Hofmeister effects at low salt concentration due to surface charge transfer

We present a theoretical comparison of the surface forces between two graphite-like surfaces at salt concentrations below 10 mM with surfaces charged by various mechanisms. Surface forces include a surface charging or chemisorption contribution to the total free energy. Surfaces are charged by charge regulation (H+ binding), site competition (H+ and cation binding) and redox charging with electrodes coupled to a countercell. Constant surface charge is also considered. Surface parameters are calibrated to give the same potential when isolated. Nonelectrostatic physisorption energies of the potential determining ions provide a specific and significant contribution to the charging energy. Consequently ion specificity is found in the surface forces at concentrations of 1–10 mM, which is not observed under constant charge conditions. The force between redox electrodes continues to show Hofmeister effects at 0.01 mM. We refer to this low concentration Hofmeister effect as “Hofmeister charging”, and suggest that the more common high concentration ion specific effects may be known as “Hofmeister screening”. Hofmeister series are considered over LiCl, NaCl, KCl and NaNO3, NaClO4, NaSCN with the cations (or H+) being the potential determining ions. A K+ anomaly is attributed to the small size of the weakly hydrated chaotropic K+ ion, with Li+ and Na+ explicitly modelled as strongly hydrated cosmotropes.