On the statistical mechanics investigation of structure and effective electrostatic force between two solid surfaces in electrolyte dissolved in non-polar solvent

Abstract

One three-site non-rigid model with two polar covalent bonds is proposed for electrolyte solvent; this model is integrated into the inhomogeneous electrolyte fluid classical density functional theory (CDFT) framework by means of the modified interfacial statistical association fluid theory. With the CDFT, we investigate surface electrostatic force (SEF) between two similarly charged planar surface and corresponding electrical double layer (EDL) in electrolyte dissolved in the model solvent. Novel properties of the EDL and SEF caused due to consideration of the model solvent are summarized as follows. (i) The EDL presents peculiar features: the solvent molecules are distributed in the slit far more uniformly than the counter-ions, adsorption of the former is less affected by the wall surface charge density strength, but increases with the site polarization charge strength and the counter-ion valence, whereas the counter-ion adsorption decreases with the counter-ion valence and exhibits anomalous stability against bulk electrolyte concentration. (ii) Abnormal characteristics exhibited by mean electrostatic potential (MEP) are insensitivity of the surface MEP strength to the inter-surface separation and the insensitivity of the MEP curve to the solvent site polarization charge strength; the former indicates that the EDL super-capacitor differential capacitance does not drop significantly with decreasing the super-capacitor size and implies one potential application in promoting the EDL energy storage by reducing the device size and dissolving the electrolyte with the present model solvent. (iii) New and novel features of the SEF include the insensitivity of the SEF curve to the bulk electrolyte concentration and like charge attraction in the 1:1 electrolyte; moreover, the SEF strength always drops definitely with the solvent site polarization charge strength. (iv) It is argued that the underlying causes of the above observations are the solvent chain structure and solvent site polarization charge, which highlights several unique effects: space congestion, interparticle association in the EDL, additional electrostatic screening caused by the polarized solvent site, and ‘bridge’ force and the entropic force due to the chain structure.