Water Clathrates in Nanostructural Organization of Hydrated Ionic

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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Water Clathrates in Nanostructural Organization of Hydrated Ionic Liquids Manifest in Peculiar Density Trend Mohammad Homaidur Rahman, and Sanjib Senapati J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b08586 • Publication Date (Web): 26 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 2018

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The Journal of Physical Chemistry

Water Clathrates in Nanostructural Organization of Hydrated Ionic Liquids Manifest in Peculiar Density Trend Mohammad Homaidur Rahman and Sanjib Senapati* Department of Biotechnology, BJM School of Biosciences, Indian Institution of Technology Madras, Chennai 600 036, India, Tel: +91-44-22574122, e-mail: [email protected] Abstract Ionic liquid-water binary solutions have significantly expanded the applications of ionic liquids (ILs) in chemical and biological research. Therefore, considerable research has focused on measuring the thermophysical properties of these binary mixtures. From low-to-moderate concentrations of water, several IL/water mixtures exhibit deviations from expected trends in thermophysical behavior. One such example is a unique density trend observed for certain IL classes, which exhibit a characteristic increase in density with the addition of small amounts of water. Since water primarily interacts with the IL anion, such deviations have always been explained in the context of anion-water associations. Surprisingly, however, IL/water mixtures containing different cations but a common Lactate anion exhibit similar peculiarities in density trends. Using atomistic level molecular dynamics simulations, we show that diverse density trends are caused by cation-mediated modulations in the IL nanostructure. Depending on its nature, the IL cation can play a dual role in modulating the IL nanostructure - (i) resist watermediated breakdown of the nanostructure by interacting with the anion very strongly, (ii) further strengthen the nanostructure by incorporating water in the IL framework. The [emim] cation fails to play both roles resulting in the density decrease, while the [tmg] cation fulfills both roles leading to a density rise. The choline cation resists the density fall by inducing the formation of ‘water-clathrates’ in the solution. Such occurrence of clathrates in IL/water binary mixtures, reported for the first time in this study, further emphasizes that the properties of ILs and its mixtures are not merely determined by the chemical nature of the component ions, but also by their unique nanostructural organizations. This unique nanostructural organizations also manifest in their unusual dynamics.

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Introduction Ionic liquids (ILs) are of immense commercial interest due to their applicability in a broad range of industrial processes. ILs are salts that exist in liquid state at ambient temperatures, which can be attributed to their poorly co-ordinating ions. The wide range of possible cation/anion combinations makes ILs the most versatile solvent class with the ability to dissolve solutes of varied chemical nature.1–3 Further, the constituent ions can be modulated for taskspecific applications designating ILs as “designer solvents”.4 Moreover, due to the high electrochemical and thermal stabilities with almost negligible vapour pressure and low toxicity, ILs are suitable alternatives to toxic and volatile organic solvents commonly used in industrial processes.5–7 Therefore, ILs are finding increasing applications in diverse areas. In material science, ILs are proven to be useful in controlling the particle morphology, growth rate and size of nanomaterials.8 Several ILs are useful as reaction media in organic and bio-organic catalysis. They help in yield, optimization, selectivity, increase in solubility of substrate, product separation and improvising enantio-selectivity.8–10 ILs are also used in separation technology for selective absorption of gases.11 Owing to the hygroscopic nature, ILs absorb considerable amounts of water. IL/water binary mixtures hold a greater significance compared to their neat counterparts in the preservation of biomolecules, as media for drug delivery, and for dissolution of sparingly watersoluble bioactive compounds.12–14 The solute amount of water present in IL/water binary mixtures aids in preserving the structural integrity of biomolecules.15,16 Moreover, increased solubility of enzymes and their substrates in certain IL/water binary mixtures make these solvent suitable reaction media for biocatalysis.10,17,18 The addition of water in ILs changes various thermophysical properties of the solution, viz. density, viscosity, surface tension, etc.19–27 As mentioned earlier, the properties of ILs intricately influence their applications, and have therefore been a subject of interest in the past decade. One such property investigated is a peculiar trend in density exhibited by ILs with introduction of low-to-moderate quantities of water. Experimental studies by independent research groups have described an unusual increment in the density of the binary mixtures of 1-alkyl-3-methylimidazolium acetate ([Rnmim]+; n=2,4,6) and water with the increasing amount of water upto certain molar concentrations, followed by an anticipated reduction at higher dilutions.19–21 Similar divergent behavior observed for other thermophysical properties of several IL/water binary mixtures has compelled researchers to explore the mechanism underlying this phenomenon. NMR


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relaxometry and Molecular Dynamics (MD) simulation studies suggested that anion-water interactions are responsible for the observed deviations.21,22,24–27 The critical role of water concentration in contributing to the maximum observed deviation has also been established.19,22,23 Senapati et. al., employed MD simulations to elucidate the molecular basis of the unique nature of the density trend shown by the [Rnmim][Ac]/water binary mixtures.24 The increase in water concentrations from low-to-moderate, results in the formation of hydrogen bond bridges among the neighboring acetate anions that brought them in vicinity, hence caused a structuring in the anionic layer. This anion layer structuring was revealed to be the cause of distinguished increase in density. Such structuring could also explain the unique trends exhibited by thermophysical properties of other IL/water mixture, such as phase behavior25

and

viscosity.26,27 While several studies have elucidated the role of anions in modulating the thermophysical behavior of IL/water binary mixtures, role of the IL cation remains elusive. The few studies that have explored the role of the cation were aimed at understanding the effect of changing alkyl tail length of the [Rnmim] family of ILs (R = 2 to 6).20,22,24,27 Their findings revealed that structuring induced by anion-water interactions can further enhanced by aggregation of alkyl chains of cations in presence of water. Thus, in the study by Senapati et. al.,24 with increasing alkyl tail length the peculiar increase in density was enhanced. In the only other study of its kind, Shi et.al.,22 observed the opposite effect on nanostructuring by replacing the [Rnmim] cation with tetrabutylphosphonium ([P4444]). Unlike the [Rnmim] cation the bulky and asymmetric [P4444] cation hindered anion-water interactions in the binary mixture of [P4444][Ac]/water, leading to a decrease in density. Evidently, a change in cation can cause profound changes in the nanostructure of IL/water binary mixtures, and therefore deserves detailed investigation. In this study we aimed at understanding the effect of cations on the density profile of three classes of lactate-anion-based IL/water binary mixtures. The selected cations, N,N,N',N'tetramethyl-guanidinium ([tmg]+), 2-hydroxyethyl-trimethylammonium ([choline]+) and 1-ethyl3-methylimidazolium ([emim]+), represent the most widely studied IL classes, guanidinium, alkylammonium and imidazolium, respectively. Moreover, first two of the three selected cations are biocompatible in nature. The choice of the anion is also guided by its biocompatibility, since lactate is a common metabolite in the living cell. MD simulations of the three IL/water binary mixtures

performed across a wide range of concentrations revealed that despite having a

common anionic counterpart, the three IL classes exhibit varying density trends. Binary mixtures


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of [tmg][lactate] exhibited increase in density with addition of water upto a mole fraction of Xw = 0.73, beyond which the density decreased rapidly. Conversely, the density of [emim][lactate]/water binary mixtures decreased rapidly at all water mole fractions. Quite unexpectedly, the density profile of [choline][lactate] remained unchanged for a large concentration range. These observations are in clear correlation with reported experimental data,28–30 and re-emphasize the tunable nature of ILs. Such profound role of the IL cation in modulating properties of IL/water binary mixtures, which has thus far been poorly explored, constitutes the primary theme of the current study. We have investigated in atomistic details the nanostructural basis for such tunable behavior of lactate-based IL/water mixtures upon changing the IL cation. Methods: Molecular dynamics simulations All-atom molecular dynamics simulations were performed on a series of pure ILs and their binary mixtures with water. Binary mixtures of three lactate ([lactate]-) anion-based ILs consisting of [tmg]+, [emim]+ and [choline]+ as cations were studied. The water content in the binary mixture in weight percentage were set from 0.0, 0.5, 1.0, 5.0, 10.0, 20.0, 50.0 and 80.0 and their corresponding mole fractions (XW) were calculated as 0.05, 0.10, 0.36, 0.54, 0.73, 0.91 and 0.98, respectively. The desired weight percentages of each system were obtained by adjusting the number of water molecules in the simulation box while fixing the number of IL ion pairs as 512. The detailed composition of the systems studied is presented in Table 1. The notations for used for ILs atoms are presented in Figure S1 to aid the discussion. The standard inter-atomic potential function used is describe as:

Where, the first three components of potential function

describe the bonded interactions:

harmonic bond stretching, angle bending, and dihedral potential, respectively. The fourth component describes the non-bonded interactions that consist of 12–6 Lennard-Jones (LJ) potential and Coulomb potential. The equilibrium parameters, e.g. bond lengths (ro), angles (θo),


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and dihedrals (ϕ) for cations and anions were estimated from ab initio-derived optimized geometries of the IL ion pairs.31–33

The Gaussian09 software package34 at the B3LYP/6-

311++G(d,p) theory level was employed for geometry optimizations.

The true minima of

optimized structures were confirmed from absence of imaginary frequencies. The atomic charges were determined using MP2/ccPVTZ theory level, through ChelPG protocol35 by fixing the total charge on the cation and anion as +1 and -1, respectively. A similar protocol was used earlier for IL charge estimation22,24,36 The remaining interaction parameters for the cation were adopted from the work of Wang et. al., for the [emim]+,33 Senapati et. al., for [tmg]+,31 Acevedo et. al., for [choline]+,37 and Yao et. al., for lactate anion.32 These ILs potential parameters are compatible with OPLS/AA parameters owing to their similar development protocol. The transferability and accuracies of these generated force fields were checked over different ILs families and were even found to reproduce the experimental liquid density as well as crystal structure. SPC/E water model was preferred in the present study over TIP3P as the latter overestimates the dynamic properties of water.38 Also, the OPLS/AA force field in conjunction with the SPC/E water model is widely being employed in recent studies25,39,40 to observe the nanostructural organization of ILs in their water binary mixture. The starting configurations of each system comprised of 512 ion pairs and a suitable number of water molecules packed randomly in cubic boxes with the help of packmol.41 The packed systems were energy minimized by using steepest descent algorithm for 2000 steps, followed by 2000 steps using conjugate gradient algorithm. Due to strong electrostatic interaction between the ion pairs, system dynamics is very slow. Therefore extra steps were needed to achieve system equilibrium. These steps involve, slow heating of the system upto 700K under constant volume followed by step-wise cooling to 600K, 500K, 400K and finally at 298K using the isothermal-isobaric ensemble. This annealing method is commonly used in several studies to ensure proper mixing of IL ions.2,24,42 After equilibration, the systems were further simulated for 35 ns using 2 fs time step at 298 K and 1 atm pressure. The Berendsen thermostat and barostat with relaxation time of 0.5 ps was employed during the NPT simulation. In addition, all these systems were further simulated in NVE ensemble for 15 ns each to calculate the mean square displacement (MSD). Average diffusion coefficient of each constituent was estimated from the final 10 ns data that was divided into 5 windows of 2 ns each. Altogether, each studied system was simulated for a total period of 50 ns. The long range non-bonded interactions were truncated at 15Å and treated using the full


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Ewald summation technique. Amber1243 simulation package was used for performing MD simulations. All the analyses were performed using in-house codes and cpptraj module of AmberTools 16.44 VMD was used for visual representation. Results: Diverse density trends in lactate-based IL/water binary mixtures Simulated densities of three lactate-based IL/water binary mixtures containing [tmg], [emim] and [chol] cations exhibited diverse trends as a function of water concentration. As shown in Figure 1, the densities of [emim][lactate]/water binary mixtures exhibited rapid decrease even with minute addition of water. When the cation was replaced by [tmg], a complete reversal of trend was observed, with the densities increasing consistently even with small additions of water upto a concentration of Xw = 0.73. The [choline][lactate]/water binary mixtures, on the other hand, largely resisted change in densities with addition of water. Simulated densities showed very good agreement with experimentally determined densities28–30 obtained from independent studies of the three IL/water binary mixtures. The simulated densities deviated the experimental ones by 2-5%, which is a commonly observed and an acceptable range.37,45 Interestingly, the diversity in density trends was primarily observed at low-tomoderate water concentrations (Figure 1, inset). At higher water concentrations of Xw>0.8, densities of all three systems showed an identical monotonous decrease, as one expects. This clearly indicates that the three binary mixtures respond in unique manner to the initial addition of water. This is surprising, since almost all earlier studies have reported that when added to IL, water first interacts with the anion counterpart. For example, a peculiar rise in density observed for [Rnmim][Ac]/water mixtures with addition of low-to-moderate water concentrations was ascribed to structuring of acetate anions by the added water.19,24 However, in the current study the lactate anion was common in all three binary mixtures, indicating that the cation counterpart plays an equally important role in modulating thermophysical behavior of the IL media. Our observations, therefore, give rise to an important question about the synergistic role of the IL cation and anion in modulating thermophysical properties of IL/water binary mixtures. Figure 1. Density change is modulated by differential persistence of IL structuring in response to added water


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To unravel the synergistic role of IL cations and anions in modulating the observed density trend, we measured the effect of added water on the distribution of the two ion components in the binary mixtures. Site-site radial distribution functions (RDF) and centre-ofmass RDFs (com-rdf) were computed between cation-anion pairs and are shown in Figure 2 and Figure S2, respectively. The sites chosen for computing the ion distributions were as follows: carboxylate oxygen (-O2) of lactate anion respectively with amine nitrogen (-N1) of [tmg], hydroxyl oxygen (-O) of [choline], and imidazolium ring carbon (-C1) of [emim] cation. For neat ILs, the site-site RDFs presented the highest peak intensities for [tmg][lactate], followed by [emim][lactate] and [choline][lactate]. The rdf peaks at distances ≤ 3.5 Å in all three systems confirmed cation-anion hydrogen bonding, mediated by the chosen atomic sites. Moreover, for neat [emim][lactate] two peaks were observed, at ~ 3.2 Å and ~ 5.2 Å with corresponding minima at ~ 4.2 Å and 6.2 Å, respectively, plausibly representing the existence of repeating cation-anion pairing in [emim][lactate] systems. With addition of very minute quantities of water the cation-anion H-bonding interactions in all three systems remained intact (see Figure 2 for RDFs upto Xw=0.10), beyond which they were differentially affected. In [tmg][lactate] systems, the cation-anion H-bonding was unperturbed even at a water concentration of Xw=0.36, while the interactions started getting disrupted in the other two systems. In [emim][lactate]/water binary mixtures, both peaks were found to decrease in intensity and the second peak exhibited a right shift. This indicates that water molecules begin to accumulate in between the first and second IL shells. We calculated the corresponding cation-anion coordination numbers in all the IL/water binary mixtures and the values are tabulated in Table S1. Similar to RDF plots, the cation-anion coordination number in [tmg][lactate] systems increased/persisted upto ~ Xw=0.5, while it monotonically decreased for the other systems. For a more vivid illustration of the cation-anion distribution, we further analyzed the center-of-mass RDFs between IL ions (Figure S2). In agreement with the above observation, [tmg] exhibited the strongest and most persistent interactions with the [lactate] anion, indicated by sharp [tmg][lactate] peak intensities even upto Xw=0.73. The already weaker cation-anion distribution in the other two systems (broad peaks) became increasingly diffused with addition of more water. Notably, in [chol]lactate] systems, akin to [emim][lactate] binary mixtures, the decreasing peak showed a right shift (at Xw>0.36) indicative of the fact that water not only hindered cation and anion interactions, but also increased the physical separation between the


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ions. We further investigated how such differential effect of water on cation-anion structuring could give rise to the divergent density trend in the following sections. Figure 2 Since the nanostructuring in ILs is primarily manifested by its ion-ion H-bonding interactions, we examine the change in cation-anion and anion-anion H-bonding in the absence and presence of water. The geometrical criteria defined in our earlier studies was used, i.e., a distance of < 3.5 Å between donor and acceptor heavy atoms and a donor-H-acceptor subtended angle greater than 135˚. As shown in Figure 3, an average of ~1.8 hydrogen bonds were formed per anion with cations in neat [tmg]lactate], which corroborated well with existing theoretical reports on the formation of two hydrogen bonds between this IL pair.46 Hydrogen bonding interactions in the other two non-protic ILs were less significant with 0.65 and 0.3 hydrogen bonds between cation-anion pairs for neat [choline][lactate] and neat [emim][lactate], respectively. As further evident from Figure 3a, the cation-anion hydrogen bonding interactions in [tmg][lactate] remained intact with addition of water at low-to-moderate concentrations. The anion-anion hydrogen bonding presented in Figure 3b presents a reverse trend to cation-anion Hbonding. In [emim][lactate], the [lactate] anion, which forms the least number of H-bonds with its cationic counterpart, forms the maximum number of H-bonds with other lactate anions. On the other hand, in [tmg][lactate] anion-anion H-bonds are barely existent. It is worth noting that despite possessing a common anion, the three neat ILs exhibit significantly diverse cation-anion H-bonding interactions due to the varying nature of the cation. It is because of this distinct inherent nanostructure that the three systems are diversely affected by added water, as described in detail in the subsequent sections. It is also important to note that the observed finite number of anion-anion H-bonds in these systems is due to the unique molecular structure of [lactate] anion containing a potential –OH group. Figure 3. Cation-water interactions are key effectors of nanostructural persistence in IL/water binary mixtures The role of intruding water in modulating ion structuring in ILs was further investigated by computing the distribution of water around the cation and anion. The sites chosen were OW of water and -N1 amine nitrogen of [tmg], -C1 ring carbon of [emim], -O hydroxyl oxygen of


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[choline], and -O2 carboxyl oxygen of [lactate]. As shown in Figures S3 and S4, water interacts in unique manner with each cation class (Figure S3), while anion-water interactions are identical in all three systems (Figure S4). Evidently, the cation-water interactions play the predominant role in the observed divergence in density trends of the three systems studied here. For a more clear visualization, the cation-water interactions at low-to-moderate water concentrations are presented in Figure 4. It is interesting to note the distinct modes of interactions the three classes of cations establish with water at different concentrations. At low water concentrations, the [emim] cation makes the strongest interaction indicated by the sharp RDF peak with a defined minima at ~ 4Å. The [tmg]-water and [chol]-water interactions, conversely, are more diffused. This is because the positive charge on the imidazolium ring is least sterically hindered compared to the other two cation moieties. With increasing addition of water, [tmg]-water interactions exhibit a steady rise, indicated by rising peak intensities upto Xw=0.73. Moreover the [tmg]water RDFs undergo a left shift, indicating strengthening of the cation-water interaction. The measured cation-water co-ordination number at these concentrations also shows a similar trend with an increase for [tmg]-water, while the water coordination number for [emim] is the largest (Table S2). For a uniform comparison, the co-ordination number was calculated upto a radial distance of 4Å for all system. At this juncture it is important to recall that the densities of [tmg][lactate]/water binary mixtures exhibited a rise in the same concentration range, clearly indicating that cation-water interactions play a significant role in the peculiar density behavior. This however, is not the case with the other two binary mixtures, where the cation-water interactions slowly decrease at these concentrations. Figure 4. Thus it is now clear that [tmg][lactate] exhibits the strongest ion pairing that can resist any changes with initial addition of water. Further, tmg-water interactions increase with low-tomoderate water addition suggesting a possible strengthening of the IL nanostructure, leading to the observed density rise. Having established the distinct nanostructuring in the three IL/water binary mixtures and, consequently, their differential response to added water, we now investigate how water initiated the observed changes in the IL nanostructure. Figure 5 presents the Hbonding interactions established by water with both IL ions. As evident from the figure, water– anion H-bonding interactions exhibit a monotonous increase with the addition of water in all three binary mixtures. Conversely, change in water–cation interactions show distinct modulations in the three systems. While added water steadily engages in H-bonding interactions with the [tmg] cation throughout the studied concentration range, a less significant interaction of


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water with [choline] and least interaction with [emim] are observed. Notably, following a small initial increase, [emim]-water hydrogen bond saturates at moderate water concentrations. A similar saturation was observed for [choline]-water H-bonding, but at high water concentration. Figure 5. These findings were validated from simulation snapshots of the three lactate based IL/water binary mixtures at concentration XW = 0.73. Figure 6 depicts a central cation with all its surrounding anion and water within a 3.5 Å radial distance. On an average eight molecules of anions and nine molecules of water were found in the first shell of a [tmg] cation, that form a mesh of inter-connected hydrogen bonds involving all three molecular species (Figure 6a). Despite being surrounded by a comparable number of eight water molecules and three anions, very few water molecules engage in hydrogen bonding with the [emim] cation (Figure 6b), and instead interact primarily with the anion. Conversely, the H-bond network in [choline][lactate] exhibited a very different pattern (Figure 6c). Most of the water molecules formed a cage-like structure around the [choline] cation, while only few were engaged in direct interactions with the cation/anion. Such bimodal hydration of choline has been previously reported.47 Figure 6. Owing to the presence of a hydrophilic -OH group and three hydrophobic methyl groups in a choline molecule, it is known to exhibit two modes of hydration. While the -OH group directly H-bonds with water, the tetramethyl ammonium moiety structures water around itself to form a clathrate cage.47 In IL/water binary mixtures containing low-to-moderate amounts of water, however, this situation is little different. The -OH group of choline cation remains Hbonded to both water and the lactate anion, while the tetramethyl ammonium headgroup structures water around itself ( with the aid of lactate anions ) to form a semi-clathrate like pattern (Figure 6C). Radial distribution of water around the choline headgroup clearly depicts formation of the water clathrate, indicated by choline NT-Ow and NT-Hw distributions peak at the same distance as shown in Figure 7a. The water clathrate structure was more prominent when the water content in the system was sufficiently high, as shown in Figure 7b that represents the [choline][lactate]/water binary systems at XW = 0.91. Such unusual structuring of water by [choline][lactate] IL plausibly reverses the effect of water-induced break down of the IL nanostructure, and results in the almost steady density that does not show significant change with water added at low-to-moderate amounts (Figure 1). Thus, water exists in different molecular states in the three IL/water binary mixtures, driven primarily by their distinct interaction with the


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IL cation. The [tmg] cation H-bonds with the added water molecules, enables incorporation of individual water molecules homogeneously into the IL nanostructure, further strengthening it. The [emim] cation, on the other hand, denounces the added water, which then forms H-bonded networks with the lactate anion and disrupts the IL nanostructure. The [choline] cation does not directly interact with the added water but passively structures it to a previously unnoticed clathrate-like pattern in the IL nanostructure. Figure 7. Persistence of IL nanostructure manifests in reduced dynamics of IL/water binary mixtures The unique molecular states of water in the three IL classes can be further probed by studying the dynamical behavior, such as H-bond dynamics of IL ion-water pairs and translational diffusivity of the constituents. For H-bond dynamics of cation/anion-water, the geometric criterion of H-bond was adopted and its dynamics was analyzed by considering hydrogen bond population variable h(t). The value of h(t) is taken to be 1 when a particular cation/anion-water pair is H-bonded at time t and 0 otherwise. The hydrogen bond time correlation functions CHB(t) were employed to characterize the structural relaxation of these hydrogen bonds. The correlation function CHB(t) describes the probability that a pair of H-bonded molecules at time t=0 is also Hbonded at time t, independent of possible breaking in the interim time. The intermittent hydrogen bond correlation function CHB(t) can be mathematically defined as

The angular brackets denote averages over the initial time values and all pairs. This intermittent hydrogen bond correlation function CHB(t) was computed for all IL/water binary mixtures (Figure 8). Figure 8. In all binary solutions, CHB(t) was observed to be non-exponential with a very long decaying tail. In order to differentiate each system with other systems, the CHB(t) function was fitted to the following multi-exponential function. ,


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where

and

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is the decay time constant for the i-th component. The decay time

constant, obtained after fitting with two/three/four exponential function for cation-water in all systems, can be deduced as the time scale required for the reorganization of H-bonds. Such Hbond reorganization gives a direct evidence for the breaking and re-formation of H-bonds between donor and acceptor pairs, viz H-bond dynamics. The average time constants from cation-water H-bonds correlation function (Table 2) show that [tmg][lactate] system exhibits the slowest relaxation followed by [chol][lactate], with [emim][lactate] exhibiting the most rapid Hbond dynamics (Table 2). The slow retardation in decay constant was due to contribution of an extremely slowly decaying tail, in all systems, particularly in [tmg][lac] that exhibits the order of 103 ps.

value in

As described above, [tmg][lactate] segregates water molecules and

incorporates into the IL nanostructure by IL-water H-bonds. These water molecules fit so well in [tmg][lactate] nanostructure that breaking of these H-bonds exhibit extremely slow dynamics in this system. On the other hand, the large water aggregates formed in the [emim][lactate] binary mixture, result in rapid breaking and forming of H-bonds and consequently a faster decay of correlation function and a smaller

. The clathrate hydrate structure induced by [chol] cation

in the surrounding water results in intermediate H-bond dynamics in this system. Thus, the cation-water hydrogen bond dynamics in IL/water binary mixtures emulate the nature of interaction between the constituents in the systems, as shown in Figures 2-7. The water-water hydrogen bond correlation shows similar trend as of the cation-water hydrogen bond correlation and hence not shown. The anion-water H-bond dynamics of the three systems showed almost no difference, as one would expect. This further reiterates that nature of IL cations and their interactions with water play a key role in modulating nanostructural re-organization in binary solutions, and subsequently affect the physico-chemical behavior of these systems. We also analyzed the translational dynamics of the binary solutions. The mean square displacements (MSD) of each component was calculated and presented in Figure S5-6. The MSD data were fitted using Einstein relation

to obtain the self-diffusivity of each

component. The diffusivity of the cations and anions followed the trend - ([choline][lactate] > [emim][lactate] > [tmg][lactate]), which is in accordance with the strength of cation-anion interactions as observed in Fig. 2. With increasing concentration of water, the computed diffusivities clearly demonstrate the existence of two regimes. The first regime (XW ≤ 0.54) defines the ballistic regime or glassy state where addition of more water shows negligible effect on cation/anion diffusivities (see Fig. 9a,b inset upto XW ≤ 0.54). The second regime is called the


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exponential regime, where more addition of water increases the ion diffusivity in an exponential manner (Fig. 9a,b with XW ≥ 0.54). At the ballistic regime, the diffusivity of cations and anions are comparable in both [tmg][lactate] and [emim][lactate] (compare Fig. 9a and 9b). This indicates that cation and anion remain together in these binary mixtures and ion-pairs are mostly unaffected by the addition of low-to-moderate quantity of water. Conversely, the cation and anion in [choline][lactate] exhibit different dynamics, indicating weakly bound ion-pairs in this IL/water mixture. At the exponential regime of XW ≥ 0.73, addition of more water increases the diffusivities of cation and anion differently in three solutions. In [tmg][lactate]/binary mixtures, the diffusivity of [tmg] and [lactate] increased only by 1.7 and 1.9 fold from neat IL, respectively. In [emim][lactate, the respective fold increase was 5.4 and 5.2, while for [choline][lactate] they were 3.0 and 1.7. This trend correlates very well with the cation-anion interaction strength as discussed previously. In accordance with the

RDFs and H-bond

dynamics, the diffusivity of water was the slowest in [tmg][lactate]/water binary solution (Fig. 9c). Figure 9. Conclusion Anomalies in thermophysical behavior of IL/water binary mixtures are commonly attributed to the nature of the IL anion and its interactions with added water. In an exception to the above norm, binary mixtures composed of a common [lactate] anion were found to exhibit similar peculiarities in their experimentally determined densities when the constituent cation was changed. Our MD simulations result revealed that the nanostructural changes in the IL network, primarily mediated by the cationic counterpart, are responsible for modulating such differential density trends. At the nanostructural level there are two counteractive events at play in the IL/water binary system, a disruption of the cation-anion interactions to break down the nanostructure and a structuring of the added water to build the nanostructure. Depending on its nature, the IL cation can modulate both forces and thereby play a key role in affecting the density behavior. In our study, the imidazolium cation ([emim]) barely interacts with the added water and is a silent spectator to water-induced breakdown of the [emim][lactate] nanostructure, and a concomitant decrease in density. The protic guanidium cation ([tmg]) interacts strongly with both anion and water, thereby preventing a breakdown as well as strengthening the IL nanostructure causing the density rise. The cholinium cation, however, displays the most unusual role of structuring the added water into “clathrate-like” structures, thereby reversing the effect of


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a breaking nanostructure. The observed differential nano-structuring in the three IL/water solutions manifests in their dynamics too with the H-bond dynamics and translational diffusivity exhibiting significant slowing down in [tmg][lactate] and least affected in [choline][lactate].

Supporting Information Figure S1 presents the molecular structures of the ILs under study. Figure S2 presents the centerof-mass RDFs of cations around anions with increasing water. Figures S3 and S4 depict the distribution of water around cations and anions, respectively. Figures S5 and S6 present the mean square displacements of cations and anions in the IL/water binary mixtures. Cation-anion and cation-water coordination numbers are tabulated in Tables S1 and S2, respectively.

ACKNOWLEDGEMENT We thank Dr. Debostuti Ghosh Dastidar for her careful reading of the manuscript. This work was funded by Science and Engineering Research Board (SERB), Govt. of India (Project No. EMR/20176/000707). High Performance Computing Resources, IIT Madras is also gratefully acknowledged.


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Figure legend 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1: Comparison of simulated densities (open circles with dotted line) of neat and IL-water binary mixtures with experimental values (filled circles). Experimentally reported densities of neat and IL-water binary mixture of [emim][lactate] (red diamond), [tmg][lactate] (blue triangle) and [choline][lactate] (black circle) are shown. Experimental density data are taken from Tian et. al28 for [tmg][lactate], Wang et. al29 for [emim][lactate] and Francisco et. al30 for [choline][lactate]. The inset highlights experimental density below Xw = 0.5. Error bars are obtained from computed density values. Figure 2: Site−site radial distribution functions (RDFs) of cations around [lactate] anions with increasing mole fraction of water (Xw) - (a) [tmg]+ (b) [emim]+ and (c) [choline]+. The site of interest includes carboxylate oxygen of lactate anion (O2), and N1 nitrogen of the [tmg], C1 carbon of imidazolium cation ring and hydroxyl oxygen O of [choline], respectively. Atom notations for the cations and anion are shown in Figure S1. Figure 3: Average number of hydrogen bonds between (a) cation per anion molecules and (b) anion per anion molecules with increasing mole fraction of water. Color codes for the three systems are included in the insets. Figure 4: Site−site radial distribution functions (RDFs) of water around cation with increasing mole fraction of water (XW) - (a) [tmg][lactate] (b) [emim][lactate] and (c) [choline][lactate]. The site of interest is oxygen of water (OW), N1 nitrogen of the tmg, C1 carbon of the imidazolium ring and hydroxyl oxygen of choline (O). For clarity, only XW = 0.05, 0.54 and 0.73 have been shown. For all other XW systems, see Figure S3. Atom notations are similar to Figure S1. Figure 5: Average no of hydrogen bonds for (a) water per cation molecules and (b) water per anion molecules with increasing mole fraction of water. Figure 6: Spatial arrangements of the constituents of IL-water mixtures from MD simulations. The representative clusters of IL ions and waters are shown, which lie within a radius of 3.5 Å in each system. Snapshots are obtained from time-averaged structures for the system - (a) [tmg][lactate], (b) [emim][lactate] and (c) [choline][lactate] at XW = 0.73. Color scheme for atoms: cyan, C; red, O; blue N. For clarity, the cation-anion hydrogens atoms are omitted. The dotted lines represent H-bonds with corresponding distances. Figure 7: (a) Site-site radial distribution of [choline] and water showing the bimodal clathrate like distribution of water around the [choline] cation. The site of interest includes NT nitrogen and C2 carbon of the [choline] with Ow and Hw of water at Xw = 0.73. (b) Simulation snapshot exhibiting a clearer water clathrate around the [choline] cation in the IL/water binary mixture of Xw =0.91.


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Figure 8: The decay of H-bond correlation function with time for cation-water at increasing mole fraction 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of water. Results are shown for (a) [tmg][lactate]; (b) [emim][lactate] and (c) [choline][lactate] in their IL/water binary mixtures. The color code for mixtures at XW = 0.05 Red, XW = 0.10 Green, XW = 0.36 Blue, XW = 0.54 Yellow, XW = 0.73 Brown, XW = 0.91 Grey and XW = 0.98 Violet is used. Figure 9: Self-diffusion coefficients of (a) cation, (b) anion, and (c) water with increasing mole fraction of water. The symbols represent: red diamond - [emim][lactate]; blue triangle - [tmg][lactate] and black circle - [choline][lactate]. The error bars are estimated from five independent calculations of mean square displacements. Insets highlight diffusion coefficients of respective constituents at XW ≤ 0.73.


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Main Figures 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1


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Figure 2

(a)

(b)


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(c)


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Figure 3


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Figure 4

(a)

(b)

(c)


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Figure 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Figure 6

(a)

(b)

(c)


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Figure 7

(a)

(b)


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Figure 8


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Figure 9

(b)

(a)

(c)


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Table 1: Summary of the IL/Water binary mixtures studied. %WAT, XWAT, NIL, NWAT are weight percentage of water, mole fraction of water, number of IL ion pairs, number of water molecules, respectively, in each solution.

%WAT 0.0 0.5 1.0 5.0 10.0 20.0 50.0 80.0

XWAT 0.00 0.05 0.10 0.36 0.54 0.73 0.91 0.98

NIL 512 512 512 512 512 512 512 512


NWAT 0 28 55 289 610 1373 5492 5490

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Table 2. Fitted parameters for the cation-water hydrogen bond time correlation function in the IL-water binary mixtures at XW = 0.73. IL [tmg][lactate] [emim][lactate] [choline][lactate]

c1

0.357 0.769 0.606

τ1

0.599 0.231 0.300

c2

0.044 0.231 0.095

τ2

20.101 938.151 42.486


c3

τ3

0.599

2733.120

0.300

835.909

τavg

1637.496 216.615 254.749

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Water Clathrates in Nanostructural Organization of Hydrated Ionic

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