Kirkwood-Buff integrals for polymer solvent mixtures. Preferential solvation and volumetric analysis in aqueous PEG solutions

The effect of the molecular size of components on preferential solvation analysis has been investigated by studying aqueous solutions of the poly(ethylene glycol) (PEG) homologous chemical series. Precise density data are analyzed for some PEG-water systems, for polymers of molecular weight up to 3400 at 25°C, at the highest practicable concentrations. The combination of density, activity, and compressibility data allows the evaluation of Kirkwood-Buff integrals for these systems. Data were discussed in terms of preferential solvation, showing that co-operativity takes part to some extent in the PEG-PEG interaction, depending on PEG concentration and molecular weight. The size mismatch between solvent and solute requires a proper reference state, the Matteoli-Shulgin-Ruckenstein state, which takes into account the excluded volume effect. This correction must be applied when considering macromolecular systems, otherwise physically meaningless analyses are developed. Density data at high concentration have been analyzed in order to evaluate progressive aggregation up to network formation, recently proposed for PEG-water systems. The study provides a new picture of aqueous PEG solutions useful for the interpretation of protein precipitation induced by this uncharged polymer. Comparisons with a preferential solvation analysis of the PEG oligomers and with a recent velocity-correlation analysis of PEG polymers have been also made. This paper provides a new picture of PEG-water media, useful to interpret the colloidal stability in the presence of PEG.