Thermodynamics of adsorption of dodecyltrimethylammonium bromide onto laponite in fused ethylammonium nitrate and its aqueous solutions

Abstract

The adsorption of a cationic surfactant, dodecyltrimethylammonium bromide (DTABr), onto laponite was studied in a molten salt at room temperature (θ_f = 14 °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.