INTERACTION BETWEEN POLY(ETHYLENE OXIDE) AND DODECYL SULFATES WITH DIFFERENT MONOVALENT METAL COUNTERIONS STUDIED BY MICROCALORIMETRY

The interactions between poly(ethylene oxide) (PEO) and dodecyl sulfat e with different monovalent metal cations (MDS, M = Li+, Na+, Cs+) and dodecyltrimethylammonium bromide (C(12)TAB) have been studied by flow microcalorimetry. The enthalpy change on dilution of a solution of su rfactant and polymer with a solution of just polymer as a function of final concentration of surfactant has been measured directly for each of the surfactants. From the variation of enthalpy change with final c oncentration the thermodynamic parameters of micellization and aggrega tion have been determined. The enthalpies of binding of MDS micelles t o PEO were found to be -3.7, -2.2, and -3.4 +/- 0.5 kJ mol(-1) for LiD S, NaDS, and CsDS, respectively, at 0.1 wt % PEO. When combined with t he free energies of binding, determined from the critical micelle and critical aggregation concentrations, which show a much greater variati on with counterion, with corresponding values of -3.9, -3.1 and -0.7 k J mol(-1), significant differences in the entropy of binding were foun d between the different counterions. Larger values of the entropy for LiDS and NaDS may result from the displacement of a larger number of w ater molecules when polymer binds to the micellar surface than in the case of CsDS.