DIFFUSION IN CONCENTRATED DISPERSIONS - A STUDY WITH FIBEROPTIC QUASI-ELASTIC LIGHT-SCATTERING (FOQELS)

A fiber-optic, quasi-elastic light-scattering instrument is described using single-mode fiber optical components, including a novel slanted exit face optode. The setup operates with homodyne signal detection. It enables the characterization of diffusion processes in concentrated dispersions up to volume concentrations of 50%. The performance of the instrument is exemplified with results obtained from latex spheres with diameters of 226 nm and 404 nm at volume fractions from Phi = 0.01 to Phi = 0.5. The correlation functions are analyzed according to the second order cumulants method and the "Contin"-procedure yielding an average and a distribution function of the short-time self-diffusion coefficient, D(eff)s, respectively. At high ionic strength the concentration dependence of D(eff)s/D0 is found to be in close agreement with theoretical predictions based on a multi-body interaction model of hard spheres up to Phi = 0.45. With decreasing ionic strength the negative slope of the virial expansion tends to increase, presumably due to enhanced repulsive electrostatic interactions. The described technology offers new experimental means for on-line remote control sensing of particle size in concentrated disperse systems.