Thermodynamics of formation of inclusion compounds in water. α-Cyclodextrin–alcohol adducts at 298.15 K

Abstract

The interaction in water of ethanol, n-propanol, n-butanol, isobutanol, s-butanol and t-butanol with α-cyclodextrin (hexacycloamylose, αCD) has been studied calorimetrically at 298.15 K. The results indicate that, with the exception of the t-butanol, these alcohols form inclusion complexes with this dextrin. The calorimetric method employed allows the determination of the thermodynamic parameters characterizing the binding process, namely the enthalpy, ΔH_B°, the apparent equilibrium constant,K′_B, and then the free energy, ΔG_B°′, and entropy, TΔS_B°′. The complexes that result are weak, as shown by the low values of the constants. The absolute values of ΔH_B° and K′_B increase with increasing length of the alkyl chain. Branching of the alkyl chain lowers the value of K′_B. Evidence is given to show that t-butanol does not form a complex with αCD, probably because of steric hindrance: for this system the calorimetric data are treated according to an approach derived from the McMillan–Mayer treatment of solutions.