The aqueous ternary systems composed of amino acid and tetraalkylammonium bromide (TAAB) were subjected to vapor-liquid and solid-liquid equilibria (VLE and SLE) studies to investigate the soluting effects in these systems. The amino acids used are S(+)-serine and S(-)-proline, and the TAABs are tetramethylammonium bromide (TMAB), tetrapropylammonium bromide (TPAB), and tetrabutylammonium bromide (TBAB). The water iso-activity curves obtained from the VLE measurements reveal the negative deviations from the linear isopiestic relation (LIR), indicating unfavorable amino acid – TAAB interactions in aqueous media. The incompatibility between amino acid and TAAB leads to solid-liquid demixing above critical concentrations. The results of SLE experiments demonstrate reducing the water-solubility of TAABs in the presence of the amino acids, while the water-solubility of amino acids doesn’t significantly change with the addition of TAABs. According to both VLE and SLE measurements, the amino acids play the role of the soluting-out agents in the investigated systems. The soluting-out strength increases with increasing the hydrophilic character of amino acids and the hydrophobic character of TAABs.
