THERMODYNAMICS OF ADSORPTION OF DODECYLPYRIDINIUM CHLORIDE ON NA-KAOLINITE

A study was undertaken to determine the influence of temperature and e lectrolyte concentration on the adsorption of dodecylpyridinium chlori de (DPC) on Na-kaolinite. Enthalpies were measured using an isothermal microcalorimeter by titration of a kaolinite suspension with a concen trated surfactant solution at 6, 20 and 60-degrees-C. Curves of cumula tive adsorption enthalpies against amount adsorbed show a break around the transition from monolayer to bilayer coverage. The curves are fai rly linear both before and after the break, and resuLt in partial mola r adsorption enthalpy values that are independent of surface coverage, confirming that the surface is homogeneous with respect to the DPC ad sorption. This is additional evidence that the surfactant is hardly ad sorbed on the edges. Adsorption isotherms at different electrolyte con centrations exhibit a common intersection point that coincides with th e isoelectric point and correponds to the transition from monolayer to bilayer coverage. Isotherms are independent of temperature up to mono layer coverage, implying a zero isosteric adsorption enthalpy, but upo n bilayer formation the adsorption has a maximum at about 23-degrees-C , implying a transition from an endothermic to an exothermic process. The calorimetric adsorption enthalpies change sign at T almost-equal-t o 24-30-degrees-C. In this respect, the adsorption process is very sim ilar to micellization. The electrolyte concentration has a minor effec t on the adsorption enthalpies of the first and second layers. There a re some qualitative agreements and some quantitative discrepancies bet ween the isosteric heats of adsorption and the directly measured heats . Our experimental data fit well to a bilayer model based on the Frumk in-Fowler-Guggenheim equation.