Liquid-liquid equilibria of aqueous two-phase systems formed by Triton X-100 surfactant and thiocyanate salts

Abstract

Liquid-liquid equilibrium data were obtained for aqueous two-phase systems (ATPSs) formed by Triton X-100 surfactant (TX-100), a salt (NaSCN, KSCN, or NH4SCN), and water, at different temperatures (298.2, 313.2, and 328.2 K). For all the systems investigated, the lower phase was rich in salt, while the upper phase was rich in surfactant. Compared with other similar ATPSs reported in the literature, the salt concentrations in the upper phase were surprisingly high and varied little with the tie-line length of the ATPS, at a particular temperature. The size of the biphasic region depended on the salt cation and increased in the order NH4+ < Na+ < K+. The temperature increase led to an increase of the biphasic region, with greater magnitude of this effect for the system formed by NaSCN, which was due to less favorable interaction of the Na+ cation with water and the more intense effect of temperature in hindering this interaction, compared with the water-K+ and water-NH4+ interactions. The ATPSs formed by TX-100 and the thiocyanate salts showed much smaller biphasic regions compared with results reported in the literature for ATPSs formed by the same surfactant and salts with other anions, confirming the low salting-out capacity of the thiocyanate anion. The results provide fundamental information for understanding ATPSs formed by salts with low phase segregation ability.