Thermodynamic stability, colloidal silica

What Derjaguin considers the central issue of colloidal solutions remains largely unresolved for silica sols. This book mentions the ideas of the proponents of both the kinetic and the thermodynamic approach to the problem of stability of silica sols and is intended to stimulate the continuation of the healthy controversy started at the R. K. Iler Memorial Symposium. In this manner a consensus should eventually be reached that will allow the establishment of common quantitative parameters in the treatment of stability of silica sols and other disperse phase materials composed of polyvalent atoms linked by strong covalent bonds and the explanation of their experimentally observed behavior. 

P. C. Yates, Thermodynamic Stabilization of Highly Dispersed Systems Prevention of Particle Growth of Colloidal Amorphous Silica by Hydroxyl Ion Adsorption, presented at the Divisional Colloid Chemistry Symposium on Colloidal Silical and Silicates. 137th Meeting of the American Chemical Society, Cleveland, Ohio, April 13, I960. 

Already reported CAV/0 microemulsion technique was used to prepare the MLPs [12-15]. Briefly, this technique consists of an oil phase, a colloidal water phase, and surfactants and possesses specific physicochemical properties such as transparency, isotropy, and thermodynamic stability, n-Heptane was used as the oil phase, BrijSO as surfactant and tetraethoxysilane (TEOS) as silica precursor. This method is based on the hydrolysis and the condensation of TEOS. There is a major importance to the concentration and the order of addition of the different species. To the mixture of heptane/BrijSO we add slowly a colloidal suspension of y-Fe203 MPS in water (13 mg in 750 pL of water). After 15 min of stirring we added the cluster units in a mixture of Et0H H20 (1 1). Afterward we introduce an aqueous ammonia solution (28%). Finally the TEOS was added and the microemulsion was stirred during 3 days before several precipitation/resuspension to transfer our MLPs in water. 

A third system that is claimed to behave as a model hard sphere fluid is a dispersion of colloidal silica spheres sterically stabilized by stearyl chains g ted onto the surface and dispersed in cyclohexane ". Experimental studies of both the equilibrium thermodynamic and structural properties (osmotic compressibility and structure factor) as well as the dynamic properties (sedimentation, diffusion and viscosity) established that this system can indeed be described in very good approximation as a hard sphere colloidal dispersion (for a review of these experiments and their interpretation in terms of a hard sphere model see Ref. 4). De Kruif et al. 5 observed that in these lyophilic silica dispersions at volume fractions above 0.5 a transition to an ordered structure occurs. The transition from an initially glass like sediment to the iridescent (ordered) state appears only after weeks or months. 

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