Surface Charge Regulation on the Concave Wall of a Spherical Silica Pore, Impact of the EDL Overlap, Potential Drop in the Stern Region, and Deviation from the Electroneutrality Principle

Surface charge regulation has been investigated for a spherical pore of silica that is submerged in an aqueous (+1, −1) electrolyte by using the modified fundamental measure theory combined with a surface reaction model. Surface charges have been obtained at different concentrations, pH, and pore sizes, and then, they are interpreted via the electric double layers that overlap through the potential drop in the Stern region and deviation from the electroneutrality principle. For all examined cases, within the pore, ions form a diffuse layer, where overlap is determined by the width of the diffuse layer and the pore radius. The results show that the width of the diffuse layer (the surface charge) reduces (increases) with concentration and curvature but increases (decreases) with a greater deviation from the electroneutrality principle. We have also shown that a more significant drop in the mean electrostatic potential in the Stern region causes the formation of a more compact and therefore a thinner diffuse layer, which leads to more surface charge. Last, it has been found that the concave walls of spherical pores had a higher surface charge than the walls of slit-like pores. This suggests that a more compact and thinner diffuse layer forms inside the spherical pore, at least at low concentrations.