Effect of salts on the aqueous two-phase system in mixed solutions of Gemini (12-3-12,2Br −) and sodium dodecyl sulfate

Abstract

Salts NaCl, KCl or NaBr shorten the phase-separation time of the ATPS (aqueous two-phase system) in mixed solutions of Gemini (12-3-12,2Br −) and sodium dodecyl sulfate (SDS). They also change the region of ATPS; it moves toward higher molar mixing ratios of Gemini to SDS and becomes wider. The minimum concentration of surfactant needed for the formation of ATPS is also lowered. Negative ions play the dominant role. Densities of the coexisting phases are strongly influenced by the total concentration of surfactants, the molar mixing ratio of Gemini to SDS, temperature and the type of salt. For some cases, phase inversion (the upper phase changes to lower and the bottom phase changes to upper) appears. Salts notably enhance extraction ability; the oil-like phase (looks like oil in appearance), a Gemini-rich solution, shows affinity for xylenol orange. Results obtained from negative-staining and transmission electron microscopies show that salts change the microstructures of the two phases. In the oil-like phase, the long rod-like micelles tend to congregate into big-rammed aggregates and form a coarse network structure that increases viscosity. The conjugate phase contains mixed multilamellar vesicles, micelles, loops and cylindrical micelles.