Sugars are renewable, biodegradable, non-toxic, and non-charged hydrophilic materials owning soluting-out aptitude in aqueous media due to their high affinity for water.[1,2] During the past decade, there have been significant efforts to study sugar-based aqueous two-phase systems (ATPSs) as alternative extraction platforms.[1-5] However, despite its importance for optimum designing of green separation processes, the mechanism of ATPS formation in a ternary aqueous system composed of a sugar and a second solute is still unclear. Aiming to study the main driving forces that control the phase behavior of these systems, this work focuses on the vapor-liquid equilibrium investigation for ternary systems of {water + sugar + tetraalkylammonium salt} via the isopiestic method. The magnitude and the type of deviation of constant water activity curves from the linear isopiestic relation (semi-ideal behavior) have been taken as a benchmark to study the soluting effect occurring in the investigated systems. The impacts of cation alkyl chain length and anion type of ammonium salts, and also the structure and stereochemistry of sugars on the deviation from the semi-ideal behavior have been scrutinized and explained based on soluting-in and soluting-out phenomena.
