(145g) Turbidity Spectra and Static Light Scattering in Mie Scattering Regime for Monitoring of Particle Formation Processes
AIChE Annual Meeting
Monday, November 13, 2006 - 5:20pm to 5:40pm
Turbidimetry and static light scattering have been used for many years to monitor particle formation processes in disperse systems. If scattering particles are small compared to the wavelength of the incident light, then the classical Rayleigh-Debye-Gans (RDG) theory of light scattering can be applied for quantitative interpretation of light scattering and turbidity measurements in order to determine detailed information about mass distribution and structure of particles and their aggregates. It is well known that as the size of primary particles becomes comparable with the wavelength of the incident light (typically hundreds of nanometers for visible light), experimental observations start to deviate from the RDG theory. In particular, the scattered intensity magnitude is no longer proportional to the second power of the particle or cluster mass and the scattered intensity patterns become more complex as well. Moreover, turbidity decreases with time in aggregating systems of large primary particles, while it would be expected to keep increasing with time according to the RDG theory.
Here we propose a general modeling framework for interpretation of turbidity and light scattering measurements based on the mean-field Mie theory calculations for aggregates of primary particles of arbitrary size, which allows us to calculate scattered intensities and turbidities of aggregates with a given mass and structure. In particular, we show that the scattered intensities of aggregates scale as a power of the aggregate mass, where the scaling exponent is a function of the primary particle size as well as the aggregate fractal dimension. We analyze spectrophotometric (turbidity spectra between 400 and 800 nm) and small angle static light scattering measurements in aggregating dispersions of particles of various sizes in order to validate scattering models developed here. This approach can be used for quantitative interpretation of turbidity spectra and/or static light scattering measurements in the Mie scattering regime for a variety of particle sizing applications involving clusters and aggregates.