Image-charge effects on ion adsorption near aqueous interfaces

Abstract

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.

Interactions between conducting surfaces in salt solutions

“…Placing fully to the left, or to the right, of C 0 will lead to a spurious electric field of the unit cell (even at zero applied voltage), which needs to be accounted for. 32–34 Thus the ensemble can be straightforwardly treated using 3-D Ewald methods. One of the consequences of our construction in that the total Ewald energy, U C , of the ions in C is equal to twice the energy of the ions in C 0 .…”

A molecular perspective on induced charges on a metallic surface

“…32 Such simulations allowed, in particular, studying the effect of the polarization of the metal on the interfacial structure, dynamics and capacitance, the adsorption of ions and biomolecules, or solid-liquid friction for a variety of electrodes such as gold, platinum, or graphite and liquids from pure water and solutions of simple salts to water-in-salt electrolytes, polyelectrolytes, and biomolecules in solution and ionic liquids. 28,[33][34][35][36][37][38] They also emphasized the combined role of the atomic structure of the metal and molecular nature of water in the overall hydrophilic/phobic behavior of the interface. 39,40 From a more fundamental point of view, the fluctuations of the charge of the electrode in constant-potential simulations reflect the statistics of the microscopic configurations in the corresponding thermodynamic ensemble.…”

Effect of the metallicity on the capacitance of gold - aqueous sodium chloride interfaces

“…Classical molecular dynamics simulations of electrode -electrolyte interfaces bring important microscopic insights for the interpretation of electrochemistry experiments 1 . They are not only relevant in the case of concentrated electrolytes, for which classical theories fail to capture many effects such as ion-ion correlations 2 , but also in cases where the molecular structure of the solvent can impact the interfacial properties 3 . Even though most of the recent works were dedicated to the study of supercapacitors 4 , in which the electrode is generally a carbon material and the electrolyte an organic or ionic liquid, systems made of good metals such as platinum or gold put in contact with water were also considered.…”