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Structure and Dynamics of Hydroxyl-Functionalized Protic Ammonium Carboxylate Ionic Liquids Dhileep Nagi Reddy Thummuru, and Bhabani S. Mallik J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05995 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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The Journal of Physical Chemistry

Structure and Dynamics of Hydroxyl-Functionalized Protic Ammonium Carboxylate Ionic Liquids Dhileep Nagi Reddy Thummuru and Bhabani S. Mallik* Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy 502285, Telangana, India ABSTRACT: We performed classical molecular dynamics simulations to investigate the structure and dynamics of protic ionic liquids, 2-hydroxy ethylammonium acetate, ethylammonium hydroxyacetate and 2-hydroxyethylammonium hydroxyacetate at ambient conditions. Structural properties such as density, radial distribution functions, spatial distribution functions and structure factors have been calculated. Dynamic properties such as mean square displacements as well as residence and hydrogen bond dynamics have also been calculated. Hydrogen bond lifetimes and residence times change with the addition of hydroxyl groups. We observe that when hydroxyl group is present on cation, dynamics become very slow and it forms the strong hydrogen bond with carboxylate oxygen atoms of the anion. The hydroxyl functionalized ILs show more dynamic diversity than the structurally similar ILs.

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1. INTRODUCTION Due to their excellent electrical and thermal properties, ILs are used in various applications such as lubricants, solvents in bioprocesses, fuel cells, heat storage and in catalysis.1–6 ILs are proving to be promising solvents as modifying the substituent on cation or anion can change their physical and chemical properties. Primarily, ILs can be involved in many types of interactions such as electrostatic, van der Waals, hydrogen bonding (HB), solvophobic effects, Coulombic and dipole-dipole interactions, etc;7 first three are being considered as the key interactions between cation and anion of ILs. It is very difficult to characterize the hydrogen bonds in ILs due to their localized and directional characters.8 The study of these hydrogen bonds becomes much more difficult when we consider protic ionic liquids (PILs) which contain proton donor and acceptor sites. Presence of these sites enables the formation of hydrogen bond network in bulk phase.4,7,9,10 Experimentally, Ludwig and co-workers11–14 have probed the strength of cation-anion interactions of imidazolium based aprotic, and ammonium based protic ILs through far-IR spectroscopy. They found an increase in contribution of the hydrogen bonding interaction to the total interaction energy from aprotic ionic liquid (APIL), [C4mim][NO3] (1-butyl 3-methylimidizolium nitrate) to PIL, propylammonium nitrate (PAN).12 This increase can enhance the fluidness of ILs, and lower the melting points, viscosities and enthalpies of vapourization.11,15 This is due to the distortion of charge symmetry of the system by strong, localized and directional hydrogen bonds which in turn leads to the formation of defects in the electrostatic network of ILs.11–15 Noak et al.16 used the IR, Raman, and NMR spectroscopy techniques to show that the methyl group on C2 position of the cation ring of imidazolium aprotic ILs distorts the hydrogen bonding between cation and anion of IL very strongly. This leads to notable changes in their melting and freezing points. Miran et al.17 have

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shown that hydrogen bond strength is very important in determining the ionicity of PILs. These experimental studies suggest that the HB interaction between cations and anions in PILs has as much importance as electrostatic and Van der Waals interactions. Ammonium and hydroxylammonium based ILs have recently attracted researchers towards their ability to be used as the alternate solvents. Using first principles molecular dynamics simulations, Mallik et al. showed that hydrous tetramethylammonium fluoride18 can be used as an alternate solvent for a hard-to-dissolve molecular crystal, triaminotrinitrobenzene, and it was found that the solubility of this molecule is enhanced due to formation of a Meisenheimer complex. The hydroxyl based ILs have been used to absorb CO2 from natural and flue gases.19 Yuan et al.20 have shown that, due to the high solubility of CO2, the hydroxylammonium ILs have high possibilities of being used as solvents for CO2 absorption.21 Hydroxylammonium ionic liquids are also very useful in dissolving insoluble polymers such as zein polymer, polyaniline and polypyrrole.22,23 It should be noted that understanding of physicochemical properties of ILs are crucial for developing the alternate solvents. To this end, Chhotaray et al.24 measured the glass transition temperature, density, speed of sound, viscosity and decomposition temperature of hydroxylammonium based ILs at different temperatures. Kurnia et al.25 reported the refractive index, density, viscosity and decomposition temperatures of many hydroxylmmonium ILs at various temperatures. Pinkert et al.26 explored the electrical density, conductivity, and viscosity of alkanolammonium ILs. Recently, Sarkar and coworkers27 performed time-resolved fluorescence anisotropy experiments on hydroxyl functionalized ionic liquid, and found that this IL was more heterogeneous as compared to other similar ILs.

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Theoretical calculations have proven to be useful for predicting the properties of hydrogen bonds of ILs at molecular level. Hunt et al.28,29 examined the electronic structure of gas phase aprotic ion pair of 1-butyl 3methylimidazolium chloride at the levels of B3LYP, MP2 and CCSD(T) methods. They found that C2 position of the cation ring decreased the HB strength between cation and anion. They further observed that it also unexpectedly decreased the entropy of the system. Based on these observations, they explained the increase in viscosity and melting point of IL on the hypothesis that loss of HB strength was over-weighed by the loss of entropy. Static quantum chemistry calculations give deep insights about the structure of HB; however, they fail to explain dynamics and effect of temperature on it. Molecular Dynamics (MD) simulations can be useful to overcome these issues. A deep knowledge at the molecular level is required in order to understand the structure and dynamics of ILs by performing computer simulations. MD simulations provide a convenient path to calculate the thermophysical properties of ILs. A number of research papers that examine the structure and dynamics of ILs using MD simulations have been published since last decade. These studies provide insights into how the computed properties of different ILs change with ion type, temperature and other conditions. Many of these studies have calculated structural properties like radial distribution functions, spatial distribution functions, and dynamic properties like diffusion coefficient, mean square displacement and ionic conductivities. Very recently, we have studied structure, dynamics and thermophysical properties of five alkylammonium carboxylate ionic liquids (ILs) from classical molecular dynamics simulations.30 These studies have shown that MD simulations are very useful to calculate many liquid state properties of ILs. Imidazolium based aprotic ILs have been studied mostly by MD simulations.31–38 Pensado et al.39 have investigated the surface properties of imidazolium based ILs with hydroxyl tail groups using MD simulations, and reported that, due to the presence of hydroxyl groups, liquid structure of ILs is less organized. Experimental studies have already proved that ILs with hydroxyl groups become less ordered40, have strong inter ionic interactions,19 and their presence increases the polarity, hydrophilicity, heterogeneity and hydrogen bonding capability.41 It was proved that hydrogen bonding has a significant effect on polarity of PILs42 that increases when hydroxyl groups present on cation or anion.41,43 Deng et al.44 have shown that hydroxyl substituent on the cationic tail of imidazolium nitrate does not permit formation of nonpolar domains. They also found that hydroxyl groups formed a large number of hydrogen bonds with anions, a few with cationic head groups, and very few with each other. Hydroxyl group which is present on cation or anion of IL prevents the segregation of IL into polar and apolar domains.40,45–48 Even though in most of the cases it is true, however, there are some exceptions where increase in local viscosity increased with inclusion of hydroxyl group. For example, Gholami and co-workers reported the organization

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of anions around hydroxylimidazolium cations. They have shown that when bulkier anions are present, cation-cation interaction increases.49 Sarkar and co-workers also investigated the viscosity of the hydroxylimidazolium IL and found the higher value as compared to non hydroxyl structurally similar IL.27 The present study aims to explore the structure and dynamics of three hydroxyl functionalized ILs, 2-hydroxyethylammonium acetate (HEA), ethylammonium hydroxyacetate (EHA), 2hydroxyethylammonium hydroxyacetate (HEHA), in order to know the effect of hydroxyl group on structure and dynamics when it is present on either cation or anion or on both the ions. We have also included another PIL, Ethylammonium acetate, in our study for the comparison.

2. COMPUTATIONAL METHODS Cations and anions used for this study are shown in Figure 1. Classical MD simulations were carried out using GROMACS 5.0.4.50,51 Bonded and non-bonded parameters were taken from the OPLS-AA force fields.52 Force field parameters used are shown in Tables S2 and S3 of supplementary information (SI). It was found that charge scaling method is a better alternative to the polarizable force fields53 when it comes to analysing the charge transfer effect in ILs. Buhl et al.54 showed in their ab-initio calculations of [C1mim][Cl] that the net charge on each ion is 0.7 to 0.8 due to charge transfer between ions. Youngs et al.55 showed that scaling of charge on each ion to 0.7 gave the best results for density and self-diffusivity. The simulations of [C4mim][Cl] were carried out by scaling charges to 0.9 to obtain the better results.56 Four ammonium based ILs were studied by Sunda et al.57 with charges being scaled to 0.78. Since we too are studying ammonium based ILs, we have also scaled the charges to 0.78. This factor also has been proved to be appropriate to provide structure and thermophysical properties satisfactorily in our earlier study involving ammonium based ILs.30 An individual ion was optimized with Gaussian software58 using the basis set B3LYP/6311+G(2d,p). The antechamber package was used to generate the partial charges.59 Packmol software package60 was used to prepare the initial configuration of 500 ion pairs. The structure of this configuration was taken for further simulation in Gromacs software package. Steepest descent method was used for initial energy minimization.61 After this, NVT simulation of 2 ns was performed at a higher temperature than the target room temperature in order to enable proper mixing of the ions. Annealing process was applied with stepwise to cool down the system from 500 K to room temperature in NVT ensemble. Periodic boundary conditions were applied in all the directions. Velocity-Verlet algorithm was used to integrate the equations of motion. LINCS algorithm was used to constrain the bonds with hydrogen atom. Then, the equilibration was done for 2 ns in

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NpT ensemble followed by 8 ns simulation for calculating density. The V-rescale thermostat62 and Berendson barostat63 were used for isothermal-isobaric simulations with coupling constant of 0.1 and 2.0 ps, respectively. The last structure of the NpT simulation was taken for a further run of 10 ns in NVT ensemble at room temperature. The Vrescale thermostat62 alone was used for this run. The nonbonded interactions were calculated using a cut-off of 1.2 nm. Langevin dynamics was used to control the temperature at collision frequency of 2 ps-1. Time steps of 2 and 1 fs were used for the equilibration and NVE simulation runs, respectively. Particle Mesh Ewald (PME) switch method was used in later simulations to treat the long-range electrostatics with a cut-off value of 1.2 nm. NVE ensemble was used for the final simulation run of 200 ns, and we used the obtained trajectory for further analysis of various structural and dynamical properties. Gromacs input along with configuration files of ionic liquids and force field parameters may be found in the supporting information (SI).

3. RESULTS AND DISCUSSION We present the comparison of experimental and simulated density data for the ILs studied in Figure 2. The maximum uncertainty in the calculation of densities of ionic liquids in our molecular dynamics simulations is ±0.0004 g.cm-3. The reported results show that the maximum deviation from experiment is 4.7% for EHA. Density of HEA from our simulations is 1.125 that matches the experimental density of 1.120 g.cm -3 with very less error percentage. This experimental value was taken from the work of Yuan et al.64 However, we found two more experimental25,26 density values for HEA - we therefore averaged all the available densities to 1.148 g.cm -3 resulting in an error percentage of 2.03. The differences in experimental values may have been due to the variation of percentage of moisture from one sample to another. Experimental density of EHA as reported by Greaves et al.65 was 1.15 g.cm-3 with a deviation of 4.69% from our calculated density (1.096 g.cm-3). The density estimated for HEHA is 1.219 g.cm-3. Experimental density of EAA is 1.017 g.cm-3 and the deviation from calculated is 1.96%. Density of HEHA is the highest among the studied ILs that can be attributed to the presence of two hydroxyl groups. This increase in density with inclusion of hydroxyl group was also observed from the work of Chhotaray et al.24 The ILs with larger number of hydroxyl groups on the cation show more density due to the possibility of forming more interionic hydrogen bonds.26 This is due to the increased propensity of hydrogen bonding with number of functionalized hydroxyl groups. We also estimated the molar volumes from our calculated densities and compared with the values obtained using experimental data; the obtained molar volumes for EAA, HEA, EHA and HEHA are 105.60, 107.56, 110.40, and 112.9 cm3.mol-1, respectively. Molar volumes from the experimental densities for EAA, HEA, EHA and HEHA are 103.24, 105.4, 105.22

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and 110.48 cm3.mol-1. In both the cases, molar volumes increase with the increase in molecular weight of IL. To know the pattern of microstructure of ILs, we first analysed the radial distribution functions66 (RDFs) and spatial distribution functions67 (SDFs). Figure 3 depicts the centre of mass (COM) RDFs of four ILs. EAA shows that first peak of cation-anion RDF (Figure 3: top panel) appears at 0.42 nm. The first peaks corresponding to cation-anion RDFs are located at 0.45 nm for both HEA and EHA; the position of this peak is moved towards longer distances(0.52 nm) for HEHA. When compared to HEA and EHA ionic liquids, the position of first peak of EAA RDF is located slightly towards shorter distances and the peak height is higher than other ILs considerd in this study. Due to the inclusion of hydroxyl (OH) group on cation, anion or on both, the cation-anion interactions decrease while the solvation shell size increases. Cation-anion RDFs are well defined up to the third solvation shell, which shows the long-range interactions present in these ILs. Peak intensities for second solvation shells of all the ILs are similar. The presence of two OH groups leads to increase in size of ions in HEHA: as a result, the minimum position of cation-anion RDF is moved to a longer distance. The minima of cationcation RDFs (Figure 3: middle panel) are located at 0.55 nm, which are at much longer distances as compared to cationanion RDFs. The first solvation shell size of cation-anion is lesser than that of cation-cation. It is observed that peak heights are high in cation-anion RDFs thus indicating stronger electrostatic interactions between opposite ions. Aparacio et al. also observed that inclusion of hydroxyl group on cation increased the cation-anion interaction.19 Cation-cation interaction of EAA is stronger than cationanion due to the relatively stronger association of cations. HEA and HEHA have similar types of cation-cation RDFs, which means that association of cations in these two ILs is similar due to the same cation being present in both the ILs. EAA and EHA have also common cation; as a result of which their cation-cation RDFs are similar. This type of behaviour can be seen from cation-anion RDFs also. EAA and HEHA cation-anion RDFs are distinct, but HEA and EHA show similar RDFs due to the presence of same number of OH. It has been found that when hydroxyl group is present on the anion, the anion-anion interaction decreased, as also the size of the first solvation shell size. The anion-anion interactions are weaker than cation-anion and cation-cation interactions. This was consistent with the previous results.24,27,39,44,45,49,68–70 From the above observations we can say that inclusion of hydroxyl group changes the structure of ILs. ILs with hydroxyl groups behave differently from the ILs without OH group. To further understand the association of ions, coordination numbers of COM RDFs (SI: Table S1) were calculated from their first minima. It was found that cationanion coordination numbers for all the studied ILs were almost same except for EHA where it showed a slightly higher number of 7.44 as compared to around 7.24 for the rest of the ILs. It means that EHA ions are more solvated by

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their counter ions than other ILs. Cation-cation coordination number increased from EAA (7.36) to HEA (7.77) and decreased to EHA (6.93). Cation-cation coordination number in HEHA is 7.25 that is lesser than EAA (7.36). Cation-cation coordination number increases when OH group is introduced on cation. With the presence of OH group on anion, the coordination number decreases significantly. However, when OH group is present on both cation and anion, coordination number decreases slightly. It suggests that when hydroxyl group present on cation, it facilitates the hydrogen bonding between cations. Anionanion coordination number is highest for EAA (8.8) and less in all other ILs: HEA (8.63), EHA (6.06) and HEHA (5.91). Anion-anion coordination number decreases when OH group is present on anion. A similar type of increase in coordination number of cation-cation RDF was observed with the increase in number of OH group on cation in the work reported by Aparacio et al.71 From the above observations it can be stated that each cation is surrounded by nearly 7 anions. ILs retain their multi association structure when hydroxyl group is introduced. Cation-anion association is increased with the presence of OH group on anion and in remaining cases the change is not so significant. It shows that hydroxyl group on anion helps in the aggregation of the counter ions. Highest cation-cation coordination number is found for HEA. When OH group is present on cations, association of cations is observed rather than the anions due to hydrogen bonding between cations. Structure Factor: RDFs can be used to calculate the structure factor I(q) from the classical MD simulations using the following equation: =

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∑ ∑

∑ ∑

where xi, and xj represent the number of concentrations of itype and j-type atoms respectively in the stoichiometric unit. Atomic structure factors are denoted by fi and fj. Hij(q) are the partial structure factors, which were calculated from Fourier transform of the pair correlation function, and q is the scattering variable,

= 4 ! − 1

42

' =

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… … … 9

sin '( … … … 10

sin 2/, 2π/L

where Lorch window function was denoted by ', a function used to decrease the effects of finite truncation of r;72,73 rmax is the half of the box edge. ρ0 (electrons2/nm3) indicates the bulk number density. Pair correlation functions are shown with gij(r). Very few experimental and computational studies can be found on the calculation of structure factor of ammonium based ILs. For example,

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Santos et al.74 studied the IL, methyltributylammoniumbis(trifluoromethylsulfonyl)amide and found a well match between calculated and experimental quantities. Mariani et al.75 calculated the structure factors of whole systems for Ethylammonium nitrate (EAN), propylammonium nitrate (PAN) and butylammonium nitrate (BAN). Anion-anion structure factor showed low q peak (LqP) between 10 and 15 nm-1. This LqP was due to long range correlations of cation-cation and anion-anion. Due to unavailability of experimental structure factors for the ILs considered for this study, it was not possible to carry out a quantitative comparisons. Figure 4 shows the structure factors of full systems (a), cation-cation (b), anion-anion (c) and cation-anion+anion-cation (d). There was no shift in the total structure factor of ILs with the inclusion of OH groups; this indicates that the overall structure of these ILs is similar. However, in cation-cation structure factors (b), EHA shows the strongest peak while the second one is of EAA. Anion-anion and cation-anion also show a similar trend. The more heterogeneity behaviour was observed for cation-anion+anion-cation structure factors showing an interesting pattern where the contribution is negative. In our calculations we also found these LqP for both cation and anion, which are due to long range correlations of ions. The differences between the work of Mariani et al.75 and our study can be due to the presence of different anions and the hydroxyl groups. Recently, we also studied the effect of alkyl chain length on structure factors of various alkylammonium ionic liquids.30 In the current study, even though the total structure factors show not much deviation with the inclusion of hydroxyl groups, the LqP of cation-cation, anion-anion and cation-anion+anion-cation are different that shows the ionic liquids are structurally distinct. Spatial distribution functions (SDFs) of anion around cation were calculated using the TRAVIS67 software package. Calculated SDFs are presented in Figure 5. From the SDFs, it is found that anionic clouds are close to acidic hydrogen atoms of cation. The cations in EAA and EHA have only one type of acidic hydrogen atoms, which are connected to the nitrogen atom; anionic clouds are found around these hydrogen atoms only. However, there are two types of acidic hydrogen atoms for HEA and HEHA in the cation (hydroxyl and ammonium groups) and the anionic density is being spread around these two types of acidic hydrogen atoms of cation. We further observe that hydroxyl hydrogen atoms on cation make strong hydrogen bonds with anion, which will be discussed in later section. The overall appearance of cation-anion RDFs changes with inclusion of OH group. (Figure 3) Earlier, Padua and co-workers also showed that the presence of hydroxyl group disrupts the ordering of the surface and changes the surface tension.39 We also find that when hydroxyl group is present on cation, anions are connected to the hydroxyl group. This distribution of anion around hydroxyl group can be seen for HEA and HEHA. The distribution of anions around the

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ammonium hydrogen atoms is different when hydroxyl group is present. This distributions of EAA and EHA show three distinct parts directed towards hydrogen atoms that are different than HEA and HEHA. Strong interaction between cation and anion found in COM RDFs (Figure 3) can be also confirmed from the SDFs of anion around cation. (Figure 5) The MSD plots along with β values were presented in Figure S1. Respective logarithmic plots of MSD values have also been shown in the same figure. The quantity β indicates the proper diffusive region of entities. HEA has the lowest MSD between all the studied ILs in this work while EHA had the highest. Surprisingly, when OH group is on anion, the dynamics of IL become faster. For example, the values of MSD at a particular time of both EHA and HEHA are higher than EAA. The self-diffusion coefficients for cations (D+) and anions (D-) calculated from our simulations are shown in SI: Table S4. For EAA, cation has a higher diffusion coefficient of 5.06×10-8 cm2.s-1 . For the remaining three ILs, the diffusion coefficient of anion was higher than that of cation. For example, in HEA, cation diffusion coefficient is 2.51×10-8 while the anion diffusion coefficient is 2.64×10-8 cm2.s-1. Decrease in diffusion coefficient is observed with the inclusion of OH group to cation. A similar type of behaviour of decrease in diffusion coefficient with the addition of OH group on cation was observed by Aparacio et al.76 This decrease in diffusion is due to increase in molecular weight of cations, which slows down the dynamics. Another reason for this behaviour could be the formation of cationic or anionic clusters due to increased van der Waals forces and strong hydrogen bonding interactions. The inclusion of hydroxyl groups on cations increases the viscosity of ILs20,25,26 and decreases electrical conductivity. This decrease in conductivity is due to the decrease of the mobility of ions with inclusion of hydroxyl group, which increases the possibility of forming more number of hydrogen bonds. Our calculated MSD results (Figure S1) also show the decrease in mobility of ions with addition of hydroxyl groups. Structure and dynamics of hydrogen bonds To explore further, we investigated the structure and dynamics of hydrogen bonds by calculating RDFs and hydrogen bond (HB) lifetimes using the hydrogen bond auto correlation function approach.77–82 Standard cut-off values were considered for the existence of hydrogen bond. Hydrogen bond existence was considered when the donor (D)-acceptor (A) distance was less than or equal to 0.35 nm, and the A-D-H angle was less than 300. Decay of hydrogen bond autocorrelation function depends on the geometric condition, which was defined for the existence of hydrogen bonds. Integral of autocorrelation function gave the rough estimation of HB lifetimes. We followed the same conditions for the existence of hydrogen bond for all the ILs studied in this work. Diffusion and orientation of entities are primarily responsible for HB bond making and breaking processes, which decide the decay of autocorrelation

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functions of hydrogen bonds. If the hydrogen bonds are allowed to break and reform, HB lifetimes can be calculated by defining the binary function h(t) which is defined as 1 when hydrogen bond is present and zero in the absence of any HB. The autocorrelation functions of CHB(t) have been calculated according to Luzar and Chandler’s78,83 definition of intermittent hydrogen bond autocorrelation function. /01 2 =

< ℎ0ℎ2 > < ℎ 0 >

66 67 The kinetics of hydrogen bond breaking and reformation 68 was analysed by defining the reactive flux correlation 69 function K(t) as 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110

62 = 7/01 2 − 7 8 92

where n(t) = the probability of breaking hydrogen bond which existed at time t = 0 and the two hydrogen bonding groups were present within the hydrogen bonding distance. 7 and 7 8 were the forward and the backward rate constant, respectively. The forward lifetime of hydrogen bond was given by :01 that was considered as the inverse of 7. Radial distribution functions (RDFs) of all possible interionic O…H interactions have been calculated to know the structure of hydrogen bonds. Spatial distribution functions of oxygen atoms of anion around cation were also calculated using TRAVIS software.67 O…H hydrogen bonding interactions are primarily divided into three categories: 1. cation-anion (CA) 2. cation-cation (CC) 3. anion-anion (AA). Each category is further divided into subcategories according to possible O…H hydrogen bonding interactions. There are four types of interactions in CA interactions: (a) N-H…O-C, (b) N-H…O-H, (c) O-H…O-C (d) O-H…O-H. Of these four types, the occurrence of type (a) is possible in all ILs. Types (b) and (c) are possible in EHA and HEA, respectively, while HEHA can contain both these types. Type (d) is possible only in HEHA. In CA category, interactions exist between ammonium hydrogen (H-N) of cation and carboxylate oxygen of anion (COO). This type of interaction shows first sharp peaks and the minima at 0.175 and 0.26 nm, respectively, in RDFs of all the ILs (Figures 6a-6d). The position of this type of peak is invariant with changing ILs. First minima of RDFs are slightly moved towards the right for HEHA, HEA and EHA as compared to EAA. This increase in distances is due to an increase in size of first solvation shell size with inclusion of OH group, which in turn increases the size of ILs. The positions of first and second solvation peaks are invariant with the addition of hydroxyl group. Inclusion of OH group on cation or anion or both cation and anion decreases the peak height due to the decrease in hydrogen bonding of NH…O-H type of interaction, which is due to the increase in other possibilities of making O…H interactions. The position of first minima located at 0.26 nm indicates strong

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hydrogen bonding between ammonium (NH) and carboxylate (COO) groups. Peaks of second solvation shell too followed the same trend as the first solvation shell but with a lower intensity. Existence of second and third peaks indicates the long range ordering of ions due to primarily hydrogen bonding interaction and electrostatic interactions. By considering the cut-off distance from first minima we calculated the coordination number of OH RDFs, which helped us in getting to know the number of interacting sites (SI: Table S5). Coordination number decreased with inclusion of OH groups, similar to the decrease in coordination number of COM RDFs. Coordination number of O…H RDFs in EAA was 1.68, and it decreased to 1.07, 1.04, and 0.94 in HEA, EHA and HEHA, respectively. Figure 6(b) depicts the interaction between ammonium hydrogen of cation and hydroxyl oxygen of anion. This type of interaction is possible only in EHA and HEHA. The appearance of RDFs does not change noticeably except for the height of peak. It is also observed that N-H…O-H interaction is stronger in HEHA as compared to EHA. The positions of first peaks of RDFs are located at 0.20 nm and the first minima are located at 0.25 nm. When compared to N-H…O-C interactions, the intensity of these interactions is far lesser which shows that these interactions are very weak. Figure 6(c) shows the interactions between hydroxyl hydrogen of cation and carboxylate oxygen of anion. This type of interaction is possible only in HEA and HEHA. The intensity and sharpness of these peaks are similar to NH…O-C. The positions of first peak and minimum are located at 0.178 and 0.25 nm, respectively. To know the orientation of different types of oxygen atoms of anions around the cation, site-site spatial distribution functions were calculated (Figure 7). Distribution of carboxylate oxygen (orange transparent) and oxygen of hydroxyl group of anion were calculated around the cation to know which oxygen bonded stronger to ammonium hydrogen atoms and hydroxyl group of cation. Two distinct distributions were observed for carboxylate oxygen (orange) in HEA and carboxylate oxygen is strongly bound to two distinct sites of cation: ammonium hydrogen atoms and hydroxyl hydrogen. Distribution of carboxylate oxygen (orange transparent) and oxygen of hydroxyl group (green) were calculated in EHA. In this case, both the oxygen atoms are mainly located around the ammonium hydrogen atoms of cation. When compared to hydroxyl oxygen (green), the distribution of carboxylate oxygen atoms was more for the same isosurface value. This indicates that carboxylate oxygen makes stronger hydrogen bonds as compared to oxygen of hydroxyl group; the same has already been observed from RDFs (Figure 6). In HEHA, both the cation and anion contain hydroxyl groups. In this IL, distribution of two oxygen atoms of anion is found around the hydrogen atoms of ammonium and hydroxyl groups of cation. Oxygen atom of carboxylate group shows more distribution as compared to oxygen of hydroxyl oxygen (green). This behaviour has already been observed from the cation-anion RDFs (Figure 6). From the above observations it is clear that carboxylate

58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114

oxygen atom makes stronger hydrogen bonds as compared to oxygen of hydroxyl group. The dynamics of hydrogen bonds were explored by calculating hydrogen bond (HB) forward lifetimes and auto correlation functions (Figure S2). The types of hydrogen bonds explored have already been mentioned in (cationanion) Figure 6. In N-H…O-C type HB lifetimes (Figure S3(a)), EAA shows second highest forward lifetime of 136.6 ps. Autocorrelation functions are also shown in Figure S2(a). In these autocorrelations, the decay of EAA is slowest and shows highest forward lifetime. From the above observation it is clear that EAA shows strongest N-H…O-C hydrogen bond among all studied ILs in this study. HEA shows lesser forward lifetime of 107.3 ps. The lifetime of HB decreases with inclusion of OH group, which in turn means that this type of HB becomes weak with the inclusion of new OH groups. EHA and HEHA shows the forward lifetimes of 25.4 and 27.2 ps, respectively. This decrease is due to inclusion of OH groups, which increases the possibilities of forming other type of hydrogen bonds like hydroxyl-hydroxyl and hydroxyl-carboxylate, which in turn weaken the strength of N-H…O-C type of interactions. In N-H…O-H type (Figure S3(b)), EHA and HEHA show forward lifetimes of 6.4 and 9.9 ps, respectively; these can be seen from their autocorrelation functions which indicate that HEHA decays slower. From RDF (Figure 6(c)) it is also clear that HEHA shows strong hydrogen bond, which is further confirmed by these autocorrelation functions. Forward life times of O-H…O-C type of hydrogen bonding are shown in Figure S3(c). HEA shows forward lifetime of 182.8 ps, which is higher than HEHA (46.0 ps). These interactions show slightly higher forward lifetimes as compared to N-H...O-C forward lifetimes. From the decay of autocorrelation functions it can be seen that HEHA decays faster than HEA. O-H...O-H interactions are possible only in HEHA (Figure S3(d)). Forward lifetime of these HB interactions is 14.0 ps which is far less as compared to NH...O-C and O-H...O-C cation-anion interactions. We have calculated RDFs of cation-cation and anionanion interactions in Figure 8. Figures 8(a) and 8(b) show the cation-cation interactions while 8(c) and 8(d) show the anion-anion interactions. Figure 8(a) shows the N-H...O-H type of interaction, which is possible in HEA and HEHA. It shows the bifurcated peak at 0.40 nm and the first minimum at 0.47 nm. This type of interaction shows similar peaks (in terms of both height and position) for both the ILs. This indicates that inclusion of OH on anion from HEA to HEHA did not affect this interaction. Another cation-cation O...H interaction is shown in Figure 8(b). This O-H...O-H interaction (hydroxyl-hydroxyl) shows broad shoulders. Peaks of these RDFs are not well defined indicating very weak interaction. Anion-anion interactions are shown in Figures 8(c) and 8(d). Figure 8(c) shows the O-H...O-C type of interaction that is stronger than cation-cation interaction. First peak and minimum are observed at 0.21 and 0.30 nm, respectively for both HEA and HEHA ILs. The peak height

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where g(t) is the residence population variable. If g(t) = 1 then given atom or molecule is present in the given distance cut-off from t=0 to t. If the given atom or molecule is not present inside the given cut-off, g(t) = 0. The decay of center of mass residence autocorrelation functions is shown in SI: Figure S5. The decay of CRes(t) is fitted with the stretched bi-exponential function that is given by equation:

54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95

/;

The atoms, Nitrogen (N) of cation and carboxylate oxygen (O) of anion, have been taken as reference atoms for calculating residence times. First minimum of N--O RDF have been taken as a radial cut-off for the calculations. The decay of CRes(t) is least for HEA IL. As we know from the O--H RDFs and hydrogen bond dynamics, when the OH group is present on cation, it makes strong hydrogen bond among all possible sites. These strong HBs help in restricting the diffusion of ions. All The four ILs have comparable residence times. Atom-atom NO residence autocorrelation functions between cation and anion are

97 98 99 100 101 102 103 104 105 106 107 108

shown in Figure S6. EAA shows the highest residence time of 3908.6 ps. This is due to the formation of strong hydrogen bond between cation and anion. HEHA shows the second highest residence time. In HEHA more number of hydrogen bonds are possible compared to other three ILs. Due to this reason, cation and anion stay more time near each other. Two ILs EHA and HEA show less residence time since the hydrogen bonding between ammonium hydrogen atoms and oxygen atoms of anion become weak due to the presence of hydroxyl group on cation or anion. This is due to the formation of other types of hydrogen bonds with the inclusion of hydroxyl group. EAA showed the highest HB lifetime as well as highest residence time. HB lifetimes have relatively short time scales as compared to later and can make and break several times within a given time span of residence time. To know the centre of mass movement we calculated the cation-anion centre of mass residence times too. HEA showed highest COM residence time of 4423.3 ps. Unlike N-O residence times, COM residence times showed different order where HEA and HEHA COM residence times were close at 4423.3 and 4415.5 ps, respectively. EHA showed lowest COM residence time at 3623.2 ps. When the OH group is present on anion, it forms the hydrogen bond with O1, O2 (carboxylate) of anion which is not as strong as in HEA; moreover, it weakens the hydrogen bond between H-N and O1, O2 which in turn makes it possible for the ions to diffuse faster. We have also calculated the atom-atom residence autocorrelation functions of cation-cation and anion-anion and shown in Figure S7. Fitted data were presented in tables S6 and S7. Samanta and co-workers have studied the solvation dynamics of ILs and they observed the biphasic solvation dynamics. According their explanation fast component is ascribed to the motion of anions and the slow component is ascribed to the collective motion of the both cations and anions.85–88 Maroncelli et al. have explained that the fast component originates from local motion of the ions and the slow component comes from the diffusive behaviour of both cations and anions.89–92 Our findings of residence dynamics are in accord with these reported observations.

We have explored the effect of hydroxyl group functionalization of ions on structure and dynamics of ILs at ambient conditions. We have calculated the density, radial distribution functions and MSD from the trajectories. The densities of ILs with hydroxyl groups were found to be higher than the ILs that does not contain hydroxyl group. We have observed the significant changes in structural and dynamical properties with the addition of hydroxyl group. Cation-anion COM RDFs show changes in peak positions and heights. From the RDFs and SDFs, it was found that structural heterogeneity exists in these ILs. The structural organization occurs through hydrogen bonding between

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

hydroxyl groups and hydrogen bonding acceptor groups. We have also explored the structure and dynamics of hydrogen bond and found that interaction between ammonium hydrogen of cation and carboxylate oxygen of anion is strong. Hydrogen bond lifetimes and residence times also change with the addition of hydroxyl groups. Dynamics of constituent ions of IL strongly depends on structural organization. The highest residence time was found for HEHA, which has higher molecular weight than all other ILs considered for this study, and also it contains two hydroxyl groups. We have found that when hydroxyl group is present on cation, dynamics become very slow and it forms the strong hydrogen bond with carboxylate oxygen atoms of anion. Calculated structure factors also reveal the structural diversity of ILs with the hydroxyl group functionalization.

SUPPLEMENTARY MATERIAL

22 23 24 25 26 27 28 29 30 31 32

coefficients; Fitting parameters for residence time autocorrelation functions; Cation-anion centre of mass residence time autocorrelation functions; Site-site residence time auto correlation functions; Mean square displacements; Hydrogen bond autocorrelation functions; Hydrogen bond forward lifetimes; Input files for GROMACS simulation.

AUTHOR INFORMATION Corresponding Author *BSM: E-mail: [email protected]; Tel: +91 40 2301 7051; Fax: +91 40 2301 6032

33 5. Acknowledgements 34 35 36 37

The financial support for this work was provided by the Council of Scientific & Industrial Research, India. (01(2728)/13/EMR-II) Th. Dhileep N. Reddy likes to thank UGC, India for his PhD fellowship.

Coordination numbers and first minima; Nonbonding force field parameters for cations and anions; Self diffusion

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in the Ionic Liquid 1-Butyl-3-Methyl-Imidazolium Hexafluorophosphate. Chem. Phys. Lett. 2004, 396, 83–91.

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Figure 1: Ball-and-stick models of constituent ions of ILs. EA: Ethylammoniumcation, HEA : Hydroxyethylammoniumcation, ACT : Acetate anion, HACT : Hydroxyacetate anion. Simulation box of EHA is shown in the right side. Green color indicates the cation and orange color indicates the anion

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Figure 2: Comparison of calculated and experimental densities of four ionic liquids. The symbols, star and circle represent experimental and calculated data, respectively. The different colors for star symbol for HEA represent the data for the same IL, but different experimental references as mentioned in text.

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Figure 3: Centre of mass radial distribution functions of HEA (Black), EHA (red), HEHA (blue) and EAA (green). The top, middle, and bottom panels depict the cation-anion, cation-cation, and anion-anion centre of mass RDFs, respectively. Dotted lines indicate running number integrals and solid lines indicate RDFs.

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Figure 4. Structure factors of HEA, EHA, HEHA and EAA ILs are represented in black, red, blue and green color lines.

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Figure 5. Spatial distribution functions of the anions around the ammonium and hydroxyl groups of cations of ionic liquids. Brown and Blue colors of distribution represent ACT and HACT anions, respectively. Isosurface density values for SDFs (number of atoms per nm3) are 9.41 (EAA), 12.5 (HEA), 16.67 (EHA), 13.31 (HEHA).

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Figure 6. Radial distribution functions of most probable cation-anion hydrogen bonding interactions between oxygen and hydrogen atoms of ions.

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Figure 7. Spatial distribution functions of oxygen atoms of anion around cation. Orange and green colours indicate the oxygen atoms of carboxylate and hydroxyl groups of anions, respectively. Isosurface density values for SDFs (number of atoms per nm3) are 12.0 for all the atoms.

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Figure 8. Radial distribution functions between oxygen and hydrogen atoms of cation-cation (a, b) and anion-anion (c, d) pairs

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Structure and Dynamics of Hydroxyl ... - ACS Publications

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