Aqueous Mixtures of Room-Temperature Ionic Liquids: Entropy-Driven Accumulation of Water Molecules at InterfacesTakeshi Kobayashi,† Andre Kemna,‡ Maria Fyta,† Bjorn Braunschweig,‡ and Jens Smiatek*,†,§

†Institute for Computational Physics, University of Stuttgart, D-70569 Stuttgart, Germany‡Institute of Physical Chemistry, University of Munster, D-48149 Munster, Germany§Helmholtz Institute Munster: Ionics in Energy Storage (HIMS − IEK 12), Forschungszentrum Julich GmbH, D-48149 Munster,Germany

*S Supporting Information

ABSTRACT: This work investigates the influence of uncharged interfaces on the distributionof water molecules in three aqueous dialkylimidazolium-based ionic liquid mixtures at variouswater concentrations. The results are based on atomistic molecular dynamics (MD)simulations supported by sum-frequency generation (SFG) experiments. All outcomeshighlight an entropically driven accumulation of water molecules in front of interfaces withslight, but technologically relevant differences. Our findings reveal that the local water densitydepends crucially on the water mole fraction, local ordering effects, and the molecularstructure of the ionic liquids (ILs). We unravel the influence of hydrophobicity/hydrophilicity and bulkiness of the ions, as wellas the effect of water in defining the role of the ILs as a main solvent, a cosolvent or cosolute. The outcome of this study allowsthe definition of reliable criteria for beneficial water-IL combinations in view of distinct applications.

1. INTRODUCTION

Neat room-temperature ionic liquids (ILs) offer a broadplethora of beneficial solvent properties including lowflammabilities, low volatilities, high ionic strengths, and wideelectrochemical stability windows.1−6 In combination with highaffinities for various polar and apolar solutes, recent applicationsof ILs thus range from electrolyte components in electro-chemical devices and via long-time stable storage media forenzymes to inert and environmentally benign solvents forchemical synthesis procedures.3,4,4,6−18 In addition to the use ofneat ILs, also the individual ionic components are oftenemployed as cosolvents or cosolutes in protic and aproticsolvent mixtures.3,10,11,17,19,20 Due to recent and upcomingtechnological challenges,3,10−13,21,22 a specific interest inaqueous IL mixtures has emerged rapidly over the lastyears.23−38

With regard to this point, various technologically motivatedresearch studies reported conflicting interpretations on the roleof water and water interfaces in ILs. On one hand, the favorablepresence of water as a cosolvent or coreactant increasesenzymatic activities11 and enhances CO2 electroreductionprocesses,14−16 whereas even a spurious amount of humidityin electrochemical storage devices considerably limits theapplicability of IL electrolyte solutions.23,35 In more detail,previous simulation and experimental studies reported thatwater interfaces at highly charged electrodes form, which initiateelectrochemical decomposition processes and hence reducesignificantly the lifetime of electrochemical cells.23,35,39 Incontrast to these shortcomings, the beneficial formation ofwater interfaces at silver electrodes was shown to foster CO2electroreduction mechanisms, thereby establishing a higherefficiency of degradation processes.14 As a consequence,

whether the occurrence of water molecules at charged oruncharged interfaces is considered as a desired or undesiredeffect depends crucially on the underlying technologicalpurpose. Despite the relevance of these findings, our under-standing of water behavior in ILs is rather shallow. Hence, it isyet not fully clarified which mechanisms or molecular propertiesinitiate water clustering effects.36−38 Our limited knowledge canbe mostly attributed to the complex interplay of distinct ioncombinations and the occurrence of structuring phenomena, asreported for bulky hydrophobic cations in combination withpolar anions.40,41 The presence of further components like polaror apolar cosolutes or confinement effects gives rise to evenmore complex solutions resulting in various distinct outcomesand unclassified observations.17,18,31,40,42−46

Up to this point, we have mostly discussed previous results onthe properties of low-concentrated water molecules in ILs.However, also the opposite case of low-concentrated ILs in bulkwater has received considerable attention. As an importantexample, a lot of research effort was directed on aqueous orhydrated ILs, which either increase or decrease the structuralstability of proteins.10−13,19,21,47−50 Recent explanations rely onthe origins of preferential binding and exclusion mechanismsaround interfaces in terms of a statistical thermodynamicsframework.10,13,20 Despite their technological relevance, most ofthese effects are also only sparsely understood.10,12,13 Withregard to previous theoretical, numerical, and experimentalapproaches for solvent-IL mixtures at various mixingratios,17,18,23,25,29,33,35−39,43,51−65 the question arises whether

Received: May 1, 2019Revised: May 7, 2019Published: May 8, 2019

Article

pubs.acs.org/JPCCCite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

© XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.9b04098J. Phys. Chem. C XXXX, XXX, XXX−XXX

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only charged interfaces favor an attraction of water mole-cules35,39,43 or whether water accumulation can also occur atuncharged interfaces. Moreover, what causes the differences inthe solvent distribution for distinct ILs,31,43−46 and what is thecontribution of concentration-dependent or specific ioneffects?36−38 It may be assumed that the answers to thesequestions pave a rational way toward an improved use of ILsolutions for various technological purposes.In order to study the properties of such mixtures in more

detail, we perform atomistic MD simulations for distinctaqueous IL solutions, namely, 1-ethyl-3-methylimidazoliumdicyanamide (EMIM/DCA), 1-ethyl-3-methylimidazolium tet-rafluoroborate (EMIM/BF4), and 1-butyl-3-methylimidazoliumtetrafluoroborate (BMIM/BF4) at various water mole fractionsbetween xH2O = 0 (neat IL) and xH2O = 0.99 (highly dilutedaqueous IL solution). We focus specifically on the technolog-ically relevant distribution of water molecules in front ofuncharged walls (solid−liquid interfaces) under the influence ofplanar and spherical anions and cations with a varying alkyl sidechain length. The presence of these interfaces introduces fixedreference positions and further allows us to study thecorresponding distributions in terms of a Kirkwood−Buff(KB) based approach with a specific focus on excluded-volumecontributions.66,67 Our study thus provides a robust analysis ofwater effects in various ILs with a clear separation between bulkand local effects. As our results will show, the molecularproperties like polarity and size play a decisive role. The distinctoutcomes for the IL-water mixtures highlight the crucial role ofspecific ion and local ordering effects. Based on our findings, weare able to define reliable criteria for tunable water-IL solutionswhose implications may help to improve recent technologicalapplications. All numerical findings are supported by experi-ments on gas−liquid interfaces using sum-frequency generation(SFG) spectroscopy. Although the experimental setup differsfrom our simulation approach, the corresponding distributionsof species in front of distinct interfaces reveals a qualitativeagreement. This can be explained by the weak interaction of ionspecies with both interfaces and the resulting comparable wateraccumulation behavior.

2. METHODS

2.1. Simulation Details. All atomistic molecular dynamics(MD) simulations were performed with the GROMACS 5.1.3package.68−70 We used OPLS/AA force fields71−73 for all ions incombination with the SPC/E force field74 for water molecules.Different water mole fractions xH2O = {0, 0.125, 0.25, 0.375, 0.5,0.625, 0.75, 0.875, 0.95, 0.96, 0.97, 0.98, 0.99} in distinct ILsEMIM/DCA, EMIM/BF4, and BMIM/BF4 were randomlyinserted into rectangular simulation boxes of dimensions 6.3 nm−6.5 nm in periodic x, y−direction and 14.5 nm in z−directionusing the software package PACKMOL.75 The z−direction wasconstrained by two impenetrable silicon walls with Lennard−Jones (LJ) 9-3 potentials. In all simulations, the temperature wasmaintained at T = 300 K by an improved velocity-rescalingthermostat,76 using a coupling time constant of 0.1 ps. Thepressure was kept constant at p = 1 bar by a semi-isotropicParrinello−Rahman barostat77 (periodic x- and y-dimensionswith fixed z-dimension) with coupling time constant 2 ps andcompressibility 4.5 × 10−5 bar−1. Electrostatic interactions weretreated through the particle mesh Ewald (PME) method,78,79

where a real-space cutoff of 1.0 nm and a grid spacing of 0.16 nmwith fourth-order interpolation scheme were used. LJ

interactions were truncated at 1.0 nm and shifted to zero. Theequations of motion were integrated by the Leapfrog algorithmwith an elementary time step of 2 fs. All bonds were constrainedby the LINCS algorithm.80 An energy minimization was firstperformed using a conjugate-gradient method, followed by anequilibration period of 10 ns under constant volume-constanttemperature (NVT) conditions, and a subsequent equilibrationrun of 10 ns under constant temperature and constant pressure(NpT) conditions. The final NpT production runs had a lengthof 200 ns each, except for the lowest water concentrations, whichhad a length of 500 ns. Positions and velocities of atoms werestored every 10 ps. A molecular snapshot of a sample mixture(EMIM/DCA with a water mole fraction of xH2O = 0.25 in thepresence of two silicon walls) and the chemical structures of allion species are shown in Figure 1.

2.2. Experimental Details. All ILs EMIM/BF4 (>99%),EMIM/DCA (>98%), and BMIM/BF4 (>99%) were purchasedfrom Iolitec (Germany) and were used as received. IL−watermixtures for SFG spectroscopy were prepared by mixing the ILswith ultrapure water (Milli-Q Reference A+, 18.2 MΩ cm, TOC< 5 ppb) and were stirred until a clear and homogeneoussolution was obtained. Glassware and all necessary equipmentwhich came in contact with the solutions were cleaned in amixture of 98% sulfuric acid (Carl Roth, Germany) andNochromix (Godax Laboratories, USA) and were subsequentlythoroughly rinsed with ultrapure water and dried in a stream ofN2 gas (>99.999, Westfalen). SFG spectra from a home-builtbroadband SFG setup as described elsewhere81 were recorded inthe frequency region of C−H and O−H stretching bandsbetween 2700−3800 cm−1. The center frequency of thebroadband (>300 cm−1) femtosecond IR pulse was tuned infour steps, and the SFG intensities of each frequency wereacquired for 2 min in an ssp (SFG/VIS/IR) polarizationcombination. All spectra were normalized to the SFG signalfrom an air-plasma cleaned polycrystalline Au film withpolarizations set to ppp.

Figure 1. Top: Snapshot of a water-EMIM/DCA mixture with a watermole fraction of xH2O = 0.25. The uncharged silicon walls arerepresented through the blue boxes on either ends of the figure.Water molecules are shown in a van der Waals representation, whereasDCA− is colored in cyan and EMIM+ in gray. Bottom: Molecularstructures of the BMIM+ and EMIM+ cations with labels for atoms inthe imidazolium ring. Molecular structures of the anions DCA− andBF4

− are depicted on the right side. Hydrogen, nitrogen, and carbonatoms are colored in white, blue, and gray, respectively. The fluorideatoms in BF4

− are represented as light blue spheres, whereas the centralboron atom is colored in light red.

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3. RESULTS3.1. Atomistic Molecular Dynamics Simulations. In

terms of the molecular structure at the solid−liquid interfacewith regard to all neat ILs (xH2O = 0), the local normalizedatomic number densities ρIL

N(z) = ρIL(z)/ρILbulk of the combined

cations and anions are shown in Figure 2, where ρILbulk denotes the

ion density in the middle of the channel (bulk phase) and ρIL(z)is the local ion number density at a distance z from the wall. Ascan be seen, conspicuous ion-shell or layer structures as markedby density peaks disappear on length scales around z ≈ 1.5−2nm, which is in good agreement with previous findings for otherILs.17,18 In a close distance of z≈ 0.25 nm to the wall, the highestion density can be observed for EMIM/DCA, followed byEMIM/BF4 and BMIM/BF4. Additionally, EMIM/DCA alsoshows the highest packing fraction through the short distancesbetween the peaks. Due to the close similarity between densityprofiles for EMIM/BF4 and BMIM/BF4, it can be concludedthat different anions are mainly responsible for distinct densitydistributions and packing densities. Further inspection alsoreveals that the peak height of the first ion shell in EMIM/DCAis significantly higher when compared to the other ILs. Thisfinding can be attributed to the spatial orientation of DCA−

anions, which enhances a more compact and combined iondistribution.The corresponding outcomes for the distribution of the

individual ion species in the neat ILs as shown in the SupportingInformation underpin this assumption. As can be seen, DCA−

ions reveal a more ordered structure at short wall-distances whencompared to BF4

−, thereby inducing a more compact IL layerstructure. In more detail, DCA− ions orient parallel to theinterface whereas BF4

−with its spherical shape leads to a broaderfirst layer with distinct ion orientations. The correspondingdouble-peak structure for BF4

− ions at z ≤ 0.7 nm whencompared to DCA− can be attributed to the various orientationsof the anion. Thus, DCA− ions can be found in a more well-ordered orientation parallel to the interfaces when compared toBF4

−. Noteworthy, the EMIM+ distribution of EMIM/DCA isalso more ordered than EMIM+ from EMIM/BF4. Hence, it canbe concluded that the distribution of anions influences thedistribution of cations and vice versa. It can be thus assumed thatthe observed differences between the density profiles can bemainly assigned to the molecular volume, as well as the planar

(DCA−) or spherical (BF4−) shape and arrangement of the

anions.With regard to this point, the values for themolecular volumes

Vm of all ions in combination with the octanol−water partitioncoefficients log10 P (Table 1) with P = cC8OH

s /cH2Os , where cC8OH

s

and cH2Os denote the corresponding concentration of the ions in

octanol and water phase, respectively, support this view. Thesmaller sizes of DCA− and EMIM+ in comparison to BF4

− andBMIM+ are eminent and thus rationalize a higher local packingfraction for EMIM/DCA when compared to the other ILs. Asexpected, smaller ions also reveal a lower hydrophobicity,thereby establishing the following ordering scheme DCA− >EMIM+ > BF4

− > BMIM+ with decreasing polarity. Hence, it canbe expected that DCA− anions and EMIM+ cations show astronger water binding behavior when compared to BF4

− andBMIM+. Interestingly, the distribution of the ions does notsignificantly change for low water contents of xH2O = 0.125 whencompared to neat ILs as can be seen in the SupportingInformation. The position of the ion shells remains nearlyidentical and the long-range decay of ordering differs slightly.In order to study the amount of ion ordering and packing

effects in the presence of water in more detail, one can define atranslational order parameter for species α, either combinedanions and cations (index “IL”) or water molecules (index“H2O”), reading

Ox

Lz z z z

2d ( ( )ln ( ) ( ) 1 )z

z

L

0

/2z∫ γ γ γ= [ ] − [ − ]α αα α α

(1)

with γα(z) = ρα(z)/ραbulk, which is closely related with the

expression for the translational entropy as introduced in refs 84and 85. Notably, for a reliable evaluation of eq 1, a bulk behaviorwith γα(z) ≈ 1 for z → Lz/2 has to be guaranteed. This isachieved in our simulations by setting large distances betweenthe walls. As a consequence, the corresponding value of Oz

α

provides an estimate for the degree of translational ordering interms of the considered species.The corresponding values in Figure 3 for neat ILs (xH2O = 0)

areOzIL = 0.82 (EMIM/DCA),Oz

IL = 0.63 (EMIM/BF4), andOzIL

= 0.51 (BMIM/BF4), which highlight the fact that the highestdegree of ion order is given for EMIM/DCA, followed byEMIM/BF4 and BMIM/BF4. Although the order parameterdecreases significantly for all ILs with increasing water content,thereby implying an increase of the translational entropy, thecorresponding values reveal that up to a water mole fraction ofxH2O = 0.9 the corresponding ordering scheme remains identical

to neat ILs. In contrast, at higher water mole fractions xH2O ≥

Figure 2. Normalized local atomic number density ρILN(z) of the

combined cation and anion species in neat ILs with xH2O = 0 at distancesz from the wall for EMIM/DCA (red line), EMIM/BF4 (blue line), andBMIM/BF4 (black line). The green horizontal line shows ideal bulkbehavior with ρIL

N(z) = 1.

Table 1. Molecular Volumes Vm and Octanol−WaterPartition Coefficients Log10 P for the Individual Ion Speciesand Water Molecules As Calculated by Ref 82a

species Vm [nm3] log10 P

EMIM+ 0.118 −3.10BMIM+ 0.152 −2.04DCA− 0.056 −3.34BF4

− 0.073 −2.60H2O 0.019 −0.29

aMolecular volumes are obtained by fitting the sum of fragmentcontributions for a training set of about 12 000 molecules afteroptimization using the semiempirical AM1 method.83

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0.92 the order parameter increases for all ILs. A more detailedexplanation for these observations and differences between theILs will be presented in the remainder of this Article. Furtheranalysis of the running order parameter Oz

α(z) (Figure 4) for

neat ILs implies that the largest contributions to the integral ineq 1 come from short distances of around 1 nm in front of thechannel walls. In contrast, the contributions from large distancesare rather negligible such that Oz

IL converges to constant valuesfor all z > 1 nm. The corresponding analysis reveals that thehighest ordering of ion species and vice versa the lowesttranslational entropy84 can be attributed to the interfacialregions. The question which now arises is what is the watercontent of distinct mixtures in close vicinity of the interfaces? Aswe have already discussed in the introduction, the outcomes areof crucial importance for various technological applications.With regard to this point, the fraction of water molecules

KH2O(Δ) in the first solvent shell within wall distances z≤Δ canbe calculated by

K ( )( )

( )H OH O

all2

2ρ

ρΔ =

Δ

Δ (2)

where Δ denotes the first minimum in the local total numberdensity ρall(Δ) = ρIL(Δ) + ρH2O(Δ) with Δ = 0.38 nm forEMIM/DCA and Δ = 0.44 nm for EMIM/BF4 or BMIM/BF4,respectively. The corresponding results for all water molefractions xH2O are depicted in Figure 5. Interestingly, the localfraction of water molecules at the interface for all mole fractions

xH2O ≤ 0.875 is the highest for EMIM/DCA, followed byEMIM/BF4 and BMIM/BF4. Accordingly, the neat IL EMIM/DCA with the highest local order is related to the highest watercontent. The corresponding net differences in the water contentat the interface between the ILs remain valid up tomole fractionsof xH2O ≤ 0.75, implying the robustness of this observation. Ascan be seen, water molecules are an integral part of the mixedfirst cation−anion layer of the solution at least for EMIM/DCA.At high water mole fractions with the definition KIL(Δ) = 1 −KH2O(Δ), it follows that the local fraction of ions KIL(Δ) at xH2O

≥ 0.98 is the highest for BMIM/BF4 (KIL(Δ) = 0.52), followedby EMIM/DCA (KIL(Δ) = 0.34) and EMIM/BF4 (KIL(Δ) =0.33). With regard to these values, a significant amount of ionsaccumulates at short distances in front of the wall. As a specificexample, the local number densities of ions and water moleculesfor a specific mole fraction of xH2O ≥ 0.98 are shown in theSupporting Information.In order to study the role of the ions at various water

concentrations, we compute the slightly modified one-dimen-sional and distance-dependent Kirkwood−Buff (KB) inte-grals10,66,67,86

G L L z zd ( ) 1x y

L

W0

/2z∫ γ= [ − ]α α (3)

whose values can be interpreted as excess volumes of specieswhen compared to bulk phase.10,13 The preferential hydrationcoefficient10,87 as an estimate for the hydration tendency of thewalls is given by Γ = ρH2O

0 ΔVH2O, where ρH2O0 denotes the total

water density and ΔVH2O = GWH2O − GWIL the correspondingdifferences between the water and the ion excess volumes.Positive values of Γ imply a water attraction behavior andnegative values a water exclusion effect of the interfaces. Perdefinition ρH2O

0 ≥ 0, thus the positive or negative sign of Γ issolely determined by the difference in the excess volumes. In thatrespect, ΔVH2O > 0 implies a preferential attraction of water

molecules, whereasΔVH2O < 0 highlights an exclusion effect10,13

in terms of strong nonideal solutions.The corresponding results in Figure 6 reveal positive values

forΔVH2O for all ILs below water mole fractions xH2O < 0.6. Thelargest values of the excess volumes can be observed for BMIM/BF4, followed by EMIM/DCA and EMIM/BF4. These findings

Figure 3. Order parameter values OzIL for combined ions at various

water mole fractions xH2O.

Figure 4. Running order parameter values OzIL for combined ions in

neat ILs.

Figure 5. Fraction of water molecules KH2O(Δ) for all water mole

fractions xH2O in the first solvent shell at wall distances z ≤ Δ. Theresults for EMIM/DCA, EMIM/BF4, and BMIM/BF4, are shown in red(squares), blue (circles), and black (triangles), respectively.

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are valid for low and moderate water content (xH2O < 0.6),

whereas for water mole fractions xH2O > 0.9 a steep decrease of

ΔVH2O to even negative values can be observed. The latterimplies that a significant portion of water molecules is replacedby the ions. The change from positive to negative values thusprovides a clear separation on the role of ILs in terms of atransient behavior from a solvent to a cosolvent or even acosolute. Based on our findings, the considered ILs can beregarded as a cosolute for IL mole fractions xIL = 1 − xH2O < 0.1,as a cosolvent for 0.1 < xIL < 0.6 and as a solvent for xIL > 0.7. Thecorresponding distinct roles thus also rationalize the increasingor decreasing values, respectively, for the order parameter inFigure 3 with regard to the water content.3.2. Sum-Frequency Generation Spectra. In the

following, we discuss the experimental results using vibrationalSFG spectra for EMIM/DCA, EMIM/BF4 and BMIM/BF4 atgas−liquid interfaces and their mixtures with water. Althoughthe spectra provide more details on the orientation of theions,88−90 we here focus mainly on the properties of the watermolecules. As can be seen in Figure 7, the neat ILs show strong

bands centered at 2850, 2880, and 2943 cm−1. These areattributable to methylene νs(CH2) and methyl νs(CH3)symmetric stretching vibrations, as well as to the methyl Fermiresonance νFR(CH3) of the alkyl side chains.

88,89,91,92 Additionalvibrational modes are observed at ∼3124 and 3166 cm−1 andcan be assigned to H−C(4)−C(5)−H stretching vibrations ofthe imidazolium ring.89,93 The shape of SFG spectra can bedescribed with the following expression for the second-orderelectric susceptibility χ(2)

IA

iq

q

q qSF

(2) 2NR(2)

2

∑χ χω ω

= | | = +− + Γ

(4)

which is zero in the isotropic bulk solution but nonzero at theinterface due to symmetry breaking at the interface. In eq 4, χNR

(2),Aq, Γq, and ωq are the nonresonant contribution to the second-order susceptibility, the oscillator strength, as well as thebandwidth and resonance frequency of the qth vibrational mode,respectively. We point out that for a more rigorous treatment ofthe inhomogeneously broadened O−H bands, the latter shouldbe treated with a Voigt rather than with a Lorentzian lineshape.81,94 More details can be found in the SupportingInformation.As a reference for the water contributions, we present a SFG

spectrum of the neat water−gas interface in the absence of ILs inFigure 7a. Broad bands at 3200 and 3450 cm−1 dominate thespectrum and can be attributed to hydrogen-bonded watermolecules at the interface. The low-frequency branch of thebroad water spectrum can be assigned to tetrahedrallycoordinated water molecules and is often referred to as ice-likewater at the interface, while the high-frequency branch at 3450cm−1 originated from asymmetrically bonded liquid-likeinterfacial water molecules.94−97 The narrow band at 3700cm−1 originates from non-hydrogen bonded water moleculesthat 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 clearlyshows that the addition of water changes both the shape and theintensity of the SFG signals with varying the waterconcentration. In particular, the contribution from interfacialwater molecules is for EMIM/DCA significantly different ascompared to EMIM/BF4 and BMIM/BF4. In fact, the SFGspectra from EMIM/DCA−gas interfaces show strong con-tributions from broad O−H stretching bands at 3200 and 3450cm−1, which dominate the spectra for water concentrationshigher than 90 mol %. These bands, though, can be noticedalready at ∼50 mol % water (Figure 7a). The SFG spectra ofEMIM/BF4 and BMIM/BF4 interfaces in Figure 7b and c,respectively, show only weak and highly dispersive bands. As it isdiscussed in more detail in the Supporting Information, thedispersive line shape of the bands in Figure 7 at low waterconcentrations are caused by the interference of resonant O−Hwith the nonresonant contributions. At comparatively low watercontents of 50 mol %, a weak signature at 3600 cm−1 is observed.This feature shifts to 3530 cm−1 and broadens with a furtherincrease of the water concentration (Figure 7 b and c). A similarbehavior at low water concentrations is evident for EMIM/DCA−gas interfaces in Figure 7a. We attribute this band toweakly hydrogen-bonded water molecules at the interface andpropose that the red-shift and broadening of the band is relatedto a more extended network of interfacial molecules which canenter the interface when the concentration is high enough. From

Figure 6. Differences in the excess volumes ΔVH2O for distinct water

mole fractions xH2O in EMIM/DCA (red line with squares), EMIM/BF4 (blue line with circles), and BMIM/BF4 (black line with triangles).

Figure 7. Vibrational SFG spectra from room temperature ionicliquid−gas interfaces as a function of H2O concentration. (a) EMIM/DCA, (b) EMIM/BF4, and (c) BMIM/BF4 mixtures with water areshown. The water concentrations in [mol %] are indicated through thelegends on the right.

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a close inspection of Figure 7, additional weak bands at 3640 and∼3700 cm−1 can be noticed. Based on recent results byCammarata et al.,24 we assign the bands at 3530 and 3640 cm−1

to interfacial water molecules interacting with BF4− anions.90

At very high water concentrations, the dangling O−H bandcentered at 3700 cm−1 which was already discussed above isadditionally observed for EMIM/BF4 mixtures with water, but isabsent for BMIM/BF4 and EMIM/DCA. This band is caused byinterfacial water in the topmost surface layer with a dangling O−H that points into the gas phase. Based on the experimentalresults from the SFG spectra of O−H stretching vibrations, wecan conclude that EMIM/DCA−gas interfaces becomedominated by water molecules at high concentrations, while inthe case of EMIM/BF4 and BMIM/BF4 coadsorption of water atthe IL−gas interface does not play amajor role. Here, the highestwater concentrations lead only to a minor modification of theinterfaces. Obviously, EMIM/DCA−gas interfaces are moresusceptible to the interaction with water in good agreement withour simulation outcomes in front of uncharged interfaces. Incombination with common trends obtained from both the MDsimulations and the experimental SFG spectra for moderatewater concentrations, we conclude that interfacial watermolecules are dominating the spectra for EMIM/DCA, butare nearly absent for EMIM/BF4 and BMIM/BF4. A comparablebehavior can be observed at lower concentrations, again in goodagreement with our simulation outcomes. Nevertheless, it needsto be noted that SFG measurements are only able to study theproperties at short distances from the liquid−gas interface.Accordingly, the corresponding results correspond to short-distance solution structures of z ≤ 0.5 nm.

4. DISCUSSIONBased on our simulation and experimental outcomes, we cannow 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 isvery high. The largest contributions to the order parametercome from the region that forms the interface. As a consequence,the translational entropy is very small here. However, thepresence of water not only lowers the ion translation entropy,but also leads to a beneficial increase in the local entropy ofmixing at the interfaces. Accordingly, as the amount of waterbecomes larger, the entropy of mixing becomes larger partiallycompensating for the unfavorable translational entropy. Thesecontributions are the largest for EMIM/DCA, followed byEMIM/BF4 and BMIM/BF4, respectively, rationalizing the highamount of water at the interface for highly ordered ILs. Thereason for the pronounced ordering and packing fraction ofEMIM/DCA thus is largely due to the small size, the planarshape, and the high polarity of the ions when compared to theother ILs (Table 1). The corresponding findings are in goodagreement with recent considerations on the properties ofresidual water molecules in IL bulk phase.38

At a high water content, the behavior of the ILs changes from asolvent to a cosolute, thereby pronouncing the accumulationtendency of the individual ions at the interface. The morehydrophobic properties of BMIM+ and BF4

− when compared tothe other species as represented by the octanol−water partitioncoefficients log10 P shown in Table 1 favor an accumulation ofthe ions at the interface in order to reduce the water-accessiblesurface area. In contrast, the EMIM+ and DCA− ions are morepolar, which is in agreement with the lower accumulationtendency 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 theresulting effects, a significant amount of water molecules isreplaced by ion clustering at interfaces, which reveals typicalhallmarks of organic ions as cosolutes.10,13 The correspondingimplications are in good agreement with recent considerationson the role of bulky ions as protein stabilizers and destabilizerswhich were recently discussed for aqueous IL solutions.10,13,20,50

Our findings reveal that the amount of water at solid-ILinterfaces is higher for small and hydrophilic ion species whencompared to hydrophobic and large ions. This result is valid forall water mole fractions and can be explained by entropy-driveneffects imposed by the molecular properties of the ions.

5. CONCLUSIONSIn summary, our findings shed more light on the distribution ofwater molecules in distinct ILs. The formation of mixed water−ion shells in front of uncharged interfaces becomes evident for allwater concentrations and for all ILs. Hydrophobic and bulkierions show a lower packing fraction when compared to small andhydrophilic ions with well-located and highly ordered individualion layers. In terms of ILs with a high packing fraction and apronounced hydrophilicity, the presence of water moleculesresults in the formation of a water-rich first shell in front of theinterface. These numerical results are confirmed by SFG spectra,despite the fact that the nature of the experimental interfacediffers substantially from the simulation setup (gas−liquidinterface vs flat wall-liquid interface) which rationalizes slightdeviations at low water concentrations.In general, the presence of water molecules at interfaces is of

crucial importance for various technological applications. Forenzyme catalysis or electro catalysis, it is favorable to usehydrophilic ILs with small ions in order to increase the numberof water molecules at the interface. On the other hand, thecontact of electrodes with water molecules for battery deviceshas to be reduced, bringing ILs with bulky and hydrophobic ionsto the forefront. Finally, though, we have studied the influence ofuncharged interfaces, similar effects can be observed for chargedwalls35 and for other ILs.20 Our study has highlighted similaritiesand differences, providing important insight into the underlyingmechanisms of IL-water mixtures at interfaces.We hope that ourresults will contribute to the recent progress in technologicalapplications for ILs.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.9b04098.

Results of normalized number densities for distinct watermole fractions and a more detailed discussion of SFGspectra (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: smiatek@icp.uni-stuttgart.de.

ORCIDMaria Fyta: 0000-0002-5425-7907Bjorn Braunschweig: 0000-0002-6539-1693Jens Smiatek: 0000-0002-3821-0690NotesThe authors declare no competing financial interest.

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■ ACKNOWLEDGMENTS

T.K., M.F., and J.S. greatly acknowledge financial support fromthe collaborative network SFB 716 “Dynamic simulations ofsystems with large particle numbers” funded by the GermanFunding Agency (Deutsche Forschungsgemeinschaft-DFG).A.K. and B.B. are grateful for the funding from the DeutscheForschungsgemeinschaft (DFG) (Project Number: BR4760/3-1) and the European Research Council within the Horizon 2020program (Grant Agreement 638278).

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