Electrolyte solution structure and its effect on the properties of electric double layers with surface charge regulation

Abstract

The physical origin of charged interfaces involving electrolyte solutions is in the thermodynamic equilibrium between the surface reactive groups and certain dissolved ionic species in the bulk. This equilibrium is very strongly dependent on the precise local density of these species, also known as potential determining ions in the solution. The latter, however, is determined by the overall solution structure, which is dominated by the large number of solvent molecules relative to all solutes. Hence, the solvent contribution to the molecular structure is a crucial factor that determines the properties of electric double layers. Models that explicitly account for the solvent structure are often referred to as "civilized" as opposed to the "primitive" ones that consider the solvent as a structureless continuum. In the present paper, we demonstrate that for a physically correct description of charged interfaces that involve electrolyte solutions (electric double layers), the full solution structure needs to be taken into account in conjunction with the precise surface chemistry governed by the thermodynamic equilibrium. The analysis shows how the surface charge depends on various experimentally relevant parameters, many of which are outside the realm of simple electrostatics. We present results on the effects of solvent molecular dimensions, ionic solvation, surface chemistry, solvophilicity and solvophobicity.

Influence of Co-Ion Nature on the Gelation Kinetics of Colloidal Silica Suspensions

“…27 For pH values much greater than 2, silica presents numerous negative sites at its surface, so that cationic counterions are attracted whereas anionic co-ions are repelled. This distribution and the random thermal motion of ions lead to formation of an electric double layer 11,28 with an inner layer rich in cations and a diffuse layer where cations and a majority of anions are present, this has to be noted. Far away from the surface, the number density of counterions and co-ions is the same to satisfy electroneutrality.…”

Electric Double Layers with Surface Charge Regulation Using Density Functional Theory

“…The electrolyte solution is described in terms of a grand thermodynamic potential functional, which for single flat EDL reads [ 14 , 15 , 16 , 17 , 18 , 19 , 20 ] …”

“…This approach includes the contributions of the interactions between the solutions species, which are present in the terms [ 21 , 22 ], which accounts for the excluded volume effects and , which captures long-range interactions. In our model, these interactions are of the Lennard-Jones (LJ) [ 25 ] and Coulombic type [ 14 , 17 , 18 , 19 , 20 ], where , is the diameter of component “ i ” and is the distance between species “ i ” and “ j ”. All non-Coulombic interactions of a molecule (or ion) of type “ i ” with the interface are assumed to be of the “hard wall” type, that is, only the excluded volume effects are taken into account.…”

“…This approximation means that the solvent molecules exhibit short-range repulsive forces and isotropic attractions (necessary to ensure the existence of a stable liquid phase) but are not involved in any orientation-dependent dipole-dipole or ion-dipole interactions. The semi-primitive models proved themselves helpful in demonstrating the effect of the liquid structure on the properties of charged electrolyte interfaces [ 14 , 17 , 18 , 19 , 20 , 54 , 55 , 56 , 57 , 58 , 59 ]. Since the semi-primitive approach does not account for any dipole effects, the dielectric permittivity has to be added ad hoc in order to properly scale all Coulombic terms.…”

Measuring surface charge: Why experimental characterization and molecular modeling should be coupled

“…However, Lee et al [169] found that various physical quantities that depend on the orientation of solvent molecules were not accurately predicted by the primitive model approach. Additionally, Vangara et al [170] recently showed, by comparing explicit and implicit solvent DFT models, that both the solvent and the ions contribute to the chemical balance between surface groups and the solution.…”