Binding between (ethylene oxide)13-(propylene oxide)30-(ethylene oxide)13 and sodium decanoate.: Volume, enthalpy, and heat capacity studies

Volume, enthalpy, and heat capacity of transfer (DeltaY(t)) of (ethylene oxide)(13)-(propylene oxide)(30)- (ethylene oxide)(13) (L64), at some concentrations, from water to the aqueous sodium decanoate (NaDec) solutions as functions of the surfactant concentration (m(s)) were determined at 298 K. The copolymer was studied in both the unassociated and associated forms. For a given L64 concentration (m(c)), the DeltaY(t) vs ins profiles for the volume and the enthalpy are equal but different from that of the heat capacity because the latter contains also the relaxation terms. The experimental data were analyzed by assuming the distribution of L64 between the aqueous and the micellar phases and the shift of micellization equilibrium induced by the copolymer. A very good fit was obtained at m(c) = 35 mmol kg(-1), which provided the L64-micelles binding constant (K-b), the volume of transfer of L64 from water to micelles (DeltaV(t,M)) and the Setchenov constant. By using K-b, DeltaV(t,M), and the L64-free surfactant interaction parameter (obtained from the study in the pre-micellar region), DeltaV(t) as a function of ins at lower m(c) was calculated. The simulations resulted unsatisfactory since neither the shape of the DeltaV(t) vs m(s) trend was reproduced. Nevertheless, by adjusting K-b and DeltaV(t,M), the maximum in the DeltaV(t) vs ins curve was generated but the agreement with the experimental points was only qualitative. The data at mc < 35 mmol kg(-1) below the cmc of NaDec were interpreted in terms of a premicellar association between n mol of NaDec and 1 mol of L64.