Thermodynamics of aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)/surfactant mixtures. Effect of the copolymer molecular weight and the surfactant alkyl chain length

A calorimetric study was performed to focus attention on the interactions between copolymers and anionic surfactants in aqueous solutions. Three aspects were analyzed: (1) the hydrophobicity of the surfactant, (2) the change of the copolymer molecular weight, and (3) the nature of the hydrophilicity of the copolymer. To this purpose, the family of sodium alkanoates (sodium octanoate through sodium dodecanoate) and the triblock copolymers EO76PO29EO76 (F68), EO103PO39EO103 and EO132PO50EO132 were investigated. Comparing F68 and EO13PO30EO13 (L64), previously studied by us, provided information on the effect of the copolymer hydrophilicity. The experimental data were analyzed by means of a thermodynamic model which assumes that in the surfactant dilute region monomers of surfactant interact with unimeric copolymer leading to the formation of surfactant-copolymer aggregation complexes; when micelles do form, they can solubilize the copolymer, and then, the copolymer-micelle aggregates formation takes place. Data modeling provided useful parameters which allowed us to discriminate the various effects contributing to the formation of such aggregates. The thermodynamic properties for the formation of both the surfactant-copolymer aggregation complexes and copolymer-micelle aggregates were not consistent with the expectation; despite the more hydrophilic sites available in F68, the enthalpy and entropy for F68 were larger and the free energy more negative than those of L64. Moreover, the role of the copolymer molecular weight was important although the ratio between the number of the hydrophilic and the hydrophobic sites of binding is constant. An effort was done to verify whether these findings are representative of the physical interactions between copolymer and surfactant or reflect the large difference in the molecular masses. Therefore, the thermodynamic properties were normalized for the number of EO and PO units. They straightforwardly showed that L64 has a larger hydrophobic character than F68 and the copolymer molecular weight plays a small role.