Interfacial tensions in heptane-aqueous NaCl systems containing Aerosol OT and a second single-chain ionic surfactant

Abstract

Water-in-oil microemulsions (Winsor II) can be made to pass to oil-in-water ones (Winsor I) by the addition of a second single-chain ionic surfactant (SDS) to the system consisting of a non-polar oil in contact with aqueous solution of Aerosol OT and a sufficient amount of NaCl (cNaCl > 0.05 M). The physicochemistry of Winsor transitions determines the multicomponent surfactant mixture compositions and, furthermore, their practical applications. We have undertaken a systematic study concerning the effects of homologous series of sodium sulfates, sodium sulfonates, and sodium carboxylates in heptane-aqueous 0.1 M NaCl containing AOT (CAOT = 4.59 × 10-4 M > c.m.c.) at 25°C upon the surfactant film curvature and geometric constraints. The results are discussed in terms of the type and molecular geometry of the ionic surfactants, the composition of the interfacial monolayers, and the thermodynamics of the transition from Winsor I to Winsor III. For homologous amphiphiles the minima in the interfacial tension, γc,min, brought about by addition of the second ionic surfactant, were achieved at different concentrations of the latter (c2,min). We have shown that there is a linear correlation between the standard free energy change of the transition from the Winsor I to the Winsor III system (ΔGtro = RTlnm2,min, where m2,min is the activity of a second ionic single-chain surfactant) and the carbon number (n) in the alkyl chain for various homologous series of amphiphiles. We have calculated the CH2 group and ionic head group contribution in ΔGtro. The first parameter is almost equal for sulfates, sulfonates, and carboxylates, but the head contribution decreases in the order: COONa, SO3Na, OSO3Na. Thus the increase in the volume of the head group promotes Winsor I-Winsor III-Winsor II transitions. © Springer-Verlag 1999.