Thermodynamics of the interaction of 18-crown-6 with K+, Ti+, Ba2+, Sr2+ and Pb2+ from 323.15 to 398.15 K

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Abstract
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 technique. Equilibrium constant (K), enthalpy change (ΔH°), entropy change (ΔS°) and heat capacity change (ΔC° p) values were determined for the interaction 18C6 + Mn+= 18C6Mn+. This reaction is exothermic for the complexation of each of the cations in the temperature range studied. The ΔH° and ΔS° values decrease (become more negative) with increasing temperature in most cases. The complexes become less stable at higher temperatures as indicated by the decrease in log K values with temperature. The stability sequences for the metal ions reacting with 18C6 are the same at all temperatures, i.e. Pb2+ > Ba2+ > Sr2+ > Tl+ > K+. The association of 18C6 with these cations is enthalpy driven. The decrease in ΔH° and ΔS° values with temperature is the result of the formation of a highly organized water structure around the complexes due to ion–dipole, dipole–dipole and hydrophobic interactions. The charge densities of the cations are important in the explanation of the trends of solvation/desolvation, and the trends in ΔH° and ΔS° with temperature. Cations with higher charge densities have larger desolvation effects as complexation occurs. These effects may result in increased ΔH° and ΔS° values with increasing temperature. The ΔH° and ΔS° 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 ΔH° and ΔS° values.

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