CO2-induced drastic decharging of dielectric surfaces in aqueous suspensions

Abstract

We study the influence of airborne CO₂ on the charge state of carboxylate stabilized polymer latex particles suspended in aqueous electrolytes. We combine conductometric experiments interpreted in terms of Hessinger's conductivity model with Poisson–Boltzmann cell (PBC) model calculations with charge regulation boundary conditions. Without CO₂, a minority of the weakly acidic surface groups are dissociated and only a fraction of the total number of counter-ions actually contribute to conductivity. The remaining counter-ions exchange freely with added other ions like Na⁺, K⁺ or Cs⁺. From the PBC-calculations we infer a corresponding pK_a of 4.26 as well as a renormalized charge in reasonably good agreement with the number of freely mobile counter-ions. Equilibration of salt- and CO₂-free suspensions against ambient air leads to a drastic de-charging, which exceeds by far the expected effects of to dissolved CO₂ and its dissociation products. Further, no counter-ion-exchange is observed. To reproduce the experimental findings, we have to assume an effective pK_a of 6.48. This direct influence of CO₂ on the state of surface group dissociation explains our recent finding of a CO₂-induced decrease of the ζ-potential and supports the suggestion of an additional charge regulation caused by molecular CO₂. Given the importance of charged surfaces in contact with aqueous electrolytes, we anticipate that our observations bear substantial theoretical challenges and important implications for applications ranging from desalination to bio-membranes.