Effect of the composition of the water-dimethyl sulfoxide solvent on the thermodynamics of the formation of the [Ag(18-Crown-6)]+ complex

The effect of a water-dimethyl sulfoxide solvent (X-DMSO = 0-0.97, where X-DMSO is the mole fraction of DMSO) on the thermodynamics of complexation between Ag+ and 18-crown-6 and the solvation of all reagents involved in this equilibrium were studied. In aqueous solutions, the complex is stable mainly because of the enthalpy contribution to Delta (r)G degrees. For X-DMSO > >0.3, the contributions from entropy and enthalpy become comparable in magnitude, but they are opposite in sign. In the binary solvent, the complex is most stable at X-DMSO = 0.2 to 0.3. Analysis of the enthalpy characteristics of reagent solvation showed that this solvent effect was due to the superposition of two opposite solvation contributions occurring with an increase in the DMSO concentration in the binary solvent, namely, the destabilization of the ligand solvate sphere and the formation of stable Ag+ complexes with DMSO.