Properties of charged non-aqueous colloids in a low conductivity regime

Integrated investigations of the electrosurface properties of nonaqueous disperse and capillary systems

Survey of the experimental results obtained in the Department of Colloid Chemistry of St. Petersburg State University on the integrated investigations of the electrosurface properties of various nonaqueous disperse and capillary systems covering a wide range of electrolyte solutions in various organic solvents: amphoteric ethanol and 1-butanol; dipolar aprotic dimethylsulfoxide, dimethylformamide, acetone and nitromethane; nonpolar n-heptane in contact with SiO2, TiO2, Al2O3 and glass. Electrokinetic potential zeta and charge sigma(zeta), adsorption of ions Gamma, surface conductivity Ks of powdery silica, titania and alumina were determined depending on the concentration 10(-5)-10(-2) M of LiBr, KBr, NaBr, CsBr, HBr, NaCl, NaOH, HCl, various tetraalkylammonium bromides and ionic surfactants in the above cited solvents. Special attention was paid to the consistency of the results of zeta-potential evaluation obtained from the streaming potential and electrophoretic mobility data using the most commonly known theoretical expressions. It was shown that the surface conductivity must be taken into account in all cases for the evaluation of correct zeta-potential values. The surface conductivity and zeta-potential for the porous quartz plugs and glass membranes were determined as the functions of concentration of the anionic surfactants in n-heptane containing controlled traces of water. It was also shown that in the cases of the less polar solvents and more polar solids the enormous values of surface conductivity are due to the adsorption of the traces of water and in the cases of more hydrophylic liquids due to the overall adsorption of all ionic components from the solutions.

Supercritical EthanolA Fascinating Dispersion Medium for Silica Nanoparticles

For the first time, the dispersion stability of silica nanoparticles has been investigated in high-temperature and high-pressure ethanol by measuring the hydrodynamic diffusion coefficient of the particles by means of dynamic light scattering. The silica nanoparticles remain stable in ethanol within a wide temperature range of 24-304 degrees C at 12.3 MPa, and they start to aggregate at T >or= 305 degrees C. Numerical analysis reveals that the net interparticle repulsive potential barrier decreases dramatically with increasing temperature due to the changes in the properties of the medium. We observed that particles remain highly stable in the nonpolar supercritical ethanol in the temperature regime 241-304 degrees C, where the DLVO potential barrier is only 5-2 k(B)T. The dispersion stability of silica nanoparticles at this low potential barrier in high-temperature and high-pressure ethanol, especially in the supercritical ethanol, is fascinating. The silica-ethanol system might be a unique and special example in the colloidal dispersions. Results suggest that silica nanoparticles may be used as a model colloid to investigate the colloidal transport phenomena in the supercritical ethanol.

The Electrochemistry of Nonaqueous Copper Phthalocyanine Dispersions in the Presence of a Metal Soap Surfactant: A Simple Equilibrium Site Binding Model

The electrophoretic mobilities of copper phthalocyanine particles, dispersed in isoparaffin solutions containing zirconium octanoate, have been determined using phase-analysis light scattering. All the samples studied contained trace concentrations of water. The mobility values were converted to zeta potentials using the Hückel equation. All the systems studied exhibited a pronounced maximum in zeta potential as the zirconium octanoate concentration increased. The maximum occurred at a bulk zirconium octanoate concentration equivalent to that required for complete coverage of the particles. The zeta potential data were converted to surface charge density values through the use of the Poisson-Boltzmann equation. The latter were in the range 0.4 to 2.5 µC m-2. A simple two equation site binding theory, which considered the dissociation of zirconium octanoate and the subsequent adsorption of ions at a generic surface site, was successfully applied to the surface charge data. It is proposed that the maximum in the zeta potential and surface charge as a function of zirconium octanoate concentration was observed due to the preferential location of ZrO2+ ions at the particle surface, followed by charge neutralization with octanoate anions. It is suggested that water facilitates the dissociation process of the zirconium octanoate, although it does not directly contribute to the surface charge itself. Two plausible qualitative mechanisms are described. The first involves the presence of water at the particle-solution interface, whilst the second considers the formation of micelles in the bulk isoparaffin phase. Copyright 1999 Academic Press.