An experimental study was conducted on aqueous triphasic systems (A3PSs) to increase molecular understanding of the soluting effect phenomenon occurring in these systems. For aqueous quaternary systems 1‑butyl‑3-methylimidazolium bromide ([Bmim][Br]) + trisodium citrate (Na3Cit) + polypropylene glycol 400 (PPG400) and [Bmim][Br] + disodium sulfate (Na2SO4) + PPG400 which were able to form A3PS, the vapor-liquid equilibria measurements were conducted at 318.15 K. For these systems, deviations from the semi-ideal behavior were addressed using the vapor pressure osmometry (VPO) measurements in monophasic (MR), biphasic (BR), and three-phasic (TR) concentration regions. The VPO data revealed that, because of unfavorable IL-salt, IL-polymer, and polymer-salt interactions as well as the preferential hydration of solutes, the amount of free water molecules in the IL + polymer + salt + water solutions is less than that expected based on the semi-ideal behavior. Therefore, under condition that solutes molalities in the quaternary solutions are the same as those in the corresponding binary solutions, and have negative values and become less negative by formation of aqueous two-phase system (A2PS) and become positive with approaching the triphasic region. The triple soluting-out effect resulted from the unfavorable interactions between all pairs of solutes leads to preferential hydration of solutes and thereby creation of three aqueous salt-rich, IL/polymer-rich, and polymer-rich phases in solution for entropy reason.
