THERMODYNAMICS OF THE INTERACTION OF 18-CROWN-6 WITH K+, TL+, BA2+, SR2+ AND PB2+ FROM 323.15 TO 398.15 K

The interaction of 18-crown-6 (18C6) with Tl+, K+, Sr2+, Pb2+ and Ba2 in aqueous solution at 323.15, 348.15, 373.15 and 398.15 K, and at 1. 52 MPa has been Investigated using an isothermal flow calorimetry tech nique. Equilibrium constant (K), enthalpy change (Delta H degrees), en tropy change (Delta S degrees) and heat capacity change (Delta C-p deg rees) values were determined for the interaction 18C6 + M(n+) = 18C6M( n+). This reaction is exothermic for the complexation of each of the c ations in the temperature range studied. The Delta H degrees and Delta S degrees values decrease (become more negative) with increasing temp erature in most cases. The complexes become less stable at higher temp eratures as indicated by the decrease in log K values with temperature . The stability sequences for the metal ions reacting with 18C6 are th e same at all temperatures, i.e. Pb2+ > Ba2+ > Sr2+ > Tl+ > K+. The as sociation of 18C6 with these cations is enthalpy driven. The decrease in Delta H degrees and Delta S degrees Values with temperature is the result of the formation of a highly organized water structure around t he complexes due to ion-dipole, dipole-dipole and hydrophobic interact ions. The charge densities of the cations are important in the explana tion of the trends of solvation/desolvation, and the trends in Delta H degrees and Delta S degrees with temperature. Cations with higher cha rge densities have larger desolvation effects as complexation occurs. These effects may result in increased Delta H degrees and Delta S degr ees values with increasing temperature. The Delta H degrees and Delta S degrees values go through a maximum as the temperature increases for the complexation of 18C6 with Ba2+ and Pb2+, suggesting the trade off between complexation and desolvation in determining the magnitude of the Delta H degrees and Delta S degrees values.