RESUMO
OBJECTIVE: We have recently established a novel method to evaluate the emulsion stability of pharmaceutical skin cream. The key technology of the method is magnetic resonance imaging (MRI). The purpose of this study was to verify the usefulness of the method in the cosmetic industry. METHODS: Milky lotion-type emulsions were employed as test samples. We note that the test samples were prepared by taking account of commercial milky lotions. After the sample preparation, a centrifugation treatment (5000 g for up to 120 min) was implemented to accelerate their destabilization processes. The centrifuged samples were monitored by using T2 relaxation time (T2 ) maps. Furthermore, the histograms generated from the T2 maps were analysed to investigate the destabilization process in more detail. In addition, small fractions of the upper and lower phases were collected from the centrifuged samples, and microscopic observations were conducted. RESULTS: T2 maps successfully visualized the destabilization process accompanying the centrifugation protocol. From the microscopic observations, it was clarified that the main mechanism of the destabilization process was creaming. The sensitivity of the T2 map to creaming was much superior to that of visible observation; the T2 map can detect a slight creaming that is not visible to the naked eye. In addition, the T2 map also enables the detection of slight reversible creaming-dispersion changes accompanied by a repeated centrifugation-vortexing treatment. By using the parameters derived from the histogram analysis, the creaming behaviour can be evaluated more precisely and more objectively. This study prepared emulsions containing different thickener contents and then compared their creaming behaviours. As a consequence of the analysis, we could fully evaluate the effect of thickener on the emulsion stability by the evaluation method. CONCLUSION: MRI is a promising tool for evaluation of the stability of cosmetic emulsions.
