Solvation of molecular cosolvents and inorganic salts in ionic liquids: A review of molecular dynamics simulations

Solvation of molecular cosolvents and inorganic salts in ionic liquids: A review of molecular dynamics simulations

Journal of Molecular Liquids 210 (2015) 178–188 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevie...

1MB Sizes 0 Downloads 68 Views

Journal of Molecular Liquids 210 (2015) 178–188

Contents lists available at ScienceDirect

Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Solvation of molecular cosolvents and inorganic salts in ionic liquids: A review of molecular dynamics simulations L.M. Varela a,⁎, T. Méndez-Morales a, J. Carrete a,b, V. Gómez-González a, B. Docampo-Álvarez a, L.J. Gallego a, O. Cabeza c, O. Russina d a

Grupo de Nanomateriais e Materia Branda, Departamento de Física da Materia Condensada, Universidade de Santiago de Compostela, Campus Vida s/n E-15782, Santiago de Compostela, Spain CEA-Grenoble, 17 Rue des Martyrs, Grenoble 38054, France Facultade de Ciencias, Universidade da Coruña, Campus A Zapateira s/n E-15008, A Coruña, Spain d Department of Chemistry, Sapienza University, Rome, Italy b c

a r t i c l e

i n f o

Article history: Received 26 February 2015 Received in revised form 8 June 2015 Accepted 9 June 2015 Available online 17 July 2015 2010 MSC: 82-02 82D15 82D30

a b s t r a c t Trying to contribute to an overall picture of solvation in ionic liquids, we report a review of the main recent results in the field of molecular dynamics simulations of structural and dynamic properties of mixtures of ionic liquids with molecular cosolvents or salts. The majority of reported results indicate that solvation of neutral molecules and salt ions takes place selectively in the bulk nanoregions of the dense ionic solvents, fitting the parts of the solute's moieties to the similar regions of the dense ionic solvent, in what has been called nanostructured solvation [Méndez-Morales et al. J. Phys. Chem. B 118 (2014) 761–770]. © 2015 Elsevier B.V. All rights reserved.

Keywords: Ionic liquids Mixtures Water Alcohols Nanostructured solvation

1. Introduction The singular properties of ionic liquids (ILs) or room temperature molten salts are responsible for their current consideration as a new class of nanostructured “green” solvents. Moreover, a formidable number of ILs could be synthesized combining the anions and cations that are currently known to form the molecular structures of one of these systems. This theoretically allows tailoring one almost for every specific applications, which ultimately leads to the conventional expression of “designer solvents”, since it is already possible to design an IL with the relevant structure for dissolving almost any substance, and having the physicochemical properties required by the given application. Up to now, solvents have traditionally formed quite an elitist club in which the rule “similia similibus solvuntur” seemed a basic, unbreakable one. The fact that these ILs are made up of ions with both organic and inorganic moieties in their molecular structures gives rise to inherent polar/nonpolar

⁎ Corresponding author at: Grupo de Nanomateriais e Materia Branda, Departamento de Fsica da Materia Condensada, Universidade de Santiago de Compostela, Spain. E-mail address: [email protected] (L.M. Varela).

http://dx.doi.org/10.1016/j.molliq.2015.06.036 0167-7322/© 2015 Elsevier B.V. All rights reserved.

microphase separation of nanoscopic polar and nonpolar networks in bulk ILs and makes them very singular, highly versatile solvents with unrivaled solvation abilities. Hence, these systems have literally revolutionized the area of solvents, with all the implications this has in fields like chemistry, physical chemistry and electrochemistry. The miscibility of ILs with other solvents depends on the dielectric constant of the latter, and it is well known that ILs are miscible with compounds with medium or high dielectric constants and immiscible with liquids with low dielectric constant [1,2]. However, contrarily to what happens in traditional solvents with water as the prime example, even the basic features of the mechanisms of solvation of different molecules and ions in this new class of dense ionic solvents are still scarcely known, despite their precise knowledge being of utmost importance both from a purely fundamental point of view and from the applied perspective. Conversely, the precise effect of these additives – some of them, such as water, practically unavoidable due to high hygroscopicity of the IL – on the microstructure of ILs is still controversial [3–5]. In this sense, the vast majority of the huge number of studies of mixtures of ILs reported so far can be classified into two main categories: mixtures with molecular cosolvents (mainly water and short-chained alcohols like ethanol and methanol), and mixtures with different salts

L.M. Varela et al. / Journal of Molecular Liquids 210 (2015) 178–188

(see references below). Besides those, but rather less abundant, results for solutions of polymers, gases such as CO2 and SO2 [6–9], biomass and biopolymers [10,11] and nanoparticles [12] have also been reported. IL/water mixtures are employed as biphasic systems in catalysis, and the presence of water in ILs is almost inevitable [13,14], and its wellknown influence on the nanostructural organization of roomtemperature molten salts is a central topic in synthetic chemistry and catalysis. The physical properties of mixtures of both protic and aprotic ILs (AILs) with water have been extensively characterized, including densities [15,16], surface properties [17], electrical conductivities [18, 19], viscosities [15,16,20], thermal conductance [21], infrared and NMR spectroscopy [22], small angle neutron scattering (SANS) [23], and so on. For an early review on measurements of pure ILs and their mixtures, as well as on their prediction with numerical methods (COSMO), see Marsh et al. [24]. More recent reviews have been reported in Cabeza et al. [25] and Segade and Cabeza [26]. Spontaneous selforganization and the formation of liquid-crystalline gel phases were shown by Firestone and coworkers [27], and the addition of water to ILs has been shown to induce classical surfactant self-organization with high levels of ordering [28]. Moreover, some theoretical efforts have been devoted to the understanding of structural and transport properties of these mixtures [13,29,30]. Although to a much lesser extent than their aqueous counterparts, mixtures with alcohols have also focused a lot of attention in the last decade due to their undoubted importance in several fields like catalysis [31] or electrodeposition [32], among others. For these mixtures, some experimental results have been reported during the past decade analyzing the phase behavior of mixtures of alcohols with imidazolium based ILs [33–36]. Moreover, conductivity and viscosity studies have been reported by Rilo et al. [18,19]. Several studies have also been reported for mixtures of protic ILs with alcohols, such as densities and viscosities [37,38], volumetric properties [39] and electrical conductivities [18]. However, in spite of this extensive experimental activity, a detailed microscopic characterization of these mixtures is usually very complicated and requires sophisticated and expensive techniques. Hence, molecular dynamics (MD) simulations are usually the optimal way to probe the properties of the mixtures. It is therefore not surprising that a truly gargantuan number of simulation studies have been undertaken over the last decade to investigate the structure and dynamics of both pure ILs and IL mixtures. The most frequent technique employed to probe the microscopic structure and dynamics of these systems is classical MD due to higher computational cost of ab initio techniques (see [40] and references therein). Up to now, MD studies have mainly concerned pure ILs, and IL + solvent mixtures have received much less attention. Among the latter, mixtures with alcohols [41,42,40, 43–46,5] have been second only to those with water [47–53,3], and studies of mixtures with other compounds such as acetonitrile [54–56], ethers and propane [41], alkanes [43] or supercritical carbon dioxide [57] can also be found. The knowledge of the interactions taking place between ILs and a gas has also motivated several studies. In this regard, not only have ILs been suggested to be useful for CO2 capture, but also the absorption of gases such as SO2 by these ionic solvents is considered to be challenging. For example, Perez-Blanco and Maginn [6] studied CO2 gas–liquid interfaces of [BMIM][NTf2] at a range of temperatures and pressures and observed the process of interfacial diffusion and absorption of CO2 in the bulk of the IL as equilibrium was approached. Additionally, they expanded their analysis and moved to more realistic systems by including water in the mixtures due to its presence in a real CO2 capture operation.[7] They found that the interfacial transport dynamics of CO2 is not strongly affected by the presence of H2O. In a similar way, Karadas et al. [8] reported a combined experimental and computational study on the absorption of CO2 on [BMIM][PF6], [EMIM][NTf2] and [BMIM][NTf2] that showed the relevance of the strength of the interactions between anions and cations on CO2 absorption. On the other hand, Siqueira et al. [9] investigated the effect of adding SO2 to [BMIM][Br] on its structure and dynamics by means of

179

low-frequency Raman spectroscopy and MD simulations. They indicated the formation of a liquid mixture with low viscosity and high conductivity as a result of the absorption of gaseous SO2 by solid [BMIM][Br]. In the present contribution, we review some of the most important MD results, including some from our own groups, on mixtures of protic and aprotic ILs with water, alcohols and salts, published during the last years. We also include a brief review of recent MD simulations of mixtures near interfaces. Computational details are briefly described in Section 2, the results are presented and discussed in Section 3, and in Section 4 we summarize the main conclusions. 2. Simulation details The whole set of simulations reported in this paper was carried out using the open source Gromacs 4.5.4 package [58,59]. Gromacs includes several force fields, among which we employed GROMOS-96 [60] (a united atom force field) for the mixtures of imidazolium-based ILs with water, and OPLS-AA [61] (an all-atom force field) for the rest of the simulations. In both cases, the potential energy includes both bonded interactions (a repulsion term, a dispersion term, and a Coulomb term) and non-bonded interactions (bond stretching, bond angle and dihedral angle interactions). The initial configurations of the molecules in the simulation box were obtained in all cases by means of PACKMOL [62]. Details about the specific calculations (box size, number of molecules) have been reported elsewhere [3,42,40,63–66,5]. The molecules were randomly placed in a box whose sides were slightly larger than the corresponding experimental size and all atoms belonging to different molecules were required to keep safe pairwise distances. The number of solute molecules was calculated for each situation by considering each ionic pair as a single unit in the calculation of mole fractions. All these initial configurations were relaxed for 106 steps using a conjugated gradients algorithm with the aim of removing bad contacts resulting from the initial random configuration of ions. Then the system was equilibrated for several nanoseconds in the isothermal–isobaric (NpT) ensemble and, after that, except for the mixtures EAN/water and EAN/alcohols, the results of an additional simulation in the isothermal–isobaric ensemble were used for the analysis of the structure of the mixtures. On the other hand, the Empirical Potential for Structural Refinement (EPSR) approach is used to model the diffraction data [67]. This is a robust approach that is based on the optimization of the empirically derived potential that leads to the best agreement with experimental diffraction data. Details about the calculated quantities and the corresponding experimental data can be found in the original publications [4,68–70]. The potential used for the EAN compound is the same that Atkin successfully used in a series of EPSR-based studies of protic ionic liquids (PILs) [71] and their mixtures with water [72]. Several observables were extracted from simulated snapshots, including radial distribution functions (rdfs), angular distribution functions (adfs), and spatial distribution functions (sdfs) using either the EPSR built-in routines or the TRAVIS software [73]. 3. Results and discussion In the present section we include some of the main MD results that have been reported concerning the structure and dynamics of IL/water mixtures. The differences between protic and aprotic ionic liquids in what water solvation is concerned are discussed, as are the main features of the single-particle dynamics of water in these bulk mixtures. Moreover, some very recent results about water at interfaces of IL/water mixtures are also discussed. The rest of the section is devoted to present the corresponding results for IL/alcohol and IL/salt mixtures. A very brief summary of some recent results on the specific features of solvation of these solutes at polarized or nonpolarized interfaces is also included in the last part of this section.

180

L.M. Varela et al. / Journal of Molecular Liquids 210 (2015) 178–188

3.1. IL/water mixtures Among IL mixtures, those with water have concentrated most of the attention of MD researchers. To the best of our knowledge, Hanke and Lynden-Bell [49] reported in 2003 the first MD study of the local structure and dynamics of mixtures of water with two ILs, 1,3-dimethyl imidazolium chloride and hexafluorophosphate ([DMIM][Cl] and [DMIM][PF6], respectively), two anions of different hydrophobicity degree. The authors found that water molecules tend to be isolated from each other in IL-rich mixtures, and that a water percolating network is formed when the molar proportion of water molecules reaches 75%. The influence of water in the nanostructural organization of ILs in mixtures of 1-octyl-3-methylimidazolium nitrate ([OMIM][NO3]) with water was further analyzed by Jiang et al. [50], who studied the evolution of polar and water networks via the radial distribution functions and found an almost invariant characteristic length of 20 Å for the mesomorphic structures in IL/water mixtures like polar networks, water networks, and micelles. Water addition was seen to continuously disrupt the polar network until a turnover point after which water–water interactions overwhelmingly dominate the phase behavior and only loose micelle structure exist. This turnover was described as resulting from the competition between the hydrophobic interactions of the nonpolar groups and the breakdown of the charged polar network with increasing water content. Feng and Voth [51] also reported MD simulations of three IL/water mixtures at various water mole fractions: [BMIM][BF4], [OMIM][BF4], and [BMIM][Cl], trying to address the effect of alkyl side chain length and anion on the structure and dynamics of IL/water mixtures. They confirmed stronger aggregation of the cations and slower diffusion of the anions for longer, more hydrophobic chains, and the deeper impact of [Cl]− anion on the water distribution and diffusion in IL-rich mixtures than that of [BF] − 4 anion. In 2011, some of us also reported MD calculations of the structure and single-particle dynamics of mixtures of water with 1-alkyl-3methylimidazolium IL mixtures [3]. We performed systematic calculations for different lengths of the cation alkyl chain (alkyl = ethyl, butyl, hexyl, and octyl) and several counterions, including two halogens of different sizes and positions in the Hoffmeister series, [Cl]− and [Br]−, and the highly hydrophobic inorganic anion [PF] − 6 throughout its whole solubility regime. Radial distribution functions, coordination numbers, hydrogen bonding degree, self-diffusion coefficients and the velocity autocorrelation functions of water molecules in mixtures were analyzed. We showed the formation of water clusters, whose size is relatively independent on the cation chain length, but strongly dependent on the hydrophobicity of the anion. According to the reported results, it is the latter which mainly controls the formation of a water network and therefore the miscibility of the IL. In this contribution the effect of cation, anion, and water concentration on the duration of the ballistic regime and on the time of transition to the diffusive regime was analyzed, and the results were very much in line with those previously reported in the literature, finding complex non-Markovian behavior at intermediate times, which become shorter the more water-rich the mixture is [3,74]. From these reported results, we can conclude that water forms relatively isolated clusters in bulk ILs, whose size increases up to a point where they form a percolating network. The size of the clusters and their precise form are very much dependent of the physical properties of the IL. In Fig. 1 we represent the radial distribution functions g i j ðr 12 Þ ¼

1 X D ! !  ! ! E δ r 1− r i δ r 2− r j ρi ρ j i; j

ð1Þ

for different species in aprotic imidazolium-based IL/water mixtures [3], where ρk is the number density of species k, the sum extends over all ions of species i and j and brackets indicate the equilibrium ensemble average. The figure shows the influence of water concentration on the

structure of the mixtures. As can be seen there, heavy clustering of water at low concentrations both for hydrophilic anions and, specially, for hydrophobic ones like [PF6]− is observed, as shown by the heights of the first peaks of the water–water radial distribution functions relative to their values in pure water. However, as water concentration increases the number of water molecules in water clusters decreases down to the value of pure water. Moreover, the same figure evidences the preferential attachment of water molecules to other water molecules or to anions, compared to that to cations. This clustering process indicated by the height of the first peaks was further confirmed and quantified in ref. [75]. The authors studied the influence of cosolvents (water, methanol, ethanol) on the formation of ionic and molecular clusters over the whole miscibility range of mixtures with [BMIM][BF4], as well as on the angular distribution of neutral polar molecules around the anion and the cation in these systems. They showed that the addition of a molecular species breaks down the polar network of the pure ionic liquid in progressively smaller clusters as the mixture is enriched with cosolvent. Moreover, contrarily to those with alcohol molecules, aqueous mixtures were found to have water aggregates that eventually become quite large at high water concentrations. These clusters are mainly placed in the polar regions of the mixtures, as can be seen in Fig. 1, where water is located near the anions in the bulk mixtures. As for water single particle dynamics, in [3] it was proved – pioneeringly calculating velocity autocorrelation functions for IL-water mixtures – that for the more hydrophilic anions water molecules are placed in cages where they undergo a “rattling” motion, which gives rise to intermediate subdiffusive regimes typical of glass formers, and which is not detected for hydrophobic anions like [PF6]–. Nevertheless, for the latter, diffusive times are considerably shorter, as can be seen in Fig. 2, indicating that water is more strongly clusterized in the polar regions of the mixtures the more hydrophobic the cation is. Moreover, and confirming the presence of water in the polar nanoregions of the mixture, we see that these diffusive times generally increase with the alkyl chain length of the IL. This figure also shows that eventually the diffusive times of pure water are reached around 75% water concentration (evidently, in those mixtures which are stable at those concentrations), in agreement with Hanke and Lynden-Bell's prediction of a percolating network in the neighborhood of that concentration [49]. In summary, there is quite strong evidence that water is solvated in AILs (thereby incapable of forming hydrogen bonds with water molecules) forming clusters in cavities inside the polar nanoregions of bulk IL mixtures, which progressively grow until they form a percolating network. The situation is slightly different if we consider hydrogen-bonded PILs, but the overall picture is quite similar. Some results on water solvation in these kind of systems have been reported by Hayes et al. [72], Bodo et al. [76] and Docampo-Álvarez et al. [5]. The first group reports X-ray measurements and EPSR calculations for aqueous mixtures of ethylammonium nitrate (EAN). The authors conclude that a well-defined interface exists also in PIL/water mixtures separating nonpolar ethyl groups on the one side, and charged groups and water on the other, which increase the curvature of these interfaces due to the enlargement of cation head groups. This changes the well-known flat L3-sponge like structure of pure EAN to a branched network in mixtures. Likewise, Bodo et al. [76] reported Raman spectra and MD simulations for butylammonium nitrate (BAN)/water mixtures, whereby they found enhanced local intermediate range order of the IL ions upon water addition, with water placed in the polar regions of the mixture closer to the anions – with which they form an extended hydrogen bond network – than to cations. This image was also reported by Docampo-Álvarez et al. [5], who found in their MD simulations a homogeneous mixing process of added water molecules, which progressively accommodate themselves in the network of hydrogen bonds of the PIL. However, no water clustering similar to that in aprotic mixtures was detected in protic aqueous mixtures. Instead, they reported what seems to be the formation of a percolating water network with a sudden replacement

L.M. Varela et al. / Journal of Molecular Liquids 210 (2015) 178–188

16 14 12 10 8 6 4 2 0 0. 0

cation − water

water − water

(a) 10.6% g (r)

g (r)

anion − water

0. 2

0. 4

0. 6

0. 8

1. 0

16 14 12 10 8 6 4 2 0 0. 0

0. 2

g (r)

g (r) 0. 4

0. 6

0. 8

1. 0

16 14 12 10 8 6 4 2 0 0. 0

g (r)

g (r) 0. 4

0. 6

0. 2

0. 8

1. 0

30 25 20 15 10 5 0 0. 0

(g) 50% g (r)

g (r)

10 5 0. 2

0. 4

0. 6

0. 4

0. 6

0. 8

1. 0

0. 2

0. 4

0. 6

0. 8

1. 0

r (nm)

20

0 0. 0

1. 0

(f) 25%

r (nm)

15

0. 8

r (nm)

(e) 10.6%

0. 2

0. 6

(d) 75%

r (nm) 60 50 40 30 20 10 0 0. 0

0. 4

r (nm)

(c) 50%

0. 2

water − water (pure water)

(b) 25%

r (nm) 16 14 12 10 8 6 4 2 0 0. 0

181

0. 8

1. 0

r (nm)

12 10 8 6 4 2 0 0. 0

(h) 75%

0. 2

0. 4

0. 6

0. 8

1. 0

r (nm)

Fig. 1. Evolution of water–water, water–anion, and water–cation radial distribution functions with water molar fraction in mixtures of [HMIM][Cl] (a)–(d) and [HMIM][PF6] (e) (h) [3]. The results for pure water are shown for comparison purposes.

of [NO3]− anions in the first hydration shell of the polar heads of the IL cations around 60% water concentration in mixtures with EAN. 3.2. Alcohol mixtures As with experimental studies, MD studies of alcohol/IL mixtures are much less abundant that those concerning aqueous mixtures. However, in the recent past several contributions have been reported. Hanke et al. [41] performed a computational study of mixtures of [DMIM][Cl] with water, methanol, dimethyl ether, and propane, finding that the alcohol (and also water) is strongly hydrogen-bonded to the IL anion, whereas for dimethyl ether and propane dispersive interactions with the imidazolium cations are more important. This is connected to the inherent polar/apolar segregation of the bulk mixtures, and was later confirmed by Canongia-Lopes et al. [43] analyzing the solvation of nonpolar, polar and associating solutes in mixtures of [BMIM][PF6] with n-hexane, acetonitrile, methanol, and water. The local structure of IL/ alcohol mixtures was also considered by Raabe and Köhler [44], who

reported a computational study for mixtures of [AMIM][Cl] (A = E, B, H) and ethanol and 1-propanol at several temperatures. In their contribution, the authors proved that the polar/nonpolar mesoscopic ordering in the bulk mixture is essentially controlled by i) strong hydrogen-bond interactions between the chloride anion and the hydroxyl groups, ii) anion– cation charged domain interactions, and iii) the hydrophobic aggregation of the nonpolar alkyl tails of the molecules in the mixture, which determines the formation of nonpolar domains whose sizes increase with the cation size. Contrarily to water, amphiphilic alcohol molecules can be placed between the polar and apolar regions of the mixture, which determines the tendency of these compounds to form more homogeneous, less segregated mixtures with ILs. This picture was later reinforced by results of Jahangiri et al. [45]. Finally, as regards single particle dynamics of IL/alcohol mixtures, very few results have been previously reported. Heintz et al. [46] calculated tracer diffusion coefficients of the IL ions in mixtures of 1-ethyl, and 1-butyl-3-methylimidazolium and bis-(trifluoromethanesulfonyl)imide ([EMIM][NTf2] and [BMIM][NTf2], respectively), with both water and methanol.

182

L.M. Varela et al. / Journal of Molecular Liquids 210 (2015) 178–188

from a representative snapshot and only the ionic moieties (ammonium and nitrate are reported)). The same behavior is confirmed for mixtures composed of EAN and longer chained alcohols, and results will be subsequently reported. This could be very well attributed to alcohol molecules being placed in the borders between polar and apolar regions, hydrogen bonding to the PIL anions and hence defining these observed mesoscopic ionic clusters. These structural differences between IL/water and IL/alcohol mixtures are also undoubtedly behind the different single-particle dynamic behavior observed for the latter, reported by Méndez-Morales et al. [40]. The normalized velocity autocorrelation function D

E ! ! v ðt Þ  v ð0Þ E; C ðt Þ ¼ D ! ! v ð0Þ  v ð0Þ

Fig. 2. Concentration dependence of the diffusive times of aqueous mixtures of different AILs throughout the different miscibility regimes [3].

For our part, some of us have previously reported studies on the structure and dynamics of mixtures of alcohols with both aprotic [42,40] and protic [5] ILs. The structural picture emerging from these studies is compatible with that previously reported in the literature [44–46]: alcohols mix with ILs in a much more homogeneous fashion than water, placing themselves approximately at the same distance from all other species in the mixture, with no evidence of alcohol clusters as large as those of water in these mixtures (see Fig. 3 and [75]). As mentioned above, this is undoubtedly connected to the amphiphilicity of alcohol molecules, which allows the partial insertion of these organic solvents in the border of the polar and apolar nanodomains in the bulk. Indeed, recently, some of us reported evidences that the polar vs apolar segregation paradigm might be challenged in PILs + alcohol mixtures [4,68]. Joint use of neutron scattering data and reverse Monte Carlo (RMC) simulations indicate that EAN–methanol mixtures are far from being mesoscopically homogeneous. There is a welldefined concentration range where structural heterogeneities can be observed experimentally that are the fingerprint of mesoscopic demixing [68,69]. While no alcohol segregation was observed, RMC indicate that ionic clusters form whose spatial extent is of the order of nm (see Fig. 4 where a representative slice 30 × 30 × 20 Å was extracted

ð2Þ

! where v ðtÞ is the velocity of the center of mass of the molecule at time t and the brackets indicate the ensemble average, is represented in Fig. 5 for cosolvent molecules in mixtures of AILs and PILs with water and alcohol. As can be seen there, contrarily to aqueous mixtures, no evidence of the rattling motion associated to a caging effect is seen for alcohol molecules, which diffuse much more easily than their water counterparts. This can be easily explained by the combination of two effects: i) the absence of heavy clustering of alcohol molecules, and ii) the greater ability of alcohol to diffuse in the apolar parts of the bulk mixtures. This further confirms the ability of alcohol molecules to be homogeneously solvated between apolar and polar nanoregions of the mixture, in marked contrast with water which, due to its greater lipophobicity, is constrained to form clusters in the polar regions of the mixture. To sum up, from MD simulations the solvation of alcohols in ILs could follow quite similar trends as that in water, which we have previously termed nanostructured solvation: the different regions of the bulk IL accommodate the similar (polar or apolar) parts of the solute molecules, which crucially determines the mesomorphic structure of the mixtures as well as their dynamics. Of course, another possible reading of these results is that these solutes do not destroy the nanosegregated structure of the ILs, but rather adapt to it, becoming solvated in the most favorable region of the mixture. In a coming subsection we will see that this picture of solvation in IL environments also applies to solutions of 1:1 salts in these dense ionic solvents.

Fig. 3. Radial distribution functions of alcohol molecules (ethanol and methanol) and the rest of the species in a mixture with [HMIM][Cl] and EAN. Water–water radial distribution functions in mixtures with the same IL are included for purposes of comparison [42,5].

L.M. Varela et al. / Journal of Molecular Liquids 210 (2015) 178–188

183

morphology of neat EAN. DMSO tends to compete with the nitrate anion in HB interacting with the ammonium head: typically one or two DMSO molecules tend to coordinate the ammonium head (Fig. 6). This interaction leads to the formation of a recurrent structural leitmotif with one central ammonium cation coordinated through two HBs by two DMSO molecules. Present studies are now focusing in the direction of detecting stoichiometric complexes in these mixtures. 3.4. Salt mixtures

Fig. 4. Ionic species cluster size distribution for a mixture of EAN/methanol with XEAN = 0.15. The inset shows a representative snapshot (30 × 30 × 20 Å) where only ionic moieties are present.

3.3. Dimethyl sulfoxide (DMSO) mixtures Aiming at expanding the range of binary mixtures of ILs with molecular liquids, some of us recently undertook the exploration of EAN and dimethyl sulfoxide (DMSO), a ubiquitous solvation medium in chemical engineering. Similarly to water–DMSO, EAN–DMSO mixtures are characterized by a much lower melting point than those of the neat components. This feature can be of great interest for several bio-oriented applications. DMSO is a well-known polar, aprotic solvent that can build strong hydrogen bonding interactions with HB donors. Accordingly, it shows hints of interactions with EAN that can behave both as HB donor and acceptor. Some of us recently presented a joint experimental (X-ray and neutron scattering) and computational (RMC) study on a binary mixture of EAN–DMSO of molar fraction X_{EAN} = 0.4 [69]. Experimentally this mixture is structurally homogeneous as, at odds with EAN–alcohol ones, it does not show any hint of critical fluctuations. Moreover, DMSO progressively disrupts the polar-vs-apolar segregated

The solvation of salts in ILs is currently under intense scrutiny due to the potential applications of these mixtures in many fields starting with electrochemistry. The practical electrochemical inertness of ILs (the upper limit of their electrochemical window has been recently fixed at 6 V [77–79]) makes them optimal candidates for use as electrolytes in advanced electrochemical devices. Hence, a detailed picture of the structural and ion transport properties of lithium salt-doped ILs is of essential importance. This has lead to many experimental [80–83] and computational [84–87] studies of the properties of IL/salt mixtures. However, the solvation of monovalent cations – starting with lithium – and their transport in dense ionic environments have not been analyzed very extensively neither in experimental studies, nor even by MD simulations. In this brief review we will focus in some recent contributions to this topic reported by some of us. With regards to experimental results, we must highlight those of Hayamizu et al. [80], who reported self-diffusion coefficients of each ion in mixtures of 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM] [BF4]) with LiBF4, and observed that ionic conductivity and ionic self-diffusion decreased with increasing concentration of added salt. Moreover, they reported, using spin echo NMR, that [Li]+ cations form associated structures in the bulk IL/salt mixtures and diffuse under the influence of their counterions. This behavior was later confirmed by Duluard et al. [81] for [BMIM][NTf2] doped with a lithium salt with common anion, and also by Lassègues et al. [88] and Umebayashi et al. [82]. In addition, Monteiro [83] confirmed by Raman spectroscopy of mixtures of [EMIM][NTf2] with a Li salt with common anion the formation of aggregates of [Li]+ cations and two [NTf2]− anions, stronger than the ones of the anion with the IL cations. Likewise, by means of MD simulations Méndez-Morales et al. [63] reported the formation of stable [Li]+ and [Na]+-anion clusters in mixtures of [BMIM]-based ILs with Li salts with common anion, and

Fig. 5. Velocity autocorrelation functions of water/[HMIM][Cl], alcohol/[HMIM][Cl], water/EAN, and alcohol/EAN [40,5].

184

L.M. Varela et al. / Journal of Molecular Liquids 210 (2015) 178–188

entities inside which a marked rattling motion of salt ions takes place in resilient cages. Fig. 7(b) shows a representation of the cage correlation function   H i j ðt ÞH i j ð0Þ ; H i j ð0ÞH i j ð0Þ

P ðt Þ ¼ 

Fig. 6. Distribution of H-bonds for the EAN–DMSO mixture (XEAN = 0.4). Black symbols refer to ammonium (nitrogen atom, N1)–DMSO (oxygen atoms), while red symbols refer to ammonium (nitrogen atom, N1)–nitrate (oxygen atoms) correlations.

they analyzed the effect of the anion considering mixtures with [PF6]−, [BF4]− and [NTf2]−. Moreover, Méndez-Morales et al. [64] and Russina et al. [70] reported experimental and computational studies of the solvation of LiNO2 in primary ammonium nitrate PILs of different chain lengths (EAN, PAN and BAN), a category of ILs which was confirmed as usable electrolytes for lithium batteries by Menne et al. [89]. In all these contributions the fundamental basis of nanostructured solvation was once again confirmed, since salt cations are mainly confined to the polar regions of the mixtures with heavy lithium-anion clustering being visible in the radial distribution functions (see Fig. 7(a)). Moreover, Méndez-Morales et al. [63] reported a strong coordination of lithium and sodium cations in two different positions (monodentate and bidentate) with the anion present in the polar regions of the mixture giving rise to an intermediate range pseudolattice-like ordering. This is clearly visible in the double first peaks of the Li-anion g(r)s. The cations of the added salt were found to form bonded-like, long-lived aggregates with the anions in their first solvation shell, giving rise to stable kinetic

ð3Þ

of 15% mixtures of Li salt mixtures with [BMIM][BF4] and EAN, where Hij(t) takes the value of 1 if [Li] + cations, i, and the center of mass of their surrounding anions, j, are closer than the position of the first minimum of the lithium-anion rdfs at time t, and zero otherwise. The brackets indicate the average over all time origins. Even though non polarizable potentials were used there [63], the authors were able to prove that these entities were on the basis of the decrease of self-diffusion coefficients and ionic conductivities also reported in other experimental and computational studies using polarizable potentials [84–86]. Moreover, Varela and coworkers later extended the study to PILs [64] and found the same structural picture: nanostructurally solvated [Li]+ cations forming [Li(Anion)n]1 − n in pseudolattice-like ordered polar nanoregions. Further evidence was reported for mixtures with PILs, indicating that the number of hydrogen bonds between IL species monotonically decreases upon salt addition, indicating the presence of salt cations in the polar regions of the mixture and their gradual disruption of the hydrogen network. The authors also report that in mixtures of LiNO3 with EAN the density of lithium cations around nitrates in the polar regions is about 4.34 [Li]+/nm3, one order of magnitude greater than that in the neighborhood of the cation alkyl groups forming the apolar regions (0.36 [Li]+/nm3) [65]. The calculated coordination numbers for [Li]+ cations were ca. 3, 3.3, and 2 anions for mixtures with [PF6]−, [BF4]− and [NTf2]−, respectively [63,87,84], and lower than 3 for nitrate anions [64,90,91], and these values were reported to increase with the concentration of added salt. On its side, [Na]+ cations were found to coordinate with nearly three hexafluorophosphates in these works. Hence, all MD calculations point to added salt cations being solvated in the polar nanoregions of the bulk mixtures. Another very interesting fact that was addressed by Méndez-Morales et al. [64] and by Russina et al. [70] was the actual state of the added salt inside the polar nanoregions of the mixture. Although it could be reasonably expected to behave like a molten salt, indeed it does behave like a solid-like

Fig. 7. Radial distribution functions (a) and cage correlation functions (b) of IL–Li salt mixtures [63,64,66].

L.M. Varela et al. / Journal of Molecular Liquids 210 (2015) 178–188

structure, showing intermediate range ordering and lithium-anion radial distribution functions with a double first peak structure that closely resemble the calcite-type structure characteristic of solid lithium nitrate [92,93] instead of that of molten lithium nitrate [94–98]. These solidlike aggregates are supposed to ultimately give rise to phase separation when salt concentration increases. In addition, it is interesting to highlight the fact that the structure of the ILs is notably resilient to the addition of any kind of solute, whether it be water, alcohols or salts. This feature can be observed in Fig. 8, in which we show the concentration dependence of cation–anion radial distribution functions of several mixtures. It can be seen that the height of the peaks is only slightly decreased with the addition of salt, whereas their positions are not modified at all. Finally, the influence of the IL cation chain length has been recently analyzed by Méndez-Morales et al. [66] by means of small angle X-ray scattering and classical MD simulations. Studying propylammonium nitrate (PAN) and butylammonium nitrate (BAN) mixtures by means of small angle X-ray scattering and MD simulations, they proved that the increase of the alkyl chain length of the IL cations produces an increase in the level of segregation of the mixtures (see Fig. 9), with better defined polar and apolar domains and lower degrees of hydrogen bonding resulting in less densely packed, less resilient structures. Once more, the solvation of salts in ILs seems to confirm the nanostructured solvation paradigm of selectively placing the added substance in the similar IL nanodomains, which is graphically summarized in Fig. 10. 3.5. IL mixtures at interfaces As we have previously mentioned in this section, probably the most important fact about solvation of water, alcohols and salts in ILs is that these substances preserve the structure of the solvent up to quite high concentrations, and adapt to it being solvated in the solvophobically similar regions of the bulk mixtures. This rule marks also the behavior of these substances in mixtures near interfaces, as has recently been shown [65]. In the presence of a wall, the IL is forced to adopt a highly inhomogeneous structure (electric double layer), which, due to specific features of these dense ionic solvents, is made up of layers of charge due to correlation-governed overscreening of charge [99] which ultimately leads to electroneutrality at long distances from the wall. Moreover, for sufficiently charged walls, the finite size of the ions eventually leads to crowding of ions in the electric double layer.

185

Fig. 9. Li-anion radial distribution functions of 15% XAN–LiNO3 salt mixtures [66].

The question of the distribution of additives in these highly inhomogeneous structures in mixtures IL-additive has been addressed previously for water [100] and for Li salts [65]. In both cases, the distribution of additives (water molecules, [Li]+ cations) near the electrode is determined by that the IL would adopt in the absence of the additive. This confirms the resilience of the IL structure to the addition of solvents, as well as the essential features of the nanostructured solvation described above. In the case of water mixtures, Feng et al. [100] reported recently a MD analysis of the distribution of water/[BMIM][A] ([A] = [PF6] and [NTf2]) in the neighborhood of neutral and charged graphene planar electrodes, and they proved that the distribution of water in the double layers is governed not only by the spatial electric field in this structure and steric considerations, but also by the association of water molecules with their ionic surroundings. The situation is similar for the solvation of Li salts in imidazoliumbased mixtures in the neighborhood of graphene electrodes, as reported by Méndez-Morales et al. [65]. In this case, the addition of salt to the mixture has little effect on the distribution of the IL ions in the proximities of charged and uncharged walls. The organization of both [Li]+ and [K]+ is mainly controlled by the formation of the ionic aggregates that lithium and potassium cations arrange with the anions in their first solvation layer and not by the electrode potential. Thus, salt cations are able to be adsorbed on the negative graphene wall only when high concentrations of salt are reached.

Fig. 8. Concentration dependence of cation–anion radial distribution functions of (a) [BMIM][PF6] + water, (b) [HMIM][Cl] + water and (c) [BMIM][PF6] + LiPF6 mixtures [63,3].

186

L.M. Varela et al. / Journal of Molecular Liquids 210 (2015) 178–188

FIS2012-33126, MAT2014-57943-C3-1-P and MAT2014-57943-C3-3-P). All these research projects are partially supported by FEDER. T. M-M. and V. G-G. thank the Spanish ministry of Education for their FPU grants. O.R. acknowledges the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities and would like to thank Dr. T. Narayanan for his kind and competent assistance in exploiting beamline ID02. O.R. acknowledges support from FIRB-Futuro in Ricerca (RBFR086BOQ) and PRIN (2009WHPHRH). Facilities provided by the Galician Supercomputing Centre (CESGA) are also acknowledged.

References

Fig. 10. Graphical representation of the nanostructured solvation of water, salts (a) and alcohols (b) in [BMIM][BF4]. The employed colors are: N: blue, O: red, C: black, B: pink, F: light blue and H: white.

4. Conclusions We have provided a review of recent literature on the solvation features of molecular cosolvents and salts in ILs. The reported results seem to indicate that the IL structure is notably resilient to the presence of these additives, and that IL polar and apolar networks are only progressively eroded by the solutes. The solvation of the solutes takes place accommodating the molecular moieties of the additive particles in the nanoregions of the amphiphilically nanostructured IL that are more similar to their molecular entities. This microscopic picture has been termed nanostructured solvation. According to this mechanism, water tends to form relatively isolated clusters in non-hydrogen bonded AILs until a continuous water network appears in the bulk mixture. On the other hand, it homogeneously mixes with PILs with which it may form hydrogen bonds. The same picture applies to IL mixtures with alcohols, which do not so clearly clusterize due to their availability to mix with both the polar and apolar regions of the IL. As regards salt mixtures, the same solvation mechanism applies, with salt cations and anions solvated in the polar regions of the IL where they form middleranged solid-like pseudolattice structures that in the end act as crystallization nuclei. Finally, this form of solvation is also behind the peculiar order the mixtures adopt at interfaces, with solutes adapting to the layered structured emerging from overscreening and crowding mechanisms near polarized and non polarized walls. Hence, this nanostructure solvation is proposed as a sort of universal solvation mechanism in amphiphilically nanostructured dense ionic solvents. Acknowledgments The authors wish to thank the financial support of Xunta de Galicia through the research projects of references 10-PXI-103-294 PR, 10-PXIB-206-294 PR and GPC2013-043. Moreover, this work was funded by the Spanish Ministry of Science and Innovation (Grants nos.

[1] C.F. Poole, B.R. Kersten, S.S. Ho, M.E. Coddens, K.G. Furton, Organic salts, liquid at room temperature, as mobile phases in liquid chromatography, J. Chromatogr. A 352 (1986) 407–425. [2] P. Bonhte, A.P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Grtzel, Hydrophobic, highly conductive ambient-temperature molten salts, Inorg. Chem. 35 (5) (1996) 1168–1178. [3] T. Méndez-Morales, J. Carrete, O. Cabeza, L.J. Gallego, L.M. Varela, Molecular dynamics simulation of the structure and dynamics of water-1-alkyl-3methylimidazolium ionic liquid mixtures, J. Phys. Chem. B 115 (21) (2011) 6995–7008. [4] O. Russina, A. Sferrazza, R. Caminiti, A. Triolo, Amphiphile meets amphiphile: beyond the polar–apolar dualism in ionic liquid/alcohol mixtures, J. Phys. Chem. Lett. 5 (10) (2014) 1738–1742. [5] B. Docampo-Álvarez, V. Gómez-González, T. Méndez-Morales, J. Carrete, J.R. Rodríguez, O. Cabeza, L.J. Gallego, L.M. Varela, Mixtures of protic ionic liquids and molecular cosolvents: a molecular dynamics simulation, J. Chem. Phys. 140 (21) (2014) 214502. [6] M.E. Perez-Blanco, E.J. Maggin, Molecular dynamics simulations of CO2 at an ionic liquid interface: adsorption, ordering, and interfacial crossing, J. Phys. Chem. B 114 (2010) 11827–11837. [7] M.E. Perez-Blanco, E.J. Maggin, Molecular dynamics simulations of carbon dioxide and water at an ionic liquid interface, J. Phys. Chem. B 115 (35) (2011) 10488–10499. [8] F. Karadas, B. Köz, J. Jacquemin, E. Deniz, D. Rooney, J. Thompson, C.T. Yavuz, M. Khraisheh, S. Aparicio, M. Atihan, High pressure CO2 absorption studies on imidazolium-based ionic liquids: experimental and simulation approaches, Fluid Phase Equilib. 351 (2013) 74–86. [9] L.J.A. Siqueira, R.A. Ando, F.F.C. Bazito, R.M. Torresi, P.S. Santos, M.C.C. Ribeiro, Shielding of ionic interactions by sulfur dioxide in an ionic liquid, J. Phys. Chem. B 112 (20) (2008) 6430–6435. [10] S. Zhu, Y. Wu, Q. Chen, Z. Yu, C. Wang, S. Jin, Y. Ding, G. Wu, Dissolution of cellulose with ionic liquids and its application: a mini-review, Green Chem. 8 (4) (2006) 325–327. [11] H. Wang, G. Gurau, R. Rogers, Dissolution of biomass using ionic liquids, in: S. Zhang, J. Wang, X. Lu, Q. Zhou (Eds.), Structures and Interactions of Ionic Liquids, of Structure and Bonding, vol. 151, Springer Berlin, Heidelberg 2014, pp. 79–105. [12] A.S. Pensado, A.A.H. Pádua, Solvation and stabilization of metallic nanoparticles in ionic liquids, Angew. Chem. Int. Ed. 50 (37) (2011) 8683–8687. [13] J. Carrete, M. García, J. Rodrguez, O. Cabeza, L. Varela, Theoretical model for moisture adsorption on ionic liquids: a modified Brunauer–Emmet–Teller isotherm approach, Fluid Phase Equilib. 301 (1) (2011) 118–122. [14] S. Cuadrado-Prado, M. Domínguez-Pérez, E. Rilo, S. Garca-Garabal, L. Segade, C. Franjo, O. Cabeza, Experimental measurement of the hygroscopic grade on eight imidazolium based ionic liquids, Fluid Phase Equilib. 278 (1–2) (2009) 36–40. [15] J. Jacquemin, P. Husson, A.A.H. Padua, V. Majer, Density and viscosity of several pure and water-saturated ionic liquids, Green Chem. 8 (2006) 172–180. [16] B. Mokhtarani, A. Sharifi, H.R. Mortaheb, M. Mirzaei, M. Mafi, F. Sadeghian, Density and viscosity of pyridinium-based ionic liquids and their binary mixtures with water at several temperatures, J. Chem. Thermodyn. 41 (3) (2009) 323–329. [17] I.B. Malham, P. Letellier, M. Turmine, Evidence of a phase transition in water-1-butyl-3-methylimidazolium tetrafluoroborate and water-1-butyl-2, 3-dimethylimidazolium tetrafluoroborate mixtures at 298 K: determination of the surface thermal coefficient, bT,P, J. Phys. Chem. B 110 (29) (2006) 14212–14214. [18] E. Rilo, J. Vila, M. García, L.M. Varela, O. Cabeza, Viscosity and electrical conductivity of binary mixtures of CnMIM-BF4 with ethanol at 288 K, 298 K, 308 K, and 318 K, J. Chem. Eng. Data 55 (11) (2010) 5156–5163. [19] E. Rilo, J. Vila, S. García-Garabal, L.M. Varela, O. Cabeza, Electrical conductivity of seven binary systems containing 1-ethyl-3-methyl imidazolium alkyl sulfate ionic liquids with water or ethanol at four temperatures, J. Phys. Chem. B 117 (5) (2013) 1411–1418. [20] S. Fendt, S. Padmanabhan, H.W. Blanch, J.M. Prausnitz, Viscosities of acetate or chloride-based ionic liquids and some of their mixtures with water or other common solvents, J. Chem. Eng. Data 56 (1) (2011) 31–34. [21] T. Murphy, L.M. Varela, G.B. Webber, G.G. Warr, R. Atkin, Nanostructure-thermal conductivity relationships in protic ionic liquids, J. Phys. Chem. B 118 (41) (2014) 12017–12024. [22] S. Cha, M. Ao, W. Sung, B. Moon, B. Ahlstrom, P. Johansson, Y. Ouchi, D. Kim, Structures of ionic liquid–water mixtures investigated by IR and NMR spectroscopy, Phys. Chem. Chem. Phys. 16 (2014) 9591–9601.

L.M. Varela et al. / Journal of Molecular Liquids 210 (2015) 178–188 [23] J. Bowers, C.P. Butts, P.J. Martin, M.C. Vergara-Gutierrez, R.K. Heenan, Aggregation behavior of aqueous solutions of ionic liquids, Langmuir 20 (6) (2004) 2191–2198. [24] K. Marsh, J. Boxall, R. Lichtenthaler, Room temperature ionic liquids and their mixtures — a review, Fluid Phase Equilib. 219 (1) (2004) 93–98. [25] O. Cabeza, L. Segade, S. García-Garabal, E. Rilo, M. Domínguez-Pérez, L.M. Varela, in: A. Kokorin (Ed.), Ionic Liquids, Theory and Applications, InTech, 2011. [26] L. Segade, O. Cabeza, Physical properties of mixtures, in: A.P. de los Ríos, J.P. Hernández-Fernández (Eds.), Ionic Liquids in Separation Technology, Elsevier, Amsterdam 2014, pp. 34–69 (Ch. 1.4). [27] M.A. Firestone, J.A. Dzielawa, P. Zapol, L.A. Curtiss, S. Seifert, M.L. Dietz, Lyotropic liquid-crystalline gel formation in a room-temperature ionic liquid, Langmuir 18 (20) (2002) 7258–7260. [28] M. Antonietti, D. Kuang, B. Smarsly, Y. Zhou, Ionic liquids for the convenient synthesis of functional nanoparticles and other inorganic nanostructures, Angew. Chem. Int. Ed. 43 (38) (2004) 4988–4992. [29] L.M. Varela, J. Carrete, M. Turmine, E. Rilo, O. Cabeza, Pseudolattice theory of the surface tension of ionic liquid–water mixtures, J. Phys. Chem. B 113 (37) (2009) 12500–12505. [30] L.M. Varela, J. Carrete, M. García, J.R. Rodrguez, L.J. Gallego, M. Turmine, O. Cabeza, Pseudolattice theory of ionic liquids, in: A. Kokorin (Ed.), Ionic liquids: Theory, Properties, New Approaches, InTech, 2011. [31] M. Khodadadi-Moghaddam, A. Habibi-Yangjeh, M.R. Gholami, Kinetic study of heterogeneous catalytic hydrogenation of cyclohexene to cyclohexane in ionic liquid–alcohols mixtures, Appl. Catal. A Gen. 341 (12) (2008) 58–64. [32] V. Lair, J. Sirieix-Plenet, L. Gaillon, C. Rizzi, A. Ringued, Mixtures of room temperature ionic liquid/ethanol solutions as electrolytic media for cerium oxide thin layer electrodeposition, Electrochim. Acta 56 (2) (2010) 784–789. [33] A.B. Pereiro, A. Rodrguez, Experimental liquid–liquid equilibria of 1-alkyl-3methylimidazolium hexafluorophosphate with 1-alcohols, J. Chem. Eng. Data 52 (4) (2007) 1408–1412. [34] K. Sahandzhieva, D. Tuma, S. Breyer, A.P.S. Kamps, G. Maurer, Liquid–liquid equilibrium in mixtures of the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate and an alkanol, J. Chem. Eng. Data 51 (5) (2006) 1516–1525. [35] J.M. Crosthwaite, S.N.V.K. Aki, E.J. Maginn, J.F. Brennecke, Liquid phase behavior of imidazolium-based ionic liquids with alcohols, J. Phys. Chem. B 108 (16) (2004) 5113–5119. [36] U. Domańska, A. Marciniak, Solubility of ionic liquid [emim][PF6] in alcohols, J. Phys. Chem. B 108 (7) (2004) 2376–2382. [37] K. Kurnia, M. Mutalib, T. Murugesan, B. Ariwahjoedi, Physicochemical properties of binary mixtures of the protic ionic liquid bis(2-hydroxyethyl)methylammonium formate with methanol, ethanol, and 1-propanol, J. Solut. Chem. 40 (5) (2011) 818–831. [38] E. Rilo, L.M. Varela, O. Cabeza, Density and derived thermodynamic properties of 1-ethyl-3-methylimidazolium alkyl sulfate ionic liquid binary mixtures with water and with ethanol from 288 K to 318 K, J. Chem. Eng. Data 57 (8) (2012) 2136–2142. [39] K. Kurnia, B. Ariwahjoedi, M. Mutalib, T. Murugesan, Density and excess molar volume of the protic ionic liquid bis(2-hydroxyethyl)ammonium acetate with alcohols, J. Solut. Chem. 40 (3) (2011) 470–480. [40] T. Méndez-Morales, J. Carrete, M. García, O. Cabeza, L.J. Gallego, L.M. Varela, Dynamical properties of alcohol + 1-hexyl-3-methylimidazolium ionic liquid mixtures: a computer simulation study, J. Phys. Chem. B 115 (51) (2011) (15322-15322). [41] C.G. Hanke, N.A. Atamas, R.M. Lynden-Bell, Solvation of small molecules in imidazolium ionic liquids: a simulation study, Green Chem. 4 (2) (2002) 107–111. [42] T. Méndez-Morales, J. Carrete, O. Cabeza, L.J. Gallego, L.M. Varela, Molecular dynamics simulations of the structural and thermodynamic properties of imidazolium-based ionic liquid mixtures, J. Phys. Chem. B 115 (38) (2011) 11170–11182. [43] J.N. Canongia-Lopes, M.F. Costa-Gomes, A.A.H. Pádua, Nonpolar, polar, and associating solutes in ionic liquids, J. Phys. Chem. B 110 (34) (2006) 16816–16818. [44] G. Raabe, J. Köhler, Thermodynamical and structural properties of binary mixtures of imidazolium chloride ionic liquids and alcohols from molecular simulation, J. Chem. Phys. 129 (14) (2008) 144503(1)–144503(8). [45] S. Jahangiri, M. Taghikhani, H. Behnejad, S.J. Ahmadi, Theoretical investigation of imidazolium based ionic liquid/alcohol mixture: a molecular dynamic simulation, Mol. Phys. 106 (8) (2008) 1015–1023. [46] A. Heintz, R. Ludwig, E. Schmidt, Limiting diffusion coefficients of ionic liquids in water and methanol: a combined experimental and molecular dynamics study, Phys. Chem. Chem. Phys. 13 (8) (2011) 3268–3273. [47] M.S. Kelkar, W. Shi, E.J. Maginn, Determining the accuracy of classical force fields for ionic liquids: atomistic simulation of the thermodynamic and transport properties of 1-ethyl-3-methylimidazolium ethylsulfate ([emim][EtSO4]) and its mixtures with water, J. Ind. Eng. Chem. Res. 47 (23) (2008) 9115–9126. [48] M.S. Kelkar, E.J. Maginn, Calculating the enthalpy of vaporization for ionic liquid clusters, J. Phys. Chem. B 111 (32) (2007) 9424–9427. [49] C.G. Hanke, R.M. Lynden-Bell, A simulation study of water–dialkylimidazolium ionic liquid mixtures, J. Phys. Chem. B 107 (39) (2003) 10873–10878. [50] W. Jiang, Y. Wang, G.A. Voth, Molecular dynamics simulation of nanostructural organization in ionic liquid/water mixtures, J. Phys. Chem. B 111 (18) (2007) 4812–4818. [51] S. Feng, G.A. Voth, Molecular dynamics simulations of imidazolium-based ionic liquid/water mixtures: alkyl side chain length and anion effects, Fluid Phase Equilib. 294 (1–2) (2010) 148–156. [52] H.V.R. Annapureddy, Z. Hu, J. Xia, C.J. Margulis, How does water affect the dynamics of the room-temperature ionic liquid 1-hexyl-3-methylimidazolium

[53] [54]

[55] [56]

[57]

[58]

[59]

[60]

[61]

[62]

[63]

[64]

[65]

[66]

[67] [68]

[69]

[70]

[71] [72] [73] [74] [75]

[76] [77]

[78] [79] [80]

[81]

187

hexafluorophosphate and the fluorescence spectroscopy of coumarin-153 when dissolved in it? J. Phys. Chem. B 112 (6) (2008) 1770–1776. X. Liu, T.J.H. Vlugt, A. Bardow, Maxwell–Stefan diffusivities in binary mixtures of ionic liquids with DMSO and H2O, J. Phys. Chem. B 115 (26) (2011) 8506–8517. X. Wu, Z. Liu, S. Huang, W. Wang, Molecular dynamics simulation of roomtemperature ionic liquid mixture of [BMIM][BF4] and acetonitrile by a refined force field, Phys. Chem. Chem. Phys. 7 (14) (2005) 2771–2779. V.V. Chaban, O.V. Prezhdo, How toxic are ionic liquid/acetonitrile mixtures? J. Phys. Chem. Lett. 2 (19) (2011) 2499–2503. F. Bardak, D. Xiao, L.G. Hines, P. Son, R.A. Bartsch, E.L. Quitevis, P. Yang, G.A. Voth, Nanostructural organization in acetonitrile/ionic liquid mixtures: molecular dynamics simulations and optical Kerr effect spectroscopy, ChemPhysChem 13 (7) (2012) 1687–1700. X. Huang, C.J. Margulis, Y. Li, B.J. Berne, Why is the partial molar volume of CO2 so small when dissolved in a room temperature ionic liquid? Structure and dynamics of CO2 dissolved in [Bmim+][PF6]−, J. Am. Chem. Soc. 127 (50) (2005) 17842–17851. D.V.D. Spoel, E. Lindahl, B. Hess, A.R.V. Buuren, E. Apol, P.J. Meulenhoff, D.P. Tieleman, A.L.T.M. Sijbers, K.A. Feenstra, R.V. Drunen, H.J.C. Berendsen, Gromacs User Manual version 4.0, http://www.Gromacs.org2005. B. Hess, C. Kutzner, D.V.D. Spoel, E. Lindahl, Gromacs 4: algorithms for highly efficient, load-balanced, and scalable molecular simulation, J. Chem. Theory Comput. 4 (3) (2008) 435–447. W.R.P. Scott, P.H. Hünenberger, I.G. Tironi, A.E. Mark, S.R. Billeter, J. Fennen, A.E. Torda, T. Huber, P. Krüger, W.F.V. Gunsteren, The GROMOS biomolecular simulation program package, J. Phys. Chem. A 103 (19) (1999) 3596–3607. W.L. Jorgensen, D.S. Maxwell, J. Tirado-Rives, Development and testing of the OPLS all-atom force field on conformational energetics and properties of organic liquids, J. Am. Chem. Soc. 118 (45) (1996) 11225–11236. L. Martínez, R. Andrade, E.G. Birgin, J.M. Martínez, Packmol: a package for building initial configurations for molecular dynamics simulations, J. Comput. Chem. 30 (13) (2009) 2157–2164. T. Méndez-Morales, J. Carrete, S. Bouzón-Capelo, M. Pérez-Rodrguez, O. Cabeza, L.J. Gallego, L.M. Varela, Md simulations of the formation of stable clusters in mixtures of alkaline salts and imidazolium-based ionic liquids, J. Phys. Chem. B 117 (11) (2013) 3207–3220. T. Méndez-Morales, J. Carrete, O. Cabeza, O. Russina, A. Triolo, L.J. Gallego, L.M. Varela, Solvation of lithium salts in protic ionic liquids: a molecular dynamics study, J. Phys. Chem. B 118 (2014) 761–770. T. Mendez-Morales, J. Carrete, M. Perez-Rodriguez, O. Cabeza, L.J. Gallego, R.M. Lynden-Bell, L.M. Varela, Molecular dynamics simulations of the structure of the graphene-ionic liquid/alkali salt mixtures interface, Phys. Chem. Chem. Phys. 16 (2014) 13271–13278. T. Mendez-Morales, J. Carrete, J.R. Rodriguez, O. Cabeza, L.J. Gallego, O. Russina, L.M. Varela, Nanostructure of mixtures of protic ionic liquids and lithium salts: effect of alkyl chain length, Phys. Chem. Chem. Phys. 17 (2015) 5298–5307. A. Soper, Empirical potential Monte Carlo simulation of fluid structure, Chem. Phys. 202 (23) (1996) 295–306. O. Russina, A. Mariani, R. Caminiti, A. Triolo, Structure of a binary mixture of ethylammonium nitrate and methanol, J. Solut. Chem. (2015)http://dx.doi.org/10.1007/ s10953-015-0311-7. O. Russina, M. Macchiagodena, B. Kirchner, A. Mariani, B. Aoun, M. Russina, R. Caminiti, A. Triolo, Association in ethylammonium nitratedimethyl sulfoxide mixtures: first structural and dynamical evidences, J. Non-Cryst. Solids 407 (2015) 333–338. O. Russina, R. Caminiti, T. Mndez-Morales, J. Carrete, O. Cabeza, L. Gallego, L. Varela, A. Triolo, How does lithium nitrate dissolve in a protic ionic liquid? J. Mol. Liq. 205 (2015) 16–21. R. Hayes, S. Imberti, G.G. Warr, R. Atkin, Amphiphilicity determines nanostructure in protic ionic liquids, Phys. Chem. Chem. Phys. 13 (2011) 3237–3247. R. Hayes, S. Imberti, G.G. Warr, R. Atkin, How water dissolves in protic ionic liquids, Angew. Chem. Int. Ed. 51 (30) (2012) 7468–7471. M. Brehm, B. Kirchner, TRAVIS — a free analyzer and visualizer for Monte Carlo and molecular dynamics trajectories, J. Chem. Inf. Model. 51 (2011) 2007–2023. S.M. Urahata, M.C.C. Ribeiro, Single particle dynamics in ionic liquids of 1-alkyl-3methylimidazolium cations, J. Chem. Phys. 122 (2) (2005) 024511(1)–024511(9). J. Carrete, T. Méndez-Morales, O. Cabeza, R.M. Lynden-Bell, L.J. Gallego, L.M. Varela, Investigation of the local structure of mixtures of an ionic liquid with polar molecular species through molecular dynamics: cluster formation and angular distributions, J. Phys. Chem. B 116 (20) (2012) 5941–5950. E. Bodo, S. Mangialardo, F. Capitani, L. Gontrani, F. Leonelli, P. Postorino, Interaction of a long alkyl chain protic ionic liquid and water, J. Chem. Phys. 140 (20) (2014) 204503. M.C. Buzzeo, R.G. Evans, R.G. Compton, Non-haloaluminate room-temperature ionic liquids in electrochemistry—a review, Chem. Phys. Chem. 5 (8) (2004) 1106–1120. F. Endres, S.Z.E. Abedin, Air and water stable ionic liquids in physical chemistry, Phys. Chem. Chem. Phys. 8 (18) (2006) 2101–2116. M. Galiński, A. Lewandowski, I. Stepniak, Ionic liquids as electrolytes, Electrochim. Acta 51 (2006) 5567–5580. K. Hayamizu, Y. Aihara, H. Nakagawa, T. Nukuda, Ionic conduction and ion diffusion in binary room-temperature ionic liquids composed of [emim][BF4] and LiBF4, J. Phys. Chem. B 108 (50) (2004) 19527–19532. S. Duluard, J. Grondin, J. Bruneel, I. Pianet, A. Grélard, G. Campet, M. Delville, J. Lassègues, Lithium solvation and diffusion in the 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquid, J. Raman Spectrosc. 39 (5) (2008) 627–632.

188

L.M. Varela et al. / Journal of Molecular Liquids 210 (2015) 178–188

[82] Y. Umebayashi, T. Mitsugi, S. Fukuda, T. Fujimori, K. Fujii, R. Kanzaki, M. Takeuchi, S. Ishiguro, Lithium ion solvation in room-temperature ionic liquids involving bis(trifluoromethanesulfonyl) imide anion studied by Raman spectroscopy and DFT calculations, J. Phys. Chem. B 111 (45) (2007) 13028–13032. [83] M.J. Monteiro, F.F.C. Bazito, L.J.A. Siqueira, M.C.C. Ribeiro, R.M. Torresi, Transport coefficients, Raman spectroscopy, and computer simulation of lithium salt solutions in an ionic liquid, J. Phys. Chem. B 112 (7) (2008) 2102–2109. [84] O. Borodin, G.D. Smith, W. Henderson, Li+ cation environment, transport, and mechanical properties of the LiTFSI doped n-methyl-n-alkylpyrrolidinium+TFSI− ionic liquids, J. Phys. Chem. B 110 (34) (2006) 16879–16886. [85] O. Borodin, G.D. Smith, P. Fan, Molecular dynamics simulations of lithium alkyl carbonates, J. Phys. Chem. B 110 (45) (2006) 22773–22779. [86] J.B. Haskins, W.R. Bennett, J.J. Wu, D.M. Hernández, O. Borodin, J.D. Monk, C.W. Bauschlicher, J.W. Lawson, Computational and experimental investigation of Li-doped ionic liquid electrolytes: [pyr14][TFSI], [pyr13][FSI], and [EMIM][BF4], J. Phys. Chem. B 118 (38) (2014) 11295–11309. [87] S. Niu, Z. Cao, S. Li, T. Yan, Structure and transport properties of the LiPF 6 doped 1-ethyl-2,3-dimethyl-imidazolium hexafluorophosphate ionic liquids: a molecular dynamics study, J. Phys. Chem. B 114 (2) (2010) 877–881. [88] J. Lassègues, J. Grondin, D. Talaga, Lithium solvation in bis(trifluoromethanesulfonyl) imide-based ionic liquids, Phys. Chem. Chem. Phys. 8 (48) (2006) 5629–5632. [89] S. Menne, J. Pires, M. Anouti, A. Balducci, Protic ionic liquids as electrolytes for lithium-ion batteries, Electrochem. Commun. 31 (2013) 39–41. [90] R. Hayes, S. Imberti, G.G. Warr, R. Atkin, The nature of hydrogen bonding in protic ionic liquids, Angew. Chem. Int. Ed. 52 (2013) 4623–4627. [91] X. Song, H. Hamano, B. Minofar, R. Kanzaki, K. Fujii, Y. Kameda, S. Kohara, M. Watanabe, S. Ishiguro, Y. Umebayashi, Structural heterogeneity and unique

[92] [93] [94]

[95]

[96]

[97] [98] [99] [100]

distorted hydrogen bonding in primary ammonium nitrate ionic liquids studied by high-energy X-ray diffraction experiments and md simulations, J. Phys. Chem. B 116 (9) (2012) 2801–2813. R.E. Miller, R.R. Getty, K.L. Treuil, G.E. Leroi, Raman spectrum of crystalline lithium nitrate, J. Chem. Phys. 51 (4) (1969) 1385–1389. K.O. Stromme, On the crystal structure of lithium nitrate above room temperature, Acta Chem. Scand. 24 (4) (1970) 1479–1481. T. Yamaguchi, I. Okada, H. Ohtaki, M. Mikami, K. Kawamura, X-ray and neutron diffraction and molecular dynamics simulation of molten lithium and rubidium nitrates, Mol. Phys. 58 (2) (1986) 349–364. Y. Kameda, S. Kotani, K. Ichikawa, Intermolecular structure around lithium monovalent cations and nitrogen atoms in molten LiNO3, Mol. Phys. 75 (1) (1992) 1–16. A.K. Adya, G.W. Neilson, I. Okada, S. Okazaki, The determination of the radial distribution functions gLi − L(r), gLi − O(r) and gLi − N(r), in molten lithium nitrate from neutron diffraction, Mol. Phys. 79 (6) (1993) 1327–1350. A.K. Adya, G.W. Neilson, Neutron diffraction results from some nitrate melts, J. Non-Cryst. Solids 205–207 (Part 1) (1996) 168–171. T. Murphy, R. Hayes, S. Imberti, G.G. Warr, R. Atkin, Nanostructure of an ionic liquid–glycerol mixture, Phys. Chem. Chem. Phys. 16 (2014) 13182–13190. M.Z. Bazant, B.D. Storey, A.A. Kornyshev, Double layer in ionic liquids: overscreening versus crowding, Phys. Rev. Lett. 106 (4) (2011) 046102(1)–046102(4). G. Feng, X. Jiang, R. Qiao, A.A. Kornyshev, Water in ionic liquids at electrified interfaces: the anatomy of electrosorption, ACS Nano 8 (11) (2014) 11685–11694.