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.