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 eth anol, present in the system due to TEOS hydrolysis, in the emulsion propert ies during silica particle formation. Evolution in phase behaviour was obse rved during the reactions. Initial emulsions were stable, but soon after TE OS addition, in presence of hydrochloric (HCl), they became unstable. Phase behaviour changed, appearing three liquid phases if agitation was interrup ted. 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 ethano l concentration. The interfacial tensions decreased until a minimum was rea ched, where three liquid phases could form. All this greatly affected emuls ion 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 reduc ed by addition of a block copolymer surfactant. This surfactant was able to both increase stability and reduce interfacial tensions. This trend was th e same for the different systems studied. Systems with slower coalescence e ventually provided smaller particle size. (C) 2001 Elsevier Science B.V. Al l rights reserved.