The thermodynamics of surfactant aggregation and adsorption at the clay-water inteface.

Abstract

Heats of adsorption indicated that at neutral pH anionic surfactants interact weakly with montmorillonite, but more strongly with alumina. The results also indicated that a calorimetric approach to studying surfactant-solid surface interactions can generate insight into the problem not easily obtained from other techniques.

Relative apparent partial molar enthalpies were determined for all three surfactants at both temperatures. A model was developed to account for the enthalpy in terms of the mass action model. All deviations from Debye-Huckel limiting law behavior were considered to be due to hydrophobic association. Four conclusions were drawn from the model: (i) the model strongly indicates the formation of premicellar aggregates, (ii) the mass action model should be used to model the behavior of surfactants, (iii) the failure of the model at high concentrations indicated that micelle-micelle electrostatic interactions must be considered in this range, and (iv) no simple relationship exists between the relative apparent partial molar enthalpy above the cmc and the enthalpy of micellization.

The thermodynamics of surfactant aggregation and adsorption at the solid surface were investigated calorimetrically. Heats of dilution were obtained for sodium octylsulfate and sodium octylsulfonate at 298.15 K and 308.15 K. Heats of solution and heats of dilution were obtained for sodium octylbenzenesulfonate at 298.15 K and 308.15 K. Heats of adsorption of these surfactants on alumina and natural and sodium exchanged homoionic montmorillonite were measured.