(181af) Modeling Thermodynamic Behavior of Non-Ideal Mixed Surfactant Systems During Isothermal Titration Calorimetry Experiments

Mixed micellar systems provide a large number of advantages over single surfactant micelles due to synergistic effects that sometimes arise due to favorable energetics of mixing between the surfactants.  Because of this synergy, mixed surfactants find many applications in both industry and the laboratory.  Here we present a study of thermodynamics in a mixed micellar cationic / nonionic surfactant system.  Cetyltrimethylammonium bromide (CTAB), the cationic surfactant,  is selected because it is  commonly used in synthesis of templated silica materials. N-octyl-β-D-glucopyranoside (C8G1) is a non-ionic detergent which is biodegradable and also potentially provides ability to create imprinted sites for glucose in a silica material.  Apart from our long-term interest in using such mixtures of cationic and carbohydrate-based surfactants to synthesize materials with selective sites for carbohydrate adsorption, this system serves as a model non-ideal mixed surfactant system which presents challenges in developing a comprehensive model of the thermodynamics of micellization.

Here, the heat evolved during isothermal titration calorimetry (ITC) of CTAB / C8G1 systems will be measured and analyzed using a model that accounts for nonideal mixing.  The objective is to not only determine the non-ideal interaction parameter (β) for the system but also to develop a model able to predict the entire shape of the heat vs. concentration curve in an ITC experiment.  The model described to do this is based on incorporating a regular solution theory activity coefficient model into Clint’s pseudo-phase separation model of micellization in mixed surfactant system.  This model calculates the change in surfactant monomer concentrations with respect to the total surfactant concentration (and thus the change in micelle composition with respect to concentration).  Consistent with the regular solution theory approach, the cmc of the system is calculated based on Rubingh’s equation.  By rigorously accounting for the heats associated with partitioning of surfactants between the bulk solution and micelle phases after each injection in the ITC experiment, quantities including individual component CMCs, enthalpies of demicellization of pure surfactants and the interaction parameter (b) are determined.  The model also allows the design of experiments for rapid determination of these paratmers.  This work would thus help in efficiently selecting surfactant mixtures best suited for synthesis of selective adsorbent materials based on mixing behavior of the constituents.