Multicomponent diffusion in systems containing molecules of different size. 1. Mutual diffusion in the ternary system poly(ethylene glycol) 2000+poly(ethylene glycol) 200+water

The multicomponent approach for the description of liquid diffusion process es includes cross-term diffusion coefficients that describe the interaction between the flows of different solute components. We demonstrate that thes e cross-term diffusion coefficients can be large and thus necessary for the description of diffusion of polydisperse polymers. In this study, the four mutual diffusion coefficients have been measured at 25 degreesC for the te rnary system PEG 2000 (1) [poly(ethylene glycol) 2000 g mol(-1)] +PEG 200 ( 2) [poly(ethylene glycol) 200 g mol(-1)] + water (0). Diffusion coefficient s were measured at solute concentrations of 0.0827 mol dm(-3) PEG 2000 and 0.076, 0.2293, and 0.4105 mol dm(-3) PEG 200. One of the cross-terms, D-21, varies linearly with the concentration of PEG 200 at a constant concentrat ion of PEG 2000 and, at the highest concentration of PEG 200, was larger th an either of the main-diffusion coefficients D-11 and D-22. The diffusion c oefficients have been qualitatively interpreted in terms of molecular inter actions and the excluded volume effect. We also report binary diffusion coe fficients, densities, and viscosities for binary solutions of both PEG 200 and PEG 2000 in water at 25 degreesC.