Description
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 interfacial free OH response and the diffuse-layer SF signal, thereby separating compact and diffuse-layer contributions within a unified framework. We then con-struct 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. These results estab-lish 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.
