Observation of counterion binding in the inner Helmholtz layer at the ionic surfactant-water interface

TL;DR

This study uses phase-sensitive sum-frequency vibrational spectroscopy to directly quantify counterion adsorption at the ionic surfactant-water interface, revealing thermodynamic mechanisms of ion pairing and interfacial ordering.

Contribution

It introduces a quantitative spectroscopic method to analyze counterion binding and thermodynamics at charged interfaces, advancing understanding of ion-specific effects.

Findings

Counterion adsorption quantified at the surfactant-water interface.

Thermodynamic parameters of ion binding derived from spectroscopic data.

Adsorption phase diagram shows ion pairing evolution with salt and surfactant concentrations.

Abstract

Understanding specific ion adsorption within the inner Helmholtz layer remains central to electrochemistry yet experimentally elusive. Here we directly quantify counterion adsorption and extract the associated thermodynamic parameters within the inner Helmholtz layer using phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS). 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, from which the adsorption thermodynamic parameters are derived. 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. 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.