CO₂-induced drastic decharging of dielectric surfaces in aqueous suspensions

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.