  A generalization of the mean-field approach will be derived that will take
into account the ion-ion as well as ion-surface non-electrostatic effects on an
equal footing, being based on the bulk and surface equations of state in the
absence of electrostatic interactions. This approach will be applied to the
calculation of the differential capacitance for several models of the specific
ion-surface non-electrostatic interactions providing a thermodynamic insight
into the characteristics of the differential capacitance. The packing
considerations due to surface adsorbed ions will be shown to be more relevant
then the bulk packing constraints for ions vicinal to the surface, and to set
in prior to the conditions where the bulk packing constraints would become
relevant.
