Ionic Conductivity of Nanostructured Hybrid Materials Designed from Imidazolium Ionic Liquids and Kaolinite

Abstract

Highly nanostructured hybrid materials were prepared by the intercalation into the interlayer spaces of kaolinite of ionic liquids based on imidazolium derivatives. Their structure, thermal behavior, and composition were characterized by a range of methods, including X-ray diffraction, solid-state NMR, thermal gravimetric analysis, and elemental analysis. Measurements of their electrical conductivity were carried out by impedance spectroscopy in the temperature range 23−250 °C. Three imidazolium derivatives were used: 1-methyl 3-propyl imidazolium bromide (Im-1), 1-methyl 3-(2-chloroethyl) imidazolium cloride (Im-2), and 1-methyl 3-(benzyl) imidazolium chloride (Im-3). The electrical conductivity depends on the size as well as on the structural organization of the salts into the interlayer space. In the case of the intercalates Im-1-K and Im-2-K, the electrical conductivity measured at room temperature is about 2 × 10−5 S cm−1. This value varies with temperature. The maximum of conductivity, 4 × 10−4 S cm−1, was obtained in a relatively short-range of temperatures, between 160 and 200 °C. No conductivity could be measured in the case of Im-3-K. An optimization of the structures of the three nanohybrid materials was performed using the PM6 semiempirical method. Alternating channels of organic cations and halide anions are formed in the cases of Im-1-K and Im-2-K, leading to the observed ionic conductivity behavior. In the presence of a bulkier substituent of the imidazolium ring, such as in Im-3-K, the anionic channels are blocked by the substituent, resulting in the absence of conductivity. To the best of our knowledge, this is the first report of electrical conductivity displayed by an interlayer modification of kaolinite.