Effect of electrolytes on discontinuous cubic phases

We investigated the effect of adding a lyotropic salt (NaCl) and a hydrotro pic salt (NaSCN) on the discontinuous cubic phase formed in highly hydrophi lic nonionic (C12EO25) and ionic (DTAC) surfactant systems by phase study a nd small-angle X-ray scattering (SAXS) measurements. In C12EO25 systems, on ly a small decrease in the thermal stability of the cubic phase was observe d upon addition of NaCl or NaSCN. It was also found that NaCl induces a sli ght 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 ot her hand, in DTAC systems, the addition of both NaCl and NaSCN leads to a t ransition from the discontinuous cubic phase to the hexagonal phase, relate d 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 d iluted systems upon addition of salt. The stability of the cubic phase incr eases when long hydrocarbon-chain oils are added to DTAC/brine systems, bec ause of the formation of swollen micelles with a more positive curvature an d a moderate axial ratio. In oil-present systems, a(s) shrinks when the cou nterion is changed from chloride to bromide. The influence of the degree of counterion dissociation on the phase behavior is discussed.