(264a) Particle Size Measurement in Aggregating Colloidal Dispersions: Comparison of Turbidity Spectra and Light Scattering

Turbidimetry has been used for many years to monitor aggregation in colloidal systems. If primary particles are small compared to the wavelength of the incident light, turbidity of aggregating systems increases with time, in agreement with predictions based on the Rayleigh-Debye-Gans theory of light scattering. However, when particles become comparable to the wavelength of the incident light, the opposite is observed: turbidity decreases with time as aggregation progresses. Moreover, multiple scattering also decreases the measured turbidity unless the dispersion is sufficiently dilute or the optical path of the light beam is sufficiently short. We present spectrophotometric (turbidity) and small angle static light scattering measurements for dispersions of polystyrene colloidal particles aggregating in diffusion limited conditions. We discuss several approaches to modeling the time evolution of measured structure factors and turbidity spectra in aggregating systems at light wavelengths between 400 and 800 nm. We present experimentally validated models for particle detection in terms of turbidity spectra and scattered light intensity of growing aggregates for two cases of primary particle size: much smaller as well as larger as the wavelength of incident light.