Phase changes during silica particle formation in water-in-oil emulsions

Silica particles have been obtained by addition of Tetraethoxysilane (TEOS) to water-in-oil (W/O) emulsions. The present paper studies the role of ethanol, present in the system due to TEOS hydrolysis, in the emulsion properties during silica particle formation. Evolution in phase behaviour was observed during the reactions. Initial emulsions were stable, but soon after TEOS addition, in presence of hydrochloric (HCl), they became unstable. Phase behaviour changed, appearing three liquid phases if agitation was interrupted. It was observed that ethanol could increase the HLB temperature (T-HLB) of the emulsion systems. It also could increase the temperature range in which three liquid phases would coexist in equilibrium. This was related to a reduction in the interfacial tension, which was also dependent on ethanol concentration. The interfacial tensions decreased until a minimum was reached, where three liquid phases could form. All this greatly affected emulsion stability. Emulsion phase separation, due to coalescence, was very fast after critical concentrations of ethanol were reached. Therefore, the size of silica particles were coalescence-controlled. Their size could be reduced by addition of a block copolymer surfactant. This surfactant was able to both increase stability and reduce interfacial tensions. This trend was the same for the different systems studied. Systems with slower coalescence eventually provided smaller particle size. (C) 2001 Elsevier Science B.V. All rights reserved.