Wetting and interfacial transitions in dilute solutions of trisiloxane surfactants

Wetting behavior on apolar solid and fluid substrates of nonionic trisiloxane surfactants BE-n (B = (Me3SiO)(2)Si(Me)(CH2)(3), hydrophobic moiety, the same for surfactants studied, n = 6, 8, and 12, ethoxy group number) aqueous solutions was studied in a wide surfactant concentration range. BE-6 and BE-8 form vesicles at the critical aggregation concentration (CAC) while BE-12 forms micelles. It was found that three different regions can be observed in wetting behavior dependence on surfactant concentration. Below the CAC, the cosine of the contact angle grows almost linearly with surfactant concentration increase. At a certain surfactant concentration, higher than the CAC, the transition from partial wetting with low but measurable contact angle to complete wetting has been observed. The critical wetting concentration (CWC) was found to be independent of subphase surface energy for the substrates studied. The CWC values increase while ethoxy chain length grows. In the intermediate region, CAC < C < CWC, the surface pressure pi(sl) on \1 - C/CWC\dependence at the subphase-solution interface can be described by a critical scaling equation with critical exponent beta = 0.3-0.4. The transition from partial to complete wetting corresponds to stepwise decrease of the equilibrium interfacial tension. At the same narrow concentration region, the equilibrium spreading coefficient for solution/tetradecane becomes positive and a transition from a lens to a rapidly spreading unstable thick film was observed for these solutions on tetradecane. The adsorption of BE-12 on Graphon study has shown that the wetting transition was related to a transition from a plateau region to a significant increase of surfactant adsorption on solid substrate. The mechanism of observed wetting behavior was discussed on the basis of modern conceptions of interfacial transition and surfactant thin-film properties. The applicability of Cahn's concept of critical wetting transition to the observed wetting behavior is also discussed.