Effective electrostatic interaction between columnar colloids: roles of solvent steric hindrance, polarity, and surface geometric characteristics

Abstract

In the present work, coupling between the electrostatic interaction and solvent steric hindrance, and solvent polarity are studied by classical density functional theory with emphasis on their influence on the effective electrostatic potential between two similarly charged and parallel cylinder surfaces immersed in electrolyte solution.Several conclusions are confirmed. First, solvent steric hindrance always enhances the effective electrostatic potential curve oscillation and its first potential well depth or induces the potential well formation even in +1:−1 electrolyte; the enhancing effects are positively correlated with the solvent packing fraction. The curve oscillation is weakened anomalously with the surface charge density strength. Second, with a certain packing fraction of the crowding agent, its diameter has no distinct effect on the oscillation intensity of the effective electrostatic potential curves, but the oscillation period is always equal to the diameter. Third, influences of solvophilicity or solvophobicity of counter- and co-ions on the effective electrostatic potential are different in strength and mode. Strong counter-ion solvophobicity greatly raises the like-charge attraction strength with a simultaneously weakening of the effective electrostatic potential oscillation, and vice versa; solvophobicity or solvophilicity of co-ion influences the effective electrostatic potential curve in a less obvious way by translating it up/down, respectively; more specifically, the curve morphology is rarely changed. Fourth, it is illustrated that high counter-ion valence and high surface charge are not necessary conditions (confirmed in previous literature) of the surface charge reversal occurrence; counter-ion solvophobicity induces the charge reversal at far milder conditions: univalent counter-ion, lower bulk concentration, and lower surface charge strength. Fifth, by comparing with previous two works on the effects of solvent granularity on the effective electrostatic potential in flat and spherical slits, we conclude that the flat slit contact potential is always repulsive, the spherical slit contact potential can be very attractive, and the present cylinder slit contact potential is between the two.