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 wetting behavior of CTAB solution on mica at concentrations from 10 −7to 1.6 × 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 × 10 −5 M(contact angle 73°), and then increases up to 3 × 10 −4 Mdue 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 force 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.