(24d) Multi-Wavelength Photo-Centrifugation for Particle Sizing without Reference to Models for the Optical Behaviour

Analytical photo-centrifugation is widely used for the determination of particle size distributions in finely dispersed suspensions and emulsions. It determines extinction-weighted size distributions of the Stokes diameter. Transformation into volume-weighted size distributions (Q₃) requires analytical models for the extinction at single particles, which depend on particle shape and the complex refractive index. These two features are typically unknown in real-life situations.

However, optical models are not required with multi-wavelength detection systems, which provide the optical model as an additional result of the analysis. The fundamental idea behind is that the wavelength dependency of turbidity for defined particle sizes is approx. equivalent to the size dependency of turbidity at fixed wavelengths. Despite this well-understood principle, there are several critical issues that may impede its transfer to practical situations (e.g. wavelength-specific detection limits).

The performance of the described approach was tested with a newly developed photo-centrifuge with a multi-wavelength detection system (LUM GmbH). Various colloidal particle systems were analysed. The approach yields optical models of the particles, which fairly well agree with the relevant analytical models (e.g. Mie theory). Correspondingly, the multi-wavelength analysis yields the same Q₃ as the traditional analysis, which is based on the particle’s refractive index.