Comparative Adsorption of Cationic and Anionic Surfactants on Fluorite

Abstract

The adsorption behaviours of cetyltrimethylammonium bromide (CTAB) and dioctyl sodium sulphosuccinate (AOT) on fluorite mineral were analyzed as examples of the adsorption of cationic and anionic surfactants on salt-type minerals. Although both surfactants gave adsorption densities well below the monolayer capacity, there were considerable differences between the behaviours of the two surfactants. Thus, while the adsorption density of CTAB increased to a plateau value, that of AOT increased to a maximum before decreasing to very low values at high equilibrium concentrations of the surfactant. Analysis of the behaviour of the surfactants was made using a modified version of the Frumkin adsorption isotherm, taking into account the cross-sectional area of the adsorbed surfactant species and the lateral interaction between their long hydrocarbon chains. The values of the free energy change indicated that both CTAB and AOT underwent adsorption via a physical process, while the lateral interaction coefficient was found to be twice as large in the case of AOT as for CTAB for small or moderate coverage. This latter behaviour was attributed to the fact that while CTAB possesses only one hydrocarbon chain per molecule, AOT has two such chains.