Molecular solvent structure, charge regulation, and colloidal interactions in confined electrolyte solutions

Charged surfaces in contact with electrolyte solutions give rise to electric double layers through redistribution of ionic species near the interface. When brought into close proximity, double-layer overlap leads to interactions commonly described using continuum electrostatic models with fixed surface charge or fixed surface potential boundary conditions. However, these idealized descriptions fail to capture the molecular solvent structure and the chemically regulated nature of surface charging under nanometer-scale confinement. Here, we investigate the interaction between charge-regulating surfaces in electrolyte solutions using classical density functional theory with an explicit molecular solvent. Surface charge regulation is treated self-consistently through chemical equilibrium between surface reactive groups and potential-determining ions, with surface charge and potential determined at each iteration from the full molecular density distributions rather than a mean-field approximation. Excluded-volume effects, electrostatic interactions, and solvent-induced structural forces are fully accounted for within a unified thermodynamic framework. Our results demonstrate that confinement-induced solvent layering couples strongly to the charge-regulation mechanism, producing oscillatory variations in surface charge, surface potential, and interaction forces with separation. At short separations, the interaction is dominated by molecular-scale structural contributions, while electrostatic screening dominates at larger distances. Extending the analysis to spherical colloidal particles, we show that the interaction energy exhibits multiple metastable minima arising from discrete solvent layering, with direct surface contact effectively precluded across the full range of Hamaker constants examined-a qualitative departure from classical DLVO theory. These findings highlight the limitations of continuum double-layer theories and underscore the essential role of solvent structure in determining surface interactions in confined electrolytes.