Due to the enormous ways ionic liquids can be designed, they offer great opportunity to be adjusted to meet the needs of various applications.Applications that do not require the anion of the ionic liquid to react with CO2 have been study extensively, with [Tf2N] anion being the most commonly studied anion.For those applications, dialkylimidazolium cations tend to be used most often because they offer low viscosities.For applications that require CO2 to have a chemical reversible reaction with the anion, our group has proposed using ionic liquids with Aprotic Heterocyclic Anions (AHAs).For those applications, trihexyltetradecylphosphonium ([P66614]) cations tend to be used most often because they are thermally stable and generally make ionic liquids that do not have a melting point.One purposed application that involves ionic liquids with Aprotic Heterocyclic Anions is the vapor-compression co-fluid refrigeration cycle.The project involves finding an anion that gives the desired enthalpy and a cation that gives a low viscosity.The viscosity of numerous [P66614][AHAs] were measured to find ones that give low viscosities.Of those, the CO2 solubilities were measured at multiple temperatures and the chemical enthalpies were estimated.Two ionic liquids that are close to what is needed for the refrigeration application are [P66614][3-Triz] and [P66614][4-NO2pyra].[P66614][3-Triz] gives a lower viscosity than [P66614][4-NO2pyra].However, [P66614][4-NO2pyra] matches the desired chemical enthalpy needed whereas [P66614][3-Triz] has a chemical enthalpy greater than desired.The viscosities of both [P66614][3-Triz] and [P66614][4-NO2pyra] are higher than desired, which required adjusting the cation in order to reduce the viscosity.It turns out by reducing the size of the cation from [P66614] to triethyloctylphosphonium ([P2228]), the viscosity can be reduced.The viscosity of [P66614][4-NO2pyra] at 278.15 K is 3560 cP, whereas the viscosity of [P2228][4-NO2pyra] at 278.15 K is 1100 cP.The addition of an ether can also reduce the viscosity.The viscosity of [P222(10201)][4-NO2pyra] at 278.15 K is 309 cP (where 10201 indicates –CH2-O-C2H4-O-CH3).The down side of the ether substitutent is that the synthesis is more challenging and the ionic liquid is more likely to decompose at lower temperatures.If the viscosity of the ionic liquid needs to be lowered, tetraglyme can be used as an additive to lower the viscosity.Tetraglyme was added to various ionic liquids to see how tetraglyme affects the viscosity.Tetraglyme always decreases the viscosity of the ionic liquids.It appears that the viscosity of higher molecular weight ionic liquids do not decrease as much as lower molecular weight ionic liquids when tetraglyme is added to the ionic liquid.The viscosity of various ionic liquids mixed with another ionic liquid were measured.For a few mixtures, the viscosity of the mixture is greater than the viscosity of the two pure ionic liquids, such as [P2228][4-NO2pyra]/[hmmim][Tf2N].For other mixtures, such as [hmmim][4-NO2pyra]/[P2228][Tf2N], the viscosity decreases greatly when a small amount of the lower viscosity ionic liquid was added; however, at no point was the viscosity lower than the viscosity of either of the two pure ionic liquids.The viscosity of the mole fraction of 0.5 of [P2228][4-NO2pyra] in [hmmim][Tf2N] matches the viscosity of the mole fraction of 0.5 of [hmmim][4-NO2pyra] in [P2228][Tf2N].This was true for three other such pairs investigated.This is because the origin of the cations and anions is unimportant once the ionic liquid is mixed with another ionic liquid.The constraint that the viscosities have to match each other at the mole fraction of 0.5, along with the viscosities of the four pure ionic liquids, can result in the log of the viscosity of both mixtures being far from linear functions of composition.