Calorimetric and Volumetric Investigations of the Effect of the Hydrophobicity of the Surfactant on the Binding between (Ethylene oxide)13-(propylene oxide)30-(ethylene oxide)13 and Sodium Alkanoates in Aqueous Solutions

The enthalpy and the volume of transfer (DeltaY(t)) of the unassociated (ethylene oxide)(13)-(propylene oxide)(30)-(ethylene oxide)(13) (L64) from water to the aqueous sodium alkanoate solutions as functions of the surfactant concentrations (m(S)) were determined at 298 K. The surfactants studied are sodium hexanoate, sodium heptanoate, sodium octanoate, sodium undecanoate, and sodium dodecanoate. As a general feature, for the short alkyl chain surfactants, DeltaY(t) describes an S-shaped curve in the range of m(S) analyzed whereas for the more hydrophobic surfactants the DeltaY(t) vs m(S) trends exhibit maxima which appear at ms values very close to the critical micellar concentration in water. The experimental properties were quantitatively treated by means of a thermodynamic model, recently proposed by us, which assumes that the processes of micellization and the formation of both the surfactant-copolymer aggregation complex and the micelle-copolymer mixed aggregate take place simultaneously. The thermodynamics for the surfactant/copolymer aggregation complex formation states that both the compounds release water molecules from their nonpolar moieties which interact through the van der Waals forces generating a hydrophobic microenvironment in the aggregation complex similar to that in the micellar state. Conformational variations of the copolymer produced by the attachment of the surfactant molecules induce the formation of some hydrogen bonds between the ether oxygen of the ethylene oxide units and water. The thermodynamics of transfer of L64 from the aqueous phase to the sodium alkanoate micelles was also determined. The interactions in the mixed micelles and the surfactant-copolymer aggregation complex are similar. However, due to the large size of the micelle, the conformational effects caused by the L64 solubilization in the micelle are quite significant.