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Effect of Macromolecular Polydispersity on Diffusion Coefficients Measured by Rayleigh Interferometry

Summary: Effect of Macromolecular Polydispersity on Diffusion Coefficients Measured by
Rayleigh Interferometry
Huixiang Zhang, and Onofrio Annunziata*,
Department of Chemistry, Texas Christian UniVersity, Fort Worth, Texas 76129, and Alcon Research Ltd.,
Fort Worth, Texas 76134
ReceiVed: October 29, 2007; In Final Form: December 17, 2007
Rayleigh interferometry has been extensively used for the precise determination of diffusion coefficients for
binary and ternary liquid mixtures. For ternary mixtures, the 2 2 matrix of multicomponent diffusion
coefficients is obtained. Polydispersity adds complexity to the meaning of these measured diffusion coefficients.
Here we discuss three important issues of polydispersity regarding the diffusion measurements extracted
from this interferometric technique. First, we report novel equations for the extraction of diffusion moments
from the Rayleigh interferometric pattern. These moments are used to define polydispersity parameters for
macromolecular systems. We have experimentally determined mean diffusion coefficients and polydispersity
parameters for aqueous solutions of poly(ethylene glycol) and poly(vinyl alcohol) at 25 C. Aqueous solutions
of poly(ethylene glycol) mixtures were used to examine the accuracy of the polydispersity parameters. Second,
we compare Rayleigh interferometry to dynamic light scattering. Specifically, we have performed diffusion
measurements on the same system using both techniques. To our knowledge, no direct experimental comparison
between dynamic light scattering and classical methods for the measurements of diffusion coefficients has
been previously reported in relation to polydispersity. We find that substantial discrepancies (i.e., 1 order of
magnitude) between the mean diffusion coefficients obtained from these two different techniques can be


Source: Annunziata, Onofrio - Department of Chemistry, Texas Christian University


Collections: Chemistry