WETTING AND INTERFACIAL TRANSITIONS IN DILUTE-SOLUTIONS OF TRISILOXANE SURFACTANTS

Wetting behavior on apolar solid and fluid substrates of nonionic tris iloxane surfactants BE-n (B = (Me3SiO)(2)Si(Me)(CH2)(3), hydrophobic m oiety, the same for surfactants studied, n = 6, 8, and 12, ethoxy grou p number) aqueous solutions was studied in a wide surfactant concentra tion range. BE-6 and BE-8 form vesicles at the critical aggregation co ncentration (CAC) while BE-12 forms micelles. It was found that three different regions can be observed in wetting behavior dependence on su rfactant concentration. Below the CAC, the cosine of the contact angle grows almost linearly with surfactant concentration increase. At a ce rtain surfactant concentration, higher than the CAC, the transition fr om partial wetting with low but measurable contact angle to complete w etting 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 descr ibed by a critical scaling equation with critical exponent beta = 0.3- 0.4. The transition from partial to complete wetting corresponds to st epwise decrease of the equilibrium interfacial tension. At the same na rrow concentration region, the equilibrium spreading coefficient for s olution/tetradecane becomes positive and a transition from a lens to a rapidly spreading unstable thick film was observed for these solution s on tetradecane. The adsorption of BE-12 on Graphon study has shown t hat the wetting transition was related to a transition from a plateau region to a significant increase of surfactant adsorption on solid sub strate. The mechanism of observed wetting behavior was discussed on th e 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 .