Thermodynamic Study of the Micellization of Zwitterionic Surfactants and Their Interaction with Polymers in Water by Isothermal Titration Calorimetry

The micellization of a homologous series of zwitterionic surfactants, a group of sulfobetaines, was studied using isothermal titration calorimetry (ITC) in the temperature range from 15 to 65 degrees C. The increase in both temperature and the alkyl chain length leads to more negative values of Delta G(mic)(0), favoring the micellization. The entropic term (Delta S-mic(0)) is predominant at lower temperatures, and above ca. 55-65 degrees C, the enthalpic term (Delta H-mic(0)) becomes prevalent, figuring a jointly driven process as the temperature increases. The interaction of these sulfobetaines with different polymers was also studied by ITC. Among the polymers studied, only two induced the formation of micellar aggregates at lower surfactant concentration: poly(acrylic acid), PAA, probably due to the formation of hydrogen bonds between the carboxylic group of the polymer and the sulfonate group of the surfactant, and poly(sodium 4-styrenesulfonate), PSS, probably due to the incorporation of the hydrophobic styrene group into the micelles. The prevalence of the hydrophobic and not the electrostatic contributions to the interaction between sulfobetaine and PSS was confirmed by an increased interaction enthalpy in the presence of electrolytes (NaCl) and by the observation of a significant temperature dependence, the latter consistent with the proposed removal of hydrophobic groups from water.