Hydration forces between inorganic particles: Crystallization of silicotungstic acid

The ubiquitous nature of colloidal suspensions in nature and industrial products has warranted much effort in understanding their stability and phase transition from a colloid-poor to a colloid-rich phase. At small particle separations in certain systems, stability is found where classical colloid theories predict flocculation. These classical colloid theories are based on a continuum model of the solvent. But at such small particle separations, it is believed that the granularity of the solvent becomes important and that this unexpected stability is attributable to solvation forces.We investigated solvation forces using a model nano-sized colloidal system (silicotungstic acid) where interparticle distances could be controlled. The solvation force was manipulated by varying the solvent chemical potential (via temperature, humidity, and added electrolyte concentration).We probed solvation forces in the colloid-rich (crystalline) phase using a thermogravimetric vapor pressure technique. We found that the dehydration curves implied a discontinuous or oscillatory interaction potential, the structuring of water around a particle was heavily influenced by the cations associated with the charged particle, and the release of solvent in going to a lower hydration state drove the phase transition via entropy. Such observations indicated the presence of solvation forces.The colloid-poor (dilute solution) phase was probed using static light scattering. The results demonstrated that upon the addition of a background electrolyte, the second virial coefficient continuously decreased beyond values predicted from the classical Derjaguin-Landau-Verwey-Overbeek theory of colloid stability. Such a trend signified that there was an additional interparticle attraction of significance (relative to van der Waals attraction) that was believed to arise from a "solvation attraction." This assertion was further supported from the success of modeling our model nano-sized particles as adhesive hard spheres whose phase behavior is governed by a strong, short-range attraction. The magnitude of this attraction, which could not be attributable solely to van der Waals attraction, was also believed to arise primarily from a "solvation attraction."U of I OnlyETDs are only available to UIUC Users without author permissio