Thermodynamic Equilibrium Constant Studies on Aqueous Electrolytic (Alkaline Earth Chlorides) Solutions Containing 18-Crown-6 at 298.15 K

Abstract

Osmotic coefficient data have been obtained for the binary aqueous solutions of alkaline-earth chlorides (MgCl2, CaCl2 and BaCl2) at 298.15 K using a vapor pressure osmometer. The measurements are extended to aqueous ternary solutions (containing a fixed concentration of 0.1 mol⋅kg−1 18-Crown-6 (18C6) having various electrolyte concentrations (0.01–0.2 mol⋅kg−1). The mean activity coefficients of the ions and of 18C6 in binary and ternary solutions were obtained through calculations of activity and osmotic coefficient data. The lowering of activity coefficients of the ions and of 18C6 in ternary solutions is attributed to the presence of host-guest type equilibria due to complexation between them in the case of solutions containing Ca2+ and Ba2+ ions. The data are further subjected to scrutiny by applying the methodology developed by Patil and Dagade based on the McMillan-Mayer theory of solutions to obtain thermodynamic equilibrium constant values through transfer Gibbs energies. It is noted that the size of the crown cavity (diameter 0.266–0.32 nm), charge density of ions (i.e., coulombic interactions) as well as hydrophobic interaction play a major role in governing the occurrence and stability of the complexed species. The results are compared with those reported earlier for alkali-halides and 18C6 complexes and discussed further from the point of view of the importance of ion-pair formation equilibria in aqueous solutions.