Thermodynamic modeling of the formation of giant micelles

Abstract

The physical-chemical study of the molecular association of surfactant monomers is essential for hydrophobic-hydrophilic control in aqueous solutions. One of the supramolecular architectures formed by these surfactants are the giant micelles, supramolecular aggregates that from the rheological point of view can be compared to polymeric solutions. Although the rheology of these supramolecular aggregates is well understood, energetic information on the morphological transition from micelles to giant micelles is scarce, since the classical thermodynamic models for surfactant aggregation cannot be easily applied. In 2016, Ito and collaborators demonstrated for the first time that the enthalpy change related to the formation of giant micelles could be determined in experiments of calorimetric titration of a cationic surfactant with aromatic anions. In this way, this post-doctoral research, linked to the FAPESP thematic project "Architecting colloids via supramolecular interactions: from fundamentals to applications", seeks to deepen this topic. It is intended to add a thermodynamic modeling for the formation of giant micelles, mediated by cationic and zwitterionic surfactants and organic and inorganic salts, using isothermal titration calorimetry. The thermodynamic parameters that will be obtained from the modeling are: entropy variation (S) and Gibbs free energy (G) of the formation of giant micelles, using enthalpy variation (H) as the starting thermodynamic parameter. The characterization of the equilibrium thermodynamics of the formation of giant micelles, under different conditions (temperature and pH), will allow proposing physical-chemical models that will rationalize the thermodynamic control of morphological transitions of supramolecular aggregates mediated by surfactants. The comparison of different systems may allow correlating the results with structural aspects of surfactants and aromatic ions. (AU)