Charging of dielectric surfaces in contact with aqueous electrolyte -- the influence of CO$_2$

TL;DR

This study investigates how dissolved CO$_2$ influences the surface charge of various dielectric materials in aqueous environments, revealing effects beyond traditional screening and pH regulation, supported by experiments, modeling, and simulations.

Contribution

It introduces the concept of dielectric regulation of surface charges due to adsorbed CO$_2$, supported by experimental and theoretical evidence.

Findings

CO$_2$ affects surface charge states of dielectrics

Screening and pH regulation do not fully explain observed effects

A dynamic CO$_2$-adsorbed layer influences surface charge

Abstract

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.