Monovalent counterion distributions at highly charged water interfaces: Proton-transfer and Poisson-Boltzmann theory

TL;DR

This study combines synchrotron-X-ray scattering and Poisson-Boltzmann theory to analyze monovalent ion distributions at charged water interfaces, revealing proton-transfer mechanisms and validating theoretical models without fitting parameters.

Contribution

It provides the first spatial distributions of Cs+ ions near charged interfaces and demonstrates the agreement with a renormalized Poisson-Boltzmann theory.

Findings

Proton transfer at low salt concentrations affects surface charge.

Cs+ distributions match theoretical predictions.

High salt reduces proton transfer and increases surface charge.

Abstract

Surface sensitive synchrotron-X-ray scattering studies reveal the distributions of monovalent ions next to highly charged interfaces. A lipid phosphate (dihexadecyl hydrogen-phosphate) was spread as a monolayer at the air-water interface, containing CsI at various concentrations. Using anomalous reflectivity off and at the $L_3$ Cs$^+$ resonance, we provide, for the first time, spatial counterion distributions (Cs$^+$) next to the negatively charged interface over a wide range of ionic concentrations. We argue that at low salt concentrations and for pure water the enhanced concentration of hydroniums H$_3$O$^+$ at the interface leads to proton-transfer back to the phosphate group by a high contact-potential, whereas high salt concentrations lower the contact-potential resulting in proton-release and increased surface charge-density. The experimental ionic distributions are in excellent agreement with a renormalized-surface-charge Poisson-Boltzmann theory without fitting parameters or additional assumptions.