ADSORPTION DYNAMICS OF SINGLE AND BINARY SURFACTANTS AT THE AIR-WATER-INTERFACE

Models of diffusion-controlled adsorption of mixed surfactants at the air/water interface reveal a rich variety of interesting behavior. The models predict the following. A maximum in the dynamic adsorption concentration for the less surface-active surfactant can occur if this component has a higher initial bulk concentration. The maximum is even more pronounced for small diffusion lengths, at which the dynamic subsurface concentration of the fast-adsorbing component can temporarily exceed that of the bulk. If an impurity is more surface-active than the main component, then small amounts of the impurity can drastically alter the adsorption behavior of the latter, as shown in some examples. These phenomena can be partly explained in terms of the timescales of adsorption for single surfactants described by the Henry's law or Langmuir isotherms, and partly by the competition of the components for surface sites. Since dynamic surface concentration data are not yet available for binary surfactants, the model predictions are used to calculate dynamic surface tensions and compare them with dynamic tension data. The results have implications for the dynamic and equilibrium selectivities of foam fractionation processes and for the dynamics of free-surface flows.