Mobile ion activities at charged interfaces

The issue of mobile ion chemical activities at charged aqueous interfaces is of historical interest. Because of experimental difficulties in measuring the surface-charge-dependent potential energies associated with moving an ion from bulk solution to the interfacial region many current practitioners assume that the energy is primarily electrostatic in nature and that it can be estimated from Poisson–Boltzmann theory and experimentally measured surface charge densities. The present effort uses the entropic balanced surface potential model (EBSPM) to assess some of the energy terms associated with moving a mobile ion to the charged interface. The findings suggest the following order of energy contributions:electrostatic>bulk solution activity, hydration, dipole>‘crowding’.Additional findings include: (1) activity based Boltzmann expressions relating bulk solution activities to interfacial concentrations rely on the assumption that the interfacial mobile ion is beyond the influence of the bulk solution Debye–Huckel term, (2) activity and concentration based interfacial pH conditions can be related to bulk solution values by: pHinterface=pHbulk+eΨ/2.303kT (where Ψ is the potential energy difference associated with moving the ion to the interface, e is the charge on the electron, k is the Boltzmann constant, and T is the absolute temperature), and (3) activity and concentration based interfacial oxidation–reduction potentials can be related to bulk solution values by the expression: Einterface=Ebulk−Ψ.