Charging of dielectric surfaces in contact with aqueous electrolyte -- 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-dependence 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 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.