Room-temperature ionic liquids (RTILs) are organic salts that are characterized by low melting points. They are considered to possess a homogeneous microscopic structure. We provide the first experimental evidence of the existence of nanoscale heterogeneities in neat liquid and supercooled RTILs, such as 1-alkyl-3-methyl imidazolium-based salts, using X-ray diffraction. These heterogeneities are of the order of a few nanometers and their size is proportional to the alkyl chain length. These results provide strong support to the findings from recent molecular dynamics simulations, which proposed the occurrence of nanostructures in RTILs, as a consequence of alkyl chains segregation. Moreover, our study addresses the issue of the temperature dependence of the heterogeneities size, showing a behavior that resembles the density one only below the glass transition, thus suggesting a complex behavior above this temperature. These results will provide a novel interpretation approach for the unique chemical physical properties of RTILs.

Here we report on the structural and dynamical properties of a series of room temperature ionic liquids, namely 1-alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([Cnmim][NTf2]), with varying alkyl chain lengths (1≤n≤10) at ambient temperature, where all the salts are stable liquids. Using small-wide angle x-ray scattering (SWAXS), three major diffraction peaks are found: two high- Q peaks that show little dependence on the alkyl chain length (n) and a low-Q peak that strongly depends both in amplitude and position on n. This low-Q peak is the signature of the occurrence of nanoscale structural heterogeneities whose sizes depend on the length of the alkyl chain and are related to chain segregation into nano-domains. Using optical heterodyne-detected Raman-induced Kerr effect spectroscopy, we access intermolecular dynamic features that suggest that chain aggregation only occurs for n≥3, in agreement with the SWAXS data. Moreover, the increase in the frequency and width of the main band of the optical Kerr effect spectra in going from n = 2 to 3 is consistent with stiffening of the intermolecular potential due to chain segregation. Multicomponent line shape analysis suggests that there are least three modes that underlie the main band in the 0–200 cm−1 region of the optical Kerr effect spectra of these ionic liquids. Given the similarity of ionic liquids to other complex fluid systems, we assign the low-frequency component to a fast β-relaxation mode and the intermediate- and high-frequency components to librational modes.

Ionic liquids represent an exciting novel class of materials with potentially enormous applicative impact; they are proposed as environmentally responsible replacements for the noxious volatile organic solvents, as smart separation and catalysis media, or to develop electrochemical devices, just to mention a few examples. Recently, compelling experimental as well as computational evidence highlighted the complexity of RTIL morphology at the mesoscopic spatial scale, as compared to traditional molecular liquids. In this Perspective, we report on our current understanding on the nature of structural heterogeneities in ionic liquids, describing new experimental data supporting a microphase segregation structural model for these systems and proposing topics for further study.

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The existence of a high degree of order over the mesoscopic spatial scale in room temperature ionic liquids is one of their most intriguing properties. Recently the possibility that such a feature, that is witnessed by the occurrence of peculiar low Q diffraction features, reflects nm-scale structural organization has been questioned on the basis of both experimental and computational studies. In this contribution we discuss these studies and present novel experimental evidence that confirm the existence of nm-scale spatial heterogeneities due to the segregation of apolar moieties dispersed in a polar network. The consequence of this scenario is that when the chain polarity gets closer to that of the charged head, the structural heterogeneities are no longer observed.