Adsorption of cationic amphiphiles on laponite in water–NaBr mixtures at 298 K. Surface pressure of adsorbed amphiphiles linked to the Freundlich isotherm

Abstract

The adsorption of amphiphilic ions on clays in aqueous solutions is treated in surface pressure terms. To take into account the fact that the surfactant-modified clay leads to a flocculated phase rather than a dispersed solid, the mass of the solid is assumed to be linked to its area by a homogeneous function of the mass, of order p (p<1). The thermodynamic relationships are illustrated by the behavior of the dodecyltrimethylammonium–laponite system in several water–NaBr mixtures with 0⩽C_NaBr⩽0.3 mol L^-1. Freundlich isotherms were obtained over the range of studied NaBr concentrations. The amount of adsorbed surfactant, Q_M, is linked to the molality ω of the free amphiphile in solution by the relationship Q_M=αω^β, where α and β are two coefficients. The established equations show that if p and β are the same, the Freundlich isotherm accounts for the model behavior of an ideal surface gas on a solid whose mass and area are linked by a homogeneous function of the mass, of order p. The p value is very low for low ionic strength media (p=0.17 for C_NaBr=0.05 mol L^-1). It increases with increasing salt concentration in the medium (p=0.31 for C_NaBr=0.3 mol L^-1).