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Published May 11, 2021 | Supplemental Material + Published
Journal Article Open

Image-charge effects on ion adsorption near aqueous interfaces


Electrostatic interactions near surfaces and interfaces are ubiquitous in many fields of science. Continuum electrostatics predicts that ions will be attracted to conducting electrodes but repelled by surfaces with lower dielectric constant than the solvent. However, several recent studies found that certain "chaotropic" ions have similar adsorption behavior at air/water and graphene/water interfaces. Here we systematically study the effect of polarization of the surface, the solvent, and solutes on the adsorption of ions onto the electrode surfaces using molecular dynamics simulation. An efficient method is developed to treat an electrolyte system between two parallel conducting surfaces by exploiting the mirror-expanded symmetry of the exact image-charge solution. With neutral surfaces, the image interactions induced by the solvent dipoles and ions largely cancel each other, resulting in no significant net differences in the ion adsorption profile regardless of the surface polarity. Under an external electric field, the adsorption of ions is strongly affected by the surface polarization, such that the charge separation across the electrolyte and the capacitance of the cell is greatly enhanced with a conducting surface over a low-dielectric-constant surface. While the extent of ion adsorption is highly dependent on the electrolyte model (the polarizability of solvent and solutes, as well as the van der Waals radii), we find the effect of surface polarization on ion adsorption is consistent throughout different electrolyte models.

Additional Information

© 2021 National Academy of Sciences. Published under the PNAS license. Edited by Monica Olvera de la Cruz, Northwestern University, Evanston, IL, and approved March 16, 2021 (received for review October 1, 2020). We thank the Dow Chemical Company for funding and for permission to publish the results. We also thank the NVIDIA Corporation for the donation of the Titan Xp GPU card used for this research. Data Availability: All study data are included in the article and/or SI Appendix. Author contributions: C.Y.S. and Z.-G.W. designed research; C.Y.S. performed research; C.Y.S. and Z.-G.W. analyzed data; and C.Y.S. and Z.-G.W. wrote the paper. The authors declare no competing interest. This article is a PNAS Direct Submission. This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2020615118/-/DCSupplemental.

Attached Files

Published - e2020615118.full.pdf

Supplemental Material - pnas.2020615118.sapp.pdf


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