Inclusion vs. micellization in the cethylpyridine chloride / β-cyclodextrin system: A structural and thermodynamic approach

Abstract

Abstract In the present work we have performed a structural and thermodynamic characterization of supramolecular structures formed by interaction between cethylpyridine chloride (CPC) surfactant and β-cyclodextrin (βCD), using several physical-chemistry methods. Initially, qualitative CPC/βCD interactions in solid state were confirmed by FTIR and TGA/DTA. 2D NMR ROESY experiment showed strong correlations between hydrogens of βCD cavity with aromatic and aliphatic hydrogens of CPC, suggesting the existence of different complexes in solution. This information was corroborated by structures assessed by computational approach and stoichiometric coefficient obtained by ITC (N\u202f=\u202f1.45). ITC experiments (at constant concentration of CPC) also showed that host-guest complexation occur with very high binding constant (Kb\u202f=\u202f38,300.0), and driven by enthalpy and entropy. However, as CPC is an amphiphilic molecule, it is able to form micelles at critical micellar concentration close to 1\u202fmM. In presence of βCD, it was observed that micellization was delayed, so that the cmc values increased according to the empiric equation: cmc\xa0=\xa01.02\xa0+\xa00.5[βCD]. Moreover, thermodynamic data obtained by combination of conductometric and isothermal calorimetry titrations showed that the presence of increasing concentrations of βCD causes an increase of free energy of micellization, specially by gradual reduction of entropy of micellization, due to the difficult of micelles encapsulate the inclusion compounds. The overall study was rationalized in terms of competition between micellization vs. host-guest complexation.