Surfactant adsorption at solid-liquid interfaces

Abstract

The purpose of this review is to introduce systematically a new adsorption theory which is particularly applicable to surfactant adsorption, but might also be useful in other cases. The adsorption of surfactant from solution onto solids generally shows two-step character. In the first step the surface-active species are adsorbed through the interactions (i.e. electrostatic interaction and/or van der Waals interaction) between surface-active species and the solid interface. In the second step the surfactants are adsorbed through the interaction (i.e. hydrophobic interaction if the adsorption is from aqueous solution, and hydrogen bonding or polar interaction if the adsorption is from non-polar solvents) between the adsorbed surfactants. Based on the two-step adsorption model and the mass action treatment, a general adsorption isotherm equation has been derived. The equation shows that the adsorbed amount Γ depends on the equilibrium concentration C, saturated adsorption Γ∞, the surface aggregation number (of surface micelle) n, and the equilibrium constants k 1 and k 2 for the first and second step, respectively. The equation has been applied to various types of surfactant adsorption isotherms on various adsorbents from both aqueous and non-aqueous solutions (including Langmuir-, S-, and LS-types) successfully. From the results of the thermodynamics of adsorption, one can conclude that when dealing with adsorption from aqueous solution, the second step of surfactant adsorption (i.e. surface micellization) is an entropy-driven process similar to the micellization in bulk aqueous solution. However, as expected, the reverse surface micelle formation from non-polar solvent is not an entropy-driven process. The isotherm equations for individual adsorptions of surfactant monomers, surface micelles and unoccupied sites at the surface have also been derived. In addition, by combining the general isotherm equation and the Gibbs adsorption equation, the interfacial pressure-concentration relationships have been derived. Therefore, one can expect that knowledge of two-dimensional states and transitional phases of adsorbed surfactant films at solid/liquid interfaces could be provided by this kind of investigation.