Observation of Counterion Binding in the Inner Helmholtz Layer at the Ionic Surfactant-Water Interface

Understanding specific ion adsorption within the inner Helmholtz layer (IHL) is fundamental to electrochemistry, yet direct, quantitative measurements of this compact-layer stoichiometry and thermodynamics remain experimentally challenging. Here, we use phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS) to quantify counterion adsorption in the IHL and to extract the corresponding adsorption thermodynamic parameters. Using sodium dodecyl sulfate (SDS) as a model ionic surfactant, we determine the Na⁺ and DS^- surface densities by simultaneously analyzing the interfacial free OH response and the diffuse-layer SF signal, thereby separating compact and diffuse-layer contributions within a unified framework. We then construct an adsorption phase diagram that maps the evolution of Na⁺ and DS^- species in the compact layer as functions of bulk NaCl and SDS concentrations, revealing a continuous increase in surface ion pairing. The DS^-:Na⁺ pairing ratio gradually decreases with increasing NaCl and approaches 2.8 at the supersaturation state prior to surface nucleation, the high-coverage limit preceding interfacial crystallization. These results establish PS-SFVS as a quantitative probe of ion-headgroup correlations in charged interfaces and reveal the thermodynamic mechanism underlying counterion-mediated interfacial ordering, with broad implications for electrolyte design, biomembrane stability, and soft-matter assembly.