Charging of Dielectric Surfaces in Contact with Aqueous Electrolytes─the Influence of CO 2 .

The charge state of dielectric surfaces in aqueous environments is of fundamental and technological importance. Here, we study the influence of dissolved molecular CO 2 on the charging of three chemically different surfaces (SiO 2 , Polystyrene, Perfluorooctadecyltrichlorosilane). We determine their charge state from electrokinetic experiments. We compare an ideal, CO 2 -free reference system to a system equilibrated against ambient CO 2 conditions. In the reference system, the salt-dependent decrease of the magnitudes of ζ-potentials follows the expectations for a constant charge scenario. In the presence of CO 2 , the starting potential is lower by some 50%. The following salt-dependent decrease is weakened for SiO 2 and inverted for the organic surfaces. We show that screening and pH-driven charge regulation alone cannot explain the observed effects. As an additional cause, we tentatively suggest dielectric regulation of surface charges due to a diffusively adsorbed thin layer of molecular CO 2 . The formation of such a dynamic layer, even at the hydrophilic and partially ionized silica surfaces, is supported by a minimal theoretical model and results from molecular simulations.