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 concentration from water to aqueous substrate solutions as a function of the substrate concentration (f(s)m(s)) was determined. The substrates studied are sodium perfluorobutanoate to sodium perfluorooctanoate, and the polymers are poly(ethyleneglycol)s with molecular weights of 400 and 35 000. For the PEG 400-sodium perfluoroheptanoate-water systems, measurements were also performed at some polymer compositions. As a general feature, the enthalpy of transfer steeply changes with f(s)m(s), reaching a maximum beyond which it decreases, tending to level off. A qualitative analysis of the enthalpy data assigned the steep increase of DeltaH(t) to the binding between the polymer and "loose" aggregates of surfactant and the maximum to the critical micellar concentration in the presence of the polymer. Theoretical approaches for the pre- and post-micellar regions were proposed. In the pre-micellar region, 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 entropy for the polymer-surfactant complex formation were determined. As far as the micellar region is concerned, an approach was proposed taking into account the following contributions: (i) the shift of micellization equilibrium induced by the polymer, (ii) the formation of the polymer-surfactant complexes in the aqueous phase, and (iii) the formation of polymer-micelle complexes. The standard free energy, enthalpy, and entropy for the polymer-micelles complex formation are consistent with the micelles being wrapped by the polymer chains.