Thermodynamics of adsorption of dodecylpyridinium chloride on Na-kaolinite

Abstract

A study was undertaken to determine the influence of temperature and electrolyte concentration on the adsorption of dodecylpyridinium chloride (DPC) on Na-kaolinite. Enthalpies were measured using an isothermal microcalorimeter by titration of a kaolinite suspension with a concentrated surfactant solution at 6, 20 and 60°C. Curves of cumulative adsorption enthalpies against amount adsorbed show a break around the transition from monolayer to bilayer coverage. The curves are fairly linear both before and after the break, and result in partial molar adsorption enthalpy values that are independent of surface coverage, confirming that the surface is homogeneous with respect to the DPC adsorption. This is additional evidence that the surfactant is hardly adsorbed on the edges. Adsorption isotherms at different electrolyte concentrations exhibit a common intersection point that coincides with the isoelectric point and correponds to the transition from monolayer to bilayer coverage. Isotherms are independent of temperature up to monolayer coverage, implying a zero isosteric adsorption enthalpy, but upon bilayer formation the adsorption has a maximum at about 23°C, implying a transition from an endothermic to an exothermic process. The calorimetric adsorption enthalpies change sign at T ≈ 24–30°C. In this respect, the adsorption process is very similar to micellization. The electrolyte concentration has a minor effect on the adsorption enthalpies of the first and second layers. There are some qualitative agreements and some quantitative discrepancies between the isosteric heats of adsorption and the directly measured heats. Our experimental data fit well to a bilayer model based on the Frumkin—Fowler—Guggenheim equation.