VAN-DER-WAALS ATTRACTION BETWEEN STOBER SILICA PARTICLES IN A BINARY SOLVENT SYSTEM

Hydrophilic Stober silica particles are stable in ethanol, but floccul ation may be induced by the addition of sufficient cyclohexane. Low-sh ear rheological measurements indicated non-Newtonian behaviour beyond the critical cyclohexane concentration. The thickness and composition of the solvation layer around the particles were calculated from the a dsorption excess isotherm on the basis of a multilayer adsorption mode l. The composition dependences of the Hamaker constants of the dispers ion medium and the adsorption layer were obtained from optical dispers ion measurements. A single-sheet, hard-sphere model predicted a weak v an der Waals attraction in the ethanolic regime, but a strong attracti on in the cyclohexane-rich region, in good accordance with the rheolog ical properties of the dispersions and visual observations.