Anomalous Freezing of Nano‐Confined Water in Room‐Temperature Ionic Liquid 1‐Butyl‐3‐Methylimidazolium Nitrate

Abstract

Non-crystal formation of ice is investigated by simultaneous X-ray diffraction and differential scanning calorimetry measurements upon cooling to -100 °C. At room temperature, size-tunable water confinement (≈20 Å size) in a room-temperature ionic liquid (RTIL, 1-butyl-3-methylimidazolium nitrate, [C4 mim][NO3 ]) exists in a water-rich region (70-90 mol % D2 O). The confined water (water pocket) is characterized by almost monodispersive size distribution. In [C4 mim][NO3 ]-x mol % D2 O (70<x<94), the absence of sharp Bragg reflections and a distinct exothermal peak indicate that crystallization/cold crystallization both of [C4 mim][NO3 ] and D2 O is suppressed, even upon slow cooling and heating.

Tuning Water Networks via Ionic Liquid/Water Mixtures

“…Convergence to the bulk water hydrogen bond distribution (dashed-line) is thus not expected until the surface to volume ratio of the water pockets is significantly reduced. As indicated by the water rotational dynamics (Figure 1) and simulation snapshots (Figure 2), the water pockets formed within hydrophobic IL/water mixtures near the saturation limit may serve as an excellent prototype for nanoconfined systems such as reverse micelles [61,63,65,69] 4 ]/water mixtures when water content is sufficiently high to promote water network percolation on macroscopic lengthscales, with this percolation threshold occurring roughly between ∼19 to 25% water content (vide infra). We note that the hydrogen bond histogram for bulk water (Figure 2) interestingly exhibits a finite probability for five hydrogen bonds as has been discussed in previous work [133].…”

“…At higher water concentration within hydrophilic IL/water mixtures, a number of different peaks were observed for the water stretch (deuterated) [50], with relative populations being significantly concentration dependent and suggesting the formation of water domains or clusters [56]. NMR [69] and SAXS/SANS experiments [61,63,65] have independently led to the same conclusion that water within hydrophilic ILs may resemble water states in nanoconfined systems. SAXS/SANS experiments indicate the formation of water pockets within hydrophilic BMIM + /nitrate ([BMIM + ][NO − 3 ]) ILs that are similar to those formed within nanoconfined systems [61], with the size of water pockets modulated by water concentration [65] and exhibiting anomalous temperature behavior [63].…”

“…Experimental spectroscopy [25,[48][49][50][51][52][53][54][55][56][57], small angle X-ray or neutron diffraction (SAXS/SANS) [58][59][60][61][62][63][64][65], and nuclear magnetic resonance (NMR) studies [66][67][68][69][70][71] have revealed a wealth of information on the structural and dynamic properties of IL/water mixtures. We restrict such synopsis to ILs composed of shorter chain cations similar to BMIM + ; it is well-documented that ILs composed of longer chain cations form micelles, reverse micelles or self-assemble when mixed with water [53,58,60,[72][73][74] but this surfactant-like regime is not our focus.…”

Ionic Liquid-Induced Unique Structural Transitions of Proteins

“…Intriguingly, the two peaks indicating an intermolecular β-sheet structure are observed for rhodanese and α-chymotrypsin. Conversely, the spectra of the other proteins do not contain these peaks, and the six remaining proteins form an α-helical structure, as in the case of [18,19]. Moreover, an increase in alkyl chain length results in an enhancement of nanoheterogeneity [40].…”

“…However, under IL-rich conditions wherein the mixtures exhibit molten-salt-like behavior, the water molecules are scattered in the polar domain and self-assemble in the ILs. The water molecules in this state are termed "confined water" [18,19] and the nanoheterogeneity of these systems is higher. As mentioned earlier, these solvent properties may contribute to unique structural transitions of the proteins.…”