Thermodynamics of aqueous solutions of 18-crown-6 at 298.15 K: Enthalpy and entropy effects

We reported previously activity and activity coefficient data for aqueous solutions of 18-crown-6 (18C6) in the concentration range of 0.1-2.0 mol-kg(-1) at 298.15 K. The results were interpreted in terms of the binding of four water molecules (two bridged and two singly H-bonded) inside the 18C6 cavity having a D-3d conformation. In this work, we report our thermodynamic analysis of the Gibbs energy and enthalpy data (obtained using enthalpy virial data from literature) in aqueous solutions of 18C6 at 298.15 K. The excess enthalpy and Gibbs energy parameters are computed and further used to obtain excess entropies of solutions as a function of 18C6 concentration. The same data are utilized to compute the partial molar entropies of solvent and solute at finite, as well as at infinite, dilution of 18C6 in water. It is observed that Delta G(mix), Delta H-mix and T Delta S-mix values are all negative, whereas Delta G(E) values show a slightly positive variation as a function of the 18C6 concentration. The partial molar excess entropy of water ((S) over bar (1)-S degrees(1))(E), decreases (becomes negative) whereas that of 18C6, ((S) over bar (2)-S degrees(2))(E), increases with a increase in the 18C6 concentration. These results are explained in terms of various effects, which include water structure making, incorporation of water molecules in the crown cavities and crown-crown hydrophobic interactions, which persist even at the lowest concentration studied.