Hydrophobic interactions in dilute solutions of poly(vinyl alcohol)

Abstract

High-precision density and light-scattering measurements in water and 10^–3 mol dm^–3 electrolyte solutions of samples of 80 and 90% poly(vinyl alcohol)(PvOH) over an extended range of concentration and temperature have revealed a complex pattern of solution behaviour; this report is restricted to the behaviour of solutions at concentrations below 0.15% w/v.

Partial molal volume (V₂°) and expansibility (Ē₂°) studies at infinite dilutions in water show the existence of a very strong temperature dependence in these partial molal properties; V₂° for 80% hydrolysed PvOH decreases by 40% between 10 and 15 °C, rising by almost the same amount between 15 and 18 °C, becoming virtually constant at higher temperatures. Ē₂° studies suggest that increasing hydrophobic hydration occurs between 10 and 15 °C, which results in a rapid conformational change in the molecule between 15 and 18 °C, producing a structure which is predominantly hydrated in the hydrophilic sense. At temperatures above 18 °C gradual erosion of this hydration appears to take place.

Corresponding studies on a sample of 90% hydrolysed PvOH show broadly similar behaviour, but modified by the presence of a smaller proportion of hydrophobic vinyl acetate groups; for example, the conformation change with temperature is not as abrupt as in the case of 80% hydrolysed PvOH.

Similar studies in 10^–3 mol dm^–3 solutions of LiCl, Li₂SO₄, NaCl and KCl illustrate the large ionic effects upon the hydrophobically dictated conformational change; these effects compare remarkably well with the calculated thermodynamic property changes, associated with salt effects, in the folding process of the globular protein ribonuclease.

Particle size and second virial coefficent values obtained from light-scattering studies confirm the behaviour pattern observed.