however, that thermodynamic relationships such as the The Wilhelmy plate method was used to study the wetting behavior Gibbs adsorption equation can be applied to the experimental of CTAB solution on mica at concentrations from 10 07 to 1.6 1 10 03 data (4). The results obtained for the CTAB/glass system M. A freshly cleaved mica sheet shows stick-jump behavior of the were compared with recent surface force investigations on meniscus. The large amplitude oscillations of the dynamic advancing the same system by Parker et al. (5,6), using a novel noninwetting tension are due to surfactant adsorption at the three-phase terferometric technique (7). Below and around the point of contact line. Desorption in pure water is slow. During relaxation zero charge (pzc), which is at about 5 1 10 05 M for CTAB/ measurements in surfactant solutions, the wetting tension reaches its glass, the wetting tension decreases with increasing CTAB equilibrium value on a time scale of several hours. The equilibrium concentration. This indicates a more extensive surfactant wetting tension first decreases with concentration due to solid/vapor monolayer adsorption, reaches a minimum at 2.4 1 10 05 M (contact adsorption at the solid/vapor (SV) interface than at the angle 73ะ), and then increases up to 3 1 10 04 M due to solid/liquid solid/liquid (SL) interface. Approximately at the pzc, the bilayer adsorption. A complete solid/liquid monolayer without outer decrease in wetting tension begins to level off. It passes layer is never observed. The solid/liquid and solid/vapor adsorption through a minimum at about half the critical micelle concenisotherms were deduced from application of the Gibbs adsorption tration (cmc), after which the wetting tension increases up equation and surface force and XPS data. The solid/liquid bilayer to the cmc ร9 1 10 04 M. In this branch, a larger adsorption contains 1.4-1.6 surfactant cations per mica charge site, 0.7-1.0 in at the SL interface than at the SV interface is indicated, the inner layer and 0.6-0.75 in the outer layer. The areas per surfacwhich is consistent with the formation of an adsorbed SL tant cation are 0.48-0.68 and 0.64-0.80 nm 2 , respectively. แญง 1996 bilayer.