Surfactant adsorption and surface micellization

Abstract

This paper briefly reviews a new adsorption theory particularly applicable to surfactant adsorption. A one-step model of surface micellization is first introduced. By using the mass-action law treatment, an S-type isotherm equation is derived which explains the adsorption of nonionic surfactants from aqueous solution onto polar adsorbents. However, the adsorption of ionic surfactant generally shows two-step character. First the surface-active species are adsorbed through electrostatic attraction and/or van der Walls interaction. The second step involves hydrophobic interaction if the adsorption is from aqueous solution, and hydrogen bonding or other polar interaction if from non-polar solvent. Under these conditions a general adsorption isotherm equation is derived. The model indicates that the size of surface micelles is usually much smaller than that of corresponding bulk micelles. From thermodynamics of adsorption, it is evident that when dealing with adsorption from aqueous solution, the second step, i.e. surface micellization, is an entropy-driven process similar to the bulk micellization.