In this study, in order to get a better understanding about the soluting effect occurring in aqueous amino acid + IL solutions, the experimental measurements of water activity for the ternary amino acid (L-proline, glycine, alanine and L-serine) + IL (1-butyl-3-methylimidazolium chloride, [C4mim]Cl, 1-hexyl-3-methylimidazolium chloride, [C6mim]Cl, and 1-butyl-3-methyl-imidazolium trifluoromethanesulfonate, [C4mim][CF3SO3]) + water systems were taken using the vapor pressure osmometry (VPO) method at 308.15 K. From the obtained water activity data, the values of the vapor pressure, deviations from the semi-ideal behavior, and Gibbs free energy changes were calculated. Due to the unfavorable IL-amino acid interactions, all the studied ternary amino acid + IL + water systems showed negative deviations from the semi-ideal state (soluting-out effect). The soluting-out phenomena in these systems appears by aqueous biphasic systems (ABS) formation for the ternary [C4mim][CF3SO3] + amino acids + water systems and precipitation of the amino acids from the solution for the ternary [C4mim]Cl/[C6mim]Cl + amino acids + water systems. The influences of the nature of the ILs and amino acids on the soluting effects have been discussed.
