Mass Action Model Applied to the Thermodynamic Properties of Transfer of Nonionic Copolymers from Water to the Aqueous Surfactant Solutions

A thermodynamic model which enables the properties of aqueous copolymer/surfactant mixtures to be fit quantitatively was proposed. Namely, a relationship between the properties of transfer of the unassociated copolymer from water to the aqueous surfactant solutions (ΔYt) and the surfactant concentration was derived. The model was based on the idea that ΔYt can be expressed in terms of the following contributions:  (1) interaction between monomers of copolymer and surfactant, (2) displacement of the monomer-micelle equilibrium induced by the copolymer, (3) formation of the surfactant-copolymer aggregation complex, and (4) formation of the mixed micelles. Such a model was applied to most literature data relative to the systems formed by (ethylene oxide)13−(propylene oxide)30−(ethylene oxide)13 (L64) or (ethylene oxide)75−(propylene oxide)30−(ethylene oxide)75 (F68) copolymers and hydrogenated surfactants, i.e., sodium octanoate, sodium decanoate (NaDec), N-octylpyridinium chloride, and N,N-dimethyloctylamine-N-oxide. New enthalpy data of both L64 and F68 in decyltrimethylammonium bromide were analyzed, as well. The appropriate comparison among the parameters generated by the fitting processes gave physical insights on the mechanisms of binding between the copolymer and the surfactant according to the quantitative model. The quantities obtained from the minimizing procedure were used to predict ΔYt of the L64/NaDec system in conditions not yet investigated. The agreement between the calculated values and the new experimental points was satisfactory.