Small angle neutron scattering from 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids ([Cnmim][PF6], n=4, 6, and 8)

Abstract

The presence of local anisotropy in the bulk, isotropic, and ionic liquid phases-leading to local mesoscopic inhomogeneity-with nanoscale segregation and expanding nonpolar domains on increasing the length of the cation alkyl-substituents has been proposed on the basis of molecular dynamics (MD) simulations. However, there has been little conclusive experimental evidence for the existence of intermediate mesoscopic structure between the first/second shell correlations shown by neutron scattering on short chain length based materials and the mesophase structure of the long chain length ionic liquid crystals. Herein, small angle neutron scattering measurements have been performed on selectively H/D-isotopically substituted 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids with butyl, hexyl, and octyl substituents. The data show the unambiguous existence of a diffraction peak in the low-Q region for all three liquids which moves to longer distances (lower Q), sharpens, and increases in intensity with increasing length of the alkyl substituent. It is notable, however, that this peak occurs at lower values of Q (longer length scale) than predicted in any of the previously published MD simulations of ionic liquids, and that the magnitude of the scattering from this peak is comparable with that from the remainder of the amorphous ionic liquid. This strongly suggests that the peak arises from the second coordination shells of the ions along the vector of alkyl-chain substituents as a consequence of increasing the anisotropy of the cation, and that there is little or no long-range correlated nanostructure in these ionic liquids.

Intermolecular vibrations and fast relaxations in supercooled ionic liquids

“…Since the position of the boson peak and sound velocity [63][64][65]81 of ionic liquids are more dependent on anion than cation, the length ξ does not change with the length of the alkyl chain in 1-alkyl-3-methylimidazolium cations, in contrast to the position of the first-sharp diffraction peak observed in static structure factor of ionic liquids. 26,[66][67][68] After normalizing the spectra by the high-frequency intramolecular bands of the common [Tf 2 N] anion, we found that the low-frequency Raman spectra of ionic liquids based on imidazolium cations are significantly less intense than ionic liquids containing non-aromatic cations. This is expected because of the more polarizable moiety of the imidazolium ring, but an interestingly finding is that the relaxational and vibrational contributions change by the same amount among the aromatic and nonaromatic ionic liquids investigated in this work.…”

“…[70][71][72][73] On the other hand, the FSDP in ionic liquids has been recently assigned to correlation of the second shell of cations, with no need of a picture of nanoscale polar/non-polar heterogeneity. 68 However, it is worth stressing that the physical picture of structural heterogeneity in ionic liquids has been proposed by MD simulation not only on the basis of the occurrence of a FSDP in the calculated S(k) but also on the basis of anions depletion in regions occupied by the alkyl chains. [70][71][72][73] Several inorganic glass-formers present correlation between the characteristic length ξ obtained from the boson peak and from the FSDP.…”

“…(1)]. It turns out that typical correlation length ξ is of the It is interesting to compare the length ξ with the characteristic size of heterogeneities that has been obtained from the wave-vector of the first-sharp diffraction peak, k FSDP , observed in neutron or X-ray spectra, 26,[66][67][68][69] and also molecular dynamics (MD) simulations, [69][70][71][72][73] 69 The FSDP in the static structure factor, S(k), indicates that an intermediate-range order (IRO) develops in the liquid, but the nature of such IRO in glass-forming liquids is another lively debated issue because different explanations have been proposed. In SiO 2 , the network of connected SiO 4 2− tetrahedral produces the correlations of the IRO.…”

“…A similar situation occurs if one compares 1-alkyl-3-methylimidazolium ionic liquids of alkyl chains of different lengths, while keeping the same anion: the nearest neighbour structure of anions around the imidazolium ring is almost not modified within polar regions, whereas the FSDP shifts to lower wave-vector for cations with longer carbon chain. 26,[66][67][68] 32 The unchanged value of ω bp in Eq. (7), together with the finding that the sound velocity of ionic liquids does not change as the length of the alkyl side chain increases while keeping the anion, [63][64][65] 32 it has been found that ω bp in these systems change by the same amount, respectively, 13 and 17 cm −1 .…”

Communication: Unusual structure and transport in ionic liquid-hexane mixtures

“…Examples of such amphiphilic ions include alkyl sulfate anions and tetraalkyl-ammonium or phosphonium cations. When the alkyl tails on such anions or cations are long enough, a characteristic peak in the observed liquid structure function, the so-called first sharp diffraction peak (FSDP) is routinely observed both in X-ray and neutron scattering 7,8 as well as in molecular simulations. 9 For a cationic hydrocarbon chain of length n = 5 or longer, the FSDP has been shown to lead to nanoscale phase segregation between polar head groups and non-polar alkyl tails.…”