Studies of Volumetric and Transport Properties of Ionic Liquid-Water Mixtures and Its Viability To Be Used in Absorption Systems

Binary systems of two ionic liquids (ILs), 1-ethylpyridinium methanesulfonate [C2Py][MeSO3] and choline dihydrogen phosphate [Chol][H2PO4], and water have been experimentally studied. Density, viscosity, electrical conductivity and proton activity have been measured at several temperatures covering all the miscibility range. From density data, isobaric coefficient of thermal expansion was calculated to study the volumetric behavior of the mixtures. All volumetric data were fit using polynomial equations. The Vogel-Fulcher-Tamman (VFT) equation accurately describes the temperature dependence of viscosity for all systems. Systems based on [C2Py][MeSO3] are less dense and viscous than those involving [Chol][H2PO4], making [C2Py][MeSO3] more suitable for absorption systems where pumping cost has a significant importance. Electrical conductivity data were adjusted using the Casteel-Amis equation. Similar trends were found for both systems, although ionic conductivity is higher for [C2(P)y][MeSO3] + H2O mixtures. The relation between viscosity and electrical conductivity was also explored. According to Walden plots, ILs present low ionicity; however, internal friction and ionic concentration does not seem to be enough to explain the behavior of ionic conductivity and IL concentration. Proton activity measurements show different tendencies with molar fraction of each IL, [C2Py][MeSO3] leads to lower pa(H+) values than [Chol][H2PO4] in the binary mixtures. Results of ionic conductivity and proton activity suggest a higher corrosive potential of [C2Py][MeSO3] + H2O; however, a further analysis needs to be done to evaluate this risk in absorption systems. Finally, a composition analysis based on ionic chromatography (IC) was carried out to obtain insight about its effect on their physicochemical properties.