Polymer-surfactant interactions. A quantitative approach to the enthalpy of transfer of poly(ethylene glycol)s from water to the aqueous sodium perfluoroalkanoates solutions

The enthalpy of transfer (DeltaH(t)) of nonionic polymers at a fixed concen tration from water to aqueous substrate solutions as a function of the subs trate concentration (f(s)m(s)) was determined. The substrates studied are s odium perfluorobutanoate to sodium perfluorooctanoate, and the polymers are poly(ethyleneglycol)s with molecular weights of 400 and 35 000. For the PE G 400-sodium perfluoroheptanoate-water systems, measurements were also perf ormed at some polymer compositions. As a general feature, the enthalpy of t ransfer steeply changes with f(s)m(s), reaching a maximum beyond which it d ecreases, tending to level off. A qualitative analysis of the enthalpy data assigned the steep increase of DeltaH(t) to the binding between the polyme r and "loose" aggregates of surfactant and the maximum to the critical mice llar concentration in the presence of the polymer. Theoretical approaches f or the pre- and post-micellar regions were proposed. In the pre-micellar re gion, the cooperative binding was treated as a simple one-step association process based on the formation of complexes between one mole of polymer and n moles of surfactant. The standard free energy and the enthalpy and entro py for the polymer-surfactant complex formation were determined. As far as the micellar region is concerned, an approach was proposed taking into acco unt the following contributions: (i) the shift of micellization equilibrium induced by the polymer, (ii) the formation of the polymer-surfactant compl exes in the aqueous phase, and (iii) the formation of polymer-micelle compl exes. The standard free energy, enthalpy, and entropy for the polymer-micel les complex formation are consistent with the micelles being wrapped by the polymer chains.