Unveiling the soluting-out effect of amino acids (S(+)-serine and S(-)-proline) on aqueous solutions of tetraalkylammonium bromide salts

Abstract

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.