Influence of sulfate ions on the interaction between fatty acids and calcite surface

The effect of sulfate ions on adsorption/displacement of long chain fatty acids onto/from calcite surface is investigated. The fatty acids used in this work are stearic acid (SA), oleic acid (OA) and 18-cyclohexyl-octadecanoic acid (CHOA). The main methods are thermogravimetric analysis for determining the extent of surface modification and vapour adsorption isotherm to evaluate the wetting behaviour. An appreciable reduction of adsorbed amount of fatty acids is observed in presence of sulfate ions. This is interpreted by the high affinity of the sulfate ions for the active sites of the calcite. In addition, they modify the calcite surface to a negative charge, as indicated by zeta potential measurements, hence reducing the availability of the active sites for carboxylate ions. The ability of sulfate ions to displace the pre-adsorbed fatty acids from the calcite surface is also demonstrated. It is mostly related to the molecular structure of the fatty acids, where the percentage of displacement by 0.1 M sulfate is found to be 7 and 10 for stearic acid and oleic acid, respectively. This work shows that the sulfate ions changed the wettability of calcite to more water-wet as indicated by water vapour adsorption isotherms.