Adsorption/aggregation of surfactants and their mixtures at solid-liquid interfaces

Adsorption of surfactants and polymers at solid-liquid interfaces is used widely to modify interfacial properties in a variety of industrial processes such as flotation, ceramic processing, flocculation/dispersion, personal care product formulation and enhanced oil recovery. The behavior of surfactants and polymers at interfaces is determined by a number of forces, including electrostatic attraction, covalent bonding, hydrogen bonding, hydrophobic bonding, and solvation and desolvation of various species. The extent and type of the forces involved varies depending on the adsorbate and the adsorbent, and also the composition and other characteristics of the solvent and dissolved components in it. The influence of such forces on the adsorption behavior is reviewed here from a thermodynamics point of view. The experimental results from microcalorimetric and spectroscopic studies of adsorbed layers of different surfactant and polymer systems at solid-liquid interfaces are also presented. Calorimetric data from the adsorption of an anionic surfactant, sodium octylbenzenesulfonate, and a non-ionic surfactant, dodecyloxyheptaethoxyethylalcohol, and their mixtures on alumina, yielded important thermodynamic information. It was found that the adsorption of anionic surfactants alone on alumina was initially highly exothermic due to the electrostatic interaction with the substrate. Further adsorption leading to a solloid (hemimicelle) formation is proposed to be mainly an entropy-driven process. The entropy effect was found to be more pronounced for the adsorption of anionic-non-ionic surfactant mixtures than for the anionic surfactant alone. Fluorescence studies using a pyrene probe on an adsorbed surfactant and polymer layers, along with electron spin resonance (ESR) spectroscopy, reveal the role of surface aggregation and the conformation of the adsorbed molecules in controlling the dispersion and wettability of the system.