Host–guest interactions between 4-sulfonatocalix[8]arene and 1-alkyl-3-methylimidazolium type ionic liquids

The inclusion of 1-alkyl-3-methylimidazolium ions (Cnmim+) in 4-sulfonatocalix[8]arene was studied by isothermal titration calorimetry in aqueous solution at 298 K. Sequential formation of 1:1 and 2:1 complexes occurred in enthalpy driven reactions. The alteration of the molecular structure of the guest barely affected the driving force of the confinement of the first Cnmim+. However, the binding affinity of the second Cnmim+ went through a maximum when the number of carbon atoms in the side chain was increased. The enthalpy and entropy change diminished linearly with the length of the aliphatic substituent of Cnmim+ for 1:1 complexation, but reached a minimum for the hexyl homologue in the case of 2:1 inclusion. Linear entropy–enthalpy correlations were found with smaller slope and intercept for the second binding step.