Effect of solvation on the thermodynamics of formation for 18-crown-6 ether complexes with glycine and triglycine in water-ethanol solutions at 298 K

Abstract

Using data from calorimetric titration, standard thermodynamic parameters logK ○, Δr G ○, Δr H ○, and TΔr S ○ of the formation of 18-crown-6 ether (18C6) molecular complex with triglycine (3Gly), [3Gly18C6] in H2O-EtOH solvents with contents of ethanol x ranging between 0.0 and 0.5 mole fractions are calculated. Increasing the concentration of EtOH in the solvent is found to raise the reaction’s exothermicity from −5.9 to −21.0 kJ mol−1 and logK ○ [3Gly18C6] from 1.10 to 2.53. A comparative analysis of the effect the composition of H2O-EtOH solvent has on the reactions of [3Gly18C6] and [Gly18C6] formation is performed. As in case of [Gly18C6] formation, the changes in the complex’s enthalpy of solvation Δtr H ○([3Gly18C6]) are close to the Δtr H ○(18C6) parameter and differ considerably from the Δtr H ○(3Gly) value, testifying to the crucial role 18C6 plays in changing the [3Gly18C6] state of solvation. The ratio between solvation contributions from reagents to Δtr G ○ of [3Gly18C6] formation is found to differ from that the one between the corresponding contributions to Δtr H r o : in transferring from water to H2O-EtOH mixtures, the increase in the positive Δtr G ○(18C6) values is slight and therefore negligible when compared to Δtr G ○(3Gly).