Dwelling on the adsorption of surfactant at the air/water interface in relation to its states in the bulk: A comprehensive analysis

Abstract

The self-aggregation (micellization) of surfactants in the bulk in relation to their adsorption at the air/water interface has been analyzed and discussed. In this respect, the thermodynamic rationales of Traube, and Rosen and Aronson have been considered and analyzed at length. The first deals with adsorption at low surface pressure, the second hypothetically deals with concentration close to the critical micelle concentration having a constant surface excess and zero surface pressure. The question of possible micelle formation prior to completion of the process of adsorption has been also addressed. The standard Gibbs free energy of adsorption by Traube's method, and the method of Rosen and Aronson for ionic, non-ionic, zwitterionic, and bolaform surfactants have been evaluated: their differences have been attempted to account for and discussed. Considering a Born–Haber type cycle, the free energies of adsorption and micelle formation have been coupled together to describe the thermodynamic manifestation of the interrelated systems with a justification.Although the standard free energy of adsorption is lower than the standard free energy of micellization, and hence it is relatively more favored, but it has been perceived that with increasing surfactant concentration (i.e., with the progress of the process), the apparent (non standard) free energy of adsorption of amphiphiles may become relatively more positive, resulting in micelle formation even before the surface saturation.