Equilibrium wetting studies of cationic surfactant adsorption on mica .1. Mono- and bilayer adsorption of CTAB

The Wilhelmy plate method was used to study the welting behavior of CTAB solution on mica at concentrations from 10(-7) to 1.6 X 10(-3) M. A freshly cleaved mica sheet shows stick-jump behavior of the meniscus. The large amplitude oscillations of the dynamic advancing wetting tension are due to surfactant adsorption at the three-phase contact line. Desorption In pure water is slow. During relaxation measurements in surfactant solutions, the wetting tension reaches its equilibrium value on a time scale of several hours. The equilibrium wetting tension first decreases with concentration due to solid/vapor monolayer adsorption, reaches a minimum at 2.4 X 10(-5) M (contact angle 73 degrees), and then increases up to 3 X 10(-4) M due to solid/liquid bilayer adsorption. A complete solid/liquid monolayer without outer layer is never observed, The solid/liquid and solid/vapor adsorption isotherms were deduced from application of the Gibbs adsorption equation and surface farce and XPS data. The solid/liquid bilayer contains 1.4-1.6 surfactant cations per mica charge site, 0.7-1.0 in the inner layer and 0.6-0.75 in the outer layer. The areas per surfactant cation are 0.48-0.68 and 0.64-0.80 nm(2), respectively. (C) 1996 Academic Press, Inc.