The surface saturation of solutions of three low-molecular-weight surface-active alcohols were identified independently using sum frequency generation spectroscopy. Tensiometry showed that solution surface tension kept decreasing uniformly and considerably even after full saturation of the adsorption monolayer. We employed Guggenheim's model of extended interface to attribute this observation to the ongoing adsorption of alcohol molecules beneath the saturated topmost adsorption layer. Our investigation of the dynamic behaviour of thin liquid films of alcohol solutions revealed the tremendous effect of sub-monolayer adsorption on the rheological characteristics of surface. Sub-monolayer region was found to function as a supplementary source of alcohol molecules. The fast diffusion of surfactants from sub-monolayer to the topmost adsorption layer imposed a buffering effect on the dynamic response of the surface through diminishing the surface tension gradient created by surface expansion. This resulted in sudden drop of the Gibbs elasticity and consequently faster decaying foam.
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