Effect of molecular structure of oil on wetting transition on surfactant solutions

Wetting behavior of butylcyclohexane on aqueous solutions of dodecyltrimethylammoniumbromide (DTAB) was studied by interfacial tensiometry, ellipsometry, and dihedral angle measurement. The air-water surface tension is the same with and without butylcyclohexane at low surfactant concentrations (partial wetting) but lowered by the presence of hexadecane with increasing surfactant concentration (pseudo-partial wetting). From ellipsometric measurements, it is confirmed that the transition between partial wetting and pseudo-partial wetting is the first-order and the thickness of the film in the pseudo-partial wetting regime is monomolecular level. The contact angle decreases rapidly with increasing surfactant concentration in the partial wetting regime then gradually increases in the pseudo-partial wetting regime, then approaches to a saturation value around the critical micelle concentration (cmc). These behaviors are qualitatively the same as those observed in our previous studies on the hexadecane system but quantitatively it is found that the wetting transition is promoted by the introduction of the cyclohexane ring to the molecular structure of oil compared to linear chain alkanes. The effect of molecular structure of oil on the formation of a microscopic oil film is rationalized by dispersion interactions between molecules in the film and in the bulk oil phase.