Diffusivities in Binary and Ternary Mixtures Containing Poly(ethylene) Glycol, Acetonitrile, and Water by Dynamic Light Scattering

In the present study, one of the eigenvalues D-1 of the Fick diffusion matrix for ternary mixtures consisting of polydisperse poly(ethylene) glycol (PEG), acetonitrile, and water as well as the mutual diffusivity D-11 in two binary subsystems were determined by dynamic light scattering. PEGs with molecular weights M-n ranging from (400 to 4000) g<middle dot>mol(-1), a solvent mixture containing acetonitrile, and an experimental temperature of 323 K were selected because of their relevance to the application of reversed-phase liquid chromatography. In general, the accessed D-1 decreases with increasing M-n and presents different concentration dependences. It was demonstrated that the accessed D-1 is associated with the diffusion of PEGs in the solvent mixture and can be considered as the self-diffusivity of the sphere-like PEG molecules in the vicinity of infinite dilution regime. Using the self-diffusivity and the dynamic viscosity of the solvent mixture, the hydrodynamic diameter of the PEG molecules was determined via the Stokes-Einstein relation. While the values match that determined in water for smaller M-n, they tend to be smaller than that in water for larger M-n. This indicates a minor influence of the solvent or temperature on the hydrodynamic diameter of the PEG molecules. The determined diffusivities and hydrodynamic diameters of PEGs under process-relevant conditions are of relevance for the development of chromatographic separations.