Ion Adsorption at Surfaces from Local and Global Electroneutrality Constraints

Ion adsorption or exclusion from surfaces plays a major role in many systems and processes. Unfortunately, thermodynamic information characterizing the relative surface adsorption of individual ions is not currently available from approaches based on the Gibbs adsorption isotherm for mixed electrolytes without approximation. Here, we address this issue for electrolyte solutions containing any number of components at any concentration in a single phase in the presence of any type of fixed charged or uncharged surface in the absence of chemisorption or other chemical reactions. This is achieved by reference to local and global electroneutrality requirements between integrals over the surface-ion distributions. The results indicate that the surface-ion distribution integrals can be decomposed into two independent contributions: one that leads to surface charge neutralization, and the other that explains the surface thermodynamics. The resulting surface-ion integral relationships obtained here indicate exactly how the presence of additional electrolytes affects the surface distribution of any target ion of interest. The validity of the resulting relationships is confirmed using classical all atom explicit solvent molecular dynamics simulations. Using these relationships, one can then obtain individual relative surface-ion adsorptions from experimental data. We illustrate how to use the approach to extract more detailed information from experimental data than was previously available for two experimental mixed electrolyte systems involving vacuum electrolyte solution interfaces. The approach is exact and does not require a particular model for the surface region or the use of single ion chemical potentials.