How accurate is Poisson-Boltzmann theory for monovalent ions near highly charged interfaces?

TL;DR

This study evaluates the accuracy of Poisson-Boltzmann theory in predicting monovalent ion distributions near highly charged interfaces, using synchrotron x-ray scattering to compare experimental data with theoretical models.

Contribution

It provides experimental ion distribution data near charged interfaces and assesses the validity of Poisson-Boltzmann theory considering surface charge modifications due to proton binding.

Findings

Ion distributions depend strongly on bulk concentration.

Proton transfer reduces surface charge density.

Poisson-Boltzmann theory's predictions are compared with experimental data.

Abstract

Surface sensitive synchrotron x-ray scattering studies were performed to obtain the distribution of monovalent ions next to a highly charged interface. A lipid phosphate (dihexadecyl hydrogen-phosphate) was spread as a monolayer at the air-water interface to control surface charge density. Using anomalous reflectivity off and at the $L_3 $ Cs$^{+}$ resonance, we provide spatial counterion (Cs$^{+}$) distributions next to the negatively charged interfaces. Five decades in bulk concentrations are investigated, demonstrating that the interfacial distribution is strongly dependent on bulk concentration. We show that this is due to the strong binding constant of hydronium H$_3$O$^{+}$ to the phosphate group, leading to proton transfer back to the phosphate group and to a reduced surface charge. The increase of Cs$^+$ concentration modifies the contact value potential, thereby causing proton release. This process effectively modifies surface charge density and enables exploration of ion distributions as a function of effective surface charge-density. The experimentally obtained ion distributions are compared to distributions calculated by Poisson-Boltzmann theory accounting for the variation of surface charge density due to proton release and binding. We also discuss the accuracy of our experimental results in discriminating among possible deviations from Poisson-Boltzmann theory.