Regensburg 2025 – scientific programme

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DY: Fachverband Dynamik und Statistische Physik

DY 23: Poster: Active Matter, Soft Matter, Fluids (joint session DY/CPP)

DY 23.20: Poster

Wednesday, March 19, 2025, 10:00–12:00, P3

Use of molecular CO₂ for surface charge regulation — Peter Vogel¹, Markus U. Witt¹, David Beyer², Christian Holm², Muhammad Navaz Qaisrani³, Marialore Sulpizi⁴, and •Thomas Palberg¹ — ¹Inst. of Physics, JGU, Mainz, Germany — ²Inst. of Computational Physics (ICP), U Stuttgart, Stuttgart, Germany — ³MPI for Polymer Research, Mainz, Germany — ⁴Dept. of Physics, RU Bochum, Bochum, Germany

In deionized water CO₂ forms carbonic acid which partially dissociates. Such ’realistic’ salt free systems contain a significant background electrolyte concentration and a pH of 5.5. Both lowers the effective charge of dielectric surfaces. Surprisingly, the remaining molecular CO₂ causes an additional drastic decharging effect, even to complete decharging in water equilibrated against pure CO₂. Molecular CO₂ acts directly on the degree of dissociation and thus lowers the bare charge, while effective charges merely follow suit. MD simulations show the formation of a diffusely adsorbed monolayer of CO₂, which locally lowers the dielectric constant. Based on this we suggested dielectric charge regulation as novel decharging mechanism. If then salts are added to the carbonized surfaces, one finds recharging by co-ion adsorption. This process is favoured by hydrophobicity, by co-ion size and, most important, also by the amount of adsorbed CO₂. Given the ubiquity of dielectric surfaces in contact with aqueous electrolytes, this very general charge regulation processes appear to be of great fundamental and practical importance.

Keywords: dielectric surfaces; charge regulation; adsorption; carbonic acid; water