Determination of Preferential Interaction Parameters by Multicomponent Diffusion. Applications to Poly(ethylene glycol)−Salt−Water Ternary Mixtures

Abstract

Poly(ethylene glycol) (PEG) is a hydrophilic nonionic polymer used in many biochemical and pharmaceutical applications. We report the four diffusion coefficients for the PEG-KCl-water ternary system at 25°C using precision Rayleigh interferometry. Here, the molecular weight of PEG is 20 kg mol -1 , which is comparable to that of proteins. The four diffusion coefficients are examined and used to determine thermodynamic preferential interaction coefficients. We find that the PEG preferential hydration in the presence of KCl is 1 order of magnitude larger than that previously obtained under the same conditions for lysozyme, a protein of similar molecular weight. In correspondence, the coupled diffusion in the PEG case was greater than that observed in the lysozyme case. We attribute this difference to the greater exposure of polymer coils to the surrounding fluid compared to that of globular compact proteins. Moreover, we observe that the PEG preferential hydration significantly decreases as salt concentration increases and attribute this behavior to the polymer collapse. Finally, we have also employed the equilibrium isopiestic method to validate the accuracy of the preferential interaction coefficients extracted from the diffusion coefficients. This experimental comparison represents an important contribution to the relation between diffusion and equilibrium thermodynamics.

On the Role of Solute Solvation and Excluded-Volume Interactions in Coupled Diffusion

“…Indeed, for ternary aqueous salt solutions containing a neutral macromolecule, it has been recently observed that only one of the two diffusioncoefficient ratios can be approximated as a thermodynamic factor. 7 In contrast, the other diffusion-coefficient ratio was found to be significantly smaller than its corresponding thermodynamic factor. This implies that frictional interactions cannot be generally neglected.…”

“…This assumption has led to the examination of (D ij ) 0 /(D ii ) 0 strictly in terms of thermodynamic parameters. However, it has been experimentally observed for the poly(ethylene glycol) (1) 7 This implies that the thermodynamic approximation is valid only for the small solute (i ) 2).…”

“…Indeed, poly(ethylene glycol) undergoes extensive hydration in aqueous solutions. 7,10,11 This implies that ˆ1 2 may be neglected in eq 37 and 12 ≈ -(ν 2 ˆ1 0 + ν 1 ˆ2 0 ).…”

“…It is worth mentioning that the dilute-solution behavior of (D ij ) 0 /(D ii ) 0 is approximately maintained even in concentrated solutions. 7,10,11 In the limit of dilute solutions, ( To evaluate the magnitude of the thermodynamic quotients, eq 17 can be used to obtain µ ij /[C i µ ii ] ) 2B 12 -V j i . For the special case of a hard-sphere mixture, V j i ) N A (4/3)πR i 3 and the excluded-volume interactions are described by B 12 ) N A (2/ 3)π(R 1 +R 2 ) 3 , 47 where N A is the Avogadro's number and R i is the radius of component i.…”

“…However, current approaches qualitatively fail to describe the observed discrepancy between diffusion-coefficient ratios and thermodynamic factors. 7 The role of excluded-volume interactions on the thermodynamic factors have been examined also. 6,[9][10][11][12] For ternary systems, thermodynamic data on solute-solute interactions are often not available.…”