Nanoscale Structure in Short-Chain Ionic Liquids

Abstract

The temperature (T) and cationic chain length (n) evolution of the nanoscale structure of the sub-layering-threshold members of a model family of room temperature ionic liquids (RTILs) is investigated by x-ray scattering. The measured curves are computer-resolved into individual Teubner-Strey-like lineshapes. The polar-apolar layering is found to start at $n=3$ . Opposite n-trends are found at $n\le 3$ for the spacings and correlation lengths associated with the diffraction patterns' two main peaks, and assigned to a shift of balance between the two main interactions, Coulomb and van der Waals, and to increasing packing constraints due to the addition of methylenes. The spacings' thermal expansion coefficients are found to deviate from the macroscopically-measured values, and to anomalously decrease with increasing temperature. Finally, the reduced temperature scale, $t=(T-T_m)/T_m$ , ( $T_m=$ melting temperature), is demonstrated to render the observed trends significantly more systematic than those on a conventional T scale.

Keywords: ionic liquids; layering; supra-molecular structure; temperature dependence; x-ray scattering.