A review on thermal analyses of cyclodextrins and cyclodextrin complexes

Cyclodextrins are valuable natural or synthetically modified cyclic oligosaccharides that are widely used for ameliorating properties of biologically active compounds such as food additives and ingredients or medicinal compounds. They protect these compounds against light and oxidative degradation and provide host–guest supramolecular complexes having controlled release properties by molecular encapsulation, enhancing the water solubility and bioavailability. Among many characterization methods that are applicable in both solution and solid state, thermal techniques were widely used for analysis and stability evaluation of cyclodextrins and cyclodextrin complexes. This updated review deals with the use of thermal methods for analysis of cyclodextrins and cyclodextrin complexes. Classical and modern thermal techniques used for evaluation of inclusion compound formation were reviewed. Special attention was gave for thermogravimetry–differential thermogravimetry, differential thermal analysis, differential scanning calorimetry, hot stage microscopy, as well as for the more recent techniques thermogravimetry–mass spectrometry, gas chromatography-time-of-flight-mass spectrometry and isothermal titration calorimetry. In order to evaluate the cyclodextrin complexation capability by thermal methods, we grouped for the first time the guest compound as drugs by anatomical therapeutic chemical classification (ATC), as well as odorants, essential oils and vegetable extracts, antioxidants, fatty acids, oils, fatty acid-based derivatives, other organic, organometallic and inorganic compounds. The formation of cyclodextrin inclusion complexes was emphasized by disappearance of the signal corresponding to the melting or boiling points of the guest compound and the behavior of the hydration water molecules in the complex in comparison with the raw host and guest compounds. The thermal stability of the guest compound after cyclodextrin complexation was also discussed. It was emphasized that thermal methods are some of the most valuable techniques for analyzing cyclodextrin complexes and together with other methods can complete the information related to the host–guest molecular inclusion process and the specific properties of these cyclodextrin complexes.