Dynamic Penetration of an Insoluble Monolayer by a Soluble Surfactant:  Theory and Experiment

Surfactants or surface active proteins adsorbing onto aqueous−gas interfaces can require many minutes to hours to attain equilibrium, depending upon the size, bulk concentration, and chemical structure of the adsorbing species. In contrast, soluble surface active molecules adsorbing into an insoluble monolayer equilibrate on a time scale that is more rapid. Here, the time dependent behavior of the adsorption of a soluble surface active molecule into an insoluble monolayer is addressed using a diffusion-controlled adsorption model with a Langmuir or Frumkin adsorption isotherm. In the Langmuir framework, the presence of an insoluble monolayer occupying the fraction x 1 of the interface reduces the amount of soluble surfactant adsorbed at equilibrium by the factor (1 − x 1). Because less surfactant must be delivered to the interface, the equilibration time scale for the adsorption process is reduced by the factor (1 − x 1)2 as compared to adsorption onto a clean interface. In a Frumkin framework, the role of cohesive or repulsive interactions in altering the equilibrium amount delivered and the equilibration time scale is probed. The trends predicted by this framework are shown to be in qualitative agreement with dynamic surface pressure data for the penetration of DPPC (dipalmitoylphosphatidylcholine) monolayers by the soluble protein lysozyme taken on a multicompartment circular penetration trough. Discrepancies between predicted and observed time scales are suggestive either of a kinetic barrier to adsorption or of intermolecular interactions between the adsorbing molecules.