Adsorption kinetics of surfactants at fluid-fluid interfaces

We review a new theoretical approach to the kinetics of surfactant adsorption at fluid-fluid interfaces. It yields a more complete description of the kinetics both in the aqueous solution and at the interface, deriving all equations from a free-energy functional. It also provides a general method to calculate dynamic surface tensions. For non-ionic surfactants, the results coincide with previous models. Non-ionic surfactants are shown to usually undergo diffusion-limited adsorption, in agreement with the experiments. Strong electrostatic interactions in salt-free ionic surfactant solutions are found to lead to kinetically limited adsorption. In this case, the theory accounts for unusual experimental results which could not be understood using previous approaches. When salt is added, the electrostatic interactions are screened and the ionic surfactant adsorption becomes similar to the non-ionic case. The departure from the non-ionic behavior as the salt concentration is decreased is calculated perturbatively.