Temperature evolution of the bulk nano-structure in a homologous series of room temperature ionic liquids

Abstract

We present x-ray scattering measurements on a homologous series of a model room-temperature ionic liquid (RTIL), [CnC1mim][NTf2]. The measurements span a broad range of cation alkyl chain lengths, n = 4 − 22, and temperatures, T = 293 − 393 K, not hitherto available for any RTIL. Thus, they support significantly deeper insights into the RTILs' structure. Resolution of the scattering curves into individual lines yields an accurate description of the RTILs' nanoscale-segregated structure, and its n and T evolution. The results strongly support a lateral packing of partly-overlapping, interdigitated, flexible alkyl chains within the segregated apolar domains. The domain thickness, and effective chain and overlap lengths, are determined for all n and T. These reveal that the n- and T-increasing overlap drives the observed counterintuitive layer-spacing contraction upon increasing T, while the effective chain lengths hardly vary. An overlap length beyond 5–6 methylenes is found to accelerate the overlap's increase rate with n and T. This marks n ≈ 12 as the borderline between slow and fast overlap-increase regimes. Finally, the results highlight the importance of the reduced temperature, t = (T − Tm)/Tm, the “distance” from the melting temperature Tm, for comparing the variation with n of the structure's T-evolution.