Phase equilibria and volumetric and viscosity behavior of the aqueous double salt ionic liquid [EMIM][SCN]x[MeSO3](1-x)

Abstract

Continuing our recent systematic studies on thermodynamic and thermophysical properties of binary aqueous ionic liquids (ILs), this work extends our investigations to aqueous double salt ionic liquids (DSILs). Specifically, the (water + [EMIM][SCN]x[MeSO3](1-x)) system is examined here: vapor-liquid and solid-liquid equilibria, volumetric and viscosity behaviors were experimentally determined at three DSIL compositions (x approx. 0.75, 0.5, and 0.25 or 0.20) and several water contents xw. Water activity was measured at seven temperatures from (288.15 to 318.15) K and for xw from 0.3 to 1, freezing temperature for xw from 0.8 to 1, density and dynamic viscosity at two temperatures (298.15 and 323.15 K) and for xw from 0 to 0.9. The acquired data were treated as those for the ternary system (water + [EMIM][SCN] + [EMIM][MeSO3]) and modeled coherently together with its binary subsystems. The ternary water activity data were successfully correlated by the extended NRTL model, adopting for the aqueous parent IL binaries the parameters we evaluated previously, and fitting just those remaining for the (IL + IL) pair. The established model was shown to predict perfectly the measured SLE data. This model further provides an otherwise hardly procurable insight into the energetics of the DSIL. As inferred from the model, the ([EMIM][SCN] + [EMIM][MeSO3]) binary system exhibits quite large positive deviations from ideality, its excess Gibbs energy stemming from the very endothermic mixing of the two ILs which is only partially compensated by the positive excess entropy. The (IL + IL) mixture exhibits a positive, but very small excess volume, only slightly increasing as temperature increases, while its viscosity is much greatly affected by both temperature and the DSIL composition. The viscosity deviation from the linear mixing rule is for the DSIL negative and large but obeying quite accurately the linear mixing rule of logarithmic viscosity. For the aqueous mixtures of both parent ILs, the logarithmic viscosity deviation is high and greatly composition asymmetric. Coherent and sufficiently accurate representation of the volumetric and viscosity data for the ternary system and all its binary subsystems was achieved by Padé approximants with temperature-dependent parameters amended with a ternary term. Compared to the neat [EMIM][MeSO3], [EMIM][SCN]x[MeSO3](1-x) with smaller contents of [EMIM][SCN] (x < 0.2) and water (xw < 0.2) shows higher water affinity, while having considerably lower viscosity. Thus, the use of this DSIL as a water absorption medium for desiccation and cooling applications was found to be very beneficial.