CHARGE EFFECTS IN THE TRANSIENT ADSORPTION OF IONIC SURFACTANTS AT FLUID INTERFACES

A rigorous model is presented for diffusive transport of ionic surfactants to an adsorbing interface. The proposed model considers both the diffusion and migration of surfactant, counterions, and background electrolyte in the electric field that develops as the charged surfactant adsorbs at an interface. The transient electrical double-layer structure arising from specific adsorption of the surfactant is calculated by solving the coupled Nernst-Planck and Poisson equations in the bulk phase using a Frumkin constitutive relation for the interfacial boundary condition. The resulting transient double-layer model is valid over all time scales and for interfacial potentials and background electrolyte concentrations of any magnitude. Therefore, the proposed model is more general than previously derived ''quasiequilibrium'' models of ionic surfactant transport to the interface that require instantaneous equilibrium potentials in the double layer.(1-4) We compare results from the proposed ionic surfactant transport model to results from the standard Ward-Tordai model for nonionic surfactants adsorbing at a fluid/fluid interface. For low concentrations of strongly adsorbing surfactant, in the absence of background electrolyte, electrostatic effects decrease the equilibrium adsorption of surfactant by an order of magnitude. Correspondingly, the time required for equilibration of the interface is decreased. When the transient adsorption is scaled to eliminate differences due solely to different equilibrium adsorption, we discover that the rate of diffusion-limited transport of an ionic surfactant is decreased by an order of magnitude compared to that of an equivalent nonionic surfactant. Addition of nonadsorbing background electrolyte, increasing surfactant concentration, or weaker adsorption of surfactants decreases the electrostatic effects. A simple quasiequilibrium model that assumes a double layer in instantaneous equilibrium with an electroneutral bulk solution is also developed. Comparison of this quasiequilibrium model to the full transient model shows that, for a typical surfactant (e.g., sodium dodecyl sulfate) adsorbing at the water/air interface, differences between the full transient model and quasiequilibrium model occur only at times inaccessible to current dynamic surface tension techniques.