Electrostatic and excluded volume effects on the transport of electrolytes in poly(ethylene glycol)-water "mixed solvents"

The transport properties of NaCl in poly(ethylene glycol)-water (PEG-water) mixtures are presented, with special regard to the dielectric and hydrodynamic effects. We report experimental data of mutual diffusion, viscosity, and conductivity for the aqueous ternary system containing PEG with an average molecular weight of 2000 a.m.u. (PEG2000, component 1) and sodium chloride (NaCl, component 2). Both PEG2000 and NaCl are used extensively as protein-crystallizing agents. The four diffusion coefficients (D-11, D-12, D-21, and D-22) that are required to describe the diffusive properties of a ternary system correctly were determined by the precise Gouy interferometric method. We also report measurements of the dielectric constant for several aqueous binary systems containing PEG with three different molecular weights (200, 400, and 2000 a.m.u.), which significantly decreases the dielectric constant of aqueous solutions. The experimental values of the four diffusion coefficients are discussed in terms of obstruction, excluded volume, and dielectric effects. The dependence of the PEG main-term diffusion coefficient, D-11, on the salt concentration is quantitatively described by the corresponding change of viscosity. The cross-term diffusion coefficients, D-12 and D-21, are described in terms of excluded volume effects and PEG-salt nonpreferential interactions. The dependence of the salt main term diffusion coefficient, D-22, on PEG concentration is essentially due to obstruction effects. Moreover, a remarkable agreement between D-22 and the salt equivalent conductivity has been determined. This correlation suggests that, even though the dielectric constant significantly changes in the presence of PEG, the electrostatic ion-ion interactions (the summation of the extent of ion-pairing and electrophoretic contributions) are not significantly modified by the polymer. This analysis of the diffusive and conductivity properties of electrolytes in the presence of PEG have allowed us to clarify important aspects of the electrostatic properties of aqueous PEG-salt systems and supplies valuable information for salt properties in "mixed solvents". A useful classification of mixed solvents is proposed, distinguishing between "viscous" and "dielectric" mixed solvents.