Accurate Determination of the Short Time Self Diffusion Coefficient by Fiber Optic Quasi-Elastic Light Scattering: New Methods for Correcting the Homodyne Effect

Abstract

Using both a monodisperse latex and a polydisperse O/W emulsion, it is shown that Fiber Optic Quasi-Elastic Light Scattering (FOQELS) only yields an apparent average diffusion coefficient and an apparent polydispersity index. In order to obtain accurate values, a correction is needed for the presence of homodyning. For that purpose, two new approaches are introduced. Keeping account of the homodyne correction, the average diffusion coefficient is shown to be linearly related to the volume fraction. By extrapolation to zero concentration, the diffusion coefficient at infinite dilution is obtained, from which the average particle size is estimated; a simulation of the detection process reveals that the observed overestimate is most probably due to hindrance of the diffusion process by the optical fiber outlet. As theoretical considerations reveal that the short time self diffusion coefficient should be obtained for the samples considered, an apparent hard sphere volume fraction can be deduced from the slope. In order to check the usefulness of our approach, the latter is determined by an independent method as well; very similar results originate from viscosity measurements.