Assessment of the surface heterogeneity of talc materials

The hydrophobic and hydrophilic components of the surface of talc materials in aqueous solution were determined using ionic surfactants and their pola r headgroup adsorption isotherms. The hydrophilic and hydrophobic surface a reas are inferred from the amount of probe molecule adsorbed and the struct ure of the adsorbed layer. Natural dispersion of talc shows at 298 K a pH o f 9.4 and the electrophoretic measurements indicate that the particles are negatively charged. The hydrophilic surface area is estimated from the adso rption of benzyltrimethylammonium ions (BTMA(+)) through electrostatic inte ractions as supported by the increase of divalent ions in the bulk phase an d the decrease in the exothermic displacement enthalpy. It was also observe d from the adsorption isotherm of benzene sulfonate anions that the density of positive surface sites is very low and is thus neglected. The adsorptio n of an anionic surfactant essentially occurs through dispersive interactio ns between the nonpolar organic tail of the molecule and the hydrophobic su rface. Furthermore, some assumptions on the structure of dodecyl sulfate su rfactant aggregates at the interface allow the hydrophobic part of the talc particles to be estimated. The cationic surfactant adsorption has been inv estigated and found to corroborate the hydrophilic and hydrophobic area val ues first obtained. (C) 2001 Academic Press.