Thermodynamics of inclusion complex formation between 1-alkyl-3-methylimidazolium ionic liquids and cucurbit[7]uril

Abstract

The thermodynamics of 1:1 inclusion complex formation between 1-alkyl-3-methylimidazolium type ionic liquids and cucurbit[7]uril was studied by isothermal titration calorimetry in aqueous solution at 298 K. The encapsulation proved to be enthalpy driven for all cations used. The enthalpy change upon binding (ΔH) became more negative when the 1-alkyl moiety of the imidazolium ring was gradually lengthened reaching the most exothermic association with the hexyl derivative. Further increase of the number of carbon atoms in the aliphatic chain led to less negative ΔH values. The much smaller entropy change followed the trend of ΔH. The slope of the linear enthalpy–entropy correlation found in the present work is significantly smaller than that reported previously for cyclodextrin complexes, because the more rigid CB7 macrocycle cannot undergo significant conformational change upon complexation.

Keywords:

• self-assembly
• host-guest complex
• isothermal titration calorimetry
• entropy
• enthalpy

Acknowledgement

The authors very much appreciate the support of this work by the Hungarian Scientific Research Fund (OTKA, Grant K75015).