Physical characterization of titanium dioxide nanoparticles

The objective of this study was to review six measurement methods (X-ray line broadening, electron microscopy, static light scattering, dynamic light scattering, X-ray sedimentation and surface area determination), which are widely used for the characterization of ultrafine inorganic oxides used in cosmetic formulations. Depending on the processing that they have received and the system in which they are examined, these oxides can exist as primary particles, strongly bound aggregates or weakly bound agglomerates.

The example of titanium dioxide, TiO2 , is used to consider which type of particle is being measured in a particular case, and the factors which influence the 'size' that is generated by a particular method. Where appropriate, a correlation is made between results of different measurements.

Results for a particular set of four cosmetic grade TiO2 's are presented and examined, in the context of a much broader set of measurements taken from the scientific literature.

In general, X-ray line broadening, electron microscopy and surface area measurements led to estimates of the size of primary particles. By contrast, both sedimentation and light scattering measurements measured the size of the secondary particles, and the figures which were generated depended on the dispersion conditions used for preparation of the measurement samples. For poorly dispersed or lightly milled samples, the size may be dominated by the presence of weakly bound agglomerates, but even when the sample is well dispersed or heavily milled, the reported size cannot be less than that of the aggregates.