree
of subdivision. Following Hardy, Zsigmondy
divided colloids into two classes, the reversible
and irreversible; the former redissolve after
* G. Bredig proposed to call colloids "microheterogeneous
systems." W. Ostwald called them "dispersed heterogeneous
systems," which expression was contracted by P. P. von
Weimarn into the term "dispersoids."
10
CLASSIFICATION OF COLLOIDS
11
desiccation at ordinary temperatures, whereas
the latter do not.
TABLE I
Dispersed
phase.
Dispersion
medium.
Example.
Gas
Gas
No example, since gases are miscible in all
proportions.
Gas
Liquid
Fine foam, gas in beer.
Gas
Solid
Gaseous inclusions in minerals (meer-
schaum, pumice), hydrogen in iron, oxy-
gen in silver.
Liquid
Gas
Atmospheric fog, clouds, gases at critical
state.
Liauid
Liquid
Emulsions of oil in water, cream, colloidal
.LJ114 UJ.U. . ......
water in chloroform.
Liquid
Solid
Mercury in ointments, water in paraffin wax,
liquid inclusions in minerals.
Solid
Gas
Cosmic dust, smoke, condensing vapors,
(ammonium chlorid).
Solid
Liquid
Colloidal gold, colloidal sodium chlorid, col-
loidal ice in chloroform.
Solid
Solid
Solid solutions, colloidal gold in ruby glass,
coloring matter in gems.
Table II, taken from Zsigmondy,* illustrates
this classification, and shows how colloids hav-
ing the same particle size or degree of sub-
division may nevertheless act quite differently
because of specific differences in the nature of
the dispersed substances.
* Colloids and the Ultramicroscope, J. Wiley & Son, Inc.
(Translation by J. Alexander.)
12 COLLOID CHEMISTRY
With the reversible colloids (gelatin, gum
arabic, albumen), there is a more intimate
union between the two phases; in fact it is
probable that with them we have really a
mixture of (1) a dispersed phase of water sub-
divided in the solid, with (2) a dispersing
phase of the solid finely subdivided in water.
The former are therefore called emulsoids and
the latter suspensoids. Colloids of the rever-
sible type are also said to be hydrophile or
lyophile, while the irreversible colloids are
hydrophobe or lyophobe.
No sharp line is to be drawn, however, for
besides intermediate or transition cases be-
tween the two classes, there may be recognized
two groups of irreversible colloids, roughly
defined by their behavior upon concentration:
First: The completely irreversible , which
coagulate while still quite dilute and separate
sharply from the solvent with the formation of
a pulverulent precipitate rather than a gel
(i.e., pure colloidal metals). Chemical or
electrical energy is needed to bring them back
again into colloidal solution.
Second: The incompletely reversible which,
when quite concentrated, form a gel that may
CLASSIFICATION OF COLLOIDS 13
be easily redissolved or peptisized by com-
paratively small amounts of reagents, unless
the evaporation has proceeded too far (i.e.,
colloidal stannic acid).
CHAPTER III
Consequences of Subdivision
As the subdivision of a substance proceeds,
the area of its effective surface increases enor-
mously, as maybe seen from the following Table
III adapted from Ostwald. Consequently sur-
face forces, such as adsorption, capillarity and
surface tension, become enormously magnified
and of primary importance. Furthermore, the
so-called radius of molecular attraction (p =
50 MM) is well within the colloidal field, so that
the specific attractive forces of the particles
also enter as a controlling factor. In fact,
before substances can unite chemically their
particles must be first brought into proper
subdivision and proximity,* by solution, fusion,
ionization or even by mere pressure, as was
demonstrated by W. Spring, who caused fine
* It is a striking fact that absolutely dry sodium is not
attacked by absolutely dry chlorin. M. Raffo and A. Pieroni
observed that colloidal- suphur reduced silver salts energeti-
cally, whereas even fine precipitated sulphur did not form
silver sulphid in the cold, and did so only partially upon boiling.
14
CONSEQUENCES OF SUBDIVISION
15
d .*
.3 g .. C
S * 8 1 S % s - 8
2 8 - - -' = -
II II II
<M C4
II II H
a a a
a s s a
g* i s 1 I
3 1
.a .a .s .s 1 1 ;
* a *
ddddddddd
*
-* o
16 COLLOID CHEMISTRY
dry powders to combine chemically by high
pressure. If the degree of subdivision is not
profound enough to permit of the combination
of isolated atoms or ions with each other,
chemical combination in the strict sense may
not occur, but there may be produced " ad-
sorption compounds " resulting from the union
of atomic or ionic mobs in indefinite or non-
stoichiometric proportions, under the influence
of more or less modified chemical forces. The
combination of arsenious acid and ferric oxid
which Bunsen regarded as a basic ferric arsen-
ite, 4 Fe 2 0s, A^Oa, 5 H 2 0, has been shown by
Biltz and Behre to be an adsorption compound;
and Zsigmondy proved "Purple of Cassius" to be
an adsorption compound of colloidal gold and
colloidal stannic acid by actually synthesizing it
by mixing the two separate colloidal solutions.
The effect of increasing subdivision upon the
particles in colloidal solutions is illustrated in
Table IV, adapted from Zsigmondy. Tables
V and VI were prepared by Zsigmondy to
illustrate visually the relation of the sizes of
colloidal particles to well-known microscopic
objects on the one hand and to the theoretical
sizes of molecules on the other.
TABLE V
LINEAR MAGNIFICATION 1 : 10,000
A. Human blood corpuscles (diameter 7.5 n, thickness 1.6 p).
B. Fragment of rice starch granule (according to v. Hohnel) 3-8 /*.
C. Particles in a kaolin suspension.
E. Anthrax bacillus (length 4-15 n, width about 1 /*).
F. Cocci (diameter about 0.5-1 n, rarely 2 /*)
f, g, h. Particles of colloidal gold solutions Au 73a , Au^, Au ( 0.006-0.015 ft),
i, k, 1. Particles from settled gold suspensions (0.075-0.2 n).
TABLE VI
LINEAR MAGNIFICATION 1 : 1,000,000
a b
? D m S
e ei e 2 f g
n
a-d. Hypothetical Molecular Dimensions
a. Hydrogen molecule dia. 0.1 nil.
b. Alcohol molecule dia. 0.5 MM-
c. Chloroform molecule dia. 0.8 pp.
d. Molecule of soluble starch dia. about 5 MM
e-h. Gold Particles in Colloidal Gold Solutions
e. Gold particle in Aui (too small to determine),
d. " " " " , about 1.7 MM-
ej. " " " " , " 3.0 MM-
/. " " Au 73a , 6 MM-
g. " " " Au92, " 10 MM-
h. " " " AUOT, " 15 MM-
u Gold particle in settled gold suspension.
CONSEQUENCES OF SUBDIVISION 17
s
.2
1
a.
3
c3
I 1 11
II I 1 If
l! 11 I
&
3 3 ^3 03
fi( O* o f^
! 1
II i i
os I I
a 1
'111 E S " "
P
CHAPTER IV
The Ultramicroscope
As this instrument revolutionized colloid
research, a brief description of it is essential.
It is a matter of every-day experience that the
unseen motes and dust particles in the air be-
come visible in a beam of bright light, espe-
cially against a dark ground, and in this simple
fact lies the principle of the ultramicroscope.
Faraday and later Tyndall made use of a
convergent beam of light to demonstrate the
optical inhomogeneity of solutions; for in
fluids not optically clear, the path of the beam
becomes more or less distinctly visible, because
of the light scattered by the particles present.
In this manner can be recognized much smaller
quantities of matter than by spectrum analysis
in fact less than 10~ 8 mg. (1/10,000,000) of
metallic gold can thus be detected with the
naked eye.
Prof. Richard Zsigmondy while experiment-
ing with colloidal solutions conceived the idea
of examining this light cone microscopically.
18
THE ULTRAMICROSCOPE 19
His preliminary experiments having demon-
strated that he could thus see the individual
particles in various hydrosols, he sought the
assistance of Dr. H. Siedentopf, scientific
director of the Carl Zeiss factory, in Jena,
where was produced the first efficient ultra-
microscope.
The ultramicroscope consists essentially of
a compound microscope arranged for examin-
ing in a dark field an intense convergent beam
of light cast within or upon the substance
under examination. The light seen by the
eye represents, therefore, the light diffracted,
scattered or reflected upward by the substance
or by particles within it.
If within a thin beam of light from a pro-
jection lantern we scatter successively powders
of different substances in various degrees of
fineness (mica ground to pass 60, 100 and 160
mesh; lampblack; powdered oxid of zinc;
flake and powdered graphite), some of them
will produce only a homogeneous illumination
of the beam in which no isolated particles can
be seen, whereas with others, the individual
particles are distinctly visible.
Passing the beam through a beaker of dis-
20 COLLOID CHEMISTRY
tilled water, nothing can be seen; but upon
the addition of a faw drops of colloidal gold
solution, which appears quite clear to trans-
mitted light, the path of the beam through the
fluid immediately becomes visible. This Tyn-
dall effect,* as it is called, might be considered
a criterion of colloidal solution were it not that
very minute traces of colloidal impurities can
produce it and it is often exhibited by solu-
tions generally regarded as crystalloidal
those of many dyestuffs for example; further-
more with increasing fineness of subdivision
the Tyndall effect decreases, disappearing as
molecular dimensions are approached.
Just as in the cosmic field our most powerful
telescopes fail to resolve the fixed stars, which
are nevertheless visible as points of light of
varying brilliancy, so, too, in the ultramicro-
scopic field, we can see particles much smaller
than the resolving power of the microscope
(that is, smaller than a wave length of light)
provided only that they diffract sufficient light
to affect the retina. Based upon the experi-
ence of astronomers we may be able greatly to
increase the sensitiveness of the ultramicro-
* Also known as the Faraday-Tyndall effect.
THE ULTRAMICROSCOPE 21
scope by fortifying the eye, so to speak, with
the photographic plate, using at the same time
tropical sunlight or ultraviolet light for illu-
mination.
In the original form of the ultramicroscope,
as perfected by Siedentopf and Zsigmondy,
which is the one best adapted for the examina-
tion of transparent solids, a side illumination
is effected by a microscope objective with
micrometer movements, which throws an
intense but minute conical beam of light into
the fluid contained in a little cell having
quartz windows at the side and top. Above
this cell a compound microscope is adjusted
vertically, so that the narrowest part of the
light cone occupies the center of the focal
plane. If the fluid under examination is op-
tically clear or if it contains particles so small
that they cannot diffract sufficient light to
create a visual impression, the light cone
cannot be seen. If enough light is diffracted,
the light cone becomes visible, being homo-
geneous if the particles are too small or too
close together to be individually seen, and
heterogeneous if the particles can be individu-
ally distinguished. Particles or dimensions
22 COLLOID CHEMISTRY
beyond the resolving power of the microscope
(about J At) are for brevity termed ultrami-
crons. Ultramicrons that can individually be
made visible are called submicrons (or hypo-
microns) while those so small that they
produce an unresolvable light cone are termed
amicrons.
Knowing the percentage of gold present in
a colloidal gold solution and assuming a certain
specific gravity and uniform shape for the gold
particles, the average size and mass of a single
particle of colloidal gold can be calculated, if
the number present in a given volume be first
counted. In this manner Zsigmondy has
shown that the smallest particles of colloidal
gold which can be individually distinguished
with bright sunlight, are approximately 5 /*/*
in diameter, that is, five-millionths of a milli-
meter; still smaller particles exist but they
produce only an unresolvable light cone.
Magnified 1,000,000 times such a tiny gold
particle would be about J inch in diameter,
while a human red blood corpuscle would be
about 25 feet across, and a hydrogen molecule
a speck barely visible. The gold particles in
the unresolvable light cone must therefore
THE ULTRAMICROSCOPE 23
closely approach molecular dimensions. In
fact, by allowing amicrons to grow into visi-
bility in a suitable solution and then counting
them, Zsigmondy has recently shown that
some of the particles of colloidal gold have
a mass of 1-5. 10~ 16 mg., indicating a size of 1.7
to 3 jjifji.
Various other types of ultramicroscopes,
mainly modifications of dark field illumination,
have been developed by Cotton and Mouton,
Ignatowski (made by Leitz), Siedentopf (car-
dioid condenser, made by Zeiss) and others,
and besides being useful in examining colloidal
solutions, they have enabled pathologists to
see and discover ultramicroscopic bacteria
(spirochetes, infantile paralysis).
Bausch & Lamb Optical Co. of Rochester,
N. Y., are now producing a useful ultramicro-
scope.
CHAPTER V!
General Properties of Colloids
The optical properties of colloids and their
simulation of chemical compounds have been
already referred to. The other general proper-
ties of colloids may be considered under the
following headings:
1. Colloidal Protection.
2. Dialysis, Ultrafiltration and Diffusion.
3. Electric Charge and Migration.
4. Pectization (Coagulation) and Peptiza-
tion.
COLLOIDAL PKOTECTION. A most inter-
esting and important fact regarding reversible
colloids is that they can communicate their re-
versible property to irreversible colloids. The
addition of gelatin (as little as 0.0001 per cent)
to a solution of colloidal gold protects the latter
against coagulation upon the addition of
electrolytes, and permits it to redissolve after
desiccation. Ultramicroscopic examination
has shown that the gelatin does not affect the
24
GENERAL PROPERTIES OF COLLOIDS 25
motility of the gold particles, thus disposing of
the suggestion of Lobry de Bruyn that it acts
by decreasing their motion. The idea ad-
vanced by Miiller (Ber., 1904, 37, 11) that
gelatin acts by increasing the viscosity and
thus preventing the deposition of suspended
particles is disproved by the fact that quince
kernel gum, notwithstanding its viscosity,
exercises no protective action,* whereas the
small quantities of gelatin necessary to pro-
duce this effect cannot appreciably increase
the viscosity, and actually permit gold par-
ticles to settle if they are large enough to do so.
The action of reversible colloids in opposing
group formation, is of great technical impor-
tance, for in many cases it permits them to
hinder, modify and even prevent coagulation,
precipitation and crystallization.
DIALYSIS. Colloid solutions possess a small
but definite diffusibility through colloidal septa
(parchment paper, bladder) as was recognized
by Graham, who found that "tannic acid
passes through parchment-paper about 200
times slower than sodium chlorid; gum arabic
* According to Zsigmondy, quince kernel gum acts as a
protector with some substances.
26 COLLOID CHEMISTRY
400 times slower/' Graham's original form of
dialyzer may be made from a wide-mouthed
bottle whose bottom has been removed.* The
mouth is closed by a piece of bladder or parch-
ment paper tightly bound on, the solution to
be dialyzed is poured in, and the bottle im-
mersed about halfway in water contained in a
larger vessel. Most of the crystalloids diffuse
through the membrane into the outer water,
which should be frequently renewed, while
most of the colloids remain in the original
bottle, and may be thus obtained in a purified
condition. Improved modern dialyzers con-
sist of parchment or collodion sacs or thimbles,
or even of whole bladders, which have the
advantage of a larger dialyzing surface.
ULTRAFILTRATION. H. Bechhold found that
he could make filtering membranes of varying
degrees of permeability by forming them from
jellies of varying concentration. He used prin-
cipally collodion dissolved in glacial acetic acid
and afterward immersed in water, and gelatin
jellies hardened in ice-cold formaldehyde. The
jellies were formed and hardened on pieces of
filter paper, which were supported from below
* A lamp chimney will answer very well.
GENERAL PROPERTIES OF COLLOIDS 27
by nickel wire cloth, and clamped between
two flanges. The liquid to be subjected to
ultrafiltration is introduced in the chamber
thus formed and forced through the prepared
septum by appropriate pressure, which may
run up to 20 atmospheres or more and may
be produced by a pump or by compressed gas
(air, nitrogen or CO 2 ). Table VII (p. 28), pre-
pared by Bechhold, shows various colloids ar-
ranged in order of the diminishing size of
their particles in solution, and was obtained
by using ultrafilters of varying degrees of
porosity or permeability.
By means of ultrafiltration through ultra-
filters of appropriate permeability, not only
may colloids be separated from crystalloids,
but colloids having particles of different sizes
may be separated from each other.
DIFFUSION. Diffusion through a septum
is, of course, involved in dialysis. If, however,
diffusion occurs into a jelly, many interest-
ing phenomena may develop, especially if the
jelly adsorbs arty of the diffusing substances
or contains substances which can react with
them.
Owing to the enormous surface they present,
28 COLLOID CHEMISTRY
TABLE VII
Suspensions.
Prussian blue.
Platinum sol (made by Bredig's method).
Ferric oxid hydrosol.
Casein (in milk).
Arsenic sulphid hydrosol.
Colloidal gold hydrosol (Zsigmondy's No. 4, particles
about 40 MM).
Colloidal bismuth oxid (Paal's "Bismon").
Colloidal silver (Paal's "Lysargin").
Colloidal silver (von Heyden's "Collargol," particles about
20 MM).
Colloidal gold hydrosol (Zsigmondy's No. 0, particles
about 1-4 MM).
Gelatin solution, 1 per cent.
Hemoglobin solution, 1 per cent (molecular weight about
16,000).
Serum albumin (molecular weight about 5000 to 15,000).
Diphtheria toxin.
Protalbumoses.
Colloidal silicic acid.
Lysalbinic acid.
Deuteroalbumoses A.
Deuteroalbumoses B (molecular weight about 2400).
Deuteroalbumoses C.
Litmus.
Dextrin (molecular weight about 965).
Crystalloids.
colloidal gels exhibit a powerful adsorptive
action. In fact, even when percolated through
such a relatively coarse-grained septum as sand,
most solutions issue with a materially reduced
GENERAL PROPERTIES OF COLLOIDS 29
content of solute, and benzopurpurin solutions
may be thus decolorized. Further, if a solute
hydrolyzes into ions having different degrees
of adsorbability or different rates of diffusibil-
ity, they may be actually separated by diffusion
through a colloidal gel.
This phenomenon is nicely exhibited by
what may be termed a " patriotic test tube,"
prepared by filling a tube about two-thirds full
with a slightly alkaline solution of agar contain-
ing a little potassium ferrocyanid and enough
phenolphthalsin to turn it pink. After the agar
has set to a firm gel, a solution of ferric chlorid
is carefully poured on top, and almost instantly
the separation becomes evident. The iron
forms with the ferrocyanid a slowly advancing
band of blue, before which the more rapidly
diffusing hydrochloric acid spreads a white
band as it discharges the pink of the indicator.
After the lapse of a few days the tube is about
equally banded in red, white, and blue.
Even then the tubes do not cease to be of
interest, for if they are allowed to stand several
weeks the pink color is all discharged and there
develop peculiar bands or striations of blue,
apparently due to the fact that the iron ferro-
30 COLLOID CHEMISTRY
cyanid temporarily blocks the diffusion pas-
sage, which are gradually opened again after
a layer of the blue salt has diffused on from the
lower surface.
f Not only may ions be thus separated, but if
two solutes in the same solvent possess differ-
ent rates of diffusion or different degrees of
adsorbability, they also may be separated from
each other by diffusion through a colloidal gel
or septum. (Differential Diffusion.)
ELECTRIC CHARGE AND MIGRATION. The
particles of practically all colloidal solutions
possess an electric charge, and under the
influence of an electric current (difference of
potential) move toward the electrode having
the opposite charge. (Electrophoresis.) In
general, when two substances are brought into
contact, the one having the higher dielectric
constant becomes positively charged, whereas
the one with the lower dielectric constant
becomes negatively charged (Cohen's Law).
Since water has a high dielectric constant (80),
most substances suspended in pure water
become negatively charged and wander to the
anode. On the other hand if suspended in oil
of turpentine, which has a low dielectric
GENERAL PROPERTIES OF COLLOIDS 31
constant (2.23), they become positively charged
and wander to the cathode.
If, however, electrolytes are present, Coehn's
law is superseded by other controlling factors,
such as the adsorption of ions, which may give
their charge to the suspended particles. In
fact Hardy found that in pure water albumen
was amphoteric; in the presence of a trace of
alkali it acquired a negative charge and
migrated to the anode; but a trace of acid gave
it a positive charge and it then migrated to the
cathode. The following table shows the usual
charge and migration tendency of a number
of aqueous colloidal solutions.
Charged + Charged
Migrate to Cathode (- Pole) Migrate to Anode (4- Pole)
1. Hydrates of Fe, Cu, Cd, Al, Zr, 1. Sulphids of As, Sb, Cu, Pb, Gd.
Ce, Th. Halides of Ag.
2. Titanic acid. 2. Stannic acid, silicic acid.
3. Colloidal Bi, Pb, Fe and Gu 3. Colloidal Pt, Au, Ag, and Hg,
(Bredig's method). I, S, Se.
4. Albumen, hemoglobin, agar. 4. Gum arabic, soluble starch,
gamboge, mastic, oil emulsion.
5. Basic Dyes: Methyl violet, 5. Acid Dyes: Eosin, fuchsin,
Bismarck brown, methylen anilin blue, indigo, soluble
blue, Hofmann violet. Prussian blue.
PECTIZATION AND PEPTIZATION. Briefly
stated pectization means the coagulation of a
colloidal sol, and peptization its redispersion.
If a small quantity of an electrolyte is added
32 COLLOID CHEMISTRY
to a pure ruby red colloidal gold solution, the
latter changes to a blue or violet color, and
deposits its gold as a fine blackish coagulum
or precipitate.* By watching in the ultra-
microscope the coagulation of very dilute
milk by dilute acid, the individual particles of
the colloidal casein may be seen to gather
gradually together into groups, whose motion
becomes progressively less as their size in-
creases, until they are no longer able to stay
afloat, and finally coagulate in large grape-like
clusters. Hardy believes that the particles of
colloids adsorb the oppositely charged ions of
electrolytes present; at the isoelectric point
(that is when there is no excess either of posi-
tive or negative charges on the particles) coag-
ulation occurs. If, however, an excess of elec-
trolyte be added all at once, the isoelectric
point may be passed before coagulation occurs,
and the particles acquire a charge opposite to
the one they had originally. Under such con-
ditions, no coagulation may result.
* The amount in milligrams of protective colloid just
sufficient to prevent the change to violet of 10 cc. of bright
red colloidal gold solution by the addition of 1 cc. of a 10
per cent solution of NaCl, is called the "gold figure" or "gold
number" of the protector.
GENERAL PROPERTIES OF COLLOIDS 33
Burton epitomizes the difference in action
of various electrolytes as follows: "Two
remarkable results are evident on comparing
the coagulative powers of various electrolytes
on colloids of different kinds; first, the coagu-
lation depends entirely on the ion bearing a
charge of sign opposite to that of the colloidal
particle; and, second, with solutions of salts,
trivalent ions have, in general, immensely
greater coagulative power than divalent ions,
and the latter, in turn, much greater than
univalent. Acids and alkalis in particular
cases act more strongly than the corresponding
salts."
High-tension electric discharges may also
effect the coagulation or precipitation of a
finely subdivided or dispersed phase; which
fact was utilized by Sir Oliver Lodge in dis-
pelling fogs, and by Cottrell hi coagulating
smelter and similar fumes.
PEPTIZATION. So strong is the analogy
between digestion and colloidal disintegration
that Thomas Graham, the father of colloid
chemistry, coined the word peptization to
express the liquefaction of a gel. He first
speaks of the coagulation or pectization of
34 COLLOID CHEMISTRY
colloids. "The pectization of liquid silicic
acid," he states, "and many other liquid
colloids is effected by contact with minute
quantities of salts in a way which is not under-
stood. On the other hand, the gelatinous acid
may be again liquefied, and have its energy
restored by contact with very moderate
amounts of alkali. The latter change is
gradual, 1 part of caustic soda, dissolved in
10,000 water, liquefying 200 parts of silicic
acid (estimated dry) in 60 minutes at 100
degrees. Gelatinous stannic acid also is easily
liquefied by a small proportion of alkali, even
at the ordinary temperature. The alkali, too,
after liquefying the gelatinous colloid, may be
separated again from it by diffusion into water
upon a dialyzer. The solution of these col-
loids in such circumstances may be looked
upon as analogous to the solution of insoluble
organic colloids witnessed in animal digestion,
with the difference that the solvent fluid here
is not acid but alkaline. Liquid silicic acid
may be represented as the 'peptone' of
gelatinous silicic acid; and the liquefaction of
the latter by a trace of alkali may be spoken of
as the peptization of the jelly. The pure
GENERAL PROPERTIES OF COLLOIDS 35
jellies of alumina, peroxide of iron and titanic
acid, prepared by dialysis, are assimilated
more closely to albumen, being peptized by
minute quantities of hydrochloric acid."
Peptization is in reality deflocculation, a
dispersion of groups into separate particles
which once more acquire active motion and
remain afloat or in solution. The detergent
action of soap and dilute alkalis is due to the
fact that they deflocculate adhering particles
of " dirt."
CHAPTER VI
Practical Applications of Colloid Chemical
Principles
The practical applications of colloid chemis-
try are so manifold and widespread that they
touch every branch of science and technology.
Whole books may be and have been written on
many of the most restricted fields, while the
scientific literature teems with monographs
and articles, directly on, or applicable to,
colloid-chemical subjects. In what follows, it
will be possible therefore to give not an ex-
haustive, but only a most general survey,
intended rather to show the ubiquity of col-
loid phenomena; and many important topics
must be dismissed with a most rudimentary
discussion, altogether incommensurate with
theu* importance.
ASTRONOMY. As matter in colloidal state
is so common on our relatively minute earth, it
is but natural to expect to find many instances
of colloidal dispersion in the immensity of the
Universe.
36
PRACTICAL APPLICATIONS 37
Cosmic dust Is widely distributed throughout
space, and as it is gathered up by the superior
attraction of the larger heavenly masses (suns,
planets, etc.), which in any system grow at the
expense of the smaller masses, fresh quantities
are continually produced by the collisions of
bodies in space, as well as the disintegration
of meteorites, comets, asteroids, etc.
The tails of comets seem to consist almost
entirely, and the nuclei and coma largely, of
colloidally dispersed matter. The great comet
of 1882 which made a transit of the sun, was
invisible against the solar disc (a position corre-
sponding to attempted observation of colloidal
particles in the ordinary microscope against a
luminous background), but became visible
again after passing beyond the sun's disc (a
position corresponding to successful observa-
tion of the same colloidal particles in the ultra-
microscope against a dark background, the eye
of the observer being protected from the source
of illumination).
The streaming of the cometary tails away
from the sun may be due to the ionization of
the constituent colloidal particles, and their
consequent electrical repulsion; or more prob-
38 COLLOID CHEMISTRY
ably, it may be due to the sun's rays, as was
pointed out by J. Clerk Maxwell. The inten-
sity of the action of the sun's rays on a particle
depends upon its surface, which varies as the
square of its diameter, whereas the gravitation
of the same particle to the sun depends upon its
mass, which varies as the cube of its diameter.
Theoretically in the case of a particle whose
density equals that of water, the repulsion
balances gravitation when the diameter reaches
0.0015 mm. (= 1.5 /*). As the diameter di-
minishes the repulsive force gains the ascend-
ancy, soon reaching a maximum and again
diminishing, until when the particle has a
diameter of only 0.00007 (= 70 MM) the two
forces again balance each other.*
These figures, which refer to a substance hav-
ing the density of water, are approximately of
colloidal dimensions; but in the case of denser
bodies the subdivision would be even more
profound. It is therefore not surprising that,
when the earth recently passed through the tail
of a comet, no disturbance of any kind was
* See Simon Newcomb's article on "Comet," Encyclo-
pedia Britannica, llth edition. Also Svante Arrhenius,
" Worlds in the Making," Harper & Bros.
PRACTICAL APPLICATIONS 39
noticed. The comet's tail is a vast celestial
camouflage its luminosity a macroscopic Far-
aday-Tyndall effect.
The nebulae, too, apparently consist of finely
dispersed matter, rendered luminous by neigh-
boring suns; although with them as with the
comets, a small part of the light may result
from self -luminescence (incandescent gas, etc.).
METEOROLOGY. What we commonly call
" weather conditions " are largely dependent
upon the degree of dispersion of water in the
atmosphere, and this dispersion is mainly
effected and maintained by solar heat and
electrical energy. When air carrying water
vapor is chilled by rising to a higher level,
meeting a colder mass of air, or even by the
alternation of night and day, the moisture
it contains assumes the colloidal state as cloud,
fog or mist; and as the coagulation of the
dispersed water proceeds, these in turn may
condense still further into dew, rain, snow or
hail, depending upon conditions. When the
dispersed water aggregates, there is naturally
set free the energy originally used in its disper-
sion, and this may appear as electricity (light-
ning) especially if the aggregation occurs
4:0 COLLOID CHEMISTRY
suddenly as is the case in thunder and hail
storms. We have all noticed how a nearby
lightning flash is promptly followed by an
increased fall of raindrops.
Were it not for our atmosphere, the sun would
appear to us like a fiery ball set in a black star-
sprinkled sky. The blue color of the sky is due
to diffraction of the sunlight by the earth's
atmosphere, a gigantic Tyndall effect. If we
look edgewise through a clear sheet of glass, we
at once notice the green color due to colloidally
dispersed iron, and in like manner, if we look
through a great length of the atmosphere the
prevailing color is blue. As the poet Campbell
beautifully puts it :
!< Tis distance lends enchantment to the view,
And robes the mountain in its azure hue."
After the tremendous explosive eruption of
the volcano Krakatoa in 1883, colloidal dust
and ashes were projected so high that they
gradually spread around the earth, causing
" golden sunsets."
We do not know to what extent electrical
conditions on the earth affect the dispersion of
substances in its atmosphere; but since half
of the earth is always heated by the sun while
PRACTICAL APPLICATIONS 41
the other half is cooler, thermoelectric currents
are continually circulating about the earth.
Variations in solar radiation due to sun-spots
and the like, cause violent electric and magnetic
storms which are intimately connected with the
aurora, and other atmospheric phenomena
(ionization, electrical charge of dispersed par-
ticles); and it is well known that sun-spots
exercise a potent influence on the weather.
GEOLOGY AND MINERALOGY. - - The ordi-
nary properties of the solid constituents of the
earth's crust depend more upon their state of
physical subdivision than upon their chemical
constitution. Atterberg classified the frag-
ments of minerals and rocks as follows:
Diameter.
Boulders 2 m. to 20 cm.
Pebbles 20 cm. to 2 cm.
Gravel 2 cm. to 2 mm.
Sand 2 mm. to 0.2 mm.
Earth 0.2 mm. to 0.02 mm.
Loam 0.2 mm. to 0.002 mm.
Clay smaller than 0.002 mm.
The smaller the particles, the greater their
capillarity and the ease with which they are
moved by wind and by water, but the less
their permeability to water. Fine defloccu-
42 COLLOID CHEMISTRY
lated clay is carried thousands of miles by
rivers until it is finally coagulated by the salts
of the ocean, as may be observed in the deltas
of the Ganges, Nile and Mississippi. Fine
particles are easily cemented by pressure or
igneous action into rocks (e.g., sandstone,
slate), or may act as a cement for large par-
ticles (e.g., pudding-stone) or as a matrix for
fossils.
Many minerals are themselves colloidal gels
(e.g., opal, flint, bauxite) or result from the
weathering of other minerals with consequent
gel formation (e.g., kaolin from kaolinite,
serpentine from diabase). Most gems owe
their colors to impurities colloidally dispersed
within them (e.g., ruby, emerald, amethyst).
Dendrites are formed by solutions diffusing
through mineral gels. Colloidal minerals usu-
ally adsorb, and are dyed by aniline dyes
(methylen blue), whereas crystalloid minerals
are unaffected.
CLAY AND CEEAMICS. The effect of vege-
table extractive matters on the working
properties of clay have been known from
ancient times in the Bible (Exodus V) it
is mentioned that brick cannot be made with-
PRACTICAL APPLICATIONS 43
out straw. Recently patents have been taken
out for " Egyptianizing " clay by adding to it
tannin, extract of straw, humus and the like.
Glue and similar protective colloids defloccu-
late or "free out " clay and make it " cover "
in paper-coating and kalsomining. The work-
ing properties of clays depend largely upon the
size of their constituent particles and their
state of aggregation. This is especially evi-
dent in ceramics. Articles molded of clay and
then burned, lose their hydrosol condition and
become hardened into pottery.
AGRICULTURE. Although from time im-
memorial farmers have classified soils on the
basis of their physical and physiological
character as "light " or "heavy," "rich " or
"poor," "productive" or "unproductive,"
etc., it is only within comparatively recent
years that chemists have begun to realize the
full importance of the role played by the
colloids, especially the organic colloids of the
soil.
Many important properties of soils, such as
permeability, capillarity, absorption, moisture
content, etc., are dependent not so much upon
the chemical composition as upon the size of
44 COLLOID CHEMISTRY
the constituent soil particles. (See Atterberg,
Schwed, landw. Akad., 1903, and Chem. Zeit..
1905, 29, 195; Patten and Waggaman, U. S.
Dept. of Agri. Bureau of Soils, Bull. No. 52,
1908). In coarse sand, for example, the
amount of water is greatest at the bottom and
smallest at the top, whereas in fine clay the
distribution is much more uniform.
Among the natural agencies tending to
increase the size of the minute soil particles
may be mentioned heat with its drying or
evaporative effect, freezing, and the coagulat-
ing or flocculating action of soluble inorganic
salts and some organic substances present in
the soil. On the other hand, included in that
little known class of substances vaguely de-
scribed as " humus," there are numerous
organic substances derived from the bacterial,
plant, or animal debris, or exuded by the roots
of plants, which act as protective colloids
(Schutzkolloide) and tend to produce and
maintain the hydrosol, or deflocculatd con-
dition. (See P. Ehrenberg, "Die Kollide des
Ackerbodens," Zeits. angew. Chem., 1908,
41, 2122.) In an excellent paper on the
mechanics of soil moisture, L. J. Briggs (U. S.
PRACTICAL APPLICATIONS 45
Dept. of Agric., Bureau of Soils, Bull. No.
10, 1897) pointed out that very small quantities
of certain organic substances, such as are con-
tinually being produced in the soil by the
decay of organic matter, greatly decrease the
surface tension of solutions, thus counteracting
to a large extent the effects of the surface
application of soluble salts which would tend
to draw moisture to the surface by increasing
the surface tension of the capillary water of
soils. It is well known, however, that an
excess of salts will ruin a soil physically, as is
evident after flooding by sea water or the
excessive application of chemical fertilizers.
Of interest in this connection is the recent work
of the Bureau of Soils, U. S. Department
of Agriculture, carried out by Cameron,
Schreiner, Livingston and their co-workers.
Thus plants grown in the unproductive Ta-
koma soil, were greatly benefitted by green
manure, oak leaves, tannin and pyrogallol.
The injurious effects of quinone and some
other organic substances may be due to their
ability to precipitate or flocculate the pro-
tective colloids of the soil; for as Lumiere
and Seyewetz have shown (Bull. Soc. Chim.,
46 COLLOID CHEMISTRY
1907, 4, 428-431; J. S. C. L, 1907, 703)
quinone renders gelatin insoluble.
The fact observed by Fickenday (J. Landw.,
1906, 64, 343) that more alkali is required to
flocculate natural clay soils than kaolin sus-
pensions, he attributes to the protective action
of the humus present (see Keppeler and Spang-
enberg, J. Landw., 1907, 55, 299).
A. S. Cushman, in his excellent work upon
the use of feldspathic rock as fertilizer (U. S.
Dept. of Agriculture, Bureau of Plant Indus-
try, Bulletin No. 104; Cushman and Hubbard,
J. Am. Chem. Soc., 30, 779), has shown that
the fine grinding of feldspar increases the
amount of potash available under the action of
water. Thus, a coarse powder having an
area of 43 sq. cm. per cc. of solid feldspar
yielded 0.013 per cent, whereas a fine powder
whose area was 501,486 sq. cm. per cc. yielded
0.873 per cent of potash and soda. These fine
particles averaged about 0.1 /* in diameter,
which is relatively large as compared with
colloidal dimensions; but under the action of
physical and chemical soil agencies they
undergo further disintegration, finally reaching
a colloidal condition in which still more of
PRACTICAL APPLICATIONS 47
their potash is available, a condition favored
and maintained by the organic protective
colloids of the soil.
With these brief and inadequate remarks we
must dismiss this subject of such vast impor-
tance and fascinating interest, referring to the
extensive literature, much of which is quoted
in Bulletin No. 52 and the other publications
of the Bureau of Soils.
ELECTROPLATING AND ELECTRODEPOSITION OF
METALS. The addition of protective colloids
to electroplating baths tends to the production
of fine-grained non-crystalline deposits. A. G.
Betts in a paper entitled "The Phenomena of
Metal Depositing " (J. Am. Electrochem. Soc.,
1905, 8, 63) has shown that there are many
factors influencing the action of the colloid,
and has suggested a number of possible ex-
planations. The correct explanation, how-
ever, has been given by Mtiller and Bahntje
(Z. Elektrochem., 1906, 12, 317; J. S. C. I.,
1906, 484) who state that the added colloid
keeps the deposited metal (copper) in an
amorphous, non-crystalline condition, gelatin
producing the most powerful effect, egg al-
bumen considerably less, while gum and
48 COLLOID CHEMISTRY
starch have comparatively little action. They
also found that the deposited copper weighed
about 0.2 per cent more than under normal
conditions, indicating that some of the colloid
had been carried down with the metal.
The relative efficiency of the colloids just
referred to corresponds to their relative effi-
ciency in protecting from coagulation solutions
of colloidal gold (see Zsigmondy, J. S. C. I.,
1902, 192; also Colloids and the Ultramicro-
scope, p. 81), which is additional evidence
that we have another instance of protective
colloidal action, by which the crystallization
forces of the metal are powerfully influenced.
METALLURGY. Since coarsely crystalline
metals are brittle, tending to split along the
lines of crystal cleavage, various physical and
chemical means are employed in technical
practice to obtain a hard, fine-grained struct-
ure. (See I. Langmuir, Iron & Steel Inst.,
Sept. 1907; J. S. C. I., 1907, 1094.) Among
the physical methods are chilling and rolling,
while the chemical methods involve the re-
moval of undesirable constituents (as in the
conversion of pig iron into steel) or the addi-
tion of desirable constituents (as in the case-
PRACTICAL APPLICATIONS 49
hardening and the manufacturing of " chrome
steel/' " nickel steel/ 7 etc.). For example,
P. Putz has shown (J. S. C. L, 1907, 614) that
the predominant effect of vanadium in steel is
to decrease the size of the ferrite grains and
make the material tougher; it renders the
ordinary structure due to pearlite fine-grained
and homogeneous (see also Beilby, Proc. Roy.
Soc. A., 79, 463; J. S. C. L, 1907, 926).
Now, while the question is one of very great
complexity, many of the facts at present
available seem to indicate that one of the
causes favoring the fine-grained structure is
the inhibition of crystallization by substances
colloidally dissolved in the molten mass. Thus
part of the carbon in iron and steel exists in the
graphitic form, and as graphite is slightly soluble
in iron (see C. Benedicks, Metallurgie, 1908,
5, 41; J. S. C. L, 1908, 406); some of it will,
under proper conditions, be found in colloidal
form (Carnegie Research Reports, J. S. C. L,
1908, 27, 570; F. Wust, J. S. C. L, 1907, 26,
412; Hersey, J. S. C. L, 27, 531). Besides
metals may dissolve each other and other
substances colloidally, but in the case of ordi-
nary metals this is not easy to demonstrate.
50 COLLOID CHEMISTRY
An observation recently made by J. Alex-
ander * is of interest here. Moissan (Comptes
rend., 144, 593, J. S. C. L, 1907, 413) has noted
that the addition of a little platinum to me-
tallic mercury causes the latter to " emulsify "
in water. Upon making up such an " emul-
sion," Alexander noticed that the supernatant
fluid remained turbid upon standing, and
therefore examined the fluid in the ultrami-
croscope, which revealed the presence of col-
loidal metallic particles in active motion.
DYEING. The difference between a physi-
cal mixture and a chemical compound is
frequently illustrated by dissolving out the
sulphur from a mixture of iron filings and
sulphur dust, and showing that the solvent,
carbon bisulphid, does not affect the compound,
ferrous sulphid. That in many cases dyeing
is due, not to chemical combination, but to an
adsorption f of the dye by the colloidal fiber, is
evident from the fact that some dyestuffs can
be extracted from the dyed fiber by means of
alcohol. Investigation has shown that many
* J. S. C. I., 1909, 28, 280.
t In some cases adsorption may be followed by undoubted
chemical combination.
PRACTICAL APPLICATIONS 51
dyes are colloidal in solution, and the selective
coloring of various fibers, tissues, cells, nuclei,
etc., is probably due to selective adsorption or
precipitation of one colloid by another. The
ultramicroscopic researches of N. Gaidukov
(Zeitsch. f. angew Chem., 21, 393) support this
view.
The phenomena of dyeing are rather numer-
ous and complicated, for the dyestuffs are
numbered by thousands, and the various fibers,
tissues, etc., such as cotton, silk, wool, linen,
jute and straw, all react characteristically. In
some cases the colloid fiber adsorbs the dye, as
with basic colors which dye silk and wool
directly; in other cases there is necessary a
mordant which is first adsorbed and then fixes
the color. Certain colors mutually precipitate
each other and may in fact serve as mordants
for each other, e.g., methylen blue and dianil
blue 2 R.; patent blue V and magenta.
Colloid chemistry also throws much light
upon many obscure points in the practical art
of dyeing. It is possible to obtain much more
level colors in old dye liquors than in fresh
ones, and here it seems that colloidally dis-
solved substances are responsible, exercising
52 COLLOID CHEMISTRY
a restraining action upon the absorption of the
color. The addition of Glaubers' salt facili-
tates level dyeing, probably by its action as an
electrolyte, producing a partial coagulation of
the dyestuff, so that the particles of the latter,
thereby made larger, are absorbed more slowly
and evenly.
SOAP. In a comprehensive paper entitled
" Modern views on the constitution of soap "
(see J. S. C. L, 1907, 26, 590) Lewkowitsch
epitomizes the views of Merklen substantially
as follows: " Commercial soap is a product
having an essentially variable composition
dependent upon (1) the nature of the fatty
acids, (2) the composition of the 'nigre' (in
the case of settled soaps), (3) the tempera-
ture at which the boiling is conducted; it
behaves like a colloid and should not be re-
garded as a compound of sodium salts of fatty
acids, with which a definite amount of water is
combined chemically, but rather as an 'ab-
sorption-product' whose composition is a func-
tion of the environment in which the salts of
the fatty acids happen to be at the moment of
the finishing operations. "
Merklen's views conflict with the views as to
PRACTICAL APPLICATIONS 53
the chemical composition of soap previously
advanced by Lewkowitsch, who states, in
conclusion: "But whatever may be the out-
come of renewed experiments, Merklen's views
cannot fail to stimulate further research into
the composition of soap, and thus help to raise
the industry of soap-making, which has too
long been looked upon as a mere art, to the
rank of a scientifically well-founded industry,
the operations of which are governed by the
laws of mass action, the phase rule and the
modern chemistry of colloids. "
The colloidal nature of soap solutions is
indicated by their turbidity and their gelatin-
ization. That the detergent action of soap is
consequent upon its deflocculating effect was
brought out in the interesting Cantor Lecture
of H. Jackson (J. Soc. Arts, 55, 1101 et seq.),
who examined microscopically the supernatant
fluid resulting from washing a dirty cloth with
soap and water, and found in it countless
particles in a state of oscillatory motion
("pedesis ") When an individual fiber was
bathed in soap solution, the dirt particles
gradually loosened and began to oscillate;
upon substituting salt solution for the soap,
54 COLLOID CHEMISTRY
the particles flocculated and the motion
ceased. An ultramicroscopic examination of
the detergent effects produced by soap should
prove of interest.
In this connection mention must be made
of the excellent paper of W. D. Richardson
on "Transparent Soap" (J. Amer. Chem.
Soc., 30, 414), which he terms a supercooled
or supersaturated solution, having distinctly
crystalline tendencies and exhibiting colloidal
properties. Having in mind the fact that
the salts of the higher fatty acids dissolve
in water as colloids, and in alcohol as crystal-
loids (S. Ya. Levites, Zeits. Chem. Ind. Kol-
loide, 2, 208, et seq., J. S. C. L, 1908, 1134;
Mayer, Schaeffer, and Terroine, Compt. rend.,
146, 484) and also the fact that the alcohol or
equivalent solvents (glycerol, sugar, etc.) are
used in transparent soap, it seems probable
that the crystals which frequently form in it
are due to the slow separation of such part of
the soap as is in crystalloid solution. This
view is supported by the fact adduced by
Richardson (loc. cit., p. 418) that the fatty
acids separated from the crystals had a higher
melting point than those separated from the
PRACTICAL APPLICATIONS 55
clear matrix. The isolation of the crystals
was difficult because of their ramifying tend-
ency, which recalls some of the crystal figures
exhibited by some mixtures of crystalloids and
colloids. What may be called the crystalloid
phase of soap is apparently governed by the
same factors as those which Tamman has
pointed out as governing the crystallization of
supercooled solutions, i.e., 1st, the specific power
of crystallization; 2nd, the speed of crystalliza-
tion; 3rd, the viscosity (see Zsigmondy, Colloids
and the Ultramicroscope, p. 128 et seq.). Thus,
gold ruby glass when quickly cooled (or super-
cooled) is colorless, but acquires a red color
upon reheating to the softening point. By
ultramicroscopic examination Zsigmondy
showed that the nuclei of metallic gold, which
in the colorless glass were amicroscopic, grew
into ultramicroscopic visibility in the red glass.
It therefore seemed to the author that a most
important factor in determining the trans-
parency of transparent soap would be the
speed of cooling, and some experiments were
made along this line.
A piece of commercial transparent soap was
melted and cast into two cups, one of which
56 COLLOID CHEMISTRY
was quickly chilled in ice, while the other was
allowed to cool slowly by immersion in hot
water. The quickly cooled piece was trans-
parent, while the other was practically opaque,
and showed upon ultramicroscopic examina-
tion much larger ultramicrons than the trans-
parent piece.
After standing three or four months, the
quickly cooled soap was still transparent to
the naked eye, whereas large opaque spots
could be seen in the slowly cooled piece. In
the ultramicroscope the former appeared as
before, whereas the latter showed large and
perfectly resolvable crystals in a clear matrix.
These experiments give us an inkling as to
what occurs during the " heat treatment " and
tempering of metals, and it is to be hoped that
some technique may be devised that will give
us even a clearer insight than does " etching,"
into the changes that occur in metals in metal-
lurgical operations (heat treatment), use, age,
and even " disease " (tin for example).
MILK. From a colloid chemical stand-
point, the main constituents of milk may be
classified as follows:
PRACTICAL APPLICATIONS
57
In crystalloid
dispersion
salts (such as NaCl, etc.)
sugar (lactose).
In colloidal C casein an unstable or irreversible colloid.
dispersion ( lactalbumin a stable or reversible colloid.
In suspension* milk fat.
Most formulas and recipes for modifying
cows' milk for infant feeding, and for that
matter, many analyses, combine the percent-
ages of lactalbumin and of casein under the
collective title of " total proteids," thereby
obscuring the highly important fact that the
lactalbumin stabilizes and protects the casein
from coagulation by acid and rennin.f
The subjoined table will show how milks
are influenced by a difference in the ratio
between the casein and lactalbumin.
AVERAGE COMPOSITION
Lact-
Kind of milk.
Casein.
albu-
Fat.
Behavior with
acid.
Behavior with
rennin.
min.
Cow
3 02
53
3 64
Readily coag-
Readily
ulates.
coagulates.
Woman
1.03
1 26
3 78
Not readily
Not readily
coagulated.
coagulated.
Ass
67
1 55
1 64
* It is probable that some of the fat is in colloidal dispersion,
t See Alexander and Bullowa, Jour. Am. Med. Assoc., Vol.
LV, p. 1196 (Oct. 1, 1910).
58 COLLOID CHEMISTRY
It is interesting to note that the milks in the
above table are arranged in order of their
digestibility, which also corresponds with their
relative colloidal protection. Thus Jacobi has
stated that asses' milk has always been
recognized as a refuge in digestive disorders in
which neither mother's milk nor cow's milk or
mixtures were tolerated.
The addition of protective colloids to cows'
milk stabilizes it and makes it act more like
mother's milk when treated with acid and
rennin. In fact, if sufficient protective colloid
be added, coagulation of the casein in the
stomach may be entirely prevented, or at least
the coagula kept in a very fine state of sub-
division.
The action of protective colloids is beauti-
fully illustrated in the ultramicroscope, which
enables us to see the individual particles of
cows' casein in active motion and watch the
course of their coagulation by acid, first into
small and then into larger and larger groups,
whose motion decreases as their size increases,
until finally they sink out of solution in coagu-
lated masses. If, however, some gelatin or
gum arabic solution be added to the cows'
PRACTICAL APPLICATIONS 59
milk before the addition of the acid, the casein
particles continue their active dance and do
not coagulate. In this connection it is in-
teresting to note that the casein particles in
mother's milk appear to be much smaller than
those in cow's milk, probably because of the
more highly protective medium in which they
are formed and exist.
Although their method of action was not
perfectly understood, protective colloidal sub-
stances have for years been used in the modi-
fication of cow's milk for infants. For over
thirty years Jacobi has advocated the addi-
tion of gelatin and gum arabic to cow's milk
and infant's diet, and the use of gruels, dex-
trinized starch and similar reversible colloids
is familiar to all. It is interesting to note that
sodium citrate, which is largely employed as
an addition to cow's milk, acts as a protective
colloid, and when going into solution actually
exhibits actively moving ultramicrons in the
ultramicroscope, a fact which indicates its
colloidal condition.
In addition to stabilizing the casein, pro-
tective colloids in milk have a very important
influence on the milk fat. In the first place
60 COLLOID CHEMISTRY
is to be considered the emulsifying and
emulsostatic action of reversible colloids. Of
much greater importance, however, is the
result of stabilizing the casein, for insufficiently
protected casein in curding carries down
mechanically most of the milk fat present,
yielding a greasy, fatty curd which is very
difficult for the digestive juices to dissolve.
ICE CREAM. It is a fact well known to
practical ice cream makers, and amply proven
by experience, that ice cream made without
eggs, gelatin or some similar colloidal ingredi-
ent, is gritty, grainy or sandy, or else soon
becomes so upon standing; whereas ice cream
made with small quantities of colloids possesses
that rich, mellow, velvety texture so much in
demand. Here the added colloid acts as an
inhibitor of crystallization or practically speak-
ing as a preserver of texture. The added
colloid, especially gelatin, which is the one
most frequently used, also serves as a protective
colloid in preventing the coagulation of casein,
apparently an irreversible hydrosol and a
normal constituent of ice cream. In view of
what has been said above, it is evident that
gelatin thus renders ice cream more digestible.
PRACTICAL APPLICATIONS 61
A very misleading impression is given by
some official food chemists referring to gelatin
in ice cream as a " filler," which naturally leads
to the idea that it is an inferior ingredient
added in quantity to cheapen the product.
But as gelatin is expensive and as but | per
cent is used, such a view is evidently erroneous.
The food value of gelatin as a protector of the
body's nitrogen being generally admitted, and
its effect in milk being very beneficial from a
digestive point of view, its use in ice cream in
the quantities referred to is necessary, legiti-
mate and scientific.
CONFECTIONARY. In gum drops, marsh-
mallows, " moonshine " and other candies, use
is made of gum arabic, gelatin, albumen, and
other colloids to prevent the crystallization of
the sugar. Thus, besides adding to the food
value, they give the candy a smooth and
agreeable taste, and preserve it in saleable
condition.
BREWING. Beer contains dextrin and al-
bumin, both colloids. In the brewing process
many factors appear which tend to coagulate
the albumen. The influence of solid surfaces
is illustrated by changing the walls of the
62 COLLOID CHEMISTRY
fermenting vessel. Thus a certain wort fer-
mented in glass or enameled vessels showed
0.2450 per cent of albumen; the same wort
fermented in a paraffin-lined vessel showed
0.1925, and in a vessel lined with pitch only
0.1750 per cent of albumen. Old-fashioned
brewers would never use any vessel unless it
had first been treated with a decoction of
malt kernels and nut leaves, or else with
"fassgelager " (barrel dregs) which acts like
the so-called "bierstein," a deposit consisting
chiefly of organic substances that forms upon
new surfaces and protects albumen from
coagulation by their influences.
The influence of fluid surfaces is evident
from the fact that in the chemical analysis of
beer, benzine, benzol, chloroform, etc., may
be used to coagulate and shake out the beer
colloids.
The formation of gas bubbles tends to
coagulate the dissolved albumen, and this fact
killed the so-called " Vacuum Fermentation
Process." The jarring due to transportation
or even to passing trains may have a deleteri-
ous effect. A slight trace of acid tends to
stabilize the albumen as do the tannin and
PRACTICAL APPLICATIONS 63
resins from the hops, the dextrins from the
mash and the inorganic colloids of calcium and
magnesium. A proper balance between the
dextrin and albumen is necessary for the
formation of a lasting foam and a desirable
"body" (Vollmundigkeit).
In America where beer is served icy cold,
the chilling produces cloudiness, consequent
upon a coagulation of albumen. This was
cleverly overcome by Wallerstein, who in-
troduced a proteolytic enzyme which increases
the degree of dispersion of the albumen and
thus prevents the clouding.
TANNING. The skins of animals (hide)
constitute an organized colloid jelly, formed of
bundles of fine fibrils, about 1 /* in diameter,
bound together by a cementing material of
similar chemical composition, which is largely
removed by the liming and other treatment,
preceding the tannage proper.
When the swollen hide is placed in the acid
tannin solution* (tan liquor), the tannin is
powerfully adsorbed by the fiber and combines
with it to form leather. It is still a moot
* In alkaline solution both the tannin and thejaide are
negatively charged and no tanning occurs.
64 COLLOID CHEMISTRY
question whether the combination is "phys-
ical " or "chemical," but since the fixation of
the tannin follows an adsorption isotherm and
is reversible in the presence of alkalis, it may
justly be called a " colloid combination"
which partakes of the nature of both. The
positively charged hide and the negatively
charged tannin mutually coagulate each other.
Gelatin when neutral and free from electro-
lytes does not precipitate pure tannin, but in
acid solution it takes a positive charge and is
tanned. The tanning process may be aided
electrically by giving the hide a suitable
potential, positive in the case of tannin and
negative in the case of chromium compounds.
RUBBER. Rubber is made by coagulating
the milky juice (latex) of various plants.
Rubber latices are emulsions stabilized by
protective colloids (proteins or peptones) and
the nature of the coagulant depends upon the
nature of the protector. Thus, formaldehyde
preserves latices whose protectors are proteins,
but coagulates Kickexia latex by precipitating
the protective peptones.
Vulcanization consists of the combination
of sulphur with rubber. At first the sulphur
PRACTICAL APPLICATIONS 65
is adsorbed; and then by heating, part of it
enters into a close combination, probably true
chemical combination.
PHOTOGRAPHY. The photographic plate
owes its sensitiveness to an " emulsion" of
colloidal silver halides stabilized by a protec-
tive colloid (gelatin, albumen or collodion).
The degree of dispersion is controlled by the
conditions of precipitation of the silver salt
and the subsequent treatment of the emulsion
(ripening). The latent image formed upon
the exposure of the plate to light is probably
an adsorption compound between colloidal
silver and the silver halides.
BOILER SCALE. In addition to containing
various salts intended to precipitate scale-
forming ingredients, most formulas for " boiler-
compounds " and scale-preventing mixtures
include such substances as glue, dextrin,
starch, potatoes, tannin, extract of hemlock,
etc. These colloids undoubtedly prevent the
formation of hard crystalline scale, either by
inhibiting to some extent the precipitation of
the scale-forming salts or by keeping the
precipitate in an extremely fine non-crystal-
line condition.
66
COLLOID CHEMISTRY
CEMENT, MORTAR AND PLASTER. When
freshly mixed, cement and mortar contain
colloidal sols or gels, which gradually coagu-
late or "set " and bind the crystalline elements
of the plaster into a coherent whole.
The setting of the plaster of Paris is delayed
by glues, gums and other colloidal substances,
and "retarders " of this character have been
in use for years. On preparing some micro-
scope slides with a mixture containing equal
parts of plaster of Paris and water, to which
had been added varying proportions of gelatin,
the following results were observed:
Per cent
gelatin.
Time to set
in minutes.
Microscopic appearance of slide.
40
Characteristic interlacing crystals of calcium
sulphate.
T&
50
No true crystals except in a few spots, where
some colloid-free solution had diffused out.
Elsewhere aborted sphero-crystals.
&
260
No true crystals.
1
910
No true crystals.
*
960
No true crystals.
1
Not set in
No true crystals.
48 hours.
2
Not set in
No true crystals.
48 hours.
FILTRATION. Successful filtration depends
upon the use of a septum or filtering medium,
PRACTICAL APPLICATIONS 67
whose pores or orifices are small enough to
hold back the particles it is desired to sepa-
rate from the fluid; or the pores may be-
come small enough by the deposit upon or in
them of some of the precipitate, or of some
added material, such as paper pulp, kieselguhr
or shredded asbestos. It is, therefore, evident
that the presence of protective colloids, by
tending to produce the finely dispersed or
"hydrosol" condition of the particles, favors
their passage through the filter. Thus a gold
hydrosol with particles of 20-30 ju^ and con-
taining albumen, passed freely through a
Pukall and a Maassen filter. In the absence
of the protective albumen, the colloidal gold
was adsorbed by the filter, gradually clogging
the pores until the filtrate, at first red, became
colorless. In technical practice, wherever
possible, a coagulated precipitate is formed,
whose large particles are held back with com-
parative ease. It is very difficult to filter glue
or gelatin solutions or precipitates formed in
the presence of protective colloids.
The successful treatment of sewage, back-
waters and trade effluents depends largely upon
the separation from them of colloidal impuri-
68 COLLOID CHEMISTRY
ties by coagulation, adsorption and filtration.
The old ABC method depended upon the use
of alum, blood and clay (whence the name) to
make a coagulum which would carry down
suspended matter. Ferrous sulphate and lime
(yielding a coagulum of ferric hydroxid) and
alum are also used as clarifiers and coagulants.
Filtration through sand, coke, etc., is made use
of to adsorb finely dispersed impurities.
Animal charcoal and fuller's earth decolorize
sugar and oils respectively, because of their
powerful adsorptive action.
CHEMICAL ANALYSIS. The presence of col-
loids, especially in technical products or solu-
tions, may lead to grave errors in analysis, so
that the chemist should destroy them by
ignition, or else nullify their effects by the
addition of a sufficient excess of coagulant or
precipitant. Reversible colloids which are
frequently referred to under the vague term
" organic matter " may act: (1) by totally or
partially preventing the formation of precipi-
tates, just as tartaric acid and tartrates prevent
the precipitation of alumina, chromic oxid, and
ferric oxid (see Yoshimoto, J. S. C. I., 1908,
27, 952); (2) by preventing the satisfactory
PRACTICAL APPLICATIONS 69
filtration of the precipitate formed (see Mooers
and Hampton, J. Am. Chem. Soc., 30, 805); (3)
by rendering precipitates difficult to wash and
purify (see Duclaux, J. S. C. L, 1906, 25, 866).
A few experiments will serve to make clear
the importance of these remarks. Three
solutions of lead acetate were taken; to the
first was added hydrochloric acid which yielded
a heavy coagulated precipitate; to the second
was added sodium chlorid (a less highly
ionized precipitant) which yielded a colloidal
precipitate of lead chlorid; to the third was
added, first, a little glue solution and then
sodium chlorid which in this case gave no
precipitate at all.
Again in the presence of glue, silver nitrate
gives with sodium chlorid only an opalescence
which passes through filter paper. Even a
large excess of hydrochloric acid fails to pro-
duce a precipitate. But upon adding silver ni-
trate solution to a chlorid solution containing no
colloid, a copious precipitation occurs at once.
PHARMACY. Colloids, such as gum arabic,
Irish moss, tragacanth, etc., are largely used
in pharmacy in the preparation of emulsions.
If ferric chlorid be added to gum arabic emul-
70 COLLOID CHEMISTRY
sion of cod liver oil, it coagulates the gum, and
the oil, no longer protected by the emulso-
static action of the gum, promptly separates
out.
Colloidal silver (collargol, argyrol), colloidal
mercury (hygrol, blue ointment), and col-
loidal sulphur (ichythol) are largely used
medicinely. Ferric salts, especially the chlo-
rid which readily hydrolyzes into the hydrate,
act as styptics or hemostatics by coagulating
the blood colloids. The action of disinfectants
is largely controlled by colloid-chemical factors
the disinfectants are adsorbed by bacteria,
and either coagulate their protoplasm or flock
them out.
FOODS AND THEIR PREPARATION. It is a
serious error to judge foods upon the basis of
a bald chemical or calorific analysis. Fat,
protein, carbohydrate and calories are not
alone the criteria of food value the physical
condition of food largely governs its useful-
ness to the organism. The experiences of
centuries has taught us the value of "light"
bread or cake, leavened by yeast or baking
powder until it presents an enormous surface
to the digestive juices; unleavened bread was
PRACTICAL APPLICATIONS 71
eaten only in time of stress, as we learn from
the Bible. The meats yielded by young
animals are more juicy and tender than those
obtained from older animals, because the latter
are formed from tissues partially dehydrated
by age.
The ancient art of cooking involves many
factors besides mere digestibility and assimila-
tion; taste, flavor, odor and variety are
important. Egg albumen when cooked is
probably more slowly absorbed and loses its
species-specificity; therefore, some people who
have an idiosyncracy against raw eggs can
eat cooked eggs. Cream is an emulsion of fat
in an aqueous medium and wets paper; butter
is an emulsion of water in a fatty medium and
greases paper.
PHYSIOLOGY AND PATHOLOGY. The changes
which occur on almost all physiological proc-
esses are remarkable not only because of
their very profound nature, but also because
they are produced at comparatively low tem-
peratures and in the presence of very dilute
reagents. The living organism disintegrates
proteins, oxidizes carbohydrates and with the
same apparent ease synthesizes substances of
72 COLLOID CHEMISTRY
great complexity. Powerful reagents and high
temperatures, which would be destructive to
life, are necessary to bring about changes of
this character under ordinary laboratory con-
ditions.
The body and plant colloids (biocolloids)
consist of carbohydrates (starch, cellulose,
glycogen), proteins (plant and animal al-
bumins), and lipoids (lecithin, cholesterin, fats
and oils). Each tissue has a normal turgor or
state of swelling which is greatly influenced
by acids, alkalis and salts. The swelling and
shrinking of tissues, together with their selective
adsorption and the differential diffusion of
solutions through them, account for or accom-
pany many physiological phenomena, both
normal and pathological. Thus, fibrin and
gelatin swell much more in very dilute acid
than in distilled water, but the swelling is
depressed by salts. Fibrin is so sensitive that
it swells in the presence of traces of acid quite
undetectable by ordinary indicators, such as
litmus; in fact fibrin itself is a most sensitive
indicator.*
* Though the normal H ion concentration of the blood is
0.37 X 10~ 7 , a concentration of 1.00 X 10~ 7 nH represents
an advanced acid intoxication.
PRACTICAL APPLICATIONS 73
Local accumulation of acid in the organism
may cause swelling (edema) ; for example, in-
sect stings, which may be imitated by stinging
gelatin with a needle dipped in acid. If acid
accumulates in an organ with a rigid capsule
(eye or kidney), the swelling tends to establish
a vicious circle (glaucoma, nephritis) by com-
pressing the blood vessels and cutting down
the alkaline blood stream, which is unable to
wash out the acids (mainly CO 2 ) formed by
living protoplasm.
If the oxidation processes of the body are
normal, the hydrogen in foods is oxidized
mainly to water and the carbon mainly to
carbonic acid a gaseous acid which is ex-
haled without demanding fixed alkali or pro-
tein of the organism for its elimination. It
would require nearly two pounds of pure caus-
tic soda to neutralize the acidity produced daily
by an average man. In the case of pathologi-
cal oxidation, however, other non-volatile acids
are formed and a condition called "acidosis "
may arise, which is in reality a diminished
alkalinity, recognizable by the fact that an
abnormally large quantity of bicarbonate of
soda is needed to render the urine alkaline.
74 COLLOID CHEMISTRY
These acids may cause disturbances of the
body colloids, disease and even death. In fact,
throughout life there is a gradual syneresis of
the biocolloids accompanied by visible shrink-
ing and loss of water compare the chubby
hand of a child with that of an old man. In
plants an analogous process occurs in lignifi-
cation.
DIGESTION. The digestive process is pre-
liminary to the actual adsorption and use of
food by the organism, and has for its object
the modification or change of the ingested food
into such forms or such substances as may be
absorbed in the lower part of the digestive
tube. To have a correct understanding of the
absorption of the products of digestion, we
must bear in mind the fact that the walls of the
digestive tract act as semipermeable colloid
membranes and that absorption involves dif-
fusion into or through these membranes or
their constituent cells. Substances in crystal-
loidal solution, and colloidal sols whose par-
ticles are sufficiently small, represent then the
two classes of digestion products which are
diffusible and therefore absorbable.
Food as ingested consists mainly of sub-
PRACTICAL APPLICATIONS 75
stances that may be grouped into two classes:
1. Crystalloids such as water, sugars,
sodium chlorid, etc.
2. Colloids such as starch, proteins, emul-
sions, etc.
The crystalloids in foods are usually absorbed
directly, although sucrose, for example, under-
goes inversion. The colloids, as a rule, are not
directly absorbable, and, for the most part,
digestion consists in^the disintegration of the
colloidal complexes of the food, so that they
can actually diffuse into the organism and
there undergo further changes. Colloidal gels
or even sols whose particles are of large size
are, practically speaking, non-diffusible, and
must, therefore, be reduced to a more finely
dispersed state.
Investigation has demonstrated that the
high efficiency of the digestive juices is mainly
due to small quantities of certain colloidal
substances called enzymes (such as ptyalin,
pepsin and pancreatin) which act as catalyzers,
enormously hastening reactions which would
otherwise proceed so slowly that, practically
speaking, they would not occur at all. The
enzymes appear to act by forming with the
76 COLLOID CHEMISTRY
substrate a combination of unstable character,
which breaks down and liberates the enzyme
again to continue the operation. Recently
W. M. Bayliss, in his interesting monograph
on "The Nature of Enzyme Action," has
shown that in all probability "the compound
of enzyme and substrate, generally regarded as
preliminary to action, is in the nature of a
colloidal adsorption compound." Anyone who
has seen in the ultramicroscope the extremely
active motion of the individual particles in
colloidal solutions, can readily imagine the
terrific bombardment a substance must un-
dergo when a colloid enzyme is concentrated
on its surface by adsorption, and indeed it
seems probable that enzymes actually produce
their effects by virtue of their specific surface
actions and the motion of their particles.
In order to find out if this idea could be
verified by actual observation, the author
watched under the ultramicroscope the action
of diastase upon potato starch grains and the
action of pepsin upon coagulated egg albumen.
In the first case, actively moving ultra-
microns in the diastase solution gradually
accumulated about the starch grains, which
PRACTICAL APPLICATIONS 77
after a time showed a ragged and gnawed
margin. While the adsorption and motion of
the larger ultramicrons was all that could be
followed, the bright appearance of the field
indicated that more numerous finer particles
were present, and some apparently of inter-
mediate size were seen.
For observations on albumen there was used
a dilute solution of white of an egg which has
been heated nearly to boiling. It was opales-
cent and in the ultra apparatus exhibited a field
full of bright and rapidly moving ultramicrons.
Upon allowing a droplet of essence of pepsin
(Fairchild's, containing 15 per cent of alcohol
by weight) to diffuse in, an immediate coagu-
lation occurred, the particles clumping into
very large masses. A droplet of decinormal
hydrochloric acid was then allowed to diffuse
in, whereupon the large masses broke up in
small groups and single ultramicrons, which
once more resumed their original motion.
Soon, however, the albumen particles began
to grow smaller and disappear, the field all the
while becoming brighter and brighter, indicat-
ing the concommitant appearance of smaller
ultramicrons or amicrons. In vitro the addi-
78 COLLOID CHEMISTRY
tion of the pepsin to the opalescent albumen
solution caused it to clear gradually, even at
room temperature.
Enzymes are inactivated to a greater or less
extent by shaking, heating, electrolytes, etc.,
all of which, as is well known, cause the
coagulation of colloidal solutions and a result-
ing decrease in the activity of the motion of
their constituent particles. Another feature
of interest is that the action of enzymes is
reversible, a fact that does not come much
into evidence because of the dilution and
removal by diffusion of the products formed.
In cells, tissues and organs, however, changes
of concentration again occur and synthetic
processes may result.
One principle of colloid chemistry is of the
utmost importance in digestion, namely: the
protective action of reversible colloids, which
stabilize or protect from coagulation irrevers-
ible or unstable colloids. Mucin and analo-
gous colloidal substances undoubtedly have a
function of this character, which may in some
cases account for the variance between the
action of natural and artificial digestive juices.
The effects of colloidal protection are in
PRACTICAL APPLICATIONS 79
evidence in almost all physiological reactions
and processes, and it is indeed extremely
doubtful if there ever occurs in vivo any
chemical reaction which is not greatly in-
fluenced by the colloids always present.
ABSORPTION, SECRETION AND EXCRETION.
These are largely affected by the swelling and
shrinking of the body colloids and by selective
adsorption and diff erential diffusion. It must
be remembered that the blood is in reality a
circulating fluid colloid, whose attraction for
water is greater in the "acid " or venous con-
dition, than it is in the " alkaline " or arterial
condition. Tissues and organs well supplied
with venous blood tend to adsorb water
(intestine); whereas those well supplied with
arterial blood tend to give up (secrete or ex-
crete) water (kidney); and as the blood is
passing in a continuous stream, the process
continues as long as the water supply permits
and until the blood is in equilibrium with the
other tissues.*
* The functioning of organs is largely controlled by nervous
influences. Thus a sudden nervous shock may by vaso-dilation
send an excessive supply of arterial blood through the mes-
enteric arteries (an "internal blush"), and result in a secre-
tion of fluid into the intestine (nervous diarrhea).
80 COLLOID CHEMISTRY
Conditions which decrease the capacity of
the blood and tissues to hold water (diuretics,
hyperglucemia and acidosis in diabetes) natu-
rally result in the elimination of the excess or
"free " water (polyuria, diarrhea).
Minute quantities of acid increase the swell-
ing capacity of colloids, which quickly reaches
a maximum; after which increasing acidity
causes shrinking. Neutral salts oppose the ac-
tion of acids apparently by driving back the
ionization of the acid and thereby reducing the
H-ion concentration which is the controlling
factor.
The action of selective adsorption and
differential diffusion in effecting secretion and
excretion must be at once manifest. Easily
hydrolyzable compounds may be thus split up
in the body, and yield secretions of acid nature
like the gastric juice, or of alkaline nature like
the pancreatic juice, depending upon the
structure of the organ, the location of its
cavity and of its afferent and efferent vessels.
Individual compounds in the blood stream or
other body juices may also be selectively
diffused out, concentrated or separated from
other accompanying substances. By selective
PRACTICAL APPLICATIONS
81
adsorption, circulating substances may be
fixed and taken from the circulation; in fact,
poisons are usually taken up selectively by
certain organs and tissues.
An insight into the mechanism of body
processes may be obtained by considering the
Convoluted
Tubule^
Vascular
Plexus
(Ohmerulus)
f|l<~7iy/ of Renal Artery
FIG. 1. Glomerular structure.*
functioning of the kidney (see Fig. 1). The
Malpighian tufts are plentifully supplied
with arterial blood having "free water/' and
* From Dr. J. G. M. Bullowa's translation of Bechhold's
"Colloids in Biology and Medicine," D. Van Nostrand Co.,
1919.
82 COLLOID CHEMISTRY
under the pulsating pressure* of the blood
stream, they ultrafilter off a very dilute but
copious blood ultra-filtrate into the long con-
voluted tubules. The tubules, however, are
plentifully supplied with venous blood, which
is unsaturated with water and which there-
fore reabsorbs most of the water together
with some of the dissolved substances contained
in the preliminary excretion ; so that there drips
into the pelvis of the kidney a concentrated
urine having in solution many of the substances
found hi the blood, but in a much higher
concentration. Bechhold estimates that the
average of two liters of urine voided daily by an
average man, represents a preliminary excretion
of fifty liters, of which forty-eight are reab-
sorbed within the kidney itself.
In plants, differential diffusion and selective
adsorption seem to be intimately bound up
with growth and the circulation of the sap.
The plant tissues are mainly colloidal gels or
finely integrated structures, and as the sap
circulates or diffuses through them, each tissue
* Since the vas defferens has a smaller lumen than the vas
efferens, a " back pressure " is created within the Malpighian
tufts.
PRACTICAL APPLICATIONS 83
selectively adsorbs and elaborates certain
particular constituents. Thus with the potato
and tapioca plants the starch forming sub-
stances are fixed in the roots; with the sago
palm they are fixed in the stem pith ; and with
cereal grains, in the seeds. As long as the
adsorptive tissues are unsaturated or are
multiplied, so long can growth continue, the
stem and branches taking up the substances
required for the upward growth, and the root
taking up those required for the downward
growth.
When we consider the great variety of bio-
colloids and their susceptibility to changes of
structure and diffusive or adsorptive capacity,
we can easily understand the almost infinite
number of reactions that may go on within
their recesses, as they swing the balance of the
law of mass action over particles reduced to
a reactive degree of subdivision.
BIBLIOGRAPHY
The following are some of the more important standards of
reference:
ENGLISH
H. BECHHOLD, "Colloids in Biology and Medicine" (trans.
by Dr. J. G. M. Bullowa). 1919.
E. F. BURTON, "The Physical Properties of Colloidal Solu-
tions." 1916.
M. H. FISCHER, "(Edema and Nephritis." 1915. "Fats
and Fatty Degeneration." 1917.
Wo. OSTWALD, "Theoretical and Applied Colloid Chemistry"
(trans, by Dr. M. H. Fischer). 1917.
Wo. OSTWALD, "An Introduction to Theoretical and Applied
Chemistry" (trans, by Dr. H. M. Fischer). 1916.
ZSIGMONDY, "Colloids and the Ultramicroscope " (trans, by
J. Alexander). 1909.
ZSIGMONDY, "Chemistry of Colloids" (trans, by E. Spear).
1917.
FRENCH
COTTON ET MOUTON, "Les Ultramicroscopes et les objets
Ultramicroscopiques." 1906.
PAUL GASTOU, " L'Ultramicroscope dans le Diagnostic Cli-
nique et les Recherches de Laboratoire." 1916.
PERRIN, Numerous Journal Articles.
85
86 BIBLIOGRAPHY
GERMAN
ARTHUR IV!ULLER, "Allgemeine Chemie der Kolloide." 1907.
Wo. OSTWALD, "Grundriss der Kolloidchemie." 1909.
THE SVEDBERG, "Herstellung Kolloider Losungen." 1909.
FREUNDLICH, "Kapillarchemie." 1909.
VAN BEMMELEN, "Die Absorption." 1911.
The "Zeitschrift fur Chemie und Industrie der Kolloide
(Kolloid-Zeitschrift)" and " Kolloidchemische Beihefte,"
published by Wo. Ostwald, are mines of information, con-
taining both original articles and references.
Abstracts of, or references to practically all current articles
and books on Colloid Chemistry are to be found under the
division "Physical Chemistry "of "Chemical Abstracts,"
published by the American Chemical Society. Furthermore,
in the books above referred to, especially Burton, are to be
found numerous valuable references.
AUTHOR INDEX
ALEXANDER, J., 50, 57, 85.
ARRHENIUS, S., 38.
ATTERBERG, 41, 44.
BAHNTJE, 47.
BAYLISS, 76.
BECHHOLD, 26, 81, 85.
BEHRE, 16.
BEILBY, 49.
BENEDICKS, 49.
BETTS, A., 47.
BlLTZ, 16.
BREDIG, 10.
BULLOWA, 57, 81, 85.
BUNSEN, 16.
BURTON, 33, 85.
CAMERON, 45.
COEHN, 30.
COTTON, 23, 85.
COTTRELL, 33.
CUSHMAN, 46.
DE BRUYN, LOBRY, 8.
DUCLAUX, 69.
EHRENBERG, 44.
FlCKENDAY, 46.
FISCHER, M. F., 85.
FREUNDLICH, H., 86.
GAIDUKOV, 51.
GASTOU, P., 85.
GRAHAM, T., 1, 25, 33.
HAMPTON, 69.
HARDY, 10, 31, 32.
HERSEY, 49.
HUBBARD, 46.
IGNATOWSKI, 23.
JACKSON, H., 53.
KEPPELER, 46.
LANGMUIR, 48.
LEVITES, 54.
87
LEWKOWITSCH, 52.
LIVINGSTON, 45.
LODGE, O., 33.
LUMIERE, 45.
MAXWELL, 38.
MAYER, 54.
MERKLEN, 52.
MOOERS, 69.
MOUTON, 23, 85.
MUELLER, 25, 47, 85.
NEWCOMB, SIMON, 38.
OSTWALD, WOLFGANG, 10, 14,
85, 86.
PATTEN, 44.
PERRIN, 85.
PIERONI, 14.
PUTZ, 49.
RAFFO, M., 14.
RAMSEY, 8.
RICHARDSON, W. D., 54.
SCHAEFFER, 54.
SCHREINER, 45.
SEYEWETZ, 45.
SlEDENTQPF, 11, 19, 23.
SPANGENBERG, 46.
SPEAR, E., 85.
SPRING, W., 14.
SVEDBERG, THE, 86.
TERROINE, 54.
VAN BEMMELEN, 86.
VAN CALCAR, 8.
VON WEIMARN, P. P., 10.
WAGGAMAN, 44.
WUST, 49.
YOSHIMOTO, 68.
ZSIGMONDY, 7, 10, 11, 16, 17,
25, 48, 55, 85.
SUBJECT INDEX
Absorption, 19, 86.
Acidosis, 73.
Adsorption, 16, 50, 82.
Agriculture, 43.
Amicrons, 22.
Astronomy, 36.
Atmosphere, 40.
Boiler scale, 65.
Brewing, 61.
Brownian motion, 8.
Cardioid Condenser, 23.
Cement, 66.
Ceramics, 42.
Chemical Analysis, 68.
Clay, 42.
Coehn's law, 30.
Colloid Chemistry, Applica-
tions of, 36.
definition of, 6.
Colloids, Classification of, 10.
Colloidal protection, 24, 78.
Comets, 37.
Confectionary, 61.
Cosmic dust, 37.
Deflocculation, 35.
Dialysis, 25.
Diffusion, 25, 27.
Digestion, 74.
Dimensions of colloidal par-
ticles, 7. 22.
89
Dispersoids, 10.
Dyeing, 50.
Edema, 73.
Electric charge of colloidal
particles, 30.
Electrophoresis, 30.
Electroplating, 47.
Emulsoids, 12.
Enzymes, 75, 78.
Excretion, 79.
Fatty Degeneration, 85.
Faraday-Tyndalleffect,20,39.
Filtration, 66.
Foods, 70.
Geology, 41.
Glaucoma, 73.
Gold number, 32.
Hydrophile colloids, 12.
Hydrophobe colloids, 12.
Ice cream, 60.
Irreversible colloids, 10, 12.
Isoelectric point, 32.
Kidney, 81.
Lyophile colloids, 12.
Lyophobe colloids, 12.
Metallurgy, 48.
Meteorology, 39.
Micron, (M) = YQQQ mm<
Migration of Colloids, 31.
90 SUBJECT INDEX
Milk, 56. Radius of molecular attrac-
, N 1 tion, 14.
Millimicron, M = 1^0 * Reversible coUoids, 10.
1 Rubber, 64.
~ 1,000,000 m Schutz kolloide, 44.
Mineralogy, 41. Secretion, 79.
Mortar, 66. Soap, 52.
Nephritis, 73, 85. Soils, 43.
Pathology, 71. Solution, 7.
Pectization, 31. Submicrons, 22.
Pedesis, 8, 53. Suspensions, 7.
Peptization, 31, 33. Tanning, 63.
Pharmacy, 69. Tyndall effect, 20.
Photography, 65. Ultrafiltration, 25.
Physiology, 71. Ultramicrons, 22.
Plaster, 66. Ultramicroscope, 17.
Purple of Cassius, 16- Weather, 39.
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TABLE II
LjO.
Suspensio n-s-
uold Sus-
pension G
Classification of Colloidal Solutions
according to the size of the particles contained in them and
according to their behavior upon desiccation.
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