Effective electrostatic forces between two neutral surfaces with atomic scale strip shape surface charge separation

Abstract

By using classical density functional theory we investigate influences of atomic scale strip shape surface charge separation on the effective electrostatic force (EEF) between two overall neutral surfaces immersed in 1:1 electrolyte solution. Main novel findings are confirmed, and summarized as follows: (i) At lower bulk concentration, the EEF is always attractive for asymmetrical surface charge pattern and repulsive for symmetrical surface charge pattern. Strength of the EEF is positively correlated with the strip shape domain width and surface charge density intensity, and generally smaller than that of the EEF between two equally and oppositely or similarly charged planar surfaces with same surface charge intensity. (ii) With increasing of the bulk concentration, the asymmetrical strip shape surface charge pattern induces two repulsion peaks, strengths of the repulsion peaks around one times and double the ion diameter surface separation are negatively and positively correlated with the strip shape domain width, respectively; the second repulsion peak position reduces with the domain charge strength. (iii) At higher bulk concentration, the symmetrical strip shape charge pattern induces two attraction wells of the EEF curve; one positioned at wall separation larger than one times and smaller than double the ion size, the other approximately 2.75 times the ion size. (iv) Considering that systems favor low-energy configurations, the present calculations provide clear evidence that effective repulsion between colloids with non-uniform surface charge distribution or charge fluctuation is less than that with uniform surface charge distribution or with fixed charges, respectively. All of the observations can be explained self-consistently by using concepts such as ion saturation adsorption capacity, arrangement compactness of the adsorption layer, formation of quasi-hydrogen-bonding, and charge reversal.