Sugar surfactants in recent years have found increasing use in research and in industrial applications due to their versatility in tail length and headgroups as well as their biocompatibility and biodegradabilitiy. It is known that certain surfactants tend to exhibit better micellar properties in mixtures when compared with the properties of each individual surfactant (i.e. they exhibit synergy). Because of the versatility of sugar surfactants and the possibility that synergy is dependent on the structure of both surfactants in a pair, it is of interest to use techniques able to quickly characterize nonideality in surfactant mixtures. In the past techniques like tensiometry, dynamic light scattering, speed of sound measurements and NMR have been used to determine critical micellar concentrations (cmcs) of surfactant mixtures to indirectly infer the synergistic interactions between surfactants. The cmc determined by these techniques has then been utilized to obtain thermodynamic data either by using an assumed model or varying thermodynamic variables (such as temperature). Over the past two decades, isothermal titration calorimetry (ITC) has found growing application to measure the binding of proteins with target molecules, and can also be used to study the thermodynamics of micellar interactions. The advantage of using ITC is its ability to give direct thermodynamic data along with the critical micellar concentration of a surfactant system.
Here, ITC experiments are performed for binary mixtures consisting of cationic quaternary ammonium surfactants and nonionic sugar-based surfactants. The heats measured by ITC are used to develop a thermodynamic framework based on regular solution theory (RST) to calculate thermodynamic interaction parameters of non-ideal binary surfactant mixtures. The model consistently accounts for the cmc dependence on surfactant composition predicted by RST as well as the excess enthalpy of mixing in the micellar state. The effectiveness of such a framework is illustrated by finding interaction parameters of other ideal and nonideal mixtures including a sugar-based surfactant. Ultimately, development of such a thermodynamic framework allows the direct use of accurate thermodynamic data to obtain the interaction parameter rather than of the dependence of cmc on composition. The minimal sets of experiments required to make this determination will be discussed.