Effects of solvent composition and temperature on K+-18-crown-6 complexation in acetonitrile-water mixed solvents

摘要

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)(4)NCl 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 Delta(r)Hdegrees were simultaneously determined from the titration data. The logK and Delta(r)Hdegrees (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 x(AN) (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, DeltaC(p)degrees, was also determined by the temperature dependence of Delta(r)Hdegrees. Whereas the DeltaC(p)degrees is ( 57 +/- 11) and (63 +/- 20) J- mol(-1)- K-1 in pure water and in the solvent mixture at x(AN) = 0.043, respectively, it decreases with increasing x(AN). The DeltaC(p)degrees values are -(48 +/- 11),-(110 +/- 25),-(354 +/- 40),-( 359 +/- 24), and-( 304 +/- 30) J- mol(-1)- K-1 at x(AN) = 0.135, 0.258, 0.448, 0.578, and 0.759, respectively. The changes in complexation thermodynamics (DeltaDelta(r)Gdegrees, DeltaDelta(r)Hdegrees, and DeltaDelta(r)Sdegrees) 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 Delta(tr)Gdegrees, Delta(tr)Hdegrees, and Delta(tr)Sdegrees for K+ with those for 18-crown-6 and [K(18C6)](+).