Multicomponent diffusion in crowded solutions. 2. Mutual diffusion in the ternary system tetra(ethylene glycol)-NaCl-water

Multicomponent diffusion properties for the tetra(ethylene glycol) (PEG4)-sodium chloride-water ternary system are investigated at constant salt concentration (0.5 mol dm(-3)) and a wide range of PEG4 concentration approaching 3.0 mol dm(-3) (60% volume fraction). Cross-velocity correlation (counterflows) for any pair of components in this ternary system is determined. Comparison between the exact multicomponent approach and the pseudobinary approximation, which is based on the assumption that the PEG4-water mixed solvent can be treated as one component, shows how the latter approach is misleading in describing the electrolyte diffusion properties in mixed solvents. Our results show strong coupling between the fluxes of NaCl and PEG4, which is attributed to the NaCl-PEG4 nonpreferential interactions in water. Moreover, the behavior of the NaCl and PEG4 chemical potentials as a function of the system composition can be quantitatively extracted from our diffusion measurements with the help of some interesting observations about the viscosity and conductivity properties of the system.