Recent study on counterion-mediated attraction between colloidal particles

Recent experimental results were reviewed. The 1D- and 2D-USAXS studies gav e higher orders of Bragg diffraction for single crystals of colloidal silic a particles, allowing more accurate determinations of the lattice constant, lattice symmetry, and direction. The closest interparticle spacing thus de termined was confirmed to be smaller than the average spacing. The most clo sely packed planes ((110) planes for bcc) of negatively charged particles w ere found to be parallel to the likewise negatively charged capillary surfa ce, inconsistently with the accepted double layer interaction theory but co nsistently with a recent experimental finding of positive adsorption. Shaki ng caused disruption of the single crystals but newly formed microcrystals retained the lattice constant and the preference of the (110) planes. The l iquid-solid-liquid transition, a re-entrant phase transition, was found for silica particles and latex particles at given particle volume fraction and salt concentration, when the charge density of particles was varied. It wa s demonstrated that the purely repulsive Yukawa potential and the concept o f renormalized charge cannot account for the re-entrant behavior. The Monte -Carlo simulation using the Sogami potential, which contains short-range re pulsion and long-range attraction, was found to account for the fcc-bcc tra nsition, which was earlier claimed to be explainable only by the Yukawa pot ential. Furthermore, the homogeneous-inhomogeneous phase transition and voi d formation could be accounted for by the simulation using the Sogami poten tial; the Yukawa potential could not reproduce the experiments. Attention w as drawn to the experimental conditions in direct measurements of interpart icle forces; only short interparticle distance and low charge density parti cles were covered, which make it practically impossible to detect the long- range counterion-mediated attraction. It is hoped that, by technical improv ements, these shortcomings may be made up and quantitative argument become possible on the attraction. (C) 1999 Elsevier Science B.V. All rights reser ved.