Pentaglyme–K salt binary mixtures: phase behavior, solvate structures, and physicochemical properties

We prepared a series of binary mixtures composed of certain K salts (KX) and pentaglyme (G5) with different salt concentrations and anionic species (X(-): (CF3SO2)2N(-) = TFSA(-), CF3SO3(-) = TfO(-), C4F9SO3(-) = NfO(-), PF6(-), SCN(-)), and characterized them with respect to their phase diagrams, solvate structures, and physicochemical properties. Their phase diagrams and thermal stability strongly implied the formation of equimolar complexes. Single-crystal X-ray crystallography was performed on certain equimolar complexes, which revealed that G5 molecules coordinate to K(+) cations in a characteristic manner, like 18-crown-6 ether in the crystalline state, irrespective of the paired anions. The solvate structures in the molten state were elucidated by a combination of temperature-dependent Raman spectroscopy and X-ray crystallography. A drastic spectral variation was observed in the K(G5)1TfO Raman spectra, indicating that solvate structures in the crystalline state break apart upon melting. The solvate stability of K(G5)1X is closely related to the ion-ion interaction of the parent salts. A stable solvate forms when the ion-dipole interaction between K(+) and G5 overwhelms the ion-ion interaction between K(+) and X(-). Furthermore, the physicochemical properties of certain equimolar mixtures were evaluated. A Walden plot clearly reflects the ionic nature of the molten equimolar complexes. Judging from the structural characteristics and dissociativity, we classified K(G5)1X into two groups, good and poor solvate ionic liquids.