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.

In the present work we report the synthesis and physico-chemical characterization in terms of the viscosity and density of a wide series of cholinium-amino acid based room temperature ionic liquids ([Ch][AA] RTILs). 18 different amino acids were used to obtain 14 room temperature ILs. Among the most common AAs, only valine did not form an RTIL but it is a liquid above 80 °C. With respect to the methods reported in the literature we propose a synthesis based on potentiometric titration which has several advantages such as shorter preparation time, stoichiometry within ±1%, very high yields (close to 100%), high reproducibility, and no use of organic solvents, thus being more environmentally friendly. We tried to prepare dianionic ILs with some AAs with two potentially ionisable groups but in all cases the salts were solids at room temperature. All the ILs were characterized by (1)H NMR to confirm the stoichiometry. Physico-chemical properties such as density, viscosity, refractive index and conductivity were measured as a function of temperature and correlated with empirical equations. The values were compared with the data already reported in the literature for some [Ch][AA] ILs. The thermal expansion coefficient αp and the molar volume Vm were also calculated from the experimental density values. Due to the high number of AAs explored and their structural heterogeneity we have been able to find some interesting correlations between the data obtained and the structural features of the AAs in terms of the alkyl chain length, hydrogen bonding ability, stacking and cyclization. Some parameters were also found to be in good agreement with those reported for other ILs. We think that these data can give an important contribution to the understanding of the structure-property relationship of ILs because they focused on the structural effect of the anions, while most data in the literature are focussed on the cations.

The temperature dependence of ethylammonium chloride structure has been investigated by in situ laboratory parallel-beam X-ray powder diffraction. A polymorphic transition from a monoclinic LT phase to a tetragonal HT phase has been observed at 358 K. Such transformation has a reconstructive character. The thermal expansion of both polymorphs is small and anisotropic as a consequence of their organization through an anisotropic interaction network. The high temperature (HT) phase (possible space group P4/n or P4/nmm, a = 5.05 Å, c = 9.99 Å) has an excess volume of ~11% as compared with the low temperature (LT) one. The HT polymorph's structure has been solved by direct methods using powder diffraction data. In the absence of clear indications, it has been refined in P4/nmm. The structural properties of an ethylammonium chloride/water mixture at ambient conditions were also studied by using an integrated approach, which combines X-ray diffraction measurements and molecular dynamics simulations carried out with both the SPC/E and TIP5P water models. By refining a single interaction potential, very good agreement between the theoretical and experimental diffraction patterns was obtained, especially in the case of the TIP5P simulation. A complex structural behavior in which cations and anions do not possess a completely closed hydration shell of their own has been highlighted. Conversely, "solvent-shared ion pairs" are formed, in which one or more water molecules act as a bridge between the chloride and ethylammonium ions. Moreover, a strong water-water correlation is found, indicating that the water molecules in the mixture tend to aggregate and form water clusters.