The interaction of water-soluble α, ω-disubstituted alkanes (bolaform compounds) with polyvinylpyrrolidone in aqueous solution

Summary The published work on the interactions between polyvinylpyrrolidone (PVP) and small-molecule solutes (cosolutes) in aqueous solution has been briefly surveyed. The further information on these types of interaction which the proposed study of,-disubstituted aliphatic cosolutes, of the general type X(CH₂)_mY (bolaform compounds), was hoped to yield has been outlined. The technique of equilibrium dialysis (employing two-compartment acrylate cells) has been used to study the interaction between PVP in aqueous solution at 25 °C and the following eight symmetrical (X=Y) bolaform cosolutes: X=SO₄Na,m=8, 10 and 12 (A, B and C); X=CO₂K,m=10 (D); X=N(CH₃)₃Br,m=10 (E); X=NH₃Cl,m=8, 10 and 12 (F, G and H). The maximum total cosolute concentrations used were between 100 and 230 millimolal. The equilibrated dialysis-cell solutions were assayed by differential refractometry; the data obtained were converted into the values ofa, the concentration of free cosolute, andr, the concurrent number of moles of cosolute bound per base mole (vinylpyrrolidone unit) of the polymer. The limit of detectable binding wasr=0.005 for all cosolutes. The results showed that of the three disulphates (A, B and C), the octamethylene compound (A) was not detectably bound while the deca- and dodecamethylene compounds (B and C) had binding isotherms of the hyperbolic (Langmuir) form:r =nKa/(1 +Ka), withK=65 (±20) molal^–1 andn=0.057 (±0.005) form=10, andK=16 (±2) molal^–1 andn=0.139 (±0.005) form=12; the other anionic cosolute (D) was not detectably bound. Of the cationic cosolutes (E, F, G and H), Decamethonium bromide (E) had a non-hyperbolic binding isotherm with a maximum ata=75 millimolal ofr~0.025; the three di(ammonium chloride)s (F, G and H) were not detectably bound. The refractive index increments and densities of the aqueous solutions of the same eight compounds have also been determined; the results did not show any anomalies which could definitely be attributed to association or micellisation, except in the case of Decamethonium bromide where ion-pairing is known to take place. The marked difference in binding behaviour between the anionic cosolutes and the cationic ones is attributed to the different environments of the positive and negative ends of the dipolar imide groups in the pyrrolidone rings of PVP; the binding data are discussed in terms of the polar (ion-dipole) and non-polar (hydrophobic) forces between cosolute and polymer, and of the relative roles played by the value of the end-to-end distance for the cosolute molecule and by any direct hydrophobic interactions in determining the dependence of binding behaviour upon cosolute chain length,m.