The zeta potential of quartz. Surface complexation modelling to elucidate high salinity measurements

The zeta potential is a measureable electrical potential of paramount importance to understand the electrochemical properties of rocks. However, the zeta potential remains poorly understood because it takes place at the nanoscale of the electrical double layer on the mineral surface. Streaming potential measurements on quartz-rich Fontainebleau and Lochaline sandstones carried out at high salinity (above 0.1 M NaCl) yield surprisingly high zeta potential values, which cannot be correctly reproduced by a traditional surface complexation model considering that the shear plane is located at the beginning of the diffuse layer. We found that placing the shear plane, where the zeta potential is defined, slightly closer to the mineral surface than the Stern plane significantly improves the predictions of the zeta potential and surface charge density of quartz at high salinity as well as the values of the equilibrium constant describing sodium adsorption in the Stern layer. Our results have strong implications for the modelling of the electrochemical properties of minerals in contact with highly saline solutions.