Is a medium-range order pre-peak possible for ionic liquids without an aliphatic chain?

Abstract

The combination of amino acids anions with a choline cation gives origin to a new and potentially important class of organic ionic liquids that might represent a viable and bio-compatible alternative with respect to the traditional ones. We present here a combined experimental and theoretical study of a choline-proline ionic liquid, using both large and small angle X-ray diffraction (WAXS-SAXS), and classical and ab initio molecular dynamics calculations, in which we are able to point out for the first time the existence of a low Q peak in the X-ray patterns in the absence of linear or branched alkyl chains. From the calculations, we can obtain theoretical scattering patterns that reproduce very nicely the experimental spectra in all Q ranges, and from detailed analysis of the radial distribution functions (RDFs) and hydrogen bond patterns, we can state that very strong ion pairs are established in the liquid and the observed pre-peak can be ascribed to the interactions between atoms belonging to different ion pairs

Choline-amino acid ionic liquids: past and recent achievements about the structure and properties of these really “green” chemicals

“…In the first investigation, Benedetto et al (Benedetto et al 2014) described in detail the cation-anion interaction with DFT methods in vacuo and in small clusters containing up to four ion pairs, for eight amino acid anions (Ala, Val, Ser, Cys, Asp, Asn, Phe, His). In some later reports (Campetella et al 2015a(Campetella et al , b, 2016a(Campetella et al , b, c, 2017(Campetella et al , 2018, the presence of strong ion pairing was confirmed in larger models of the liquid phase (32 ion pairs). In one of these studies on cholinealanine (Campetella et al 2015a(Campetella et al , 2015b, infrared spectroscopy was chosen as experimental technique to support the computational results.…”

“…In some later reports (Campetella et al 2015a(Campetella et al , b, 2016a(Campetella et al , b, c, 2017(Campetella et al , 2018, the presence of strong ion pairing was confirmed in larger models of the liquid phase (32 ion pairs). In one of these studies on cholinealanine (Campetella et al 2015a(Campetella et al , 2015b, infrared spectroscopy was chosen as experimental technique to support the computational results. The huge red shift (Δυ=900-1000 cm −1 ) of hydroxyl stretching vibration observed in the experiment was correctly reproduced by the model and was interpreted by calculating the Wannier centers along the AIMD trajectory, as due to the large number of different conformations and energies experienced during the simulation by the hydrogen bond-mediated pairs.…”

Bio ionic liquids and water mixtures: a structural study

“…Unfortunately, due to the wat-IL interaction, this kind of interaction is not sufficient to establish long range aggregation phenomena, as shown for the neat liquid in previous works. 36,39 To better characterize the H-bond network of the simulated boxes, we have extracted the H-bonds contacts lifetime from the trajectories. For the calculation, we exploited the autocorrelation function tool of TRAVIS, 40…”

Cholinium amino acid-based ionic liquids

“…From the earliest computational studies, it was clear that the ChAAILs structure was not as simple as initially thought. In particular, the emergence of low-Q peaks in the X-ray diffraction patterns seemed to indicate some kind of aggregation phenomena at the nanoscale which previous calculations on isolated dimers were not able to grasp (Campetella et al 2015b(Campetella et al , 2016b. The authors of these studies focused on Proand Phe-based ChAAILs where the presence of structured side chains could induce and enhance the formation of transient structures at the nanoscopic level.…”