Time-temperature superposition principle for the kinetic analysis of destabilization of pharmaceutical emulsions

The time-temperature superposition principle (TTSP) was applied to the destabilization kinetics of a pharmaceutical emulsion. The final goal of this study is to predict precisely the emulsion stability after long-term storage from the short-period accelerated test using TTSP. As the model emulsion, a cream preparation that is clinically used for the treatment of pruritus associated with chronic kidney disease was tested. After storage at high temperatures ranging from 30 to 45 °C for designated periods, the emulsion state was monitored using magnetic resonance imaging, and then the phase separation behaviors observed were analyzed according to the Arrhenius approach applying TTSP. The Arrhenius plot showed a biphasic change around 35 °C, indicating that the separation behaviors of the sample were substantially changed between the lower (30-35 °C) and higher (35-45 °C) temperature ranges. This study also monitored the coalescence behavior using a backscattered light measurement. The experiment verified that the destabilization was initiated by coalescence of oil droplets and then it eventually led to obvious phase separation via creaming. Furthermore, we note the coalescence kinetics agreed well with the phase separation kinetics. Therefore, in the case of the sample emulsion, the coalescence behavior has a dominant influence on the destabilization process. This study offers a profound insight into the destabilization process of pharmaceutical emulsions and demonstrates the promising applicability of TTSP to pharmaceutical research.

Nondestructive Monitoring of the Dispersion State of Titanium Dioxide Nanoparticles in Concentrated Suspensions Using Magnetic Resonance Imaging

The purpose of the present study is to demonstrate the applicability of magnetic resonance imaging, especially T₂ relaxation time mapping, for nondestructive monitoring of the dispersion state of nanoparticles (NPs) in concentrated suspensions. TiO₂ 15-nm-diameter NPs, for use in sunscreen lotion products, were examined as a test NP. First, this study investigated whether T₂ is sensitive to the NP concentration. In experiments with pulsed nuclear magnetic resonance on TiO₂ NP suspensions with different organic solvents (ethanol, acetone, and decamethylcyclopentasiloxane), the T₂ of each solvent varied in the suspensions according to the NP concentration. This study also confirmed that T₂ mapping was effective for visualizing differences in NP concentration. Subsequently, gravitational sedimentation of the test suspensions was investigated. T₂ mapping exhibited better detection sensitivity to sedimentation occurring in concentrated suspensions than visual observation, as it enabled the detection of changes in NP distributions that could not be visible to the naked eye. In addition, measurements of backscattered light enabled the full understanding of the dispersion stability of the TiO₂ NPs in each solvent. Finally, the present study evaluated the centrifuge sedimentation of a commercial TiO₂ NP suspension. T₂ mapping clearly showed the complicated sedimentation behavior induced by the centrifugation treatment. The simulated fluid flow was consistent with the particle distribution in the centrifuged sample; thus, the sedimentation was believed to have developed in accordance with the vorticity generated by the centrifugation.