How Water Permutes the Structural Organization and Microscopic

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

How Water Permutes the Structural Organization and Microscopic Dynamics of Cholinium Glycinate Bio-Compatible Ionic Liquid Aditya Gupta, Supreet Kaur, and Hemant K. Kashyap J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b10235 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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The Journal of Physical Chemistry

How Water Permutes the Structural Organization and Microscopic Dynamics of Cholinium Glycinate Bio-compatible Ionic Liquid Aditya Gupta, Supreet Kaur, and Hemant K. Kashyap∗ Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India E-mail: [email protected] Phone: +91-(0)11-26591518. Fax: +91-(0)11-26581102

Abstract We investigate the structural organization and microscopic dynamics of aqueous cholinium glycinate ([Ch][Gly]), a bio-compatible ionic liquid (IL), by employing all-atom molecular dynamics (MD) simulations. Herein, we observe the effect of water content on the molecularlevel arrangement of ions in the IL-water mixture through simulated X-ray scattering structure function, their partial components and real-space correlation functions. The study reveals the presence of a principal peak in the total structure function of the neat [Ch][Gly] IL at around q=1.4 ˚ A−1 . The corresponding correlation tends to decrease and shift towards shorter length scales with increase in the water content. It is found that the principal peak mainly originates from the correlations between counter ions. Hydrogen bond analysis reveals that water molecules compete with the anions to form hydrogen bond with the hydroxyl hydrogen of cation. Concomitantly, strong hydrogen bonding is also observed between [Gly]− anion and water, which depreciates with increasing hydration level. Hydrogen bond autocorrelation function analysis manifests that average lifetimes of different possible hydrogen bonds decrease with increase in mole fraction of water. The mobilities of the ions are also significantly affected by water, showing a non-linear increase with increasing water content. The [Gly]− anion is found to show faster dynamics on addition of water as compared to [Ch]+ cation.

1

Introduction

poses threat to aquatic organisms. 15 Various experimental studies have been performed on enzymes like acetylcholinesterase, 16 adenosine monophosphate (AMP) deaminase, 17 antioxidant enzymes like superoxide dismutase, catalase, glutathione peroxidase and glutathione-Stransferase, 18 revealing the toxic effects of ILs on environment. Stock et al. through half maximal effective concentration (EC50 ) analysis revealed that imidazolium and pyridinium cations based ILs inhibit acetylcholinesterase, indicating the toxicity of these IL cations for environment. 16 Wang and coworkers reported through

The proficiency of ionic liquids (ILs) in industry as well as technological applications is well known over recent times. 1–6 Although these solvents have various advantages over conventional solvents such as negligible vapor pressure and inflammability, 7–11 there is still a remarkable effort needed in order to design greener ILs. Their bio-compatibility, biodegradability, and toxicity have been major issues in the recent past. It has been found that some ILs are soluble in water 12–14 which

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The Journal of Physical Chemistry 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

median lethal dose (LD50 ) analysis that the exposure of 1-octyl-3-methylimidazolium bromide ([C8 mim][Br]) can cause enlargement and disorganization of mouse liver. 18 Also, it decreases the activity of the enzymes responsible for detoxification of hydrogen peroxide. 18 Antibacterial activity of ILs, 19 their cytotoxicity 20 and phytotoxicity, 21 their toxicity on algae, 22 invertebrates, 23 and vertebrates 24 have also been studied in depth. Ferraz et al. measured minimum inhibitory concentration values to show that the 1-hexadecylpyridinium ampicillinate ([C16 Pyr][Amp]) has high activity against gram-negative resistant bacteria. 19 Chemical degradation 25 and bio-degradability 26 of ILs has also been studied because of their high chemical and thermal stability. It has been shown through half maximal inhibitory concentration (IC50 ) investigation that cations impose higher toxicity than anion 17,20 and increase in alkyl chain increases the toxicity of the cation. 16 It has been also shown that fluoride anion has the most toxic effect in comparison to all other anions used in ILs, 20 due to which perfluoronated anion-based ILs are also environmentally unfriendly, as they get easily hydrolysed to give HF. 15,17,20 These major issues of concern such as biocompatibility and toxicity have encouraged the researchers to design the so called biocompatible ILs. As it is known that IL properties can be modulated by tweaking their ionic components, there are vast number of possibilities of ion combinations in order to design greener media. The molecular components of these ILs are of benign origin and hence they are renewable and non-toxic. 27 Also, these ILs are synthesized by following green procedures such as ion exchange. 28 Therefore they follow the green chemistry principles. 29 It has been found that quaternary ammonium based cations have the least toxicity, 30 for example, cholinium cation, which is already established as biodegradable and inexpensive ion. Choline is very important biological molecule because of its presence in phosphatidylcholine, which is lipid monomer, and acetylcholine, which functions as neurotransmitter i.e. helps in sending signals from nerve cells to other cells. Choline

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has been recommended to be included in daily diet as well. 31 The bio-compatible anions based ILs were introduced in 2005 by Ohno group 28 in which the anions were derived from 20 natural amino acids. Cholinium-amino acid based ILs were first synthesized by Calvino-Casilda and co-workers. 32 They have been proved to have low toxicity and high biodegradability. 33 Due to this they are used in different biological and industrial applications such as enzymatic hydrolysis of cellulose 34 and in aqueous two-phase systems for protein separation. 35,36 They are also used to increase the tensile strength of the aged cellulose paper. 37 Cholinium cation based ionic liquids are continuously finding more attention in research and observed to be promising in various applications 38 and clubbing it with glycinate anion enhances the bio-compatibility of such ILs. Thermophysical properties such as density, conductivity, and thermal analysis of cholinium ([Ch]+ ) glycinate ([Gly]− ) (see Figure 1 for their chemical structures) along with other cholinium-amino acid ILs have also been studied. 39 Considering the bio-nature of cholinium glycinate IL, the mixture of this IL with water have been employed in various applications in recent past. Yuan et al. did experiment on aqueous solution of this ionic liquid to study CO2 absorption and they observed that absorption loading increases with increase in IL concentration. 40 Among the wide biochemical applications of the aqueous mixture of this IL, the enhancement of catalytic action has been studied the most. Rodr´ıguez and co-workers performed differential scanning fluorimetry (DSF) and differential scanning calorimetry (DSC) and found that the mixture of cholinium glycinate IL and water increases the enzymatic activity of lipase, due to which they can be used in enzyme extraction and biocatalysis. 41 Recently Das et al. explained the reason for increase in catalytic activity of thermomyces lanuginosus lipase, a catalyst used for biodiesel production, when cholinium glycinate IL along with water is added to it. 42 By employing molecular dynamics (MD) simulations they showed that the presence of IL results into interfacial activation of enzyme, without the presence of any sub-


2

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CHARMM 46 force-field parameters were used for the IL molecules 42 whereas TIP3P 47 was used to model water molecules. All the parameters used in this study for cation and anion are given in Tables S1, S2, and S3 of the Supporting Information and partial charges were scaled down to +0.85e and -0.85e for cation and anion, respectively, as done in literature. 42 Periodic boundary conditions in all the three directions and minimum image convention were properly applied on the simulation boxes. Equations of motion were integrated using leap-frog algorithm with 1 fs time step. The cut-off radius for short-range interactions and real-space part of Coulomb interactions was set to 1.2 nm. Switching functions for the short-range interactions were used from 1.0 to 1.2 nm. Coulomb interactions were calculated using Particle Mesh Ewald 48,49 summation technique in which interpolation order of 6 and Fourier grid spacing of 0.08 nm were used. NPT simulations were carried out first for structural analysis. Equilibration for 10-30 ns (longer simulation for lower xw systems) was carried out at 298 K temperature and 1 bar pressure using Berendsen thermostat and barostat, respectively and subsequently extended for 20-30 ns at the same temperature and pressure with Nos´e-Hoover 50–52 thermostat and Parrinello-Rahman 53 barostat, respectively. Last 10 ns of trajectory saved at every 100 fs was used to compute all the structural properties. In order to study the dynamics of the system, the last configuration from NPT simulation was subjected to NVT simulation. Total simulation of 120 ns was run at same temperature (298 K) using same parameters and thermostat (Nos´e-Hoover). Trajectory from 3080 ns saved at every 50 fs was used for the analysis of mean-square-displacement (MSD). For calculation of velocity autocorrelation function (VACF), 80-120 ns of the trajectory, saved at every 10 fs was used. For xw =0.5 and 0.75, last 40 ns trajectory was divided into two blocks of 20 ns each and average conductivity was calculated. In case of xw ≤0.25, two independent simulations of 40 ns were carried out, frames saving frequency being the same, and both trajectories were averaged to determine the conductivity. Radial distribution function (RDF or g(r))

strate. Due to this the enzyme did not need to be activated again when substrate is present, which increases the catalytic rate. 42 This mixture has also been used by Zong group in rice straw pretreatment for removal of lignin and they observed that after the pretreatment, enzymatic digestion of lignocellulosic biomass increases, which is used for biofuels production. 43 As previously discussed, though the mixture of cholinium glycinate and water has been used for various applications, an extensive understanding of their structural and microscopic dynamics is scarce in literature. In this spirit, we have investigated the detailed structure and dynamics of mixture of cholinium glycinate ionic liquid with water for a wide composition range by utilizing all-atom MD simulation. We have mainly focused on X-ray scattering structure, hydrogen bonding interactions as in these systems all the species are capable of forming hydrogen bonds, and transport properties. Aim of our study is to observe the effect of water on the interactions between cholinium cation and glycinate anion. This has been achieved through the computation of structural correlation functions, mean square displacement, diffusion coefficient, velocity autocorrelation function, and conductivity. O

N

H2N

HO

O

(a)

(b)

Figure 1: Chemical structures of (a) cholinium ([Ch]+ ) cation and (b) glycinate ([Gly]− ) anion.

2

Simulation Details

All the MD simulations were performed using the GROMACS-5.1.1 44 program. Initial atomic coordinates for all the systems were generated by using Packmol 45 package. A total of 1000 molecules of IL + water (W) were taken in each system, covering range of mole-fractions of water (xw ) from 0.0 to 0.75.


3


and structure function (S(q)) provide an idea of density-density correlations in real-space and inverse-space, respectively. 54 X-ray scattering S(q)s sheds light on electron density-density correlations in inverse-space. The total X-ray scattering static structure function, S(q), and its partial components were computed using the methodology given in the literature. 55–64 Firstly, RDF, gij (r), between atoms of type i and j is calculated, which includes both intramolecular and intermolecular interactions. Then S(q) is computed by using eq 1.

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where h(t) is hydrogen bond population variable which corresponds to 1 if a particular hydrogen bond was originally hydrogen bonded at time t=0 and is still hydrogen bonded at time t, otherwise it corresponds to 0. In order to get an accurate estimate of average hydrogen bond lifetime, due to error in statistical averaging at longer time, the calculated C(t) is fitted by using combination of an exponential function and a stretched exponential function given as, C(t) = a1 exp[(−t/τ1 )] + a2 exp[(−t/τ2 )α ],

S(q) = ρ0

n P n P

xi xj fi (q) fj (q)

i=1 j=1

L/2 R

4 π r2 [gij (r) −

0

[

n P

xi fi (q)][

i=1

n P

1] sinq rq r ω(r) dr

xj fj (q)]

where α is stretching exponent and 0≤α≤1 and a1 +a2 =1. τ1 and τ2 are two time constants. Analytical integration of eq 4 gives us average lifetime as,

j=1

(1)

In eq 1, ρ0 is the total number density and equals to Natom / < V >. i and j are type of atoms whereas n is number of type of atoms. xi and xj are the mole-fractions of atoms of type i and j respectively. fi (q) and fj (q) denotes the X-ray atomic form factors for the calculation of X-ray scattering structure function of atoms of type i and j respectively. 65 L is the box length. ω(r) is Lorch window function, which is given by sin(2πr/L)/(2πr/L), 66,67 which dampens the effects of finite truncation of r. Which pair of ionic correlations are contributing to the principal peak of the total S(q)? To answer this question, we dissected the total S(q) as,

S(q) = S +

+ 2S [Ch]

[Ch]+ −[Ch]+

−[Gly]

−

(4)

(q)+S

[Gly]− −[Gly]−

+

(q) + 2S [Ch]

−W

(q)+S

W −W

1 , α

(5)

where Γ is the gamma function. Self diffusion coefficients of the ionic species for all the xw were computed by using MSD and VACF given as, 54 E 1 d D 1 2 |~ri (t) − ~ri (0)| = Di = lim 6 t→∞ dt 3

Z

∞

h~vic (t) · ~vic (0)i ,

0

(6)

where ~ri (t) is the vector coordinate of ith ion at time t and ~vic (t) is center of mass velocity of ith ion at time t. In order to confirm the diffusive regime in the MSD of ionic species for different systems, we have also computed β(t) function which is given as, 71 D E 2 d log |~ri (t) − ~ri (0)|

(q)

β(t) =

−

(q) + 2S W −[Gly] (q) (2)

d log t

(7)

This function tends to 1 for t in diffusive regime. The ionic conductivities of the mixtures were calculated by computing the current autocorrelation functions and using liner-response GreenKubo formulation as, 54,72

Hydrogen bond autocorrelation was calculated using, 68–70

C(t) = hh(0) h(t)i / h(0)2 ,

a2 τ2 Γ hτ i = a1 τ1 + α

(3)


4

Page 5 of 23

1 3 kB T V

∞

Z

D E ~ · J(0) ~ J(t) dt

(8)

0

2

S(q)

~ is electric current which is given In eq 8, J(t) by, ~ = J(t)

N ion X

qi ~vic (t),

(9)

i=1

1

1

0

0

-1

-1

-2 -3

-3 -4 0.5

1

1.5

2

-0.5

S(q)

S(q)

-1 -2

-1

-3

-3 -4 1.5

2

2.5

-5 0

3

0.5

1

xw=0.2

1

1

0

0

-1 -2

-1 -2

-3

-3

-4

-4 1.5

2

2.5

-5 0

3

0.5

1

2

2.5

3

-1

S(q)

q(Å ) Figure 2: Simulated total X-ray scattering structure function, S(q) for aqueous cholinium glycinate ([Ch][Gly]) at different mole-fractions of water.

xw=0.5

1

0

0

-1 -2

-3 -4 1.5 -1

q(Å )

(g)

2

2.5

3

xw=0.75

-1

-3

1

3

-2

-4 0.5

2.5

(f) 2

1

-5 0

2

-1

(e)

1.5

1.5

q(Å )

2

1

3

xw=0.25

-1

0.5

2.5

(d) 2

q(Å )

-1.5

2

-1

(c)

1

1.5

q(Å )

2

0.5

3

-2

-4

-5 0

2.5

xw=0.15

-1

-1

-2 0

0

S(q)

0

0

S(q)

0.5

1

1

2

(b) xw=0.1

0.5

1.5

2

q(Å )

S(q)

1

1

-1

1

-5 0

xw=0.0 xw=0.05 xw=0.1 xw=0.15 xw=0.2 xw=0.25 xw=0.5 xw=0.75

-

q(Å )

(a)

Simulated X-ray Scattering Structure Functions, S(q) 1.5

0.5

-1

Results and Discussion

3.1

-5 0

3

-

[Gly] -[Gly] + [Ch] -[Gly] + + [Ch] -[Ch] + [Ch] -W W-[Gly] W-W total

q(Å )

2

3

2.5

xw=0.05

-2

-4 -5 0

where qi is charge on the ith ion and Nion is the number of ions.

2

xw=0.0

S(q)

σCACF =

S(q)

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


-5 0

0.5

1

1.5

2

2.5

3

-1

q(Å )

(h)

Figure 3: Partial structure functions for xw = (a) 0.0, (b) 0.05, (c) 0.1, (d) 0.15, (e) 0.2, (f) 0.25, (g) 0.5, and (h) 0.75. Respective total structure functions have also been plotted.

Figure 2 shows the simulated total X-ray scattering structure function, S(q), at different mole-fractions of water. Below 3 ˚ A−1 , we observe two peaks for all the mole-fractions of water studied. The first or principal peak position lies in between 1.3 and 1.8 ˚ A−1 and the second peak position varies in between 1.8 and 2.6 ˚ A−1 .


5


(a)

(b)

(e)

Page 6 of 23

(c)

(f)

(d)

(g)

Figure 4: Equilibrium snapshots of the simulation boxes of aqueous cholinium glycinate for xw = (a) 0.0, (b) 0.25, (c) 0.5, and (d) 0.75. Figures (e), (f) and (g) depict equilibrium snapshots rendered with water only for xw =0.25, 0.5, and 0.75, respectively. [Ch]+ is shown in violet color, [Gly]− is shown in red color, and water arrangement is shown in cyan colored isosurface. Hydrogen atoms of the cation are excluded for clarity.

For pure IL, the principal peak occurs at around q = 1.4 ˚ A−1 , which corresponds to a characteristic distance of 4.5 ˚ A in real-space. We observe that with increase in the mole-fraction of water, this peak shifts towards higher q value and its intensity also goes down. The shift of this peak towards higher q values while ascending towards the water-rich regime depicts decrease in corresponding length scale. However, the second peak position and intensity are observed to be less sensitive to the water content. This gradual change in intensity of the principal peak of S(q) with dilution manifests changes in the IL native structure, signifying the importance of water content on the structural organization of the mixture. On reaching xw =0.75, S(q) renders a broad peak, showing that the correlations that give rise to principal peak have been affected to a large extent. In order to further understand the molecular basis for the existence of the peaks observed in

total S(q), we have also computed the partial structure functions using the scheme given in eq 2. We observe from Figure 3 that major contribution to the principal peak is by [Ch]+ [Gly]− pair correlations which decreases with increase in mole-fraction of water, due to which the intensity of the principal peak in the total S(q) decreases. One of the interesting features in partial S(q)s shown in Figure 3(a) is the observation of peaks and antipeaks at around 1 ˚ A−1 for correlations involving only ionic species; positive contributions from [Ch]+ -[Ch]+ and [Gly]− -[Gly]− and negative contribution from [Ch]+ -[Gly]− . These observations clearly demonstrate the presence of charge ordering, which is a generic feature of ionic liquids and high-temperature ionic melts. 1,56,58,73–77 These peaks and anti-peaks in partial S(q)s arise due to some kind of charge density alternations or fluctuations that are off-phase. 55,58,59 These alternations are further


6

8

3.2

xw=0.0 xw=0.05 xw=0.1 xw=0.15 xw=0.2 xw=0.25 xw=0.5 xw=0.75

g(r)

6

4

2

0 0

2

HO[Ch]+-O[Gly]-

0.25

0.5

0.75

1

HO[Ch]+-OW

1.5

g(r)

supported by center of mass radial distribution function calculated between charge centers (group of atoms where major positive and negative charge is localized) of cation and anion for all xw shown in Figure S2 of the Supporting Information. Cationic (N(CH3 )3[Ch]+ ) and anionic charged groups’ (COO[Gly]− ) self correlations are observed to be oscillatory in the same phase, while their cross correlation fluctuates in different phase. This difference in phase has also been reported in the literature 78 and it gives rise to peak and anti-peak in the corresponding partial structure functions. However, in many cases 1,60,73,77 they cancel out each other and therefore one does not observe any charge ordering peak in the total structure function. This phenomena is true for [Ch][Gly] IL and it persists even after dilution upto xw =0.75. Further, we can see that on dilution of neat IL, there is no significant change in peak intensity or peak position of the correlations till xw =0.25, which corroborates with our observation in total S(q). On diluting the IL further, peaks and anti-peaks due to ionic species starts shifting to lower q values and the amplitudes of the peaks begin to decrease. This means that on addition of water beyond xw =0.25, ionic correlations shifts to long-distance but with weaker strength. Correlations due to water ([Ch]+ -W, [Gly]− -W, and W-W) also begin to emerge in lower q regime as well as in higher q regime (0.75≤q/˚ A−1 ≤1.2). Thus water plays a major role in deciding the structure of IL in higher water mole-fraction regime, whereas for lower xw , the structure of the mixtures is mostly ILlike. A qualitative understanding of how ions and water molecules are arranged in these mixture can be gleaned through the representative snapshots of the boxes for xw =0.0, 0.25, 0.5, and 0.75 shown in Figure 4.

1

0.5

0 0

1.25

0.25

r (nm)

(a) 1.2

0.5

0.75

1

1.25

r (nm)

(b) 1.2

HN[Gly]--O[Ch]+

0.8

HN[Gly]--OW

0.8

g(r)

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


g(r)

Page 7 of 23

0.4

0 0

0.4

0.25

0.5

0.75

r (nm)

(c)

1

1.25

0 0

0.25

0.5

0.75

1

1.25

r(nm)

(d)

Figure 5: Radial distribution function, g(r), for cholinium hydroxyl hydrogen (HO[Ch]+ ) with (a) glycinate oxygens (O[Gly]− ) and (b) water oxygen (OW ), and glycinate amine hydrogen (HN[Gly]− ) with (c) cholinium oxygen (O[Ch]+ ) and (d) water oxygen (OW ) for all the mole fractions of water.

tion is observed between [Ch]+ and [Gly]− for the whole composition range studied here (Figure 5(a)). While cholinium cation can act as hydrogen bond donor through its hydroxyl hydrogen, the glycinate anion acts as acceptor through its carbonyl oxygens. This interaction is diminished with increase in the water content. In Figure 5(b), we observe that at the same time, cation hydroxyl group is also capable of acting as hydrogen bond donor to water oxygen, thus rendering strong hydrogen bond interaction with water as well. However, unlike the previous one, the strength of this hydrogen bonding interaction is found to be increasing with increase in xw . Figures 5(c) and 5(d) depict that glycinate amine group shows slight hydrogen bonding interaction with water, whereas that with cation is almost negligible. Hence, [Gly]− is not able to act as significant hydrogen bond donor. As we can see in Figure 6, water molecules form strong hy-

Hydrogen Bonding and Real Space Correlations

Since all the species in the mixtures studied are capable of forming hydrogen bonds, we first focused on the atomic RDFs involving hydrogen bonding and depicted those in Figures 5 and 6. Significant hydrogen bonding interac-


7


1

0.5

0.5

0.25

40

30

0.2

30

0.15 20 0.1 10 0

0.05 0

0.1

0.2 0.3 r (nm)

0.4

0.5

0.05 0.04

20

0.03 0.02

10 0

0

0.06

0.01 0

0.1

0.2 0.3 r (nm)

(a) 1

0 0

1.25

0.25

0.5

r (nm)

0.75

1

1.25

(a) 12

HW-O[Gly]-

g(r)

30 20 10 0

9

0

0.1

0.2 0.3 r (nm)

0.014 0.012 0.01 0.008 0.006 0.004 0.002 0

0.4

0.5

40 30 20 10 0

0

0.2 0.3 r (nm)

1

(c)

20 10 0

0

0.1

0.2 0.3 r (nm)

0.4

0.5

40 30 20 10 0

0

0.1

0.2 0.3 r (nm)

(e)

Figure 6: Radial distribution function, g(r), of water hydrogens (HW ) with (a) water oxygen (OW ), (b) cholinium oxygen (O[Ch]+ ), and (c) glycinate oxygens (O[Gly]− ) for all the mole fractions of water.

0.5

0.009 0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0

g(r, θ)

0.8

0.035 0.03 0.025 0.02 0.015 0.01 0.005 0

g(r, θ)

0.6

r (nm)

30

θ (degree)

0.4

θ (degree)

0.2

0.4

xw = 0.5 HW-OW-O[Ch]+

40

3

0.018 0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0

(d)

xw = 0.5 HW-OW-OW

0 0

0.1

(c)

6

0

xw = 0.5 H[Gly]--N[Gly]--OW

40

(b)

0.5

(b)

xw = 0.5 H[Gly]--N[Gly]--O[Ch]+

r (nm)

0.4

g(r, θ)

0.75

g(r, θ)

0.5

θ (degree)

0.25

θ (degree)

0 0

g(r, θ)

0.75

0.25

g(r, θ)

1

xw = 0.5 H[Ch]+-O[Ch]+-OW

40

0.4

0.5

(f) xw = 0.5 HW-OW-O[Gly]-

θ (degree)

40

0.3 0.25

30

0.2

20

0.15 0.1

10 0

g(r, θ)

g(r)

1.5

xw = 0.5 H[Ch]+-O[Ch]+-O[Gly]-

HW-O[Ch]+

θ (degree)

2

1.25

HW-OW

θ (degree)

xw=0.05 xw=0.1 xw=0.15 xw=0.2 xw=0.25 xw=0.5 xw=0.75

g(r)

2.5

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0.05 0

0.1

0.2 0.3 r (nm)

0.4

0.5

0

(g)

drogen bond network within themselves, which enhances with increasing water concentration. This signifies that the hydrogen bond network of water is also altered in the presence of ionic liquid. On the other hand, the hydrogen bond donor capability of water is negligible with the cation, but it is very significant with the anion. Also, the hydrogen bonding between water and [Gly]− tends to decrease at higher water molefraction. Hence, we conclude by noting that water molecules interact with cation and anion in an opposite manner. All the aforementioned observations can also be gleaned from Figure S10 in which we have shown the change in number of hydrogen bonds (per IL/water according to the observed hydrogen bond donor species) with time for all the mentioned hydrogen bonding RDFs in the Supporting Information. We have also investigated distance-angle distribution function, g(r,θ), following the methodology present in the literature, 79 to quantify the presence of hydrogen bonding as observed through real space correlations. Figure 7 shows

Figure 7: Distance-angle distribution function, g(r,θ), for different possible hydrogen bonding pairs present in xw =0.5 system. Refer to the Figures S3-S9 for the g(r, θ) plot for other compositions of the mixture.

the distribution of H-D· · · A for all the possible hydrogen bonded groups in xw =0.5 system. The maximum distance-angle distribution function probability is found to be in accordance with the hydrogen bond geometrical criteria i.e 6 HDA≤30◦ and r(DA)≤0.35 nm, confirming the presence of hydrogen bond interactions predicted through RDFs shown in Figures 5 and 6. Figures 7(a), 7(b), and 7(g) show that the intensities of g(r,θ) for H[Ch]+ -O[Ch]+ -O[Gly]− , H[Ch]+ -O[Ch]+ -OW , and HW -OW -O[Gly]− triplets around r∼0.27 nm and 0◦

How Water Permutes the Structural Organization and Microscopic

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