Counterion Ordering at Ionized Carboxylate Self-Assembled Monolayers on Au(111) and Its Influence on the Potential of Zero Charge

The electrostatic properties of the Au(111)/self-assembled monolayer (SAM)/electrolyte interface were investigated using density functional theory combined with the effective screening medium and reference interaction site model (DFT-ESM-RISM) with explicit counterions. Calculations for deprotonated carboxyl-terminated SAMs show that Na+, K+, and Cs+ counterions localize within the SAM termination region, occupying interstitial positions between COO– groups rather than forming a diffuse layer governed by electrolyte concentration. The resulting interfacial structure can be described as a quasi-two-dimensional carboxylate–counterion ionic crystal. The calculated potential of zero charge (PZC) shifts exhibit weak dependence on bulk electrolyte concentration and are consistent with available experimental data for Na+ systems. To clarify the microscopic origin of the PZC shifts, the electrostatic potential profiles were decomposed into contributions arising from the SAM dipole, the carboxylate–counterion ionic layer, and polarized interfacial water. The analysis reveals that the water contribution dominates for Na+, whereas the ionic contribution becomes increasingly important for K+ and Cs+. The results indicate that the electrostatic response of the interface is governed primarily by local counterion–carboxylate organization within the SAM termination region and the resulting polarization of interfacial water. These findings provide a molecular-level explanation of experimentally observed PZC shifts and establish the electrostatic consequences of local counterion–carboxylate organization and the resulting polarization of interfacial water.