Effects of Solvent Composition and Temperature on K+–18-Crown-6 Complexation in Acetonitrile–Water Mixed Solvents

Complexation of K+ by 18-crown-6 ether (18C6) in pure water and in acetonitrile–water mixed solvents containing 0.1 mol-dm− 3 (C2H5)4NCl has been systematically studied by isothermal titration calorimetry (ITC) at 293, 298, and 303 K. The formation constant K of the 1:1 [K(18C6)]+ complex and the complexation enthalpy Δ rH were simultaneously determined from the titration data. The logK and Δ rH(kJ-mol− 1) values at 298 K are 2.04, −26.2 in pure water and 2.23, −25.0; 2.61, −24.2; 2.95, −23.8; 3.48, −21.0; 3.85, −19.4; 4.36, −18.7; and 5.73, −17.0 in the mixed solvents at xAN (mole fraction of acetonitrile) of 0.043, 0.135, 0.258, 0.448, 0.578, 0.759, and 1.0, respectively. The change in heat capacity for the complex formation, Δ Cp°, was also determined by the temperature dependence of Δ rH. Whereas the Δ Cp° is (57 ± 11) and (63 ± 20) J-mol− 1-K− 1 in pure water and in the solvent mixture at xAN = 0.043, respectively, it decreases with increasing xAN. The Δ Cp° values are −(48 ± 11), −(110 ± 25), −(354 ± 40), −(359 ± 24), and −(304 ± 30) J-mol− 1-K− 1 at xAN = 0.135, 0.258, 0.448, 0.578, and 0.759, respectively. The changes in complexation thermodynamics (Δ Δ rG, Δ ΔrH, and Δ Δ rS) are discussed in terms of the corresponding transfer thermodynamics of K+, 18-crown-6, and [K(18C6)]+ upon transferring from water to acetonitrile–water mixed solvents. It was found that hydrophobic solvation of the complex [K(18C6)]+ plays an important role in complex formation occurring in water and in the water-rich mixed solvent. Moreover, changes in solvent structure significantly affect the transfer enthalpy and entropy of each species, i.e., K+, 18-crown-6, and [K(18C6)]+. The observed monotonous changes in the complexation Gibbs energy, enthalpy, and entropy with solvent composition are due to the effective compensation of the Δ trG, Δ trH, and Δ trS for K+ with those for 18-crown-6 and [K(18C6)]+.