Thermodynamic properties of association colloids

Abstract

The single components of association colloids are amphiphilic molecules, which are held together by van der Waals forces and/or hydrogen interactions. The thermodynamic parameters associated with the aggregation behaviour of amphiphiles depend on the chemical nature of the molecules as well as their environmental conditions (temperature, ionic strength, etc.). The systems studied and presented in this paper are association colloids, also known as micellar systems, which are formed of biological relevant detergents (e.g. bile salts, fatty acids). Information concerning the self-organisation (self-aggregation) of such micellar colloids can be easily obtained from isothermal titration calorimetry (ITC), as the critical micelle/aggregation concentration, the number of molecules (n) associated in the aggregate and the heat of aggregation. A complete thermodynamic description (ΔH, ΔS, ΔG, ΔCp) of the micellar colloids can be derived from the ITC data, allowing insights into the formation and stability of these colloidal systems. Based on the mass action model, taking into account counter ion binding, the ITC titration curves were simulated, and the aggregation number n of the aggregates derived. Isothermal titration calorimetry has a considerable advantage compared to other methods, because the critical micellisation concentration, the thermodynamic parameters of the aggregation process, and the aggregation numbers can be determined directly from one experiment.