Surfactant Effects on Nonpolar Surfaces

When one considers the effects of low-molecular-weight surfactants on wetting, it is helpful to divide the subject into two regimes the effect on nonpolar surfaces and that on polar surfaces the reason is that, for nonpolar surfaces, it is often possible to assume that adsorption at the S2 interface will be negligible. If surfactant adsorption at the S2 interface is small (e.g., F2 0), as is usually observed for the situation where fluid 2 is air, Equations (17.41) and (17.42) indicate that 

Theoretically, one can determine the adsorption at the SI interface from the change in 0° with surfactant concentration. Using the Gibbs equation, it is possible to determine the adsorption of surfactant at the 1-2 interface (from dcTuld In c). It is found experimentally that for most hydrocarbon surfactants on nonpolar surfaces, the adsorption at SI is the same as that at the 1-2 interface, or that Fsi F.  

A more quantitative picture of the situation can be obtained by looking at the effect of variations in ti2 on the reversible work of adhesion, Wasn-Differentiating Equation (17.20) by an yields 

Assuming that adsorption at the S2 interface is zero, and setting (dasjldan) = 

the reversible work of adhesion will be independent of surfactant concentration. On the basis of these assumptions, a number of interesting relationships can be derived relating the effects of surfactant concentration on wetting phenomena. 

---