Complexation of gaseous guests by solid host: I. Quantitative thermodynamic approach for the reactions of β-cyclodextrin with amines using data in aqueous solution

The stability constants and reaction enthalpies for complexes of different primary, secondary and tertiary amines with β-cyclodextrin and the protonation constants of the amines in aqueous solution have been measured at 298.15 K using potentiometric and calorimetric titrations. Pronounced selectivity of the inclusion process in aqueous solution is shown. Free energies of solution of the complexes in water determined from solubility measurements appear to be nearly identical with that of β-cyclodextrin. By using a thermodynamic cycle and thermodynamic parameters of solution of the reaction participants, the Gibbs energy, enthalpy and entropy of the complexation reactions between solid β-cyclodextrin and several gaseous amines are calculated. Complex stabilities in aqueous solution and at the solid–gas interface are comparable. Unlike the equilibria in aqueous solution, the reaction enthalpies for the “solid–gas” complexation are much higher and the interactions are unfavorable in terms of the entropy changes.