Amino acids are the building blocks of proteins, which are a type of the most important biological macromolecules. The thermodynamic investigation of aqueous amino acid solutions in the presence of second solutes, especially electrolytes, provides valuable information about the complex behavior of proteins in biological systems. Further, distinct pairs of amino acids and a second solute in aqueous media can undergo the soluting-out phenomenon as aqueous biphasic systems (ABS) formation[1,2] or precipitation[2]. Such systems have a significant place in designing green extraction and separation strategies.[1-3] Quaternary ammonium salts own a tunable hydrophobic/ hydrophilic character because of the possibility of changing the ammonium alkyl chain length. Therefore, this kind of salts can be a good choice for a comprehensive study of soluting effects of electrolytes on aqueous amino acid solutions.[4,5] This work addresses soluting-out and soluting-in effects occurring in ternary aqueous solutions composed of amino acid and quaternary ammonium salts. For this purpose, the vapor-liquid equilibrium properties of several ternary systems of {water + amino acid + quaternary ammonium salt} have been measured by the use of the isopiestic method at 298.15 K. The effects of amino acids type, alkyl chains length of ammonium, and anion type on vapor pressure, osmotic coefficients and the slope of constant water activity lines have been determined. The concave or convex shape of constant water activity curves has been taken as a benchmark to study the type and magnitude of deviation from the semi-ideal behavior and therefore the soluting effects occurring in the investigated systems.
