An electron microscope study has been made of the aggregation of small particles of gold (ca. 2-nm radius) supported on the surfaces of different solid materials (amorphous carbon, silicon monoxide, and layer-lattice silicates) when immersed in aqueous electrolyte solutions. Particle aggregation on relatively rough surfaces of carbon and silicon monoxide, where weak particle-substrate adhesion allows reasonably high particle migration rates, occurs by a primary minimum interaction. On the smooth surfaces of silicate minerals, particle aggregates formed at room temperature in solutions of sodium chloride having concentrations greater-than-or-equal-to ca. 5 x 10(-2) mol dm-3 and pH values within the range 8-10 were found to exhibit a reasonably uniform particle separation distance (1.7 +/- 0.7 nm), indicating a secondary minimum interaction. Similar dispersions immersed in aqueous sodium nitrate under comparable conditions remained colloidally stable, even at electrolyte concentrations in the region of 4 mol dm-3. Secondary minimum aggregation was, however, found to occur in these latter systems at elevated temperatures (greater-than-or-equal-to 40-degrees-C). In the case of silicate-supported dispersions, lowering the pH of the electrolyte solution within the range of pH 7-2 was found to be accompanied by an increasing tendency toward particle coalescence. This effect became dominant in the region of the pH (PZC) of the gold surface. Whilst the aggregation behavior of particles supported on the surfaces of carbon and silicon monoxide can be described reasonably well in terms of the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO), this theory is entirely inadequate to account for particle aggregation on the surfaces of the silicate minerals. In these latter cases, particle aggregation behavior can be modeled satisfactorily by including an interparticle repulsive force, attributed to the structuring of solvent molecules at the particle surfaces, which decays in a simple exponential manner with increasing distance of separation. The observed effects of surface electrical potential, pH, and temperature on the colloid stability of these systems support the view that counterion hydration plays a major role in the development of structural interactions between electrically charged particles dispersed in aqueous media. (C) 1994 Academic Press, Inc.