Static and dynamic properties of nano-confined water in room-temperature ionic liquids

Accepted Manuscript Static and dynamic properties of nano-confined water in roomtemperature ionic liquids

Hiroshi Abe, Takahiro Takekiyo, Yukihiro Yoshimura, Akio Shimizu PII: DOI: Article Number: Reference:

S0167-7322(19)32008-2 https://doi.org/10.1016/j.molliq.2019.111216 111216 MOLLIQ 111216

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

8 April 2019 2 June 2019 18 June 2019

Please cite this article as: H. Abe, T. Takekiyo, Y. Yoshimura, et al., Static and dynamic properties of nano-confined water in room-temperature ionic liquids, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111216

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ACCEPTED MANUSCRIPT Static and dynamic properties of nano-confined water in room-temperature ionic liquids

Hiroshi Abe a,*, Takahiro Takekiyo b, Yukihiro Yoshimura b, and Akio Shimizu c

a

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Department of Materials Science and Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka

239-8686, Japan b

Department of Applied Chemistry, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka 239-8686, Japan

c

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Graduate School of Environmental Engineering for Symbiosis, Soka University, 1-236 Tangi, Hachioji, Tokyo

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192-8577, Japan

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Nano-confined water (“water pocket”) was spontaneously formed in room-temperature ionic liquid (RTIL). The static structure of the self-dispersed “water pocket” was determined by a complementary use of small-angle X-ray scattering and small-angle neutron scattering. The size of

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the “water pocket” is tuned by the water concentration and temperature. The “water pocket”

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suppressed the crystallizations both of the RTIL and water at low temperature. The solidifications in the intermediate concentration region were quite sensitive to the cooling rate, and non-equilibrium degree in the mixture was enhanced by the “water pocket”. Hydrogen bonding of water molecules

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in the “water pocket” was weakened by the nano-confinement. Slow dynamics of the “water pocket” in the RTIL was clarified by quasielastic neutron scattering. The loosely packed and size-tunable “water pocket” is utilized for next generation nano-heterogeneity engineering.

Keywords: Room-temperature ionic liquids, spontaneous formation of nano-confined water, weak hydrogen bonding, slow dynamics of “water pocket”, non-equilibrium liquid

1

ACCEPTED MANUSCRIPT 1. Introduction Water is a fascinating molecule and a typical non-equilibrium liquid, although a molecular structure of water is simple. Under high pressure, anomalous behaviors of water such as proton order/disorder and hydrogen bonding appeared on the pressure-temperature phase diagram [1]. A significant finding is that low density amorphous (LDA) and high density amorphous (HDA) were

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discovered under high pressure [2, 3]. Recently, the polyamorphism of water was summarized in the literature [4]. Other HDA ices including metastable phases were obtained by thermodynamic pathways. One is very high density amorphous of water, which was found firstly in wide-angle

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X-ray scattering (WAXS) pattern and Raman spectrum [5]. The non-equilibrium nature of water is explained by the polyamorphic transitions. On the extension of LDA-HDA, a low-density liquid

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(LDL) and a high-density liquid (HDL) are predicted on the phase diagram, and a liquid-liquid

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critical point (LLCP) was hypothesized [3]. The nano-heterogeneity of the LDL was demonstrated by molecular dynamics (MD) simulations [6]. The scenario of the LLCP is still discussed with proposing a lot of models. As the reason of experimental difficulties, crystal ice easily appears at

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around the LLPC. As another ice polymorph, using the MD, aeroices were demonstrated under

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negative pressure in the simulation box [7]. The structure of the aeroices resembled to the aerogel, and density of the aeroices was extremely small. Nano-confinement of water has been investigated in the same manner with the nano solid

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materials. Nano-particles, thin layer and nano-multilayer systems have quite different characters compared with the bulk state. In case of water, water was confined in the nano porous silica, which has the uniform pore size [8, 9]. If water was confined in the pore size of 21 Å, the crystallization was suppressed completely. Moreover, self-diffusion constant of the nano-confined water was obtained by quasielastic neutron scattering (QENS). The diffusion coefficients were smaller than that of bulk water. It was interpreted that slow dynamics of the nano-confined water is caused by the weakened hydrogen bond network. Room-temperature ionic liquids (RTILs) are promising liquids for various applications using their inherent properties; nearly zero vapor pressure, thermal, electrochemical, and chemical 2

ACCEPTED MANUSCRIPT stabilities [10-13]. To resolve the outstanding features of the RTILs, a liquid structure of the RTILs reflecting molecular interactions has been investigated. Theoretically [14, 15] and experimentally [16, 17], the liquid structures were characterized by nano-heterogeneity even in the liquid state. The nano-heterogeneity of the RTILs developed proportionally to the alkyl chain length, n, of the 1-alkyl-3-methylimidazolium, [Cnmim]+, cations. By a combination of a cation and an anion, liquid

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morphologies on the nano-scale varied sensitively. Liquid structures possessing nano-heterogeneity or hierarchy were summarized in the recent review papers [18-21]. A fourth evolution of the RTILs is in progress by mixing with solutions [22]. The salt–solvent mixtures enlarge the usages of the

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further industrial applications. Particularly, water was combined widely with the hydrophilic RTILs. In binary systems, the liquid structures were modified extensively by the hydrogen bonding of

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water including proton transfer [21]. For instance, N, N-diethyl-N-methyl-N-(2-methoxyethyl)

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ammonium tetrafluoroborate ([DEME][BF4])-water mixtures, hierarchy structure was connected with the electrochemical instabilities [23-25]. [DEME][BF4] is classified as the aprotic RTIL. The AC impedance anomalies [23] and rhythmic pH oscillations [24, 25] of [DEME][BF4]-water

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appeared in the water-rich region. The additional fluctuations caused by proton network over the

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medium-range could cause the hierarchy structure [21]. In case of protic RTIL-water systems, anomalous behaviors were also induced by the water additive. Ethylammonium nitrate, [EAN][NO3], and propylammonium nitrate, [PAN][NO3], are the representative protic RTILs.

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Bicontinuous nano-structures of [EAN][NO3] were visualized in the simulation box by referring the WAXS data [26]. The proton-dominant electrochemical anomalies were enhanced by the water additive [27]. In the specific water concentration of [EAN][NO3]-water system, electrochemical anomalies coincided with amorphization at low temperature [28]. The proton-mediated fluctuations could suppress crystallizations both of [EAN][NO3] and water. In this study, static and dynamic properties of nano-confined “water pocket” inside the RTIL were investigated by a complementary use of X-ray and neutron. By the nano-confinements of water, hydrogen bonding and diffusion process of water were modified. The size-tunable water pocket could be a new target for next generation nano-heterogeneity engineering. 3

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2. Experiments 2.1 Materials Hydrophilic [C4mim][NO3] was purchased in Sigma Andrich Co. Distilled water (99.9%, Kanto Chemical Co.) was used as additive. Distilled D2O (99.9%, Merck) was selected in this study.

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The concentration of water in [C4mim][NO3] as-received samples was determined and verified to be 130−150 ppm using the Karl Fischer titration method. Sample preparation was done in a dry box

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under a flow of helium gas to avoid atmospheric H2O.

2.2 Small-angle X-ray scattering and small-angle neutron scattering

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Small-angle X-ray scattering (SAXS) experiments were carried out using a Kratky camera

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system (BioSAXS-1000, Rigaku Co.). The wavelength (λ) of incident X-ray was 1.542 Å. The incident X-ray was collimated using a parabolic multilayer mirror and was focused by a converting optical tool (CBO-f, Rigaku Co.). A 2D detector (PILATUS 100K/R) was used for rapid data

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acquisition. Here the scattering vector, Q, is defined as 4π(sin θ)/λ (Å−1). The vacuum chamber was

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set to remove air scattering. The sample holder was a quartz capillary with a diameter of 1.0 mm and thickness of 0.1 mm.

Small-angle neutron scattering (SANS) data were collected in the BL15 (TAIKAN) instrument

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at the Japan Proton Accelerator Research Complex (J-PARC) [29]. To simultaneously detect the scattering data from the wide Q range, the small- and medium-angle detector banks were installed in the beamline. Temperature was controlled with a circulating bath (Ministat 125, Huber Co.) The mixtures were placed into a quartz cell (Starna Scientific Co.) with low neutron absorption ability. The thickness of the cells was 2.0 mm. For data corrections, empty-cell and D2O-filled-cell were measured. By removing the long-wavelength components of incident neutron radiation, multiple scattering was almost suppressed.

2.3 Simultaneous wide-angle X-ray scattering and differential scanning calorimetry 4

ACCEPTED MANUSCRIPT In situ WAXS and differential scanning calorimetry (DSC) (SmartLab, Rigaku Co.) can probe microscopic and macroscopic changes [30]. Simultaneous measurements of WAXS and DSC can determine complicated phase diagrams more precisely by removing ambiguity. A one-dimensional detector (D/teX, Rigaku Co.) was integrated into the diffractometer for rapid scanning. The incident wavelength of X-ray was Cu Kα (λ = 1.542 Å). Dry nitrogen gas was flowed through the DSC

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attachment. The temperature range of the simultaneous measurements was 50 to −100 °C, and the cooling and heating rates were 1-10 °C/min.

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2.4 Quasielastic neutron scattering

The QENS experiments were carried out using a time-of-flight near backscattering DNA

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spectrometer at beamline BL02 at J-PARC [31]. The spectra over an energy transfer range from −0.04 meV to 0.10 meV were measured at an energy resolution of 3.6 μeV. The Q range was 0.025–

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1.975 Å−1. Double cylindrical Al cells were used in the QENS experiments. The inner diameter of the outer cylindrical cell (0.25 mm thickness) was 14.0 mm, while the outer diameter of the inner

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cylindrical cell (0.25 mm thickness) was 13.6 mm. A sample thickness of 0.2 mm was selected to

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reduce multiple scattering during the experiments. The temperature range was −20 °C to 30 °C, controlled using a top-loading cryo-furnace (Suzuki Shokan Co.). Cooling rate was 1.5 °C/min. The

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spectra obtained were then normalized to those of a vanadium standard.

3. Results and discussion 3.1 Phase diagram of [C4mim][NO3]-water [C4 mim][NO3] was selected for the following reasons: (i) three stable conformers of the [C4 mim]+ cation (TT, GT, and G’T) [32] in Fig. 1 are observable clearly by Raman spectroscopy [33], (ii) H/D exchange of [C4 mim][X]-D2O in Fig. 1 was suppressed completely in the [C4 mim][NO3]-D2O system [34], and (iii) the conformation fraction of [C4mim][[NO3] did not depend on the water concentration [34]. Associating with (i), recently, 6 stable conformers of the [C4 mim]+ were calculated by the DFT calculations [35]. The kinetic phase diagrams of 5

ACCEPTED MANUSCRIPT [C4 mim][NO3]-x mol% D2O upon cooling and heating were determined by the simultaneous WAXS and DSC measurements (Figs. 2(a) and 2(b)) [36]. The cooling and heating rates were fixed at 5.0 o

C/min. Pure [C4mim][NO3] did not crystallize upon cooling, and the cold crystallization (CI phase)

occurred upon heating as reported previously [37]. Furthermore, crystal (CI phase) - crystal (CII phase) phase transition was also detected by a change of the WAXS pattern. Next, phase mixing

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effect in the kinetic phase diagrams is mentioned. Below 70 mol% D2O (xIL), the phase transitions, which are the same as those of the pure IL, appeared continuously on the kinetic phase diagrams. The solid phases in the mixtures below 70 mol% did not include crystal ice because of no Bragg

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reflections of the crystal ice. Therefore, excess water transformed the amorphous ice at low temperature. In contrast, above 90 mol% D2O (xI), crystal ice appeared upon cooling without the

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RTIL crystallization.

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DSC thermal traces also provide a significant information about phase transition. The enthalpy difference, H, from the liquid state to the solid state was obtained on the water concentration scale (Fig. 3). In the specific water concentration at xIL < x < xI, both [C4mim][NO3] and D2O were

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almost amorphized upon cooling. In fact, upon cooling, glass transition temperatures were

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determined in the water concentration (Fig. 2(a)). Tg fluctuated on the phase diagram at xIL < x < xI. Recently, crystallization/amorphization depending the cooling rate were observed in the specific water concentration [38]. Here, we predict that the specific water concentration is regarded as a crossover point from RTIL-like to water-like behaviors. In order to interpret the discontinuous

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region of H, peculiar nano-structure, which is different from the nano-heterogeneity of the pure RTILs, could be formed. The additional fluctuations could disturb the crystal nucleation both of the RTIL and water. The discontinuity of H was also observed in the protic [EAN][NO3]-water system [28].

3.2 Static structure of “water pocket” Hyper structure of lysozyme was changed in the [C4 mim][NO3]-x mol% mixtures by SAXS [39]. Partial refolding was observed in the 80 mol% mixture (xIL < x < xI). At 95 mol% (xI < x), an 6

ACCEPTED MANUSCRIPT unfolded state of lysozyme was obtained. On the other hand, the MD simulations of [C8 mim][NO3]-water demonstrated the nano-confined water [40]. The nano-confined water existed near the nano-domain boundaries. Above 95 mol% water (xI < x), water percolated over the whole simulation box. The percolated water state is equivalent to the bulk water one. Here, it is emphasized that the size of lysozyme is double of the “water pocket”. From the both experimental

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and simulation results, we hypothesize that double sized-lysozyme could be captured by the “water pocket” at 80 mol%, and half of lysozyme is immersed in the “water pocket”. The immersed part of lysozyme becomes the folding state. In order to prove the hypothesis, a complementary use of

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SAXS and SANS was carried out. Using a contrast of cross sections between X-ray and neutron, aggregations of D2O in the RTIL was extracted [39, 41].

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SAXS [39] and SANS [41] data were collected separately. Figure 4 reveals the SAXS and

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SANS patterns at room temperature. A distinct SANS peak appeared in the 77.8 mol% mixture, although there was no SAXS peak. The observed SANS peak cannot be fitted by the conventional models for small-angle scattering using the SASfit program, which has a lot of form factors [43].

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Recently, ab initio bead modeling has been developed to reconstruct the hyper structures of protein

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solutions. ATSAS program package [44] can correspond to the various mixtures and flexible systems. By introducing pair-distance distribution functions, P(r), the SANS peak was well fitted. Using indirect transform methods [45], the observed small angle scattering was transformed into the

inter-atomic

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smooth P(r), which contains size and shape of a domain. P(r) corresponds to distribution of distance

inside

a

domain

as

a

form

factor.

P(r)

is

applied

to

mono-disperse/poly-disperse systems of domains with an arbitrary form factor. P(r) is sensitive to the shapes of domains. While, the structure factor as the interference between domains might be neglected [46]. Background subtraction of PRIMUS [46] in the ATSAS package is carried out by assuming zero concentration as no domain (no interference). P(r) in an inset of Fig. 4 was also calculated for the best fitting. The SANS peak at room temperature depended on the water concentration (Fig. 5). With decreasing the water concentration, the SANS peak shifted to higher Q positions. This implies that size of the “water pockets” changed, depending on the water 7

ACCEPTED MANUSCRIPT concentration. Using the bead modeling, shapes and distributions of the “water pockets” were reconstructed in the simulation box (Fig. 6) [42]. The formations of the “water pocket” resemble to the MD simulation results [40]. Cations and anions are not visualized in the figure. The simulated “water pockets” are derived only from the SANS peak. Therefore, isolated waters, which cannot contribute to the SANS peak, are not displayed. This is because the D-enhanced SANS peak

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contains information only about D2O aggregation. Even in the liquid state, the almost mono-dispersive distribution of the “water pockets” was simulated. Moreover, coarsening of the “water pockets” was obtained in the lower temperature and higher water concentration. A

changing the water concentration and temperature.

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significant finding is that the loosely packed “water pocket” is size- and distribution-tunable by

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Cation effect of “water pocket” was clarified by comparing [C4 mim][NO3] with [DEME][NO3]

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[41]. In [DEME][NO3]-D2O system, there was no SANS peak. Since both systems contain the same [NO3]- anion and D2O, presence or absence disappearance of the “water pocket” could be alternated by [C4 mim]+ or [DEME]+ cation. The MD simulation of [C4 mim][BF4]-water suggested that

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molecular interaction between the [C4mim]+ cation and H2O is repulsive (Fig. 7) [47]. Therefore,

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we predict that water additives could be excluded from the [C4 mim]+-dominant nano-domains. The excluded water might form the “water pocket” at the boundaries between polar and non-polar nano-domains. On the other hand, [DEME]+ cation possesses ether bond, which is electro-negative part (Fig. 7) [48]. Since the oxygen of the [DEME]+ cation is regarded as a water capture site, water

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molecules could migrate between the [DEME]+ cation and [NO3]- anion. Then, we deduce that the “water pocket” is not formed in [DEME][NO3]-D2O system. Hydrogen bonding of water is a key to interpret the outstanding properties of water. Recently, H/D exchange rate of the nano-confined water in the RTILs were investigated by NMR [49]. As mentioned earlier, the H/D exchange of [Cn mim][X]-water system occurs at hydrogen linked with C2 (Fig. 1). Peak splitting on the NMR spectra indicates that the H/D exchange rate among the water molecules inside the RTIL was very slow. The slow rate could be caused by the “water pocket”. On the other hand, strength of the hydrogen bonding is probed directly by Raman 8

ACCEPTED MANUSCRIPT spectroscopy. In [C4mim][NO3]-x mol% D2O, the OD stretching Raman bands of weak hydrogen bonding (WHB), medium hydrogen bonding (MHB), and strong hydrogen bonding (SHB) were measured at room temperature [21]. After peak profile fitting of WHB, MHB, and SHB, the decomposed SHB portion decreased at 70-90 mol% D2O. The region of water concentration coincides with that of presence of the “water pocket”. Hence, in the nano-confined circumstance,

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hydrogen bonding of water could be weakened. Compared with bulk water, extremely slow exchange rate and weak hydrogen bonding are characterized as a peculiar feature of the

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nano-confined water.

3.3 Dynamic properties of “water pocket”

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Nano-domains of pure RTILs fluctuate with some lifetime in the simulation box by the MD

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simulations [14, 15]. Experimentally, dynamic hierarchy of the pure RTILs was investigated systematically by QENS [50, 51]. Relaxation time at each scale was explained, relating to nano-domains and anion effect. Based on the nano-heterogeneity, slow and fast relaxations were

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connected with the anion and alkyl chain length dependences. The experimental facts correspond to

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glass transitions of the pure RTILs as a non-equilibrium phenomenon. Generally, the non-equilibrium behavior is understood by introducing free energy landscape (FEL) [52]. The idea of FEL is described by many basins and saddle points on the potential energy surface. Furthermore,

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the cage energy landscape (CEL) is applied into the RTILs [53]. In the CEL, local minima on the free energy surface depend on types of solvents: shape of basin for ionic solvent is deep and steep, that of organic solvent is shallow and wide. The local minimum of the RTILs is estimated as an intermediate basin between them. Recently, non-equilibrium degrees of pure RTILs were evaluated at a viewpoint from the crystal polymorphs and crystal multiple pathways [28, 54-56]. Both at low temperature and high pressure, the non-equilibrium properties were influenced by the cation conformers. Not only in the pure RTILs but also in the mixed system, the non-equilibrium phenomenon was observed at low temperature. Solidifications of [C 4 mim][NO3]-80.0 mol% H2O were sensitive to the cooling rate (Fig. 8) [38]. For instance, crystallinity of the mixture was 9

ACCEPTED MANUSCRIPT reduced by increasing the cooling rate. Above 4.0

o

C/min, crystallization was completely

suppressed upon cooling. Here, we confirm that the non-equilibrium degree in the mixtures is enhanced by the “water pocket” formation. Incoherent QENS extracted diffusion of the confined water inside solid porous materials in the previous studies [8, 9]. In order to clarify diffusion of water inside the “water pocket”, QENS

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experiments were also carried out. Using a contrast of incoherent scattering cross sections, inc, between H and D, diffusion of the confined water was evaluated, where inc of H and D are 80.27 and 2.05 barn, respectively [57]. The incoherent dynamic structure factors, 𝑆i𝑖𝑛𝑐 (𝑄, ω) , of

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[C4 mim][NO3]-80.0 mol% H2O and [C4 mim][NO3]-80.2 mol% D2O at −20 °C are shown in Figs. 9(a) and (b) [38]. The [C4mim][NO3]-80.0 mol% H2O mixture provides total scattering of

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𝑖𝑛𝑐 𝑖𝑛𝑐 ( (𝑄, ω) (Fig. 9(a)), while [C4mim][NO3]-D2O mixture described by 𝑆IL 𝑆IL+H 𝑄, ω) has no 2O

D2O, the difference between The

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information of D2O (Fig. 9(b)). Considering negligible contributions of incoherent scattering of 𝑖𝑛𝑐 𝑖𝑛𝑐 ( (𝑄, ω) and 𝑆IL 𝑆IL+H 𝑄, ω) becomes ∆𝑆 𝑜𝑏𝑠 (𝑄, ω) (Fig. 9(c)). 2O

∆𝑆 𝑜𝑏𝑠 (𝑄, ω) is given by [51, 58, 59],

(𝑄, 𝜔) , ∆𝑆 𝑜𝑏𝑠 (𝑄, ω) = 𝑅(𝑄, 𝜔)⨂𝐴H2O 𝑆H𝑖𝑛𝑐 2O

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(1)

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where ⊗ represents the operator of the convolution. R(Q, ) is the instrumental resolution function, (𝑄, ω) contains dynamic which was determined using a vanadium standard sample. 𝑆H𝑖𝑛𝑐 2O information about the self-diffusion of the water molecule in the “water pocket”.

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The Kohlrausch-Williams-Watts (KWW) function is utilized to describe dynamic processes such as liquids and glass [60-62]. The KWW function, f(t), is expressed as; 𝑡

𝛽

𝑓 (𝑡) = exp {− (𝜏 ) } .

(2)

𝑅

β represents the stretching exponential and is restricted to between 0 and 1. R is the characteristic relaxation time. Using the Fourier transform of f(t), ℱ [𝑓(𝑡)], and the Lorentzian function, Li(i, ) [63], Eq. (1) is rewritten by, ∆𝑆 𝑜𝑏𝑠 (𝑄, ω) = 𝑅(𝑄, 𝜔)⨂[𝐴IL ℱ[𝑓(𝑡)] + 𝐴H2O ∙ LH2O (ΓH2O , 𝜔)] ,

(3)

where i represents the half width at half maximum (HWHM) of the Lorentzian function of the 10

ACCEPTED MANUSCRIPT 𝑖𝑛𝑐 ( i-component. To estimate the 𝑆IL 𝑄, ω) component, the QENS profiles of [C4 mim][NO3]-80.2

mol% D2O at -20 ºC were fitted firstly (Fig. 10(a)). By fixing the obtained parameters of the D2O-based mixture, the QENS profiles of [C4 mim][NO3]-80.0 mol% H2O were fitted secondary (Fig. 10(b)). Finally, the parameters derived only from water in the water pocket were obtained. Quantitative analysis of the calculated HWHM was carried out using a jump diffusion model

𝐷

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[8, 51, 58, 59, 63]. In a jump diffusion model, (Q) is provided by; 𝑄2

ΓH2O (𝑄) = 1+𝐷H2O 𝑄2 𝜏 , H2O

0

(4)

where 𝐷H2O and 0 correspond to the diffusion coefficient of water and mean residence time of

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water, respectively. The Q2 dependence of ΓH2O (𝑄) of water in the “water pocket” at -20 oC is

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plotted in Fig. 11. The diffusive motion inside the “water pocket” are well described by the solid line in Fig. 11, based on Eq. (4). The broken line in Fig. 11 corresponds to the bulk(Q) value of bulk

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water at the same temperature. By fitting the (Q), 𝐷H2O was found to be 1.8 × 1010 Å2s-1, and 0 was calculated to be 20.0 ps (Table 1). In case of bulk water at -20 oC, the Dbulk and 0 values of

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bulk water were found to be 4.2 × 1010 Å2s-1 and 22.7 ps in Table 1 [64]. The water molecules in the “water pocket” is characterized by slower dynamics, compared with bulk water. The 0 value of the

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“water pocket” is comparable to that of the bulk water. Since the hydrogen bonding of water in the “water pocket” is weaker than that in the bulk water by Raman spectroscopy, some other attractive interaction could affect in the “water pocket. However, the additional interaction in the “water

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pocket” could provide the large diffusion constant. Thus, the inconsistency among the diffusion coefficient, the residence time, and weak hydrogen bonding implies that transportation mechanism in the “water pocket” is different from that in the bulk water. Generally, in the bulk liquid, Grotthuss mechanism as an indirect mass transfer has been discussed to explain the experimental results [65, 66]. We assume that, in the nano-confined circumstance, the Grotthuss exchange mode is not activated due from the spatial constrains. Then, a slow diffusion process described by a direct mass transfer might be realized in the “water pocket”. For comparison, one dimensional (1D) diffusion of water involved inside solid nano-pores was investigated by QENS [9]. The pore size of sample C14 11

ACCEPTED MANUSCRIPT of mesoporous MCM-41 silica was 28.4 Å in diameter [9]. The diffusion coefficient of water, 𝐷H2O , in C14 was 6.9 × 1010 Å2s-1 at -5 oC, and 0 was 7.0 ps at -5 oC (Table 1). The values of bulk water at -5 oC [64] is also listed in Table 1 for comparison. Compared with bulk water, the smaller diffusion coefficient of 1D water was induced by the interaction with the pore wall. The dimensional constrains for the diffusion process and solid matrix should be considered in additions

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to the spatial constrains. In the confined water on the nano-scale, not only a static average size can be determined by SANS but also slow dynamics are observed by QENS. The size, distribution, and slow dynamics of

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the “water pocket” contribute to a disturbing of the crystal nucleation of water at a low temperature. If we hypothesize that the Grotthuss mechanism is not permitted by the spatial constrains, small

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diffusion coefficient of water does not contradict with the slow H/D exchange rate [49] and the

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weak hydrogen bonding in the “water pocket” [21]. Here, we summarize three effects for dynamics of the nano-confined water; (i) Limit of application for the Grotthuss mechanism, (ii) spatial size

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and dimensional constrains as a geometrical factor, and (iii) liquid or solid matrix.

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3.4 Aggregation model of water in RTIL

We introduce the specific aggregation model as shown in Fig. 12 to interpret the discrete amorphous region (xIL < x < xI) at low temperature [36]. In recent Raman spectroscopy, the water

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concentrate regions were decomposed into three [67]. Water-poor, intermediated, and water-rich regions were determined consistently by water concentration dependence of the various Raman bands. At x < xIL, single water molecules and small water clusters exists isolatedly. Since crystal ice was not formed in this region, the size of the water clusters could be smaller than the critical size of crystal nucleation. The small water clusters could appear randomly near the nano-domain boundaries with some lifetime. The picture in the water-poor region (Fig. 12) is similar to the MD simulation results [40]. Amount of amorphous ice increased proportionally to the water concentration. While, H of the crystal RTILs decreased monotonically with increasing the water concentration (Fig. 3). H is proportional to crystal volume of the RTIL because of no Bragg 12

ACCEPTED MANUSCRIPT reflections of crystal ice at x < xIL. Crystalline of [C4 mim][NO3] did not involve water, and the excluded water clusters were amorphized at low temperature. In the water-rich region (xI < x), the opposite tendency was observed. Water concentration achieved to the percolation threshold at x = xI. Then, crystal ice was formed at xI < x. The RTIL was confined inversely by the percolated water. Then, crystallization of the RTILs was suppressed at low temperature.

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“Water pocket” appeared in the intermediate region (xIL < x < xI) at room temperature. As a crossover phenomenon from the RTIL- to water-dominant region, some additional fluctuations could be induced spontaneously. Here, we introduce the correlated “water pocket” having long

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lifetime to interpret the experimental results (Fig. 12). Some “water pocket” might interfere with others. The cooperative fluctuations among the “water pockets” cause developing network over the

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medium-range. The network forming of the “water pockets” enforces them to fluctuate

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synergistically. The additional fluctuations in the mixtures change the system to be non-equilibrium in the same manner with multiple relaxation processes of the pure RTILs as a glass forming factor [51]. In fact, cooling rate dependence of the solidifications [38] reflects the non-equilibrium state of

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[C4 mim][NO3]-water. Here, we emphasize that dynamic hierarchy at each scale prevents

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crystallizations. The non-equilibrium state is derived from weak hydrogen bonding and slow diffusion on the nano-scale, correlated fluctuations of the “water pocket” on the mesoscopic scale,

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and on the macroscopic scale.

4. Summary

Nano-confined

“water

pocket” was

spontaneously

formed

inside

the

hydrophilic

[C4 mim][NO3]. At the intermediate region (xIL < x < xI), the “water pocket” suppressed the crystallization both of the RTIL and water. The discrete region of fully amorphized solids corresponds to that of presence of the “water pocket” at room temperature. The characteristic features of the “water pocket” are as follows: (i) spontaneous formation. (ii) weak hydrogen bonding of water, (iii) slow diffusion of water, (iv) mono-dispersive distribution, (v) variable shape, (vi) size-tunable, (viii) long lifetime of the “water pocket”, and (vii) a non-equilibrium state. The 13

ACCEPTED MANUSCRIPT intrinsic properties of the “water pocket” were enhanced by the nano-confinement in the RTIL. Cooling rate dependence of solidifications indicates that the correlated “water pocket” enhances the non-equilibrium state. Entirely different behaviors between bulk water and the “water pocket” are originated from the dynamic hierarchy. In the nano-heterogeneity engineering for next generation, using a non-crystal form, loosely packed “water pocket” might be applied into the cryo

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preservation of the proteins.

Acknowledgements

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We thank Professor N. Hamaya of Ochanomizu University, Dr. T. Matsumoto of Rigaku Co., Ms. M. Shigemi, and Dr. H. Kishimura of National Defense Academy for experimental supports and

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helpful discussions. Also, we appreciate Dr. S. Takata of JAEA, Dr. K. Ohishi of CROSS, and Dr. J.

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Suzuki of CROSS for SANS experimental supports in TAIKAN of J-PARC (Proposal No.: 2012B0001). For QENS experiments (Proposal No: 2017B0011), Dr. T. Yamada of CROSS, and Dr.

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K. Shibata of J-PARC Center supported our experiments and analyses.

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Table 1

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[67] J. Kausteklis, M. Talaikis, V. Aleksa, V. Balevičius, J. Mol. Liq. 271 (2018) 747–755.

“Water pocket”

Bulk water

1D water

in liquid matrix -20 °C D

Bulk water

in solid matrix (C14) -20 °C

-5 °C

-5 °C

1.8

4.2

6.9

8.5

20.0

22.7

7.0

4.66

(× 1010 Å2s-1) 0 (ps) 16

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Table caption

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Table 1. Diffusion coefficient, D, and mean residence time, 0, of water determined by QENS. The values of bulk water [64] and 1D water [9] are quoted from the previous studies.

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Figure captions

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Fig. 1 H/D exchange site of [C4mim]+ cation. Three stable conformers of the [C4mim]+ cation.

Fig. 2 Kinetic phase diagrams determined by the simultaneous measurements. (a) cooling process;

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(b) heating process. L and A are, respectively, liquid and amorphous phases in the [C4 mim][NO3]-D2O mixtures. CI and CII are two forms of the [C4 mim][NO3] crystal phases. The I

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phase reveals the crystal ice of D2O.

Fig. 3 Water concentration dependence of enthalpy. Enthalpy for the crystallization of

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[C4 mim][NO3] and D2O was calculated from the DSC traces. Below x = 70 mol% (xIL), crystal [C4 mim][NO3] existed obviously. Above xI (94-100 mol%), a single phase of crystal ice was formed without the IL crystalline.

Fig. 4 SAXS of [C4mim][NO3] - 80.0 mol% D2O and SANS of [C4mim][NO3] - 77.8 mol% D2O. Blue curve reveals ab initio simulation results (ATSAS program package). Pair-distance distribution function, P(r) of [C4mim][NO3]-77.8 mol% D2Ois inserted.

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ACCEPTED MANUSCRIPT Fig. 5 Water concentration dependences of SANS intensities at room temperature.

Fig. 6 Simulated “water pocket” referring to the observed SANS peak. [C4 mim]+ cations and [NO3]anions are omitted. With increasing water concentration, larger water pocket is visualized in real

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space. Coarsening of the “water pocket” is simulated at low temperature.

Fig. 7 Molecular interactions of [DEME][BF4]–H2O and [C4 mim][BF4]–H2O.

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Fig. 8 Cooling rate dependence of WAXS patterns of [C4 mim][NO3]-80.0 mol% H2O. Blue closed

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sample pan are represented by red closed circles.

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circles reveal the ideal positions of Bragg reflections of crystal ice. Bragg reflections from Al

Fig. 9 The dynamic structure factors, 𝑆i𝑖𝑛𝑐 (𝑄, ω) , of (a) [C4 mim][NO3]-80.0 mol% H2O, (b) [C4 mim][NO3]-80.2 mol% D2O, and (c) the difference between the H2O- and D2O-based mixtures.

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The intensity is plotted by a logarithmic scale.

Fig. 10 QENS spectra and fitted results of (a) [C4mim][NO3]-80.2 mol% D2O and (b) [C4 mim][NO3]-80.0 mol% H2O at -20 ºC. Q is fixed at 1.18 Å-1.

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Fig. 11 Q2 dependence of HWHM, ΓH2O (𝑄), at -20 ºC. The solid curve reveals that the fitted values are based on a jump diffusion model (see text for details). The broken line represents the Γbulk (𝑄) of bulk water at -20 ºC for comparison.

Fig. 12 Schematics of three kinds of the water states in the [C4 mim][NO3]. “Water pocket” at xIL
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Fig. 1

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(a)

partial TC

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(b)

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Water pocket

L

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Temperature (°C)

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I

A or (C+A)

L + CII

L

partial TCC

0

L + CI

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scL -100

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A or (C+A) 0

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8000

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2000

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P(r)

1.5 1.0 0.5

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10-3 10-2

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r (Å)

10-1

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0.2

(a)

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I(Q)

0.4

0.3

69.6 % 74.8 % 77.8 % 79.9 % 85.0 % 89.9 % 96.6 %

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Q (Å )

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3.0 °C/min

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ACCEPTED MANUSCRIPT 1. Spontaneous formations in room-temperature ionic liquids.

2. An ideal non-equilibrium state in the mixtures.

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3. Weak hydrogen bonding and slow diffusion.

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12