Aqueous Mixtures of Room-Temperature Ionic Liquids: Entropy-Driven

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Aqueous Mixtures of Room-Temperature Ionic Liquids: Entropy-Driven Accumulation of Water Molecules at Interfaces Takeshi Kobayashi, Andre Kemna, Maria Fyta, Björn Braunschweig, and Jens Smiatek J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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The Journal of Physical Chemistry

Aqueous Mixtures of Room-Temperature Ionic Liquids: Entropy-Driven Accumulation of Water Molecules at Interfaces Takeshi Kobayashi,† Andre Kemna,‡ Maria Fyta,† Björn Braunschweig,‡ and Jens Smiatek∗,†,¶ †Institute for Computational Physics, University of Stuttgart, D-70569 Stuttgart, Germany ‡Institute of Physical Chemistry, University of M¨ unster, D-48149 M¨ unster, Germany ¶Helmholtz Institute M¨ unster: Ionics in Energy Storage (HIMS – IEK 12), Forschungszentrum J¨ ulich GmbH, D-48149 M¨ unster, Germany E-mail: [email protected]

Abstract This work investigates the influence of uncharged interfaces on the distribution of water molecules in three aqueous dialkylimidazolium-based ionic liquid mixtures at various water concentrations. The results are based on atomistic molecular dynamics (MD) simulations supported by sum-frequency generation (SFG) experiments. All outcomes highlight an entropically-driven accumulation of water molecules in front of interfaces with slight, but technologically relevant differences. Our findings reveal that the local water density depends crucially on the water mole fraction, local ordering effects, and the molecular structure of the ionic liquids (ILs). We unravel the influence of hydrophobicity/hydrophilicity and bulkiness of the ions, as well as the effect of water in defining the role of the ILs as a main solvent, a co-solvent or co-solute. The

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outcome of this study allows the definition of reliable criteria for beneficial water-IL combinations in view of distinct applications.

1. Introduction Neat room-temperature ionic liquids (ILs) offer a broad plethora of beneficial solvent properties including low flammabilities, low volatilities, high ionic strengths and wide electrochemical stability windows 1–6 . In combination with high affinities for various polar and apolar solutes, recent applications of ILs thus range from electrolyte components in electrochemical devices and via long–time stable storage media for enzymes to inert and environmentally benign solvents for chemical synthesis procedures 3,4,4,6–18 . In addition to the use of neat ILs, also the individual ionic components are often employed as co–solvents or co–solutes in protic and aprotic solvent mixtures 3,10,11,17,19,20 . Due to recent and upcoming technological challenges 3,10–13,21,22 , a specific interest in aqueous IL mixtures has emerged rapidly over the last years 23–38 . With regard to this point, various technologically motivated research studies reported conflicting interpretations on the role of water and water interfaces in ILs. On one side, the favorable presence of water as a co-solvent or co-reactant increases enzymatic activities 11 and enhances CO2 electroreduction processes 14–16 , whereas even a spurious amount of humidity in electrochemical storage devices considerably limits the applicability of IL electrolyte solutions 22,35 . In more detail, previous simulation and experimental studies reported that so-called water interfaces at highly charged electrodes form, which initiate electrochemical decomposition processes and hence reduce significantly the life time of electrochemical cells 22,35,39 . In contrast to these shortcomings, the beneficial formation of water interfaces at silver electrodes was shown to foster CO2 electroreduction mechanisms, thereby establishing a higher efficiency of degradation processes 14 . As a consequence, whether the occurrence of water molecules at charged or uncharged interfaces is considered as a desired or undesired 2


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effect depends crucially on the underlying technological purpose. Despite the relevance of these findings, our understanding of water behavior in ILs is rather shallow. Hence, it is yet not fully clarified which mechanisms or molecular properties initiate water clustering effects 36–38 . Our limited knowledge can be mostly attributed to the complex interplay of distinct ion combinations and the occurrence of structuring phenomena, as reported for bulky hydrophobic cations in combination with polar anions 40,41 . The presence of further components like polar or apolar co-solutes or confinement effects gives rise to even more complex solutions resulting in various distinct outcomes and unclassified observations 17,18,31,40,42–46 . Up to this point, we have mostly discussed previous results on the properties of lowconcentrated water molecules in ILs. However, also the opposite case of low-concentrated ILs in bulk water has received considerable attention. As an important example, a lot of research effort was directed on aqueous or hydrated ILs, which either increase or decrease the structural stability of proteins 10–13,19,21,47–50 . Recent explanations rely on the origins of preferential binding and exclusion mechanisms around interfaces in terms of a statistical thermodynamics framework 10,13,20 . Despite their technological relevance, most of these effects are also only sparsely understood 10,12,13 . With regard to previous theoretical, numerical, and experimental approaches for solvent-IL mixtures at various mixing ratios 17,18,23,25,29,33,35–39,43,51–65 , the question arises whether only charged interfaces favor an attraction of water molecules 35,39,43 or whether water accumulation can also occur at uncharged interfaces? Moreover, what causes the differences in the solvent distribution for distinct ILs 31,43–46 , and what is the contribution of concentration-dependent or specific ion effects 36–38 ? It may be assumed that the answers to these questions pave a rational way towards an improved use of IL solutions for various technological purposes. In order to study the properties of such mixtures in more detail, we perform atomistic MD simulations for distinct aqueous IL solutions, namely 1-ethyl-3-methylimidazolium dicyanamide (EMIM/DCA), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM/BF4 ) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM/BF4 ) at various water mole fractions

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between xH2 O = 0 (neat IL) and xH2 O = 0.99 (highly diluted aqueous IL solution). We focus specifically on the technologically relevant distribution of water molecules in front of uncharged walls (solid-liquid interfaces) under the influence of planar and spherical anions and cations with a varying alkyl side chain length. The presence of these interfaces introduces fixed reference positions and further allows us to study the corresponding distributions in terms of a Kirkwood-Buff (KB) based approach with a specific focus on excluded-volume contributions 66,67 . Our study thus provides a robust analysis of water effects in various ILs with a clear separation between bulk and local effects. As our results will show, the molecular properties like polarity and size play a decisive role. The distinct outcomes for the IL-water mixtures highlight the crucial role of specific ion and local ordering effects. Based on our findings, we are able to define reliable criteria for tunable water-IL solutions whose implications may help to improve recent technological applications. All numerical findings are supported by experiments on gas-liquid interfaces using sum-frequency generation (SFG) spectroscopy. Although the experimental setup differs from our simulation approach, the corresponding distributions of species in front of distinct interfaces reveals a qualitative agreement. This can be explained by the weak interaction of ion species with both interfaces and the resulting comparable water accumulation behavior.

2. Methods 2.1. Simulation Details All atomistic molecular dynamics (MD) simulations were performed with the GROMACS 5.1.3 package 68–70 . We used OPLS/AA force fields 71–73 for all ions in combination with the SPC/E force field 74 for water molecules. Different water weight fractions xH2 O = {0.000, 0.125, 0.250. 0.375, 0.15, 0.500, 0.625, 0.750, 0.875, 0.950, 0.960, 0.970, 0.980, 0.9904} in distinct ILs EMIM/DCA, EMIM/BF4 and BMIM/BF4 were randomly inserted into rectangular simulation boxes of dimensions 6.3 nm – 6.5 nm in periodic x, y–direction and 14.5 nm in z– 4


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direction using the software package PACKMOL 75 . The z–direction was constrained by two impenetrable silicon walls with Lennard-Jones (LJ) 9-3 potentials. In all simulations, the temperature was maintained at T = 300 K by an improved velocity-rescaling thermostat 76 , using a coupling time constant of 0.1 ps. The pressure was kept constant at p = 1 bar by a semi-isotropic Parrinello-Rahman barostat 77 (periodic x– and the y–dimensions with fixed z–dimension) with coupling time constant 2 ps and compressibility 4.5 · 10−5 bar−1 . Electrostatic interactions were treated through the Particle Mesh Ewald (PME) method 78,79 , where a real-space cut-off of 1.0 nm and a grid spacing of 0.16 nm with fourth-order interpolation scheme were used. LJ interactions were truncated at 1.0 nm and shifted to zero. The equations of motion were integrated by the Leapfrog algorithm with an elementary time step of 2 fs. All bonds were constrained by the LINCS algorithm 80 . An energy minimization was first performed using a conjugate-gradient method, followed by an equilibration period of 10 ns under constant volume-constant temperature (NVT) conditions, and a subsequent equilibration run of 10 ns under constant temperature and constant pressure (NpT) conditions. The final NpT production runs had a length of 200 ns each, except for the lowest water concentrations, which had a length of 500 ns. Positions and velocities of atoms were stored every 10 ps. A molecular snapshot of a sample mixture (EMIM/DCA with a water mole fraction of xH2 O = 0.25 in the presence of two silicon walls) and the chemical structures of all ion species are shown in Fig. 1.

2.2. Experimental Details All ILs EMIM/BF4 (>99 %), EMIM/DCA (>98 %) and BMIM/BF4 (>99 %) were purchased from Iolitec (Germany) and were used as received. IL–water mixtures for SFG spectroscopy were prepared by mixing the ILs with ultrapure water (Milli-Q Reference A+, 18.2 MΩ cm, TOC < 5 ppb) and were stirred until a clear and homogeneous solution was obtained. Glassware and all necessary equipment which came in contact with the solutions were cleaned in a mixture of 98% sulfuric acid (Carl Roth, Germany) and Nochromix (Godax Labs, USA) 5



3

2

1 3

2 4

5 EMIM+

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4

1 5

BMIM+

DCA-

BF4-

Figure 1: Top: A snapshot of a water-EMIM/DCA mixture with a water mole fraction of xH2 O = 0.25. The uncharged silicon walls are represented through the blue boxes on either ends of the figure. Water molecules are shown in a van-der-Waals representation, whereas DCA− is colored in cyan and EMIM+ in gray. Bottom: The molecular structures of the BMIM+ and EMIM+ cations with labels for atoms in the imidazolium ring. The molecular structure of the anions DCA− and BF− 4 is depicted on the right side. Hydrogen, nitrogen, and carbon atoms are colored in white, blue, and gray, respectively. The fluoride atoms in BF− 4 are represented as light blue spheres, whereas the central boron atom is colored in light red.

and were subsequently thoroughly rinsed with ultrapure water and dried in a stream of N2 gas (>99.999, Westfalen). SFG spectra from a homebuilt broadband SFG setup as described elsewhere 81 were recorded in the frequency region of C-H and O-H stretching bands between 2700 – 3800 cm−1 . The center frequency of the broadband (> 300 cm−1 ) femtosecond IR pulse was tuned in four steps, and the SFG intensities of each frequency were acquired for 2 min in an ssp (SFG/VIS/IR) polarization combination. All spectra were normalized to the SFG signal from an air-plasma cleaned polycrystalline Au film with polarizations set to ppp.

3. Results 3.1. Atomistic Molecular Dynamics Simulations In terms of the molecular structure at the solid-liquid interface with regard to all neat bulk ILs (xH2 O = 0), the local normalized atomic number densities ρN of the IL (z) = ρIL (z)/ρIL

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combined cations and anions are shown in Fig. 2, where ρbulk denotes the ion density in IL the middle of the channel (bulk phase) and ρIL (z) is the local ion number density at a distance z from the wall. As can be seen, conspicuous ion-shell or layer structures as marked by density peaks disappear on length scales around z ≈ 1.5 – 2 nm, which is in good agreement with previous findings for other ILs 17,18 . In a close distance of z ≈ 0.25 nm to the wall, the highest ion density can be observed for EMIM/DCA, followed by EMIM/BF4 and BMIM/BF4 . Additionally, EMIM/DCA also shows the highest packing fraction through the short distances between the peaks. Due to the close similarity between density profiles for EMIM/DCA EMIM/BF4 BMIM/BF4

2.5 2 ρNIL(z)

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


1.5 1 0.5 0 0

0.5

1

1.5

2

2.5

z [nm]

Figure 2: Normalized local atomic number density ρN IL (z) of the combined cation and anion species in neat ILs with xH2 O = 0 at distances z from the wall for EMIM/DCA (red line), EMIM/BF4 (blue line), and BMIM/BF4 (black line). The green horizontal line shows ideal bulk value behavior with ρN IL (z) = 1. EMIM/BF4 and BMIM/BF4 , it can be concluded that different anions are mainly responsible for distinct density distributions and packing densities. Further inspection also reveals that the peak height of the first ion shell in EMIM/DCA is significantly higher when compared to the other ILs. This finding can be attributed to the spatial orientation of DCA− anions, which enhances a more compact and combined ion distribution. The corresponding outcomes for the distribution of the individual ion species in the neat ILs as shown in in the supplementary material underpin this assumption. As can be seen, DCA− ions reveal a more ordered structure at short wall-distances when compared to BF− 4 , thereby inducing a more compact IL layer structure. In more detail, DCA− ions orient parallel 7



to the interface whereas BF− 4 with its spherical shape leads to a broader first layer with distinct ion orientations. The corresponding double-peak structure for BF− 4 ions at z ≤ 0.7 nm when compared to DCA− can be attributed to the various orientations of the anion. Thus, DCA− ions can be found in a more well-ordered orientation parallel to the interfaces + when compared to BF− 4 . Noteworthy, the EMIM distribution of EMIM/DCA is also more

ordered than EMIM+ from EMIM/BF4 . Hence, it can be concluded that the distribution of anions influences the distribution of cations and vice versa. It can be thus assumed that the observed differences between the density profiles can be mainly assigned to the molecular volume, as well as the planar (DCA− ) or spherical (BF− 4 ) shape and arrangement of the anions. With regard to this point, the values for the molecular volumes Vm of all ions in combination with the octanol–water partition coefficients log10 P (Tab. 1) with P = csC8 OH /csH2 O where csC8 OH and csH2 O denote the corresponding concentration of the ions in octanol and water phase, respectively, support this view. The smaller sizes of DCA− and EMIM+ in comparison Table 1: Molecular volumes Vm and octanol–water partition coefficients log10 P for the individual ion species and water molecules as calculated by Ref. 82. Molecular volumes are obtained by fitting the sum of fragment contributions for a training set of about twelve thousand molecules after optimization using the semiempirical AM1 method 83 . Species EMIM+ BMIM+ DCA− BF− 4 H2 O

Vm [nm3 ] log10 P 0.118 -3.10 0.152 -2.04 0.056 -3.34 0.073 -2.60 0.019 -0.29

+ to BF− are eminent and thus rationalize a higher local packing fraction for 4 and BMIM

EMIM/DCA when compared to the other ILs. As expected, smaller ions also reveal a lower hydrophobicity, thereby establishing the following ordering scheme DCA− > EMIM+ > + BF− with decreasing polarity. Hence, it can be expected that DCA− anions 4 > BMIM

and EMIM+ cations show a stronger water binding behavior when compared to BF− 4 and BMIM+ . Interestingly, the distribution of the ions does not significantly change for low water 8


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contents of xH2 O = 0.125 when compared to neat ILs as can be seen in the supplementary material. The position of the ion shells remains nearly identical and the long-range decay of ordering differs slightly. In order to study the amount of ion ordering and packing effects in presence of water in more detail, one can define a translational order parameter for species α, either combined anions and cations (index ’IL’) or water molecules (index ’H2 O’), reading

Ozα

2xα = Lz

Lz /2

Z

dz ([γα (z) ln γα (z)] − [γα (z) − 1])

(1)

0

with γα (z) = ρα (z)/ρbulk α , which is closely related with the expression for the translational entropy as introduced in Refs. 84,85. Notably, for a reliable evaluation of Eqn. (1), a bulk behavior with γα (z) ≈ 1 for z → Lz /2 has to be guaranteed. This is achieved in our simulations by setting large distances between the walls. As a consequence, the corresponding value of Ozα provides an estimate for the degree of translational ordering in terms of the considered species. The corresponding values in Fig. 3 for neat ILs (xH2 O = 0) are OzIL = 0.82 (EMIM/DCA), OzIL = 0.63 (EMIM/BF4 ) and OzIL = 0.51 (BMIM/BF4 ), which highlight the fact that the highest degree of ion order is given for EMIM/DCA, followed by EMIM/BF4 and BMIM/BF4 . Although the order parameter decreases significantly for all ILs with increasing EMIM/DCA EMIM/BF4 BMIM/BF4

1 0.75 0.5 OzIL

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


0.25

0.1

0.25

0.5

0.75

1

xH O 2

Figure 3: Order parameter values OzIL for combined ions at various water mole fractions xH 2 O .

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water content, thereby implying an increase of the translational entropy, the corresponding values reveal that up to a water mole fraction of xH2 O = 0.9 the corresponding ordering scheme remains identical to neat ILs. In contrast, at higher water mole fractions xH2 O ≥ 0.92 the order parameter increases for all ILs. A more detailed explanation for these observations and differences between the ILs will be presented in the remainder of this article. Further analysis of the running order parameter Ozα (z) (Fig. 4) for neat ILs implies that the largest contributions to the integral in Eqn. (1) come from short distances of around 1 nm in front of the channel walls. In contrast, the contributions from large distances are rather negligible 8

EMIM/DCA EMIM/BF4 BMIM/BF4

7 6 OzIL(z) [nm]

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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5 4 3 2 1 0 0

0.5

1 z [nm]

1.5

2

Figure 4: Running order parameter values OzIL for combined ions in neat ILs. such that OzIL converges to constant values for all z > 1 nm. The corresponding analysis reveals that the highest ordering of ion species and vice versa the lowest translational entropy 84 can be attributed to the interfacial regions. The question which now arises is what is the water content of distinct mixtures in close vicinity of the interfaces? As we have already discussed in the introduction, the outcomes are of crucial importance for various technological applications. With regard to this point, the fraction of water molecules KH2 O (∆) in the first solvent shell within wall distances z ≤ ∆ can be calculated by

KH2 O (∆) =

10

ρH2 O (∆) ρall (∆)


(2)

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where ∆ denotes the first minimum in the local total number density ρall (∆) = ρIL (∆) + ρH2 O (∆) with ∆ = 0.38 nm for EMIM/DCA and ∆ = 0.44 nm for EMIM/BF4 or BMIM/BF4 , respectively. The corresponding results for all water mole fractions xH2 O are depicted in Fig. 5. Interestingly, the local fraction of water molecules at the interface for all mole fractions xH2 O ≤ 0.875 is the highest for EMIM/DCA, followed by EMIM/BF4 and BMIM/BF4 . Accordingly, the neat IL EMIM/DCA with the highest local order is related to the highest water content. The corresponding net differences in the water content at the interface between the ILs remain valid up to mole fractions of xH2 O ≤ 0.75, implying the robustness of this observation. As can be seen, water molecules are an integral part of the mixed first cation-anion layer of the solution at least for EMIM/DCA. At high water mole fractions with the definition KIL (∆) = 1 − KH2 O (∆), it follows that the local fraction of ions KIL (∆) at xH2 O ≥ 0.98 is the highest for BMIM/BF4 (KIL (∆) = 0.52), followed by EMIM/DCA (KIL (∆) = 0.34) and EMIM/BF4 (KIL (∆) = 0.33). With regard to these values, a significant 1

EMIM/DCA: ∆=0.38 nm EMIM/BF4: ∆=0.44 nm BMIM/BF4: ∆=0.44 nm

0.1

2

KH O(∆)

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


0.01 0.1

0.2

0.3

0.4

0.5 0.6 xH O

0.7

0.8

0.9

1

2

Figure 5: Fraction of water molecules KH2 O (∆) for all water mole fractions xH2 O in the first solvent shell at wall distances z ≤ ∆. The results for EMIM/DCA, EMIM/BF4 , and BMIM/BF4 , are shown in red (squares), blue (circles), and black (triangles), respectively. amount of ions accumulates at short distances in front of the wall. As a specific example, the local number densities of ions and water molecules for a specific mole fraction of xH2 O ≥ 0.98 are shown in the supplementary material. In order to study the role of the ions at various water concentrations, we compute the slightly

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modified one-dimensional and distance-dependent Kirkwood-Buff (KB) integrals 10,66,67,86 Lz /2

Z

dz [γα (z) − 1]

GWα = Lx Ly

(3)

0

whose values can be interpreted as excess volumes of species when compared to bulk phase 10,13 . The preferential hydration coefficient 10,87 as an estimate for the hydration tendency of the walls is given by Γ = ρ0H2 O ∆VH2 O , where ρ0H2 O denotes the total water density and ∆VH2 O = GWH2 O − GWIL the corresponding differences between the water and the ion excess volumes. Positive values of Γ imply a water attraction behavior and negative values a water exclusion effect of the interfaces. Per definition ρ0H2 O ≥ 0, thus the positive or negative sign of Γ is solely determined by the difference in the excess volumes. In that respect, ∆VH2 O > 0 implies a preferential attraction of water molecules, whereas ∆VH2 O < 0 highlights an exclusion effect 10,13 in terms of strong non-ideal solutions.

50 0 ∆VH O [nm3] 2

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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-50 -100 -150 -200

EMIM/DCA EMIM/BF4 BMIM/BF4 0

0.2

0.4

0.6

0.8

1

xH O 2

Figure 6: The differences in the excess volumes ∆VH2 O for distinct water mole fractions xH2 O in EMIM/DCA (red line with squares), EMIM/BF4 (blue line with circles) and BMIM/BF4 (black line with triangles).

The corresponding results in Fig. 6 reveal positive values for ∆VH2 O for all ILs below water mole fractions xH2 O < 0.6. The largest values of the excess volumes can be observed for BMIM/BF4 , followed by EMIM/DCA and EMIM/BF4 . These findings are valid for low and moderate water content xH2 O < 0.6 whereas for water mole fractions xH2 O > 0.9 a 12


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steep decrease of ∆VH2 O to even negative values can be observed. The latter implies that a significant portion of water molecules is replaced by the ions. The change from positive to negative values thus provides a clear separation on the role of ILs in terms of a transient behavior from a solvent to a co-solvent or even a co-solute. Based on our findings, the considered ILs can be regarded as a co-solute for IL mole fractions xIL = 1 − xH2 O < 0.1, as a co-solvent for 0.1 < xIL < 0.6 and as a solvent for xIL > 0.7. The corresponding distinct roles thus also rationalize the decreasing or decreasing values, respectively, for the order parameter in Fig. 3 with regard to the water content.

3.2. Sum-Frequency Generation Spectra In the following, we discuss the experimental results using vibrational SFG spectra for EMIM/DCA, EMIM/BF4 and BMIM/BF4 at gas–liquid interfaces and their mixtures with water. Although the spectra provide more details on the orientation of the ions 88–90 , we here focus mainly on the properties of the water molecules. As can be seen in Fig. 7, the neat ILs show strong bands centered at 2850, 2880, and 2943 cm−1 . These are attributable to methylene νs (CH2 ) and methyl νs (CH3 ) symmetric stretching vibrations, as well as to the methyl Fermi resonance νF R (CH3 ) of the alkyl side chains 88,89,91,92 . Additional vibrational modes are observed at ∼3124 and 3166 cm−1 and can be assigned to H-C(4)-C(5)-H stretching vibrations of the imidazolium ring 89,93 . The shape of SFG spectra can be described with the following expression for the second-order electric susceptibility χ(2)

ISF

2 X A q (2) = |χ(2) |2 = χN R + ωq − ω + iΓq

(4)

q

which is zero in the isotropic bulk solution but non-zero at the interface due to symmetry (2)

breaking at the interface. In Eqn. (4), χN R , Aq , Γq and ωq are the nonresonant contribution to the second-order susceptibility, the oscillator strength, as well as the bandwidth and resonance frequency of the q-th vibrational mode. We point out that for a more rigorous

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treatment of the inhomogenously broadened O-H bands, the latter should be treated with a Voigt rather than with a Lorentzian line shape 81,94 . More details on ion motion can be found in the supplementary material. As a reference for the water contributions, we present a SFG spectrum of the neat water-gas interface in the absence of ILs in Fig. 7a). Broad bands at 3200 and 3450 cm−1 dominate

Figure 7: Vibrational SFG spectra from room temperature ionic liquid–gas interfaces as a function of H2 O concentration. In (a) EMIM/DCA, (b) EMIM/BF4 , and (c) BMIM/BF4 mixtures with water are shown. The water concentrations in [mol %] are indicated through the legends on the right.

the spectrum and can be attributed to hydrogen-bonded water molecules at the interface. The low-frequency branch of the broad water spectrum can be assigned to tetrahedrally coordinated water molecules and is often referred to as ice-like water at the interface, while the high-frequency branch at 3450 cm−1 originated from an asymmetrically bonded liquid-like interfacial water molecules 94–97 . The narrow band at 3700 cm−1 originates from non-hydrogen 14


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bonded water molecules that have one hydroxyl group pointing into the gas phase (dangling O-H bonds) 98,99 . A close inspection of the spectra for all aqueous ILs clearly shows that the addition of water changes both the shape and the intensity of the SFG signals with varying the water concentration. In particular, the contribution from interfacial water molecules is for EMIM/DCA significantly different as compared to EMIM/BF4 and BMIM/BF4 . In fact, the SFG spectra from EMIM/DCA–gas interfaces show strong contributions from broad O-H stretching bands at 3200 and 3450 cm−1 , which dominate the spectra for water concentrations higher than 90 mol%. These bands, though, can be noticed already at ∼50 mol% water (Fig. 7a)). The SFG spectra of EMIM/BF4 and BMIM/BF4 interfaces in Fig. 7b) and c), respectively, show only weak and highly dispersive bands. As it is discussed in more detail in the supplementary material, the dispersive line shape of the bands in Fig. 7 at low water concentrations are caused by the interference of resonant O-H with the non-resonant contributions. At comparatively low water contents of 50 mol%, a weak signature at 3600 cm−1 is observed. This feature shifts to 3530 cm−1 and broadens with a further increase of the water concentration (Fig. 7 b) and c)). A similar behavior at low water concentrations is evident for EMIM/DCA–gas interfaces in Fig. 7a). We attribute this band to weakly hydrogen-bonded water molecules at the interface and propose that the red-shift and broadening of the band is related to a more extended network of interfacial molecules which can enter the interface when the concentration is high enough. From a close inspection of Fig. 7, additional weak bands at 3640 and ∼ 3700 cm−1 can be noticed. Based on recent results by Cammarata et al. 24 , we assign the bands at 3530 and 3640 cm−1 to interfacial water molecules interact90 ing with BF− 4 anions .

At very high water concentrations, the dangling O-H band centered at 3700 cm−1 which was already discussed above is additionally observed for EMIM/BF4 mixtures with water, but is absent for BMIM/BF4 and EMIM/DCA. This band is caused by interfacial water in the top-most surface layer with a dangling O-H that points into the gas phase. Based on the ex-

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perimental results from the SFG spectra of O-H stretching vibrations, we can conclude that EMIM/DCA–gas interfaces become dominated by water molecules at high concentrations, while in the case of EMIM/BF4 and BMIM/BF4 co-adsorption of water at the IL–gas interface does not play a major role. Here, the highest water concentrations lead only to a minor modification of the interfaces. Obviously, EMIM/DCA–gas interfaces are more susceptible to the interaction with water in good agreement with our simulation outcomes in front of uncharged interfaces. In combination with common trends obtained from both the MD simulations and the experimental SFG spectra for moderate water concentrations, we conclude that interfacial water molecules are dominating the spectra for EMIM/DCA, but are nearly absent for EMIM/BF4 and BMIM/BF4 . A comparable behavior can be observed at lower concentrations, again in good agreement with our simulation outcomes. Nevertheless, it needs to be noted that SFG measurements are only able to study the properties at short distances from the liquid-gas interface. Accordingly, the corresponding results correspond to short-distance solution structures of z ≤ 0.5 nm.

4. Discussion Based on our simulation and experimental outcomes, we can now propose a rationale for the results. At a low water content, the packing density as well as the ordering of the ions for all ILs is very high. The largest contributions to the order parameter come from the region that forms the interface. As a consequence, the translational entropy is very small here. However, the presence of water not only lowers the ion translation entropy, but also leads to a beneficial increase in the local entropy of mixing at the interfaces. Accordingly, as the amount of water becomes larger, the entropy of mixing becomes larger partially compensating for the unfavorable translational entropy. These contributions are the largest for EMIM/DCA, followed by EMIM/BF4 and BMIM/BF4 , respectively, rationalizing the high amount of water at the interface for highly ordered ILs. The reason for the pronounced ordering and packing

16


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fraction of EMIM/DCA thus is largely due to the small size, the planar shape, and the high polarity of the ions when compared to the other ILs (Tab. 1). The corresponding findings are in good agreement with recent considerations on the properties of residual water molecules in IL bulk phase 38 . At a high water content, the behavior of the ILs changes from a solvent to a co-solute, thereby pronouncing the accumulation tendency of the individual ions at the interface. The more hydrophobic properties of BMIM+ and BF− 4 when compared to the other species as represented by the octanol-water partition coefficients log10 P shown in Tab. 1 favor an accumulation of the ions at the interface in order to reduce the water-accessible surface area. In contrast, EMIM+ and DCA− ions are more polar, which is in agreement with the lower accumulation tendency of EMIM/DCA when compared to the other ILs. Thus, the accumulation behavior of the ions at the uncharged interfaces is mainly driven by hydrophobicity. With regard to the resulting effects, a significant amount of water molecules is replaced by ion clustering at interfaces, which reveals typical hallmarks of organic ions as co-solutes 10,13 . The corresponding implications are in good agreement with recent considerations on the role of bulky ions as protein stabilizers and destabilizers which were recently discussed for aqueous IL solutions 10,13,20,50 . Our findings reveal that the amount of water at solid-IL interfaces is higher for small and hydrophilic ion species when compared to hydrophobic and large ions. This result is valid for all water mole fractions and can be explained by entropy-driven effects imposed by the molecular properties of the ions.

5. Conclusions In summary, our findings shed more light on the distribution of water molecules in distinct ILs. The formation of mixed water-ion shells in front of uncharged interfaces becomes evident for all water concentrations and for all ILs. Hydrophobic and bulkier ions show a lower packing fraction when compared to small and hydrophilic ions with well-located and highly

17



ordered individual ion layers. In terms of ILs with a high packing fraction and a pronounced hydrophilicity, the presence of water molecules results in the formation of a water-rich first shell in front of the interface. These numerical results are confirmed by SFG spectra, despite the fact that the nature of the experimental interface differs substantially from the simulation setup (gas–liquid interface vs. flat wall-liquid interface) which rationalizes slight deviations at low water concentrations. In general, the presence of water molecules at interfaces is of crucial importance for various technological applications. For enzyme catalysis or electro catalysis, it is favorable to use hydrophilic ILs with small ions in order to increase the number of water molecules at the interface. On the other hand, the contact of electrodes with water molecules for battery devices has to be reduced, bringing ILs with bulky and hydrophobic ions to the forefront. Finally, though, we have studied the influence of uncharged interfaces, similar effects can be observed for charged walls 35 and for other ILs 20 . Our study has highlighted similarities and differences, and has provided important insight into the underlying mechanisms of IL-water mixtures at interfaces. We hope that our results will contribute to the recent progress in technological applications for ILs.

6. Supporting Information Supporting information is available and includes results of normalized number densities for distinct water mole fractions and a more detailed discussion of SFG spectra.

7. Acknowledgments TK, MF, and JS greatly acknowledge financial support from the collaborative network SFB 716 ”Dynamic simulations of systems with large particle numbers” funded by the German Funding Agency (Deutsche Forschungsgemeinschaft-DFG). AK and BB are grateful for the funding from the Deutsche Forschungsgemeinschaft (DFG) (Project number: BR4760/3-1) 18


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and the European Research Council within the Horizon 2020 program (grant agreement 638278).

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Aqueous Mixtures of Room-Temperature Ionic Liquids: Entropy-Driven

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