Evaluation of the effect of ionic-liquids as soluting-out agents on the solubility of carbohydrates in aqueous solutions

Abstract

In order to study the soluting-effect of ionic liquids on carbohydrates in aqueous solutions and to better understanding of its molecular mechanism, systematic investigations on the vapor–liquid (VLE) equilibria and solid–liquid equilibria (SLE) of ternary ionic liquid + carbohydrate + water systems were carried out at various temperatures. The ionic liquids 1-butyl-3-methylimidazolium bromide ([Bmim][Br]) and 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO4]) with different hydrophilicity and seven carbohydrates including monosaccharides (L-(+)-arabinose, D-(+)-xylose, D-(+)-glucose and D-(−)-fructose), disaccharides (maltose and sucrose), and a polyol (maltitol) were selected. The water activity data showed that the hydrophilicity of the investigated solutes obey the following order: [Bmim][HSO4] > [Bmim][Br] ≥ carbohydrates. In these systems, the more hydrophilic solutes [Bmim][Br] and [Bmim][HSO4] acted as soluting-out agent and precipitated the sugars from the aqueous solutions. The negative deviations of the constant water activity lines from the linear isopiestic relation for [Bmim][HSO4]/carbohydrate systems were somewhat larger than [Bmim][Br]/carbohydrate systems. The order of these deviations was agreement with the obtained soluting-out coefficients, ks, from the SLE experiments.