Long-Range Electrostatic Attraction Between Macroions Mediated by Oppositely Charged Counterions: Experimental Supports, Past and Present

摘要

Recent experimental observations are reviewed, which testify to the existence of electrostatic attraction between colloidal particles. The ultra-small-angle X-ray scattering technique shows five orders of Bragg diffraction for colloidal silica particle dispersions, indicating that a (bcc) single crystal of a lattice constant of 3000 A is formed. The closest interparticle spacing can be accurately determined and is definitely smaller than the average spacing calculated from the particle concentration. This implies that contraction takes place during crystallization and the dispersion is no longer homogeneous but contains the single crystal, voids and/or free particles. Two types of symmetries, six-fold and four-fold, are observed from the same dispersion with {110} planes (most densely packed planes of the bcc structure) parallel to the capillary surface. This implies that the presumably negatively charged surface does not repel the anionic particles but actually attracts them. The void formation, which is a direct evidence of the attraction, is further confirmed for rather small latex particles under the density-matched condition. Furthermore, the same latex particles are found to show the macroscopic vapor-liquid condensation when the density difference between the particle and the medium is not adjusted. These two phenomena are ther-modynamically the same. The results are analyzed in terms of the Sogami potential. The recent measurements of interparticle potential are critically reviewed. It is pointed out that only rather short distances are covered in the surface force measurements and the atomic force microscopy so that the long-range attraction in question cannot be detected. A method based on the determination of the distribution function demonstrates the presence of a long-range attraction in addition to the widely accepted repulsion, and the results are successfully reproduced by the Sogami potential. The positive adsorption of ionic entities near like-charged interface is in direct contradiction to the standard double layer interaction theory and shows the presence of electrostatic attraction between the interface and particles. The attraction is generated through the intermediary of counterions present in the space between particles or between particles and plate. Considering the nature of the Sogami treatment, it is proposed to call the attraction the counterion-mediated Gibbs attraction.