General isotherm equation for adsorption of surfactants at solid/liquid interfaces. Part 1. Theoretical

Abstract

Based on the two-step adsorption mechanism (first step : the surface-active species are adsorbed through the interactions between the surface-active species and the solid surface; second step: through the hydrophobic interaction between the adsorbed surface-active species) and the mass-action treatment, the following general adsorption isotherm equation is derived: Γ=Γ_∞k₁C([graphic omitted] +k₂C^n–1)// 1 +k₁C(1 +k₂C^n–1) where Γ is the amount of surfactant adsorbed at concentration C, Γ_∞ is the limiting adsorption at high concentrations, k₁ and k₂ are the equilibrium constants involved in the first and second step, respectively, and n is the aggregation number of surface hydrophobic aggregates or hemimicelles. The equation can interpret various types of surfactant adsorption isotherms (including L-, and LS-types) published in the literature.