ADSORPTION OF ALKYLTRIMETHYLAMMONIUM BROMIDES ON SILICA - CALORIMETRIC STUDY OF EFFECT OF COIONS

Adsorption isotherms and differential enthalpies of adsorption have been determined for dodecyl- and tetradecyltrimethylammonium bromide on silica at initial pH 9 and on mica in sodium, tetramethylammonium, and calcium bromide solutions of low concentrations. The adsorption isotherms show a clear difference between the influence of the various colons at low coverage and the effect is also seen in the calorimetric results. The initial adsorption is discussed in terms of a competition between surfactant ions and ions of the same charge. The colon effect disappears at higher adsorption densities. The relation between the shape of the adsorption isotherm and the charge properties of the surface and their variation with experimental conditions are discussed. The general trend of the change of the differential adsorption enthalpies with coverage is the same for the adsorption on silica and mica except in the initial stage. As the surface coverage increases, there is a steady decrease toward more exothermic values so, as the maximum adsorption is reached, the adsorption enthalpy for tetradecyltrimethylammonium bromide on silica is -10 kJ/mol more exothermic than the enthalpy of micelle formation. The change of the adsorption enthalpy with surface coverage is discussed in terms of changes in the structure of the aggregates formed at the surface. The calorimetric results are consistent with the interpretation of the adsorption of surfactants on highly charged surfaces expressed in a newly developed thermodynamic model implying adsorption of small discrete aggregates at the lowest adsorption densities and bilayer formation in the later stages of adsorption. (C) 1995 Academic Press, Inc.