Anomalous dynamic surface tension of mixtures of nonionic surfactants with different partial molar areas at the water-air interface

On the basis of a thermodynamic model, the adsorption isotherm and a surface tension equation are derived for mixtures of nonionic surfactants with different partial molar surface area. The ratio of these partial molar areas of the two surfactants omega(1)/omega(2) has significant influence on the form of the dynamic surface tension curve. For omega(2) > omega(1) a maximum in the gamma(t) dependence can be expected, even under the assumption of a pure diffusion-controlled adsorption and desorption of both mixture components. The anomalous behavior can be observed when a significant desorption of the more concentrated surfactant sets in due to a starting competitive adsorption of the less concentrated but more surface active component of the mixture. Experiments are performed with different surfactant mixtures chosen such that all three cases could be studied: omega(2) > omega(1), omega(2) approximate to omega(1), or omega(2) < omega(1). The surfactants used are Triton X-405 (large molecular area), alkyl dimethyl phosphine oxides (medium molecular area), and n-alkanols (small molecular area). Anomalous effects are experimentally observed for omega(2) > omega(1).