Monolayer penetration by a charged amphiphile: equilibrium and dynamics

The penetration of a soluble surfactant into an insoluble monolayer provides a means of understanding intermolecular interactions and their impact on equilibrium and dynamic surface pressures. In this paper, the adsorption of an ionic surfactant into an insoluble monolayer is studied theoretically and numerically. The equilibrium increase in surface pressure Δπ caused by the surfactant adsorption is derived for a Davies adsorption isotherm using a Gibbs adsorption equation properly constrained for the presence of the insoluble monolayer. The dynamic surface pressure is studied using this surface equation of state for Δπ assuming either diffusion controlled or mixed kinetic-diffusion controlled mass transfer. Several trends are predicted with variations of the surface coverage of the insoluble component, the concentration of soluble surfactant and the ionic strength of the surfactant subphase. Experiments in the literature have shown that, for an uncharged monolayer, Δπ at equilibrium is greater the higher is the surface coverage of the insoluble monolayer into which the ionic, soluble surfactant adsorbs. Our results show that this trend can be attributed to the role of the insoluble component in presenting an entropic barrier to adsorption, thereby reducing the repulsive surface charge density at a given net surfactant coverage, allowing more surfactant to adsorb. Greater surface pressures result. Signature trends in the timescales for diffusion controlled mass transfer of an ionic surfactant as a function of initial surface coverage of the insoluble monolayer are derived. This timescale is longer than predicted by simply accounting for the area blocked by the insoluble component using a Langmuir argument, and approaches the Langmuir argument with increasing ionic strength. For mixed kinetic-diffusion controlled mass transfer, because the insoluble component blocks interface, it reduces the amount of surfactant that can adsorb. This decreases the diffusion timescale, and allows adsorption-desorption kinetics to play a controlling role. Since electrostatic repulsion reduces the adsorption of the soluble component, kinetics also play a stronger role the lower is the ionic strength or the higher the surfactant valence. Considering a surfactant with fixed physicochemistry, a shift of controlling mechanism from diffusion control to kinetic control is demonstrated with increasing bulk concentration, surface charge density or surface coverage of insoluble component.