Monolayer penetration by a charged amphiphile: equilibrium and dynamics

The penetration of a soluble surfactant into an insoluble monolayer provide s 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 a nd numerically. The equilibrium increase in surface pressure Delta pi cause d 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 Delta pi assuming either diffusion controlle d or mixed kinetic-diffusion controlled mass transfer. Several trends are p redicted with variations of the surface coverage of the insoluble component , the concentration of soluble surfactant and the ionic strength of the sur factant subphase. Experiments in the literature have shown that, for an unc harged monolayer, Delta pi at equilibrium is greater the higher is the surf ace coverage of the insoluble monolayer into which the ionic, soluble surfa ctant adsorbs. Our results show that this trend can be attributed to the ro le of the insoluble component in presenting an entropic barrier to adsorpti on, thereby reducing the repulsive surface charge density at a given net su rfactant coverage, allowing more surfactant to adsorb. Greater surface pres sures result. Signature trends in the timescales for diffusion controlled m ass transfer of an ionic surfactant as a function of initial surface covera ge of the insoluble monolayer are derived. This timescale is longer than pr edicted by simply accounting for the area blocked by the insoluble componen t using a Langmuir argument, and approaches the Langmuir argument with incr easing ionic strength. For mixed kinetic-diffusion controlled mass transfer , because the insoluble component blocks interface, it reduces the amount o f surfactant that can adsorb. This decreases the diffusion timescale, and a llows adsorption-desorption kinetics to play a controlling role. Since elec trostatic repulsion reduces the adsorption of the soluble component, kineti cs also play a stronger role the lower is the ionic strength or the higher the surfactant valence. Considering a surfactant with fixed physicochemistr y, a shift of controlling mechanism from diffusion control to kinetic contr ol is demonstrated with increasing bulk concentration, surface charge densi ty or surface coverage of insoluble component. (C) 2001 Elsevier Science B. V. All rights reserved.