The interaction of aromatic compounds with poly(vinylpyrrolidone) in aqueous solution. Part 6.—Polymer precipitation and viscosity studies with phenols and O-substituted phenols

Abstract

The work reported in the previous Part on the binding of phenols and O-substituted phenols by poly(vinylpyrrolidone)(PVP) in aqueous solution has been extended by studies of their effects on the solubility and solution viscosity behaviour of the polymer, using the same thirteen cosolutes, i.e. PhOH, PhOMe, PhOEOH, PhOGl, HOPhNO₂, HOPhOH, HOPhOMe, MeOPhOMe, HOEOPhOEOH, HOPhOGl, Ph(OEOH)₂, PhPhOH and 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’)]. Only four of these cosolutes precipitate the polymer, their critical precipitation concentration c^* in mol m^–3, for 10 g dm^–3(1% w/v) PVP K-90 at 20 °C, being: PhOH, 85; HOPhNO₂, 17; HOPhOH, 50; HOPhOMe, 69. The viscosity measurements show that in general the intrinsic viscosity, [η], is reduced in the presence of the cosolutes (most markedly so by the precipitant cosolutes); paralled increases are seen in the Huggins slope parameter, k_H. The importance of the acidic phenolic hydroxy group in these phenomena is shown by the small viscosity effects seen with PhOGl (which is not bound by the polymer) and with the three hydroxyethyl compounds PhOEOH, HOEOPhOEOH and Ph(OEOH)₂, by the low value of c^* for the highly acidic cosolute HOPhNO₂, and by c^* decreasing with more phenolic hydrogen-bond donor/acceptor groups (PhOH → HOPhOMe → HOPhOH). The absence of precipitation with HOPhOGl, despite its phenolic hydroxy group and its high solubility, and the small viscosity effects it produces, parallel the ‘inhibitory’ effect of the glucosyl group seen in the binding behaviour with this cosolute and with PhOGl. With PhOMe and MeOPhOMe the solubility is evidently too low to show up any effects. Correlating the intrinsic viscosity with the binding ratio r(the average number of cosolute molecules bound per monomer unit of the chain) shows that there are two main forms of behaviour: (i) a simple linear dependence of [η] on r, which is seen with six cosolutes (HOPhNO₂, PhOEOH, HOEOPhOEOH, Ph(OEOH)₂, PhPhOH and HOPhOH); and (ii) a linear dependence of [η] on r², which is seen with four cosolutes (PhOH, HOPhOH, HOPhOMe and HOPhOGl). The results are interpreted on the basis of the reversible non-covalent cross-linking of the polymer chains by bound cosolute molecules, which explains the observed reductions in [η] and the parallel increases in k_H, as well as the precipitation seen with four of the cosolutes. A statistical-mechanical model derived by Kuhn and coworkers, based on the cross-linking concept, is applied to the viscosity data, enabling the equilibrium constant for cross-linking to be calculated.