Binding of short alkyl chain surfactants to the (ethylene oxide)13-(propylene oxide)30-(ethylene oxide)13 and (ethylene oxide)75-(propylene oxide)30-(ethylene oxide)75 copolymers studied by microcalorimetry

The enthalpy of transfer (DeltaH(1)) of neutral copolymers, at a fixed and low concentration ca. 10(-3) mol kg(-1), from water to the aqueous surfactant solutions as a function of the surfactant concentration (fsms) was determined at 25 degreesC. The surfactants studied are sodium octanoate, sodium decanoate, N-octylpyridinium chloride (OPC), and N,N-dimethyloctylamine-N-oxide (ODAO). The copolymers are (ethylene oxide)(13)-(propylene oxide)(30)-(ethylene oxide)(13) (L64) and (ethylene oxide)(75)-(propylene oxide)(30)(ethylene oxide)(75) (F68). The two copolymers show different hydrophilic-hydrophobic ratios, the larger being for F68. As a general feature, the shape of the DeltaH(1) vs fsms curve depends on the nature of the surfactant whereas the magnitude of the enthalpy depends essentially on the nature of both the surfactant and the copolymer. A quantitative analysis of the experimental data of the systems investigated was done. L64 and F68 in ODAO solutions behave like classical hydrophobic nonionic additives. OPC does not exhibit specific interactions toward F68. A cooperative binding between the copolymer and the surfactant is present in the other systems studied. The standard free energy, enthalpy and entropy for the copolymer-surfactant complex formation were obtained. From these data, the isotherm bindings, were calculated in the range 20-30 degreesC; temperature is a crucial parameter in determining the copolymer-surfactant interactions.