Here, z is the ion valence (1:1 for NaCl), C is the ion concentration in the EDL, 𝑘 9 𝑇 is the thermal energy, the s's are the charge density in the diffuse and Stern layers, e is the elementary charge, 𝜀 $ and 𝜀 : are the vacuum permittivity and the relative permittivity of water, respectively, and the curly bracket indicates the GCS potential. We can evaluate eqn. 2 by using a Stern layer capacitance of 0.2 F m -2 from Sahai and Sverjensky, 50 a Stern layer thickness of 2Å from Brown and co-workers, the ionic strength-dependent interfacial charge densities from Hore and coworkers, and the notion that 𝜎 12??7@) =𝜎 >#)A< due to charge neutrality in the EDL. Fig. shows that in the ionic strength regime studied here, the GCS potential due to the mobile charges contributes only about 50% of 𝛷(0) !"! for an aqueous salt solution in contact with fused silica at pH 5.8. For brine conditions, over 90 % of 𝛷(0) !"! for silica is due to the contribution from the bound charges. In other words, the GCS model recapitulates only 50% (resp., 10%) of the total surface potential at dilute (resp., concentrated) solution conditions. A modified Poisson-Boltzmann model with hydration repulsion may provide better agreement between experiment and theory, especially when further modified with effective ion diameters at charged interfaces, as its good agreement with XPS measurements has been established for silica colloids to account for non-ideal behavior (short-range ion correlations, site availability, etc.) of aqueous electrolytes at surfaces. A related issue is the spatial variation of the (field-dependent) relative permittivity, er, 60 which these models neglect, i.e. the solvent is modeled as a uniform continuum, despite large differences in reported er. These considerations point to the possibility that the total potential we employ to compute cation surface coverages from eqn. 1 should in fact be reduced by about 50%. Fig. shows the resulting surface coverages, recomputed with eqn. 1 using half of the Ftot values reported in Fig. (n.b., Ftot,0 was kept unchanged in this calculation).