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The desorption rate stands between the theoretical curves for pure barrier control (intensive convection) and mixed barrier + diffusion control (no convection).

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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.

Aqueous mixtures of cationic hexadecyltrimethylammonium chloride (CTAC) and nonionic pentaethyleneglycol monododecyl ether (C12E5) are investigated. Adsorption layer properties are systematically studied within a wide concentration range for a 1:1 molar ratio of the surfactants. Surface tension and dilatational rheology measurements are conducted by profile analysis tensiometry. The interfacial data are juxtaposed to drainage kinetics and stability results for microscopic foam films, investigated by microinterferometric thin liquid film instrumentation. The obtained results give experimental evidence of synergistic interactions in the studied solutions, as compared to the corresponding single surfactant systems. Specific runs of dynamic and equilibrium surface tension curves are registered against the total surfactant quantity; the surface dilatational elasticities for the mixtures are systematically higher. A clear correlation is established between adsorption layer performance and foam film characteristics. The maxima of the film lifetimes are well outlined, and the respective values are shifted towards lower overall concentrations. The reported results substantiate the key role of the adsorption layers, and the surface dilatational properties in particular, for foam film drainage kinetics and stability. The well-expressed synergy observed in adsorption layer and foam film properties suggests the substantial benefits of using mixed surfactant systems in the design and fine-tuning of foam systems for innovative applications.

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