Effects of interionic non-hard sphere neutral interaction and solvent crowding on differential capacitance curve of electrical double layer

Abstract

By using classical density functional theory, we study the effects of interionic nonhard sphere neutral interaction (NHSNI) and solvent crowding on curve of differential electrical capacitance (Cd) vs surface charge strength |σ| of an electrical double layer (EDL) formed inside a cylindrical pore electrode. Main new findings are summarized as follows. (i) Increasing the intercounterion neutral attractive interaction helps in raising the Cd − |σ| curve and vice versa; the lowering or enhancing effect continues until rather high |σ|, such as |σ| = 1 C m−2. (ii) Attractive NHSNI between coion and counterion lowers the Cd − |σ| curve and vice versa; the lowering or enhancing effect quickly becomes very weak with |σ|. (iii) The lowering or enhancing effect strength of the intercounterion NHSNI potential range is negatively correlated with the bulk electrolyte concentration, whereas the correlation becomes positive for the potential range of the NHSNI between coion and counterion. (iv) Whether the bulk concentration is high or low, one stronger intercounterion attractive NHSNI tends to induce one more obvious peak of the Cd − |σ| curve at higher |σ|; only after this peak, the intercounterion NHSNI does not influence the Cd − |σ| curve obviously. However, the peak does not come with one intercounterion repulsive NHSNI and any kind of NHSNI between coion and counterion. (v) The solvent crowding induced by its granularity generally raises the Cd curve when |σ| is low and moderate, but the enhancing effect tends to become unobservable with further rising of |σ|. The above findings are explained reasonably by analyzing the ions local distributions in the EDL, their adsorption capacities, and the peak of change rate of the total ion adsorption capacity with |σ|.