(121f) Development of a Thermodynamic Framework for Isothermal Titration Calorimetry Studies of Mixed Surfactant Systems

Sugar surfactants are finding increasing number of applications in industry and research due to their biodegradability and multifunctional headgroups. It is also known that certain surfactant mixtures tend to exhibit better properties than single surfactants due to synergism. In the past the methods used to study surfactant mixtures utilized critical micellar concentration (cmc) to infer thermodynamics of a micellar system, which can be a time-consuming process that requires the assumption of activity coefficient models for interpretation. In the last two decades, the use of isothermal titration calorimetry (ITC) has allowed researchers to obtain thermodynamic data more directly based on heat flows during micellization and mixing. With the possibility of designing new surfactants for specific applications, it has become desirable to devise rapid characterization techniques for surfactant mixtures that consume small amounts of a synthesized compound. A new type of experiment to study a range of mole fractions in a binary surfactant mixtures using a single ITC experiment has been developed in our laboratory and is described here. A thermodynamic framework will be described to compare the traditional ITC data obtained from demicellization experiments (with mole fraction in the surfactant mixture varied in a series of experiments) with data obtained by the new technique, called re-equilibration (in which one surfactant is added to the other). ITC measurement of various mixtures consisting of cationic and sugar surfactants will be reported under this thermodynamic framework, to study the effects of changes in sugar surfactant architecture on mixed surfactant properties. Ultimately, the thermodynamic framework developed here will help in rapid characterization of binary surfactant mixtures used for templating of selective adsorbent materials.