Effect of dilution on particle size analysis of w/o emulsions by dynamic light scattering

Dynamic Light Scattering (DLS) is a well-known (sub)micron particle size analysis technique. As such, it should be a valuable technique for water droplet size determination in (sub)micron water-in-oil emulsions. In this study, the sample pretreatment conditions were optimized in order to enable accurate droplet size determination by DLS. Whereas DLS was indeed capable of detecting differences between differently produced (sub)micron sized w/o emulsions, it was observed that the estimated droplet size was highly influenced by the measurement conditions used. First of all, the experimentally determined diffusion coefficient decreased with increasing water droplet concentration, which is thought to be due to interaction effects between the water droplets. To minimize this effect, the used concentration should be as low as possible. However, in doing so, the water droplets were frequently observed to disappear during storage or even during analysis, due to their dissolution in the dilution medium. The latter disturbing effect could be avoided by (sufficient) water saturation of the dilution medium and preventing water evaporation by closing the diluted sample and by restricting the head space. Summarizing, (sub)micron water droplets can be characterized using DLS. However, careful selection of the sample preparation and measurement conditions is necessary to obtain reliable data.