Effects of Particle Size and Stabilizing Agents upon Dielectric Properties of Water-in-Oil Type Emulsions

抄録

The dielectric relaxation due to the interfacial polarization of water-in-oil type emulsions (W/O emulsions) was investigated at frequencies ranging from 10 kHz to 3 MHz. In preparing W/O emulsions, differences of mixing or micronizing efficiency arise between bench-scale and production-scale apparatus owing to the difference of mixing or shear rate. Therefore, the size of dispersed particles can be regarded as an index of the mechanical effect arising from the mixing apparatus. Firstly, the relationship between the size of dispersed particles and the dielectric properties of W/O emulsions was investigated. It was found that 1 d after preparation, the degree of particle aggregation leveled off and became fairly steady, and the values, for W/O emulsions with coarse particles, of the limiting dielectric constant at low frequency, ε_l, and parameter α, indicating the distribution of relaxation frequencies, determined from complex plane plots, were greater than for those for emulsions with fine particles. The value of ε_l is thought to be related to the thickness of the surfactant layer between aggregated particles rather than to the size of particle clusters, but the relation becomes more ambiguous as the concentration of surfactant becomes higher. Relaxation frequency, f₀, decreases with increasing particle aggregation. Secondly, in order to obtain good emulsification in the W/O emulsion, the stabilizing effects of 23 stabilizing agents upon the state of dispersion of W/O emulsion were evaluated by dielectric measurement. Primary emulsifiers for micronizing dispersed particles, such as polyoxyethylene (POE) (6) sorbitan monooleate, secondary emulsifiers for protecting interface membranes, such as phytosterol, and lipojelling additives, such as aluminum stearate, decreased the value of ε_l of W/O emulsions. From the present study, it was concluded that dielectric evaluation can be applied to examine the mechanical effects on emulsification and to as an aid in the selection of optimum additives and vehicles for a W/O emulsion.