Effect of electrolytes on discontinuous cubic phases

We investigated the effect of adding a lyotropic salt (NaCl) and a hydrotropic salt (NaSCN) on the discontinuous cubic phase formed in highly hydrophilic nonionic (C12EO25) and ionic (DTAC) surfactant systems by phase study and small-angle X-ray scattering (SAXS) measurements. In C12EO25 systems, only a small decrease in the thermal stability of the cubic phase was observed upon addition of NaCl or NaSCN. It was also found that NaCl induces a slight reduction on the effective surface area per surfactant molecule (a(s)), whereas in the case of NaSCN, a(s) increases. These results are attributed to changes in the hydration of the poly(oxyethylene) (EO) chain. On the other hand, in DTAC systems, the addition of both NaCl and NaSCN leads to a transition from the discontinuous cubic phase to the hexagonal phase, related to a more pronounced shrinkage of a(s) and to the micellar growth within the cubic phase. Changes in micellar structure resemble that occurring in diluted systems upon addition of salt. The stability of the cubic phase increases when long hydrocarbon-chain oils are added to DTAC/brine systems, because of the formation of swollen micelles with a more positive curvature and a moderate axial ratio. In oil-present systems, a(s) shrinks when the counterion is changed from chloride to bromide. The influence of the degree of counterion dissociation on the phase behavior is discussed.