title: Interaction of ionic liquids ions with natural cyclodextrins.
abstract: 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 × 10(5), 0 < -Δ(r)H°/(kJ·mol(-1)) < 44, and -28 < TΔ(r)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 -C(6)F(13) 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.
authors.affiliation: Department of Physical Chemistry, Institute of Chemical Technology, Technická 5, 166 28 Prague 6, Czech Republic.
authors.forename: Daniel
authors.initials: D
authors.lastname: Ondo
authors.forename: Marcela
authors.initials: M
authors.lastname: Tkadlecová
authors.forename: Vladimír
authors.initials: V
authors.lastname: Dohnal
authors.forename: Jakub
authors.initials: J
authors.lastname: Rak
authors.forename: Jaroslav
authors.initials: J
authors.lastname: Kvíčala
authors.forename: Jochen K
authors.initials: JK
authors.lastname: Lehmann
authors.forename: Andreas
authors.initials: A
authors.lastname: Heintz
authors.forename: Nikolai
authors.initials: N
authors.lastname: Ignatiev
mesh_terms: D003505:Cyclodextrins / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D052578:Ionic Liquids / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D013816:Thermodynamics
chemical_list: D003505:Cyclodextrins; D052578:Ionic Liquids