Thermodynamics of adsorption of dodecyltrimethylammonium bromide onto laponite in fused ethyl-ammonium nitrate and its aqueous solutions

The adsorption of a cationic surfactant, dodecyltrimethylammonium bromide (DTABr), onto laponite was studied in a molten salt at room temperature (theta(f) = 14 degrees C), ethylammonium nitrate (EAN), and in mixtures of this salt with water at 298 K. Adsorption isotherms were determined by potentiometric methods using DTA(+) and Br--selective electrodes. At low EAN concentrations (less than 1 mol L-1), the shape of the adsorption : isotherm is classical and all curves share a common intersection point. In more concentrated salt solutions, the quantity of adsorbed amphiphile decreases with increasing EAN concentration. The activity coefficients of transfer of DTABr between water, EAN-water solutions and molten EAN were determined. A comparison of adsorption isotherms at a given surfactant chemical potential shows that the,quantity of DTA+ adsorbed onto laponite is approximately four time less in molten salt than that in the 1 mol L-1 EAN aqueous solution. This may be caused by formation of an adsorbed layer constituted of both dodecyltrimethylammonium and ethylammonium ions.