Thermodynamic study of the soluting effect in aqueous ionic liquid - monosaccharide solutions by the vapor pressure osmometry

Abstract

In this work, to study vapor-liquid equilibria of several ternary ionic liquid+carbohydrate+water and binary ionic liquid+water systems, water activity measurements have been made using the vapor pressure osmometry (VPO) technique at 308.15K. Three imidazolium-based ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]), 1-butyl-3-methylimidazolium bromide ([Bmim][Br]), and 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO4]) and four monosaccharide carbohydrates l-(+)-arabinose, d-(+)-xylose, d-(+)-glucose, and d-(−)-fructose were selected. The hydrophilicities of the investigated ILs follow the order: [Bmim][HSO4]>[Bmim][Br]≫[Bmim][BF4]. Our results showed that the unfavorable IL-carbohydrate interactions can be lead to reducing the water activity and vapor pressure of ternary solution relative to the semi-ideal solution model (Δaw<0 and ∆p−(∆p°IL+∆p°C)<0). For the systems that had extreme negative departure from the semi-ideal behavior ([Bmim][BF4]/carbohydrate) aqueous biphasic system (ABS) is formed in the concentrations higher than the critical values. For these systems, magnitudes of the negative departures from the semi-ideal behavior were in agreement with the soluting-out strength of the monosaccharides that follow the order: d-(+)-glucose>d-(−)-fructose>l-(+)-arabinose>d-(+)-xylose. Aqueous [Bmim][Br]/carbohydrate and [Bmim][HSO4]/carbohydrate systems with smaller negative departure from the semi-ideal behavior are unable to form the ABS and the soluting-out effect of these ILs on aqueous carbohydrate solutions is appeared in the form of precipitation of sugars from the aqueous solutions. In these systems, for a certain carbohydrate the magnitude of the departure from the semi-ideal behavior increased from [Bmim][Br] to [Bmim][HSO4] which is in agreement with the hydrophilicity of the ILs.