Interaction of Ionic Liquids Ions with Natural Cyclodextrins

• Published in: The journal of physical chemistry. B Vol. 115; no. 34; pp. 10285 - 10297
• Main Authors: Ondo, Daniel, Tkadlecová, Marcela, Dohnal, Vladimír, Rak, Jakub, Kvíčala, Jaroslav, Lehmann, Jochen K, Heintz, Andreas, Ignatiev, Nikolai
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 01-09-2011
• Subjects: Affinity; B: Statistical Mechanics, Thermodynamics, Medium Effects; Bearing; Binding; Cyclodextrins; Cyclodextrins - chemistry; Enthalpy; Entropy; Hydrophobic and Hydrophilic Interactions; Ionic liquids; Ionic Liquids - chemistry; Models, Molecular; Molecular Conformation; Stability; Thermodynamics
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp205039g

The interaction of natural α-, β-, and γ-cyclodextrins (CDs) with 14 hydrophobic ionic moieties of ionic liquids (ILs) was systematically examined in dilute aqueous solutions using isothermal titration microcalorimetry (ITC) and NMR spectroscopy. The studied cationic and anionic moieties involved some recently developed heavily fluorinated structures, as well as some others of common use. To isolate the effect of a given ion, the measurements were performed on salts containing the hydrophobic IL ion in question and a complexation-inactive counterion. Additional ITC experiments on ILs whose both cation and anion can interact appreciably with the CD cavity demonstrated that to resolve the effect of individual ions from such data is generally a tricky task and confirmed the superiority of the isolation strategy adopted for the purpose throughout this work. The binding constant, enthalpy and entropy determined at 298.15 K for the 1:1 (ion:CD) inclusion complex formation range in broad limits, being 0 < K < 2 × 105, 0 < −Δr H°/(kJ·mol–1) < 44, and −28 < TΔr S°/(kJ·mol–1) < 14, respectively. The stabilities of complexes of perfluorohexyl bearing ions with β-CD belong to the highest ever observed with natural CDs in water. The established binding affinity scales were discussed in both thermodynamic and molecular terms. The concepts of hydrophobic interaction and guest–host size matching supported by simple molecular modeling proved useful to rationalize the observed widely different binding affinities and suggest possible binding modes. Enthalpy and entropy contributions to the stability of the ion-CD complexes were found to compensate each other considerably obeying more or less the linear compensation relationship marked by existing literature data on binding other guests to natural CDs. As outliers to this pattern, the most stable complexes of −C6F13 bearing ions with β-CD were found to receive an enhanced inherent entropy stabilization due to extraordinarily high extent of desolvation occurring in the course of binding.