Surfactant-Mediated Transfer of Colloidal Silica Particles from Water into an Immiscible Weakly Polar Solvent.

Spontaneous transfer of discrete colloidal silica particles from water into 2,6-dimethyl-4-heptanone (DH) is obtained upon preliminary adsorption of dodecylbenzyldimethylammonium bromide (DBDAB) from the aqueous phase onto the solid. Partitioning of the surfactant between the silica surface and both liquid phases is evaluated through the determination of the isotherm in the aqueous phase, taking into account their mutual solubility. The organosol particles are found to be covered by a statistical monolayer of surfactant molecules presumably hydrated to the extent of 20% on a mole ratio basis. Monodispersion of this stable suspension is ascertained by dynamic light scattering. Evidence of DBDAB association in the monolayer emerges from the conductimetric behavior of the amphiphile in DH and from the reversal of the silica native charge in the organosol, as shown by electrophoretic mobility determinations. Disorganization of the water layer contacting silica upon adsorption of DBDAB is argumented to originate from the presence of sparsely distributed dodecyl chains. Copyright 1998 Academic Press.

Transfers of Colloidal Silica from Water into Organic Solvents of Intermediate Polarities

Dispersions of discrete metal-oxide submicroparticles in organic solvents of medium polarities are uneasy to generate and weakly documented. We address this topic along two general methods focusing on silica. Successive transfers of colloidal particles from water into n-propanol and then into 1,2-dichloroethane by azeotropic distillation yield a stable organosol. The particles are found to be propanol-coated by surface esterification to the extent of 0.40 nm2 per molecule. Alternatively, centrifugation-redispersion cycles make it possible to obtain stable suspensions of unaltered silica in methanol and acetonitrile starting from an aqueous silicasol. Particles are characterized by various methods including nitrogen adsorption, transmission electron microscopy, dynamic light scattering, and electrophoresis. The stabilities of these suspensions in various organic solvents are investigated with special concern for the role of residual water. Stabilization of silica in methanol is inconspicuously related to solvent permittivity and prominently dependent on the presence of adsorbed water. In contrast, the acetonitrile silicasol, which is unaffected by residual water, displays electrophoretic behavior compatible with electrostatic stabilization. Copyright 1998 Academic Press. Copyright 1998Academic Press