Chain length effects in the adsorption of surfactants at aqueous interfaces: Comparison of existing adsorption models with a new model

Abstract

Adsorption isotherm equations for surfactant or collector adsorption, such as the Stern-Langmuir equation and the regular behavior model are briefly discussed. It is concluded that for a correct description of the adsorption process, chain length effects must be taken into account far more stringently than has been the practice to date. To this end a general model for the adsorption of flexible long-chain surfactants with a charged head group is developed on the basis of the polymer adsorption theory of Scheutjens and Fleer, in which electrostatic interactions are incorporated. The general model is applied for two specific adsorbed chain conformations: flat adsorption and end-on or perpendicular adsorption. These conformations represent the extremes for adsorption of a surfactant onto a hydrophobic and a hydrophilic surface, respectively. The isotherm equations quantitatively show how the chain length effects contribute to the adsorption free energy. In the case of pseudoideal behavior, which is observed if the solvent is poor, both equations reduce to the Stern-Langmuir isotherm. For r = 1 the equation reduces to the regular behavior model. Moreover, the general form of the equations is the same as that obtained in the case of regular behavior. The form of the master equation suggests that it will be quite useful in describing surfactant adsorption. A brief discussion is given of Traube's rule and the hemi-micelle concept in the light of the newly developed model.