Wave damping and gibbs elasticity for nonideal surface behavior

Abstract

In existing theories for capillary ripples on surfactant solutions and for Gibbs elasticity of thin films the thermodynamic surface characteristics of the surfactant have been introduced explicitly only for cases of ideal surface behavior. The present study quantitatively evaluates the effect of the ideality deviations occurring under many practical conditions. The influence of these ideality deviations resides in their effect on the surface tension gradients that determine both capillary ripple properties and Gibbs elasticity of thin films. The ideality deviations treated here are those of regular behavior, implying higher values of surface pressure and surfactant adsorption at a given concentration than for ideal behavior. As a consequence, the surface compression modulus, to which the surface tension gradients are proportional, reaches higher values that are less sensitive to diffusional surface-bulk interchange than in the ideal case. The result of this is three-fold: the maximum through which the wave-damping coefficient generally passes upon surfactant addition is much sharper, damping for very soluble monolayers is much higher, and Gibbs elasticity of thin films also reaches much higher values than for ideal surface behavior under otherwise similar conditions. In particular, the peak in damping coefficient is most sensitive to variations in nonideality of a surface; it is therefore suggested that wave-damping measurements are eminently suitable for quantitative assessment of “specific interaction” between surfactants.