Winsor transitions and interfacial film compositions in systems containing sodium dodecylbenzene sulphonate and alkanols

Abstract

In alkane + water systems, containing the single chain surfactant sodium dodecylbenzene sulphonate (SDBS), inversion of microemulsion type (from o/w to w/o) cannot be effected by addition of salt alone. The addition of alkanols as cosurfactants can promote the Winsor I—III—II transitions and reduce the oil-water interfacial tension to ultralow values. The compositions of mixed monolayers of surfactant and alkanol have been determined tensiometrically and (in the case of butanol) by direct analysis. Long chain alkanols are more effective at promoting microemulsion phase inversion than the shorter homologues. The ratios of cosurfactant to surfactant at the microemulsion drop surfaces at the condition of minimum tension are 3.0, 1.3 and 1.0 for butanol, octanol and dodecanol, respectively. The effects of alkane chain length on monolayer compositions incorporating butanol are discussed in terms of the differing extents of alkane penetration into the chain region of the film. Estimated rigidity constants for films containing SDBS and butanol are low (approximately 0.7 kT) in line with measured values reported recently for a number of surfactant+cosurfactant systems.