The interaction of aromatic compounds with poly(vinylpyrrolidone) in aqueous solution. Part 5.—Binding isotherms for phenols and O-substituted phenols

Abstract

In an extension of previous work, and as part of an investigation of the mechanisms by which phenols precipitate water-soluble polymers from aqueous solution, the binding isotherms have been determined for poly-(vinylpyrrolidone)(PVP) in aqueous solution at 25 °C with the following 13 cosolutes: PhOH; PhOMe; PhOEOH; HOPhNO₂; PhOGl; HOPhOH; HOPhOMe; MeOPhOMe; HOEOPhOEOH; HOPhOGl; Ph(OEOH)₂; PHPhOH; HOPhPhOH [where Ph = phenyl or phenylene (1,4-substitution in XPhY and 1,3-substitution in PhXY), E = ethylidene (CH₂CH₂) and Gl = 1-deoxy, 1-β-D-glucopyranosyl (‘glucosyl’)]. The three O-substituents used (i.e. Me, EOH and Gl) have been chosen to reveal the contribution to the binding from the phenolic hydrogen without affecting that from the phenoxy or phenylenedioxy part of the cosolute molecule. The two complementary experimental techniques of equilibrium dialysis and cosolute solubility have been used for these measurements. With most of the cosolutes the binding could be interpreted in terms of the uniform site-binding model which leads to the hyperbolic (Langmuir) form of binding isotherm; with HOPhNO₂, and possibly with HOPhOMe, the binding involves cooperative effects resulting from attractive interactions between bound cosolute molecules. The binding parameters are discussed on the basis of group-contribution theory, in relation to the four main types of interaction forces in these aqueous systems, i.e. hydrophobic effects, dipole/induced dipole (van der Waals–Debye) forces, dipole/dipole (van der Waals–Keesom) forces and hydrogen bonds. The correlations observed have enabled a group-contribution table to be drawn up for the binding constants of six aromatic consolutes: PhH, PhOH, HOPhOH, PhPh, PhPhOH and HOPhPhOH. The results for the glucosides, PhOG1 and HOPhOG1, showed there to be a net repulsion between the glucose ring and the PVP chain in aqueous solution.