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# Formation of Aqueous Biphasic Systems with an Ionic Liquid Induced by Metallic Salts: Nanoscopic Views from Molecular Dynamics Simulations

Rachel Schurhammer, Georges Wipff

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Rachel Schurhammer, Georges Wipff. Formation of Aqueous Biphasic Systems with an Ionic Liquid Induced by Metallic Salts: Nanoscopic Views from Molecular Dynamics Simulations. Journal of Physical Chemistry B, 2018, 122 (44), pp.10143-10157. 10.1021/acs.jpcb.8b06193. hal-02329986

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Distributed under a Creative Commons Attribution 4.0 International License
Formation of Aqueous Biphasic Systems with an Ionic Liquid Induced by Metallic Salts:

Nanoscopic views from MD simulations

Rachel Schurhammer and Georges Wipff *

Chimie de la Matière Complexe, UMR CNRS 7140,
1, rue B. Pascal, 67000 Strasbourg, France.

rschurhammer@unistra.fr

wipff@unistra.fr
**Abstract.** The evolution of ion distributions in concentrated aqueous ionic liquid (IL)/salt mixtures when forming aqueous biphasic systems (ABSs) has been investigated via molecular dynamics simulations starting with randomly dispersed ions (with IL butyl-methyl-imidazolium triflate; salts NaCl, CsCl, SrCl₂, EuCl₃). The analysis of ion distributions, solvation and mutual interactions during the dynamics reveals the heterogeneity of all solutions, due to ion segregation into mutually exclusive IL and salt domains, even in monophasic solutions ("ionic sociology"). Ion segregation and ABS formation are found to increase (i) with the salt content (ii), with the IL content, (iii) in the order Na⁺ < Sr²⁺ < Eu³⁺, and (iv) when the IL ions' "polarity" is diminished, following experimental trends. The concomitant restructuration of the solution is rationalized as a synergistic transfer from the best donating ion pair (first hydration shell of hydrophobic moieties of IL ions) to the best accepting pair (Mⁿ⁺ and Cl⁻ ions, beyond the first shell). As for hydrophobic dehydration, this process should to be entropically favored. In ABSs, the IL and salt containing phases are linked by a well-defined "interface" that decreases in width when MClₙ becomes more hydrophilic and/or more concentrated. In the IL-rich phase of ABSs, the hydration of IL ions and their mutual interactions are shown to be similar to those displayed at aqueous interfaces.
**Introduction**

Aqueous biphasic systems “ABSs”, based on water as single solvent, play an important role in many domains such as the separation, extraction and purification of biological systems, or the treatment of polluted water.¹,² Generally, ABSs are formed by mixing two aqueous phases containing either two polymers, or a polymer plus a salting-out inducing salt, or specific salt-salt combinations, at appropriate concentrations and temperature. An interesting class of ABSs is based on aqueous solutions of ionic liquids (ILs) to which salting-out inorganic salts can be added to cause separation into two phases. In practice, such IL-based ABSs have many advantages like low viscosity, resistance to emulsification, quick phase separation, high extraction capabilities, and benefit from a potentially “infinite” panoply of IL components designed for a given use. In 2003, Rogers *and coll.* reported that addition of a water structuring salt, K₃PO₄, to an aqueous concentrated solution of 1-butyl-3-methylimidazolium chloride induced the formation of an upper IL-rich phase and a lower K₃PO₄-rich phase,³ as for classical PEG/salt based ABSs.⁴,⁵ Since then, many combinations of IL components and salting-out agents have been studied, both to define the underlying driving forces and to establish general guidelines for specific purposes.²,⁶,⁷ Similar salt-specific effects have also been observed with the related biphasic extraction systems based on hydrophobic ILs.⁸ Generally, for a given MₓXᵧ salt, ABS formation is enhanced by the size and hydrophobicity (or reduced hydrophilicity) of ionic components of the IL ⁹,¹⁰ and the hydrophilic and salting-out character of the dissolved MₓXᵧ salt.¹¹⁻¹⁴ Salting-out properties follow the Hofmeister series, whose interpretation in terms of thermodynamic and structural features remains controversial.¹¹, ¹⁵, ¹⁶ The hydrophilic / hydrophobic character ¹⁷, ¹⁸ is often related to the Gibbs free energy of ion hydration,¹⁹ and to concepts like structure breaking (“chaotropic”) / making (“cosmotropic”) features in water.¹⁷, ²⁰⁻²⁴ Recently, Shahriari *et al*. ¹⁴ correlated the amount of IL necessary to form an ABS and the entropy of hydration of the salt ions, drawing
the implication of a dominant entropic driving force for ABS formation, as for hydrophobic ILs<sup>11, 25, 26</sup> or polymer-based ABSs,<sup>27, 28</sup> presumably due to the formation of “water-ion complexes”. Insights into ion solvation and into their mutual interactions have been gained from NMR spectroscopy and simulations in aqueous salt solutions containing IL ions,<sup>11, 26, 29</sup>,<sup>30</sup> but these may not suffice to fully describe the “organization” of the solution, the distribution of all ions, and finally the formation of ABSs in conditions when it occurs.

Likewise, the nature of the aqueous- aqueous interface of ABSs has neither been characterized so far nor compared, for instance to aqueous interfaces with hydrophobic (water immiscible) ionic liquids.<sup>31-36</sup>

This led us to consider a well-documented case, and to explore by molecular dynamics, “MD”, simulations to which extent homogeneous aqueous mixtures of IL and inorganic salts separate at the nanoscopic level, and to analyze what are the related solvation patterns and driving forces. The selected IL, based on BMI⁺ (butyl-methylimidazolium⁺) and TfO⁻ (triflate CF₃SO₃⁻) ions, is readily soluble in water, as are the MClₙ metallic salts, whose ternary diagrams, adapted from ref <sup>14</sup> are shown in Figure 1. The systems at the right hand side of a curve are biphasic, those of the left hand side are monophasic, while a given curve corresponds to the intermediate states (“cloud points”). Here, ABS formation increases with the IL and / or salt content, and in the order CsCl < NaCl < SrCl₂, i.e. with the Mⁿ⁺ charge.

The simulated solutions are marked **B** to **K** in Figure 1. Additionally, a “neat” IL aqueous solution **A** (without added salt) was studied as a reference, as well as the EuCl₃ solutions **L** to **P**, expected to display the strongest ABS promoting effect. Their characteristics are given in Tables 1 and S1 of the Supporting Information. Simulations were conducted in a consistent manner, in terms of water and ion content, and of computational protocol, starting from aqueous solutions of randomly dispersed IL and salt ions (see e.g. Figure S1). During the dynamics, ions segregated and, in some cases, separated into two distinct phases (ABS
formation), affording microscopic insights into the ion segregation and solution reorganization presented and analyzed below.

## Methods.

The solutions have been simulated by classical MD simulations using the AMBER16 37 software, in which the potential energy U is empirically represented by a sum of bond and angle harmonic deformations, of dihedral torsions, and by pairwise additive 1-6-12 (electrostatic + van der Waals) interactions between non bonded atoms (equation 1):

$$ U = \sum_{\text{bonds}} k_r (r - r_0)^2 + \sum_{\text{angles}} k_{\theta} (\theta - \theta_0)^2 + \sum_{\text{dihedrals}} k_{\phi} [1 + \cos(n\phi + \phi_0)] + \sum_{\text{atom i}} \sum_{j \neq i} 4\epsilon_{i,j} \left[ \left( \frac{\sigma_{i,j}}{r_{i,j}} \right)^{12} - \left( \frac{\sigma_{i,j}}{r_{i,j}} \right)^{6} \right] + \sum_{\text{atom i}} \sum_{j \neq i} \frac{q_i q_j}{\epsilon_0 r_{i,j}} \quad (1) $$

Solute and solvent atom pairs i and j display Coulomb interactions via their charges $q_i$ and $q_j$. Cross terms in van der Waals interactions were constructed using the Lorentz-Berthelot rules. Water was represented with the TIP3P model.38 For the ion parameters, we used the consistent $Na^+$, $Cs^+$ and $Cl^-$ values from Joung and Cheatham et al. 39, $Sr^{2+}$ and $Eu^{3+}$ from refs 40 and 41, respectively. The BMI+ parameters are from Andrade et al. 42 and the TfO- ones from Baaden et al.43. A first set of simulations, conducted with integral +/-1 charges on IL ions (IL$^{+/-1}$ model), produced some instances in conflict with experiment. We thus decided to use reduced charges of +/- 0.9 (IL$^{+/-0.9}$ model with atomic charges scaled by 0.9), as in previous work on IL/ water biphasic mixtures.44-47 Formally reducing the ionic charges to a degree mimics the anion to cation electron transfer in the condensed IL phases (e.g. [BMI][PF6] or [BMI][[Tf2N],34, 48 and also better accounts for the solubility of IL ions in an aqueous phase.45 The 1-4 van der Waals and 1-4 coulombic interactions were scaled down by 2.0 and 1.2, respectively. The solutions were simulated with 3D-periodic boundary conditions, using an atom-based cutoff of 12 Å for non-bonded interactions, and correcting for the long-range electrostatics by using the Ewald summation method. The characteristics of all simulated systems are given in Tables 1 and S1 of the Supporting Information. The MD simulations were performed at 300 K starting with random velocities. The temperature was monitored via a coupling to a thermal bath using the Berendsen algorithm 49 with a relaxation time of 0.2 ps. In the (NPT) simulations, the pressure was similarly coupled to a barostat with a relaxation time of 0.2 ps. A time step of 2 fs was used to integrate the equations of motion via the Verlet leapfrog algorithm.
For each system, a “random” mixture of MCl<sub>n</sub> and IL ions (M<sup>n+</sup>, Cl<sup>-</sup>, BMI<sup>+</sup> and TfO<sup>-</sup>) was prepared in a box of 8000 or 32000 H<sub>2</sub>O molecules (“standard” box and “big” box, respectively). After 1000 steps of energy minimization, 0.25 ns of dynamics were performed with the ions in order to allow the solvent to relax around the solute. This was followed by a dynamics of 0.25 ns at constant volume and of 1 ns at a constant pressure of 1 atm. Note that the relaxed box sizes may somewhat differ from one system to the other (see Table S1 of the Supporting Information). Furthermore, in some cases, some salt reorganization already occurred at this preparation stage. The evolution of the mixture was then followed for 500 ns of dynamics (NVT ensemble), sometimes extended up to 1000 ns.

**Analysis of results.** The trajectories were analyzed using our MDS software.⁵⁰ Snapshots were drawn with the VMD software.⁵¹ The radial density functions “RDFs” were calculated, unless otherwise specified, between selected atoms: N<sub>butyl</sub> or C<sub>TFO</sub> of IL ions, M<sup>n+</sup> or Cl<sup>-</sup> ions, O<sub>H2O</sub>, H<sub>H2O</sub> atoms. The distributions of M<sup>n+</sup> and Cl<sup>-</sup>, BMI<sup>+</sup> and TfO<sup>-</sup> ions, H<sub>2</sub>O molecules in the box and the corresponding mass-weighted densities d were calculated along the z-coordinate, and averaged over different time periods (typically the last 5 or 50 ns) specified in the relevant Figures. The total potential energy E<sub>pot</sub> and interactions between selected neutral groups (IL ions, MCl<sub>n</sub> ions, water) were calculated using Ewald summation. In order to simply “measure” the extent of ion segregation and phase separation at a given time, a “demixing index” ξ<sup>□□□□□</sup><sub>x</sub> defined by 1/ξ<sup>□□□□□</sup><sub>x</sub> □□□□□d<sub>IL</sub> + 1/d<sub>wat</sub>, where d<sub>IL</sub> and d<sub>wat</sub> are the densities of IL and water, respectively, was used, as in previous work,⁴⁵,⁵² in “nano” boxes of 0.8x0.8x1.0 nm³ volume. The simulation box was thus split into such “nano-boxes”, and the corresponding normalized ξ<sup>□□□□□</sup><sub>x</sub> □ as averaged over all boxes. Thus, in principle, ξ<sup>□□□□□</sup><sub>x</sub> ranges from 1 (for randomly dispersed components) to 0 (for two juxtaposed immiscible components (e.g. water and “oil”).

**Results.**

In the following, we first describe the “neat IL” aqueous solution A as a reference. The effects of salt concentration (mainly focused on NaCl) and of IL concentration are presented in Section 2. In Section 3 we compare the different MCl<sub>n</sub> salts (CsCl, NaCl, SrCl<sub>2</sub> and EuCl<sub>3</sub>) at the same metal and IL content. Ion / ion interactions and hydration in the above series are then analyzed and compared to the purely aqueous solutions (Section 4). Although all
simulations have been conducted with both IL<sup>+/-0.9</sup> and IL<sup>+/-1</sup> models, we restrict the
presentation of IL/IS mixtures (Sections 2 to 4) to the IL<sup>+/-0.9</sup> results. The IL<sup>+/-1</sup> results are
given in the Supporting Information. The ionic content of a simulated box is noted in short,
e.g. 128 IL<sup>+/-0.9</sup>/ 296 NaCl for 128 BMI<sup>+</sup> TfO<sup>-</sup> (IL<sup>+/-0.9</sup> charges) and 296 Na<sup>+</sup> Cl<sup>-</sup> salt ions. The
evolution of the demixing index ξ□□□□□ during the first 100 ns of the different dynamics is
plotted in Figures 2 for the NaCl and for the MCl<sub>n</sub> containing solutions, and the final average
ξ□□□□□ values are collected in Table 2.

1) The « neat » aqueous IL solution is heterogeneous, due to hydrophobic contacts.

Solution A was simulated at the same IL and water content (148 BMI⁺ TfO⁻, 8000 H₂O per box) as in salt containing ones. The main results are shown in Figure 3 and Figure S2 of the Supporting Information. As seen in the final snapshots, in the absence of MClₙ salt, the solution is monophasic and the IL ions are dispersed in the box. The water density profile along the z-axis of the box, averaged over the last 100 ns, is flat. The BMI⁺ and TfO⁻ density curves are quasi-superimposable but not flat, indicative of IL heterogeneities. This feature is also visible by averaging over different time periods: 10-200, 100-200, 150-200 or 190-200 ns (Figure S3 of the Supporting Information).

According to the RDFs (see integration numbers in S2 of the Supporting Information), the BMI⁺ and TfO⁻ ions are mostly separated and well hydrated, by ca. 8 to 10 H₂O molecules in their first shell. They also sometimes form loose pairs, corresponding to a first peak in the BMI...TfO RDFs that integrates to 0.32 (within 5 Å). Of relevance for the formation of ABSs, we note that the shortest BMI⁺...TfO⁻ contacts do not correspond to electrostatic, but to hydrophobic attractions (between CF₃(TfO) atoms and either the π-ring or the butyl chain of BMI⁺), whereas the more polar SO₃⁻ and ring(imidazolium)⁺ moieties are well hydrated and separated. Hydrophobic contacts are likewise observed between TfO⁻ anions (via their
CF<sub>3</sub>(TfO) groups; see snapshots and the C<sub>TfO</sub>...C<sub>TfO</sub> RDF that integrates to 0.27), in spite of their Coulombic repulsions. Similar features are observed in the “standard” (A) and “big” boxes (A2), pointing to the critical role of hydrophobic interactions for IL ions' association.

On the other hand, there are no π–π stacking contacts between imidazolium cations.

Comparing now the IL<sup>+/-0.9</sup> to the IL<sup>+/-1</sup> models (Figure S2 and Table S2 of the Supporting Information) we note that there is a little bit more BMI<sup>+</sup>...TfO<sup>-</sup> and TfO<sup>-</sup>...TfO<sup>-</sup> pairing (and hence presumably more IL association) with the less “polar” IL<sup>+/-0.9</sup> one than with the IL<sup>+/-1</sup> one, whereas the IL<sup>+/-0.9</sup> ions are somewhat less well hydrated than the IL<sup>+/-1</sup> ones.

## 2) Adding NaCl, CsCl or SrCl<sub>2</sub> at different concentrations to an aqueous solution of BMI<sup>+</sup> TfO<sup>-</sup> ions: from mixed to separated phases.

In this section, we first compare solutions at fixed IL content (148 BMI<sup>+</sup> TfO<sup>-</sup> per box) and different MCl<sub>n</sub> salt concentrations), and then solutions at different IL concentrations, depicted with the IL<sup>+/-0.9</sup> model.

**Effect of salt concentration.** Three NaCl solutions (B, C, D, of salt:IL ratios of 1:1, 2:1 and 3:1, respectively) were first considered. As seen from final snapshots (Figure 4) and average density curves (Figure 5), all three are heterogeneous, and display increased ion segregation from solutions A (without NaCl) to B, and from B to D. Accordingly, the ξ□□□□□ index decreases in the A–D series: ≈ 0.74, 0.65, 0.62 and 0.48, respectively.

In solution D, the ions partition into two well-defined slabs, indicating phase separation. At 500 ns, the salt slab contains the majority of the Na<sup>+</sup> Cl<sup>-</sup> ions and only ca. 3 IL ion pairs. In contrast, the IL slab contains only ca. 10 NaCl ion pairs but still much water (≈ 1200 of the 8000 H<sub>2</sub>O molecules), supporting the view that the two juxtaposed phases are mainly aqueous. This picture is consistent with the density curves: IL and salt (or water) densities, averaged over the last 5 ns, display anti-correlated plateaus. The IL density drops close to
zero in the NaCl slab, as does the NaCl density in the IL slab. Note that distribution curves of
individual Na⁺ and Cl⁻ ions are superimposable, as are the BMI⁺ and TfO⁻ curves (Figure S5
of the Supporting Information), indicating that each phase is overall neutral, with neutral
contributions of Na⁺Cl⁻ ions and of IL ions, respectively. Densities averaged over a longer
time period (50, instead of 5 ns) afford the same features, but the high IL density profile
becomes somewhat “bell-shaped” (Figure S4 of the Supporting Information), due to ion
passage across the “interface”.

In the more dilute **C** and **B** solutions, ions also segregate, without forming clear slabs,
however, because the mutual IL / NaCl exclusion has decreased together with the NaCl
concentration. **B** is overall monophasic, but contains distinct NaCl-rich and IL-rich domains:
the IL density peak (near z ≈ 120 Å) correlates with a minimum of salt and water densities.
These domains are expected to exchange with each other at timescales longer than the
simulated ones, yielding, on average, flat density curves. For solution **C** the density profiles
are more “**D**-like” than “**B**-like”, showing that **C**, experimentally monophasic, is quite
heterogeneous at the nanoscopic scale.

The effect of salt concentration is likewise observed with the CsCl solutions **G** and **H** (of CsCl:IL ratios 2:1 and 3:1, respectively), or the SrCl₂ solutions **J** and **K** (of SrCl₂:IL ratios 1:1 and 2:1, respectively): the ξ□□□□□ index decreases from 0.61 (in **G**) to 0.53 (in **H**), and from 0.58 (in **J**) to 0.49 (in **K**). Snapshots (Figure 6) and density curves (Figure 5) also support enhanced ion segregation and phase separation when the salt gets more concentrated. For instance, in the IL domain from **J** to **K**, the IL density profile becomes narrower and of higher amplitude, while the water intensity decreases. The highest IL and lowest salt densities, “bell-shaped” in **J**, become rather uniform in **K** where the interfacial domain is sharper, on the average. From **J** to **K**, the IL density increases in the IL slab (from ≈ 0.7 to 0.8), while the lowest SrCl₂ density remains close to zero, supporting full phase separation. This contrasts
with the CsCl solutions **G** and **H** (experimentally monophasic), where the IL density maxima correspond to a significant CsCl salt content, more in **G** than in **H**, and the curves are rather “bell shaped”. There is thus less IL / salt separation with CsCl than with SrCl₂ at both concentrations, following experimental trends.¹⁴

**Effect of IL concentration.** Phase separation increases also with the IL concentration. To investigate this feature, we considered the NaCl solutions **C** and **D** described above, but doubled their IL content (solutions **E** and **F**, respectively). As seen in snapshots (Figure 7) and densities (Figure 5), the IL domain becomes now broader and displays a wider plateau of density. The IL density *d*<sub>IL-max</sub> is higher when NaCl is most concentrated ( **D** and **F** solutions: *d*<sub>IL-max</sub> ≈ 0.8) than when NaCl is less concentrated ( **C** and **E** solutions: *d*<sub>IL-max</sub> ≈ 0.7).

Regarding the ξ□□□□□ index, it decreases at small NaCl concentration (from 0.62 in **C** to 0.56 in **E**), indicating enhanced phase separation. At the higher NaCl concentration, however, ξ□□□□□ remains similar (0.48 in **D** and 0.47 in **F**), since both simulated solutions are biphasic.

Simultaneously increasing the IL and salt content also enhances segregation: compare the CsCl solution **G** (148 IL/296 CsCl) to **I** (296 IL/700 CsCl). They both contain an IL slab, but the latter is better structured and contains less CsCl in **I** than in **G** (Figures 5 and 7).

Accordingly, from **G** to **I**, the interface between the two phases becomes sharper, whereas ξ□□□□□ decreases (from 0.61 to 0.41).

### 3) Effect of M<sup>n+</sup> hardness on phase separation: adding NaCl, CsCl, SrCl<sub>2</sub> or EuCl<sub>3</sub> at a same concentration to an aqueous solution of BMI<sup>+</sup> TfO<sup>-</sup> ions.

Let us now compare solutions of identical IL and water content to which a fixed amount of salt species (NaCl, CsCl, SrCl<sub>2</sub> and EuCl<sub>3</sub>) has been added (148 IL<sup>+/-0.9</sup>/296 MCl<sub>n</sub> solutions **C**, **G**, **K** and **L**, respectively). Their M<sup>n+</sup> metal content is twice the IL content, whereas the Cl<sup>-</sup>
content is proportional to the Mⁿ⁺ charge. The evolution seen from snapshots (Figures 4 and 6) and density curves (Figure 5) fully supports increased phase separation when the Mⁿ⁺ charge increases, in accord with experiment.¹⁴, ²⁹ In parallel, the ξ□□□□x index decreases in the order CsCl (0.61) ≈ NaCl (0.62) > SrCl₂ (0.49) > EuCl₃ (0.42).

The EuCl₃ solution **L** is clearly biphasic: at 500 ns all IL ions concentrate in an IL-rich slab from which EuCl₃ ions are excluded, while all Eu³⁺ and Cl⁻ ions are in an aqueous salt phase containing only ca. 3 BMI⁺ TfO⁻ ion pairs. On the average, the EuCl₃ density in the IL slab is zero, as is the IL density in the EuCl₃ slab. In the SrCl₂ solution **K**, ions partition similarly as in **L**, supporting complete phase separation.

This contrasts with the CsCl solution **G** that does not display complete phase separation, in agreement with experiment,¹⁴ but the ions still segregate into different domains. Comparing now CsCl to NaCl, we observe no clear distinction between the two salts, in spite of the higher ABS promoting effect of NaCl: final snapshots, density curves and ξ□□□□x values are similar.

The water content of the IL domain also decreases upon ion segregation along the NaCl, SrCl₂ and EuCl₃ series, where the lowest *d*<sub>wat-min</sub> water densities are ≈ 0.5 (in **C**), 0.4 (in **K**) and 0.2 (in **L**), respectively (Figure 6). A similar evolution is observed when the salt concentration increases: for instance, in the NaCl series, *d*<sub>wat-min</sub> drops from ≈ 0.8 (in solution **A**) to 0.6 (in **B**), 0.5 (in **C**) and 0.4 (in **D**). Compare also solutions **J** to **K** or **G** to **H** (Figure 5). Thus, the higher the degree of ion segregation, the less humid is the IL phase, reflecting the “water dragging effect” of the salt on aggregation (see Section 4).

### 4) IL and salt ion hydration and mutual interactions in the MClₙ containing solutions.
Comparison with the pure aqueous IL or salt solutions.
In this section, we analyze how the IL segregation induced by MCl<sub>n</sub> salts modulates the
solvation of the M<sup>n+</sup>, Cl<sup>-</sup>, TfO<sup>-</sup> and BMI<sup>+</sup> ions and their mutual interactions in the final
MCl<sub>n</sub>/IL mixtures, as compared to the aqueous solutions of MCl<sub>n</sub> alone (no IL) and of IL
alone (solution A). For consistency, we first select solutions containing same numbers of
MCl<sub>n</sub> units (296) and of IL<sup>+/-0.9</sup> ones (148), i.e. solutions C, K and L.

The M<sup>n+</sup> and Cl<sup>-</sup> ions being quite hydrophilic, their first hydration shell in a given mixture is
very similar to the one in pure water (see RDFs in Figure S6 and Table S3 of the Supporting
Information). For instance, Na<sup>+</sup> binds 5.5 and Cl<sup>-</sup> 2.8 OH<sub>2</sub> in mixture C, versus 5.3 and 2.9,
respectively, in pure water. Likewise, Sr<sup>2+</sup> binds 6.9 and Cl<sup>-</sup> 0.9 H<sub>2</sub>O in mixture K, versus 7.2
and 0.8, respectively, in pure water. One also observes some M<sup>n+</sup>...Cl<sup>-</sup> pairs, similar in the
mixture as in water (ca. 0.3 pairs per Na<sup>+</sup>, 0.9 per Sr<sup>2+</sup> and 2.8 per Eu<sup>3+</sup>). All RDFs display a
second peak, also similar in the mixture to the one in water. In the context of ABS formation,
note that the M<sup>n+</sup>...M<sup>n+</sup> and M<sup>n+</sup>...Cl<sup>-</sup> RDFs have higher intensities in a given mixture (red
curves of Figure S6 in the Supporting Information) than in water (blue curves), reflecting the
MCl<sub>n</sub> salt segregation induced by IL ions.

The hydration of TfO⁻ and BMI⁺ ions and their mutual interactions are markedly modified by
the added salts (see Figure 8 and Table 3). Their RDFs with water display a peak indicating
that they are coordinated to H₂O molecules in their first shell, in a decreasing number, nH₂O,
along the CsCl, NaCl, SrCl₂ and EuCl₃ salt series. For instance, when going from the NaCl to
the EuCl₃ containing mixture, nH₂O(BMI⁺) drops from 14.7 to 9.7, while nH₂O(TfO⁻) drops
from 13.4 to 6.7. Thus, although most Mⁿ⁺ and Cl⁻ ions sit in salt-rich domains, they indirectly
influence the hydration of IL ions, in keeping with the diminished water content of the IL
domain. As a result, mutual interactions between IL ions (and hence segregation) are also
strengthened by the added MClₙ salt: see the evolution of the TfO⁻...BMI⁺ and TfO⁻...TfO⁻
RDFs, where the first peaks (at ≈ 5 - 6 Å) increase in magnitude when MClₙ becomes more hydrophilic (see also integration numbers in Table 3).

The influence of the added salt on the “IL structure” is likewise observed in the NaCl series (Figure 8 and Table 3): when going from solutions **A** (without NaCl) to **D**, the IL ions lose ca. 5 to 8 H₂O molecules in their first shell and display more frequent TfO⁻...BMI⁺ and TfO⁻...TfO⁻ contacts, as seen from the increasing magnitude of first peak of the corresponding RDFs.

The cross-interactions between M⁺ and Cl⁻ ions and the polar components of the IL were analyzed more precisely in the NaCl series **B-D** and in the MClₙ series **C, K, L**, from the S<sub>TfO</sub>...M<sup>n+</sup> and N<sub>butyl</sub>(BMI)...Cl⁻ RDFs (a summary is given in Table 3). The S<sub>TfO</sub>...M<sup>n+</sup> RDFs display a first peak that integrates within 4.4 Å to less than 0.2 in the Na⁺ series, and to 0.04 in the M<sup>n+</sup> series, indicating the limited occurrence of direct SO₃⁻...M<sup>n+</sup> contacts with the divalent or trivalent ions. The second peaks, corresponding to water-mediated SO₃⁻...H₂O...M<sup>n+</sup> contacts, increase in intensity when the NaCl salt gets more concentrated (ca. 0.50, 0.65, 0.68 respectively within 7 Å in the NaCl series **B-D**). In the case of SrCl₂ or EuCl₃ salts, the closest interactions are water-mediated, and limited (0.25 with Sr²⁺, 0.04 with Eu³⁺ within 7 Å), since their more charged cations increasingly prefer more remote water. One observes also some Cl⁻...BMI⁺ interactions, increasing in the NaCl series (from ca. 0.08 to 0.13 within 5.5 Å), but less obvious with SrCl₂ or EuCl₃ (0.09 and 0.03, respectively) that induce higher segregation.

In the RDFs along the whole MClₙ series at fixed concentration, there is no marked discontinuity between mono- and biphasic systems, indicating that short-range (first shell) interactions with the ions are not sufficient to distinguish ABS from heterogeneous monophasic solutions.
Evolution of ion hydration and mutual interactions at different distances when
"organization" proceeds.

How the IL and salt ions, initially randomly dispersed in the mixtures, progressively segregate, and how in parallel water rearranges during the dynamics around these ions has been analyzed at different distances for selected monophasic and biphasic systems. For this purpose, we followed the average number nA(X; cutoff) of X species around A at different cutoff distances, 5, 10, and 20 Å, respectively. The corresponding curves are given in Figure 9 and Figures S21-S22 of the Supporting Information).

Regarding the hydration of MCl<sub>n</sub> salts, the n<sub>H2O</sub>(M<sup>n+</sup>; 5) or n<sub>H2O</sub>(Cl<sup>-</sup>; 5) curves are rather flat, indicating that the hydrophilic M<sup>n+</sup> and Cl<sup>-</sup> ions rapidly satisfy their first coordination requirements in the different solutions. This contrasts with the n<sub>H2O</sub>(BMI<sup>+</sup>; 5) or n<sub>H2O</sub>(TfO<sup>-</sup>; 5) curves that decay rapidly (indicating rapid loss of water), and become flat after ca. 10 ns.

Hydration changes at longer distances (10 or 20 Å) are slower and more spectacular, in keeping with the water migration from (forming) IL domains towards (forming) salt domains.

The water transfer increases when MCl<sub>n</sub> becomes more salting-out. For instance, during the first 100 ns of dynamics, n<sub>H2O</sub>(M<sup>n+</sup>; 20) increases in the order Cs<sup>+</sup> ≈ Na<sup>+</sup> < Sr<sup>2+</sup> < Eu<sup>3+</sup>, by ≈ 100 H<sub>2</sub>O in the Eu<sup>3+</sup> case. In parallel, n<sub>H2O</sub>(BMI<sup>+</sup>; 20) and n<sub>H2O</sub>(TfO<sup>-</sup>; 20) drops in the same M<sup>n+</sup> order, by ≈ 300 H<sub>2</sub>O in the case of Eu<sup>3+</sup>. Ion segregation and phase separation, concomitant with water transfer, are reflected by the populations of BMI...BMI, BMI...TfO and TfO...TfO "pairs" that increase (at the three distances) when MCl<sub>n</sub> becomes more salting-out (Figure 9 and Figures S21-S22 of the Supporting Information), with marked changes at long distances. It is thus crucial to look beyond the first coordination shells of the different partners.
**Discussion.**

These MD “demixing” simulations of aqueous solutions of MClₙ and IL, their components initially placed in random positions, have enabled exploration of as to what extent the solutions become mono- or biphasic at the nanoscopic level, and thereby provide microscopic insights into ion segregation and ABS formation. In order to achieve “apples with apples” comparisons, the MD experiments were conducted in a systematic manner, comparing different salt / IL concentrations, and MClₙ metallic salts along the Hofmeister series. They reveal in all cases marked heterogeneities, with segregation of the IL ions into a water domain from which the Mⁿ⁺ and Cl⁻ ions tend to be excluded, and the converse. Ion segregation (and in some cases, full phase separation) is found to increase (i) when the added Mⁿ⁺ ions are higher charged (and hence more hydrophilic) and (ii) more concentrated, and (iii) with the IL content. This is consistent follows experimental observations.¹⁴ In the following, we first address computational and related physical issues, and compare the calculated to the experimental phase behavior of all systems, comparing the IL⁺⁻¹ to IL⁺⁻⁰.⁹ models. In order to discern the driving forces for ABS formation, we then analyze changes in ion hydration, ion-ion interactions and the water migration, also comparing these concentrated IL mixtures to IL diluted ones. This is followed by a discussion of some related energy features. Ion organization and hydration patterns in the IL phase of ABSs are then discussed, drawing analogies with IL ions at aqueous interfaces. Finally, the main characteristics of liquid-liquid interfaces of ABSs are compared to those of “oil”/water interfaces.

**Computational and related physical issues.**

Owing the simplicity of the force field used here, quantitative agreement with thermodynamic partition data is not anticipated. As will be seen below, the charges born by IL ions critically influence the extent of IL segregation, and the tested IL⁺⁻⁰.⁹ and IL⁺⁻¹ models likely bracket
the best compromise. Interactions with water, and thereby the segregation behavior, are also influenced by the water and ion models<sup>53</sup> (see for instance refs <sup>54-57</sup> for IL ions, and ref. <sup>58</sup> for salt ions), and by polarization effects.<sup>59, 60</sup>

Simulation times also matter. Note that those used here (500 to 1000 ns) are longer than those used to separate classical water /"oil" random mixtures (< 0.2 ns when "oil" is an organic molecule or SC-CO₂,<sup>52, 61</sup> or 20 to 40 ns when the "oil" is a hydrophobic IL<sup>45</sup>). The purely aqueous mixtures separate more slowly, indicating that the underlying driving forces are weaker, thereby requiring to sample more configurations. Short-range reorganization of the random mixtures generally occurs during the first ≈ 10-20 ns. After that time period, the demixing index ξ<sub>□□□□x</sub> (Figure 3) and the different energy components (see below) stay fairly constant, indicating that segregation does not significantly change at the nanometer scale.

Looking at specific cases illustrates typical steps of ABS formation. For instance, for the EuCl₃ solution **L** (see S8 of the Supporting Information), the ions first form loose aggregates during the first ca. 5 – 10 ns, and then condense (between 10 - 25 ns) into an IL slab and a small IL droplet, ≈10 nm apart. At ≈ 225 ns, these collapse into a single IL phase that remains stable until the end. In the case of the NaCl solution **D**, the two distinct phases formed earlier (at ≈160 ns), indicating that the different landscapes obtained at 500 ns are overall meaningful. In the case of the SrCl₂ solution **K**, cumulated trajectories of individual species during 100 ns (Figure S7) also show that the sampling is sufficient to study ABSs.

Another issue concerns the size of solvent heterogeneities: are they representative of
the macroscopic ones, or induced by the box size and content? In particular, is the number of
IL ion pairs (148) in the “standard” simulations not too small to finally depict a bulk phase?
This is not clearly the case for the NaCl, SrCl₂ and EuCl₃ solutions (D, K L, respectively) that
form two phases in the “standard box”. For the NaCl solution D, we however decided to test a
“bigger box” (solution D2), of size and content scaled up to 4.0. Snapshots in Figure 10 and
density curves in Figure 6) confirm the separation of IL and NaCl ions into distinct domains, forming now two slabs per ion type. These differ in size and composition, however: the first one is wider and contains more IL (and hence less water) that the second one, suggesting that the system is metastable. Because these two slabs are ≈10 nm apart, there is no driving force inducing their collapse during 500 ns of dynamics.

The shape of ionic phases can also be influenced by the shape of the box, when simulated with 3D periodic conditions: rectangular elongated boxes favor slabs, whereas cubic boxes rather favor cylindrical or “spherical” IL arrangements (see e.g. the case of the EuCl₃ solution L; Figure S9 of the Supporting Information). Whether and how these domains collapse after longer simulations remains to be investigated with bigger systems. Beyond being a computational issue, this may become of importance for experiments carried out in confined conditions. Furthermore, as experimentally observed for the big “spherical” dicarbollide anions that also form nano-aggregates in water,⁶² the structure of the solutions (mainly the monophasic ones) may evolve with time and aging.⁶³

**The monophasic / biphasic state of IL / MCl<sub>n</sub> mixtures, simulated with IL<sup>+/-1</sup> versus IL<sup>+/-</sup> ⁰.⁹ charge models. Comparison with experiment.**

For the MCl<sub>n</sub> containing mixtures, increasing the IL ions “polarity” (from the IL<sup>+/-0.9</sup> to the IL<sup>+/-1</sup> model) diminishes segregation, as indicated by the ξ□□□□x index that decreases by ≈ 0.1 units, on the average (see evolutions with time in Figure S10 of the Supporting Information and average values in Table 2). See also snapshots (Figures S11-S14 of the Supporting Information) and density curves (Figures S15-S17 of the Supporting Information) obtained with the IL<sup>+/-1</sup> charges. With the latter, segregation also increases with the salt content (compare ξ□□□□x for NaCl solutions **B** to **D**, CsCl solutions **G** to **H**, SrCl<sub>2</sub> solutions **J** to **K**) and with the M<sup>n+</sup> charge (compare ξ□□□□x for solutions **C**, **K** and **L**). The effect of IL content
is less clear, however: ξ□□□□x is similar for the **C** and **E** NaCl solutions, as for the **D** to **F** ones that do not display ion partitioning with that model (Figure S11 of the Supporting Information).

We now aim at comparing the simulated phase behavior to that experimentally observed (Figure 1). Biphasic and monophasic systems at points near the coexistence curve (and hence the cloud point) are noted “cloud-bi” or “cloud-mono”, respectively, on the different snapshots and density curves. On the simulation side, the distinction is rarely clear-cut, in particular when broad density peaks (also corresponding to some salt/IL mixing), instead of plateaus, are observed. Furthermore, the IL or salt domains are labile and fluctuate with time, as seen from the evolution of density profiles with averaging time (e.g. 50, instead of 5 ns).

Notwithstanding these issues, tentative assignments (Table S4 of the Supporting Information) yield the following trends. With the IL<sup>+/-1</sup> charges, most monophasic states (e.g. NaCl solutions **B** and **C**, CsCl solutions **G** and **H**) are correctly predicted to be so: the density profiles are more or less flat, with significant IL/salt mixing. Most biphasic solutions (e.g. NaCl solution **F**, SrCl<sub>2</sub> solution **K**), however, are predicted instead to be monophasic, though with significant IL/salt mixing. Biphasic solutions sitting near the coexistence curve (e.g. NaCl solutions **D** and **D2**, CsCl solution **I**) also look monophasic. The only clear-cut “success” of the IL<sup>+/-1</sup> model concerns the EuCl<sub>3</sub> solution **L** that remains biphasic. On the other hand, with the IL<sup>+/-0.9</sup> model, monophasic solutions (e.g. NaCl solution **C**, CsCl solution **H**), some of them sitting near the coexistence curve (e.g. NaCl solution **E**) are instead found to be biphasic, which suggests somewhat exaggerated segregation and phase separation. Thus, to summarize, the IL<sup>+/-1</sup> model correctly predicts monophasic states, but underestimates segregation and phase separation, while the IL<sup>+/-0.9</sup> model somewhat magnifies segregation.

More fundamentally, the comparison of the IL<sup>+/-1</sup> to the IL<sup>+/-0.9</sup> results illustrates the subtle influence of IL ions’ “polarity” in formation of ABSs. See for example the case of the NaCl
solution **E** that becomes biphasic with the IL<sup>+/-0.9</sup> charges, and monophasic with the IL<sup>+/-1</sup> ones.

For the EuCl₃ solution **L**, there are no experimental data. However, the simulations make clear that this salt has the strongest ABS inducing effect. Even in the more dilute EuCl₃ solutions **M, N, O, P**, one observes distinct IL and salt domains, i.e. phase separation, with either IL<sup>+/-1</sup> or IL<sup>+/-0.9</sup> models (see Figures S18 and S19 of the Supporting Information).

**Water reorganization and ion segregation: water migration from the best “water donating” ion pair to the best “water accepting” ion pair.**

Ion-specific effects on ABS formation phenomenologically follow the Hofmeister series established for the precipitation / solubilization of proteins,<sup>15, 16, 64, 65</sup> and the order of ion effects on aqueous solubilities of hydrophilic and hydrophobic IL ions,<sup>14, 25</sup> biomolecules,<sup>11</sup> or sugars,<sup>66</sup> for instance. They result from collective phenomena and from the interplay between all ion...water, ion...ion and solvent...solvent interactions at a given concentration.

The formation of ABSs with hydrophilic ILs induced by inorganic salts increases with the salting-out properties of the salt, i.e. with its concentration and hydrophilicity.<sup>7, 14, 25</sup> It results from the competition between the salt ions and IL ions to capture water, which is won by the most hydrophilic ones.<sup>3, 29</sup> Cross salt / IL interactions may also come into play. Studying the effect of organic and inorganic salts (including alkali and alkaline earth metal chlorides) on the solubility of the (weakly soluble) [BMI][TF<sub>2</sub>N] IL, Freire *et al*. mentioned “specific ion effects that result from the direct or indirect (water-mediated) interactions between the salt ions and the solute and not from a rearrangement of the water molecules caused by the ions”.

They also assigned the resulting favorable effect to the tendency to form “hydration complexes” which cause the “dehydration of the solute”. In another study with the water soluble [BMI][TfO] IL, Kurnia *et al*. reported negative NMR chemical shifts on H<sub>BMI</sub> and
$F_{TfO}$ IL atoms upon addition of NaCl, $MgCl_2$ and $AlCl_3$ salts, interpreted in terms of “unfavorable interactions between the IL and salt leading to a salting-out phenomena”.²⁹ Our simulations consider higher IL concentrations, high enough to form ABSs (≈ 1-2, instead of 0.01 $mol.kg^{-1}$ in their studies), and cannot be directly compared because at high IL concentration the IL and salt ions tend to be located in different domains of the solutions.

In order to analyze what happens prior to segregation (at low IL concentration), we simulated 2 M salt / 0.02 M IL aqueous solutions with NaCl, $SrCl_2$ and $EuCl_3$ salts and analyzed the corresponding RDFs (their main characteristics are given in Table S5 of the Supporting Information). There are, as expected, no contacts between the dilute IL ions. Their first hydration shells are similar in the NaCl, $SrCl_2$ and $EuCl_3$ solutions (ca. 21 $H_2O$ within 6 Å), and similar to that in the absence of salt, in keeping with the lack of significant IL / salt direct contacts. The closest contacts are water-mediated: the $S_{TfO}...M^{n+}$ and $N_{Butyl}...Cl^-$ RDFs integrate to ca. 1.5 (at 7 Å) and to ca. 0.3 – 0.6 (at 5.5 Å), respectively for the three salts.

Looking at longer distances, one finds a similar number of $S_{TfO}...M^{n+}$ pairs for the three salts (ca. 4.5 $M^{n+}$ within 10 Å from $TfO^{-}$). The main perturbations on the IL ions' environment thus occur mainly via water (e. g. through reorientation and / or polarization of their dipoles, and modulation of their H-bonding interactions), in an increasing manner when the $M^{n+}$ charge (and, accordingly, the $Cl^-$ concentration and number of $Cl^-$BMI$^+$ contacts) increases. This is overall consistent with the reported shifts in NMR spectra and with their interpretation.²⁹ Such short range interactions are not sufficient, however, to induce ion segregation (and finally ABS formation). When the IL concentration is increased from 0.02 to 2 M, the IL ions lose much first shell water (ca. 7, 10, and 11 $H_2O$ within 6 Å from $BMI^{+}$, 12, 13, and 18 $H_2O$ around $TfO^{-}$, respectively in the NaCl, $SrCl_2$ and $EuCl_3$ mixtures) in keeping with the mutual attractions between IL ions and their segregation.
Overall, the structure of the studied IL / salt mixtures can be represented in terms of synergistic water-donor / water-acceptor interactions between the different ion pairs present in the solution. Here, the BMI⁺ and TfO⁻ IL behave as water-donors (from their hydrophobic butyl and CF₃ moieties, respectively), whereas the more hydrophilic Mⁿ⁺ and Cl⁻ ions attract water, in the first shells and much beyond, when the ionic charge increases. Finally, the salt phase contains more water than does the IL phase. Note that ions segregate into distinct IL and salt domains whatever their origin. For instance, the NaCl solution E that contains the same number (296) of Na⁺ Cl⁻ and BMI⁺ TfO⁻ ions, can alternatively be viewed as a mixture of the NaTfO salt with the BMICl ionic liquid, equivalent to “counterion exchange” in water: with both IL¹⁻/² and IL³⁻¹ models, ions are found to segregate into BMI⁺ TfO⁻ domains and into Na⁺ Cl⁻ ones. Interestingly, in the IL domain of solution E, the association of BMI⁺ and TfO⁻ ions is reminiscent of BMI⁺ ClO₄⁻ contacts observed in [BMI][Tf₂N] + NaClO₄ aqueous mixtures, i. e. of anion exchange between the IL and salt.²⁶ More generally, instead of focusing on a single ion type, it is worth considering all ion pairs (taking into account the precise speciation of all MₓXᵧ salts in the case of multicharged ions).²⁶, ²⁹

Evolution of energy components when ion segregation and water transfer proceed.

In order to follow the energy impact of ion and solvent reorganization occurring along the
dynamics, we cut the systems into three overall neutral groups (IL ions, MCl<sub>n</sub> ions, water) and
followed their interactions (see curves in Figure 11 and in Figures S23-25 of the Supporting
Information). Regarding interaction energies with water, the salt/water one is fairly constant
along the dynamics for the NaCl and CsCl solutions (that remain monophasic), but becomes
much more attractive at the beginning of the dynamics for SrCl<sub>2</sub> and EuCl<sub>3</sub> solutions (that
become biphasic). In parallel, the IL/water interaction energies, much smaller in magnitude
than the salt/water ones, remain stable with NaCl and CsCl, but get destabilized with SrCl<sub>2</sub>
and EuCl₃, reflecting water migration from the ions of IL to those of MClₙ. The IL “self-energy” also contributes to the stabilization of ABSs: it is fairly constant along the dynamics in the NaCl and CsCl solutions, but becomes more attractive with SrCl₂ or EuCl₃, due to enhanced BMI⁺ / TfO⁻ attractions when the IL ions segregate.

On the thermodynamic side, the stabilization of a system corresponds to a decrease in its Gibbs free energy ΔG (in standard conditions), a feature not amenable to simulations.

Insights into the evolution of its ΔH component can however be obtained by looking at the total potential energy Epot (Figure 11 and in Figures S23-25 of the Supporting Information). In the NaCl series **B** to **D**, Epot stays fairly constant along the dynamics, even for the most concentrated solution (**D**) that becomes biphasic. In the MClₙ series, Epot first diminishes during ≈10 ns with SrCl₂ and EuCl₃ salts, and then remains fairly constant. Note that the demixing index ξ□□□□x follows a similar evolution, indicating that during that time period the initially random solute/solvent mixtures have become more or less homogeneous at the nanometer scale, but not yet separated into two phases (for the biphasic ones). Later on, there is thus no dominant enthalpic driving force for further phase separation, which is consistent with entropy driven segregation and separation processes.

The origin of any entropy gain is unclear. Experimentally, hydration of metallic salts is an exergonic process (negative ΔG), dominated by high negative ΔH enthalpic energies, somewhat reduced by unfavorable ΔS entropic costs.⁶⁷ Enriching the water environment of the salt ions upon ABS formation should thus not be a source of entropic gain. On the other hand, water is also highly structured around hydrophobic species ⁶⁸ and displays (weak) attractions with them, overcompensated by an unfavorable entropic cost.⁶⁹ As a result, hydrophobic species tend to “expel” water from their surface and to aggregate with each other, contributing to a favorable entropic gain. The latter likely operates already in IL solution **A** (without MClₙ salt) where significant hydrophobic contacts (TfO⁻...TfO⁻,
BMI⁺...TfO⁻) between IL ions are observed. They might explain the ability of this IL to be salted-out, even by weakly salting-out species like NaCl, carbohydrates ⁶⁶ or aminoacids.⁷⁰ On the other hand, like ionic liquids used for ABSs, it does not precipitate in concentrated conditions.

**The IL phase of ABSs: ions hydration and organization into “dry” / “humid” domains.**

We have seen that the IL-rich phase of ABSs is mainly aqueous: even in the most dramatic cases (NaCl solution **D**, SrCl₂ solution **K**, EuCl₃ solution **L**), water is in excess over IL ions (more than ≈ 800 H₂O molecules for at most 148 BMI⁺ TfO⁻ ions). It is thus interesting to look at the state of water in the IL phase, focusing on solution **L**. Typical snapshots (Figure 12) make clear that water is mainly connected to the SO₃⁻ moiety of TfO⁻, via three to four strong hydrogen bonds, on the average. Conversely, there are nearly “dry” IL regions where CF₃(TfO) and methyl(BMI) or butyl(BMI) hydrophobic moieties segregate, whereas the polar interactions between SO₃(TfO)⁻ and ring(imidazolium)⁺ are most often mediated by water.

Hydration of the “IL phase” of these ABSs is thus reminiscent of the one observed in hydrophobic, but hygroscopic ILs.⁴⁵, ⁷¹, ⁷² In particular, the IL anion is better hydrated than the cation, due to H-bonding interactions.⁷³ In the IL phase of ABSs, solvating water around polar groups tends to form filaments (in the SrCl₂ solution **K**) or aggregated fingers (in the EuCl₃ solution **L**). The remarkable feature of ABSs is the fact that dehydration of the hydrophobic moieties is simply induced by the remote added salt, at a sufficient concentration. When the added the MClₙ salt becomes less salting-out (less hydrophilic and / or less concentrated), the water content of the IL phase increases, and the phase breaks up into smaller domains, droplets or aggregates. As mentioned above, dehydration of hydrophobic moieties of the IL is entropy favored,¹¹, ⁷⁴ indirectly contributing to the salting-out effect.
In the IL phase of the L solution, IL ions display some “organization”, induced by water. We simulated the pure dry [BMI][TfO] IL for comparison (see Figure S2 of the Supporting Information). In the latter, the apolar groups also aggregate with each other, as do the polar ones. For instance, each SO₃(TfO)⁻ is surrounded by a “cage” of 3-4 ring(imidazolium)⁺ groups, whereas a ring(imidazolium)⁺ is in contact with 2-3 SO₃(TfO)⁻ groups. Neither the TfO⁻ anion nor the BMI⁺ cation are amphiphilic enough, however, to induce longer range ordering.⁷², ⁷⁵, ⁷⁶ In the presence of water, these polar interactions are disrupted, whereas the apolar ones are strengthened.

## Analogies between ionic liquid patterns in ABSs and at an aqueous interface.

The analogy between IL ions' hydration patterns in ABSs and at aqueous interfaces is striking. First note that both IL ions prefer the water surface over bulk water.⁷⁷ This feature is illustrated by a MD experiment we performed on the aqueous solution of BMI⁺ TfO⁻ ions at the air interface: density profiles and snapshots (Figures 13 and S27 of the Supporting Information) show that IL ions are “attracted” by the interface (more with the IL<sup>+/-0.9</sup> charges than with the IL<sup>+/-1</sup> ones) where they adopt amphiphilic orientations: the hydrophobic Butyl(BMI) and CF₃(TfO) moieties are “dry” and point towards air, while the polar (ring)imidazolium⁺ and SO₃(TfO)⁻ ones prefer water. In particular, SO₃⁻ is hydrated by 3 to 4 H₂O molecules, connected to bulk water, as in ABSs.

Calling attention to interfacial properties helps to understand why trends in ABSs formation from IL / salt mixtures follow the ions' surface activity and their Hofmeister series classification.¹⁵, ²⁵, ⁶⁵ The studied IL ions behave as small amphiphiles, as do components of polymers like PEGs, dextrans or water soluble macromolecules that also form ABSs. Even “big spherical ions”, some of them being components of ILs (polyoxometallate,⁷⁸, ⁷⁹ dicarbollide,⁶², ⁸⁰ ClO₄⁻ anions ⁸¹ or NR₄⁺ quaternary ammonium cations) display hydrophobic
features and surface activity. They also tend to aggregate in water, and thus to form ABSs. By contrast, the salting-out $M^{n+}$ $Cl^-$ ions that increase the surface tension of water are instead “repelled” by the interface and prefer a bulk water environment.

## The liquid-liquid “interface” of IL/salt ABSs

Generally, liquid-liquid interfaces are defined with respect to the adjacent solvents (e.g. water and a hydrophobic ionic liquid).³⁴, ⁸²⁻⁸⁶ In the case of the studied ABSs, the average z-position of the interface $\langle z \rangle_{ITF}$ might be defined by the z-plane where the water density $d_{wat-ITF}$ is the mean of densities in the IL slab and in the salt slab: $d_{wat-ITF} = 1/2 (d_{wat-IL} + d_{wat-salt})$. Since the real partitioning occurs between IL and salt ions, the interface can alternatively be defined by the crossing of the density curves of IL and salt ions. Focusing on the interfaces of **D**, **K** and **L** biphasic solutions (with NaCl, $SrCl_2$, and $EuCl_3$ salts, respectively), one finds similar $\langle z \rangle_{ITF}$ positions: with the first one, the interface is shifted by only ≈ 0.3 nm from the second one, towards the IL-phase on the average. In the three solutions, the interface is rather sharp and narrow: defining its “width” by the distance between z-planes where the IL-density and the salt-density attain 90% of their values in the bulk adjacent phases, one obtains ca. 2.2 nm, 1.6 nm and 1.2 nm, respectively for NaCl, $SrCl_2$, and $EuCl_3$ containing ABSs. Thus, the more hydrophilic the metal salt is, the sharper and narrower is the interface. Snapshots (Figure 14) and density curves (Figure 5) show that there is more salt-penetration on the IL-side than IL-penetration on the salt-side of the interface. Near the interface, the IL density slope is much the same with the three salts, while the salt slope is weaker, and decreases from $EuCl_3$ to NaCl. Comparing now these ABSs interfaces to those of hydrophobic ILs /water solutions, the former are found to be comparable in size. The main difference with the studied ABSs is that the $BMI^+$ $TfO^-$ ions are better hydrated, and that the cohesion forces of the IL phase are induced by the added hydrophilic salt, and by water. On
the other hand, neither the butyl chain of the BMI⁺ cation nor the perfluoro group of the TfO⁻ anion are long enough to induce organized structures, like micelles in water.⁸⁷ The simulated interfaces can be viewed as zooms of “flat” ABSs at rest, as well as interfaces of IL droplets formed in the solutions, near the cloud point compositions.

## Conclusions.

We report MD simulations of concentrated aqueous mixtures of IL and MCl<sub>n</sub> salt ions, selecting solutions that are experimentally monophasic and biphasic. They reveal in all cases collective phenomena yielding marked heterogeneities and segregation features into mutually exclusive neutral IL and salt domains (“ionic sociology”). These domains range, depending on the concentration, from locally fluctuating nano-regions (in monophasic solutions) to completely separated IL and salt aqueous phases (in ABSs). Segregation stems here from two main features: (i) IL ions tend to “attract” each other, due to hydrophobic forces (dehydration of their apolar moieties) and to IL/IL attractions upon segregation. Even without added salt, the IL ions display significant anion... cation and anion...anion hydrophobic contacts. (ii) The salt ions trap water, by increasing amounts and at longer distances when their salting-out character (concentration and / or M<sup>n+</sup> hydrophilicity) is increased. The water transfer from the IL to the salt ions can be viewed as a water-donor / water-acceptor synergistic process. MCl<sub>n</sub> ions attract water, mainly in their second coordination shell, and much beyond when the cationic charge increases. This contrasts with the IL ions whose hydrophobic moieties can lose already up to ca. 50% of first shell water in the presence of MCl<sub>n</sub> salts.

Thermodynamically, this should be entropically favored. Consistent comparisons show that ion segregation and ABS formation increase with the salt concentration, the IL concentration, the M<sup>n+</sup> charge and the “polarity” of IL ions, in good agreement with experiment. In the ABSs, the aqueous-IL/aqueous-salt interface is found to be quite sharp, and of decreasing
width when the MClₙ salt becomes more strongly “salting-out”. Finally, we point to analogies
between ion “organization” in ABSs and the contrasting behavior of IL and MClₙ salts at
aqueous interfaces, and thus with the Hofmeister classification of IL and salt ions. From the
analysis of snapshots, density curves, the “demixing index” ξ<sub>demix</sub>, RDFs, energy components,
no clear cut transition is observed, however, between heterogeneous monophasic systems and
biphasic ones, calling for physical and simulation studies on nano-organization in these
phases as well as on their “cloud point” intermediate state, focusing on interactions and
structuration at “long distances”. Comparisons with other ILs (e.g. [BMI][BF₄] or related
ABSs (e.g. with PEGs) combining dry ILs<sup>88</sup> or “humid” ILs<sup>89</sup> should should enable further
analysis of the specificity of the presently studied ones.

**Supporting Information.** Tables contain the description of all simulated systems, the characteristics of RDFs, and the assignment of the different “phases”. The Figures afford final snapshots of the simulated systems with the IL<sup>+/- 1</sup> and IL<sup>+/-0.9</sup> models, density curves along the z-axis, extended results for the neat solution **A**, for the aqueous IL/ vacuum interface and for the pure dry IL.

**Acknowledgements.** The authors are grateful to Alain Chaumont and Etienne Engler for assistance and useful discussions, to Jack Harrowfield for linguistic assistance, and to the Université de Strasbourg for computational resources.

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<table><caption>Table 1: Characteristics of the simulated solutions A to P. Unless otherwise specified the box is “standard” and contains 8000 H₂O; the “big” box (A2 and D2 systems) contains 32000 H₂O. An extended version of the Table is given in Table S1.</caption><thead><tr><th rowspan="2"></th><th>IL</th><th colspan="2">Salt</th><th>[IL]</th><th>[MCl<sub>n</sub>]</th></tr><tr><th>number of pairs</th><th>type and number of pairs</th><th>mol.kg<sup>-1</sup></th><th>mol.kg<sup>-1</sup></th></tr></thead><tbody><tr><td>A</td><td>148</td><td>0</td><td>-</td><td>1.03</td><td>0.00</td></tr><tr><td>A2</td><td>592</td><td>0</td><td>-</td><td>1.03</td><td>0.00</td></tr><tr><td>B</td><td>148</td><td>148</td><td>NaCl</td><td>0.97</td><td>0.79</td></tr><tr><td>C</td><td>148</td><td>296</td><td>NaCl</td><td>0.92</td><td>1.59</td></tr><tr><td>D</td><td>148</td><td>444</td><td>NaCl</td><td>0.87</td><td>2.38</td></tr><tr><td>E</td><td>296</td><td>296</td><td>NaCl</td><td>1.83</td><td>1.29</td></tr><tr><td>F</td><td>296</td><td>444</td><td>NaCl</td><td>1.74</td><td>1.94</td></tr><tr><td>D2</td><td>592</td><td>1776</td><td>NaCl</td><td>0.87</td><td>2.38</td></tr><tr><td>G</td><td>148</td><td>296</td><td>CsCl</td><td>0.76</td><td>1.59</td></tr><tr><td>H</td><td>148</td><td>444</td><td>CsCl</td><td>0.68</td><td>2.38</td></tr><tr><td>I</td><td>296</td><td>700</td><td>CsCl</td><td>1.13</td><td>3.05</td></tr><tr><td>J</td><td>148</td><td>148</td><td>SrCl<sub>2</sub></td><td>0.88</td><td>0.79</td></tr><tr><td>K</td><td>148</td><td>296</td><td>SrCl<sub>2</sub></td><td>0.77</td><td>1.59</td></tr><tr><td>L</td><td>148</td><td>296</td><td>EuCl<sub>3</sub></td><td>0.67</td><td>1.59</td></tr><tr><td>M</td><td>148</td><td>148</td><td>EuCl<sub>3</sub></td><td>0.81</td><td>0.79</td></tr><tr><td>N</td><td>148</td><td>74</td><td>EuCl<sub>3</sub></td><td>0.91</td><td>0.40</td></tr><tr><td>O</td><td>148</td><td>50</td><td>EuCl<sub>3</sub></td><td>0.94</td><td>0.27</td></tr><tr><td>P</td><td>148</td><td>25</td><td>EuCl<sub>3</sub></td><td>0.98</td><td>0.13</td></tr></tbody></table>

<table><caption>Table 2: Final demixing index ξ<sub>demix</sub> for systems A to L (averages during the last 10 ns; fluctuations are about 0.02).</caption><thead><tr><th>Solution</th><th>A</th><th>B</th><th>C</th><th>D</th><th>E</th><th>F</th><th>G</th><th>H</th><th>I</th><th>J</th><th>K</th><th>L</th></tr></thead><tbody><tr><td>IL<sup>±1</sup></td><td>0.73</td><td>0.74</td><td>0.70</td><td>0.64</td><td>0.71</td><td>0.65</td><td>0.71</td><td>0.68</td><td>0.63</td><td>0.70</td><td>0.65</td><td>0.45</td></tr><tr><td>IL<sup>±0.9</sup></td><td>0.74</td><td>0.65</td><td>0.62</td><td>0.48</td><td>0.56</td><td>0.47</td><td>0.61</td><td>0.53</td><td>0.41</td><td>0.58</td><td>0.49</td><td>0.42</td></tr></tbody></table>

<table><caption>Table 3: Characteristics of RDFs of IL ions in 148 IL<sup>±0.9</sup>/ MCl<sub>n</sub> solutions (see Figure 8): Integration numbers (up to indicated distances).</caption><thead><tr><th>Solution</th><th></th><th>A</th><th>A2</th><th>B</th><th>C</th><th>D</th><th>J</th><th>K</th><th>L</th><th>P</th></tr><tr><th></th><th>dry IL</th><th>0 NaCl</th><th>0 NaCl</th><th>148 NaCl</th><th>296 NaCl</th><th>444 NaCl</th><th>148 SrCl<sub>2</sub></th><th>296 SrCl<sub>2</sub></th><th>296 EuCl<sub>3</sub></th><th>25 EuCl<sub>3</sub></th></tr></thead><tbody><tr><td>BMI···BMI (12Å)</td><td>17.9</td><td>3.5</td><td>3.7</td><td>4.9</td><td>5.6</td><td>8.6</td><td>8.7</td><td>8.6</td><td>10.3</td><td>9.7</td></tr><tr><td>BMI···TfO (8Å)</td><td>6.1</td><td>1.8</td><td>1.8</td><td>1.6</td><td>1.9</td><td>2.7</td><td>2.9</td><td>2.6</td><td>3.1</td><td>3.9</td></tr><tr><td>TfO···TfO (8Å)</td><td>4.0</td><td>1.1</td><td>1.1</td><td>2.3</td><td>2.7</td><td>3.6</td><td>2.2</td><td>3.5</td><td>4.1</td><td>2.9</td></tr><tr><td>BMI···wat (6Å)</td><td>-</td><td>17.6</td><td>17.3</td><td>15.4</td><td>14.7</td><td>12.1</td><td>13.8</td><td>11.9</td><td>9.7</td><td>10.1</td></tr><tr><td>TfO···wat (6Å)</td><td>-</td><td>17.9</td><td>17.6</td><td>15.3</td><td>13.4</td><td>9.8</td><td>12.4</td><td>10.1</td><td>6.7</td><td>7.5</td></tr><tr><td>S<sub>TfO</sub>···wat (5Å)</td><td>-</td><td>9.0</td><td>9.1</td><td>8.5</td><td>7.5</td><td>6.6</td><td>7.4</td><td>6.2</td><td>4.5</td><td>5.1</td></tr><tr><td>S<sub>TfO</sub>···wat (7Å)</td><td>-</td><td>30.0</td><td>30.6</td><td>27.5</td><td>23.1</td><td>19.3</td><td>23.1</td><td>18.6</td><td>12.4</td><td>14.8</td></tr><tr><td>S<sub>TfO</sub>···M<sup>n+</sup> (4.5Å)</td><td>-</td><td>-</td><td>-</td><td>0.13</td><td>0.16</td><td>0.18</td><td>0.04</td><td>0.04</td><td>0.00</td><td>0.00</td></tr><tr><td>S<sub>TfO</sub>···M<sup>n+</sup> (7Å)</td><td>-</td><td>-</td><td>-</td><td>0.50</td><td>0.65</td><td>0.68</td><td>0.29</td><td>0.25</td><td>0.03</td><td>0.01</td></tr><tr><td>N<sub>butyl</sub>(BMI)···Cl<sup>-</sup> (5.5Å)</td><td>-</td><td>-</td><td>-</td><td>0.08</td><td>0.12</td><td>0.13</td><td>0.09</td><td>0.09</td><td>0.03</td><td>0.00</td></tr></tbody></table>
<figure><img src="image_3.png" /><figcaption>Figure 1: Ternary phase diagram of the [BMI][TfO] IL, water and MCl<sub>n</sub> salts: CsCl (+), NaCl (•), SrCl<sub>2</sub> (■), adapted from ref. [11]. Simulated solutions are marked **B** to **F** (NaCl; red), **G** to **I** (CsCl; yellow), **J** and **K** (SrCl<sub>2</sub>; green).</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 2: IL<sup>±0.9</sup> salt solutions: demixing index ξ<sub>demix</sub> as a function of time (0 to 100 ns). Left: All NaCl solutions. Right: MCl<sub>n</sub> solutions.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 3: Neat IL aqueous solution A (IL$^{±0.9}$ model). Snapshot at 200 ns with the IL only (line 1). Line 2: Average densities along the z-axis of the box (distances in Å) during 100-200 ns (BMI$^+$ orange, TfO$^-$ purple, water blue) ); C$_{TfO}$, N$_{Butyl}$ and O$_{H2O}$ are the reference centers. Line 3 : IL···IL RDFs as a function of the distance (in Å): TfO···BMI (purple), TfO···BMI (gray), BMI···BMI (blue). Line 4: snapshots of IL...IL contacts. An extended version of the Figure, including IL$^{±1}$ results is given in Figure S2. Density curves averaged over different time periods are shown in Figures 5 and S3.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 4: 148 [BMI][TfO]⁰.⁹/NaCl solutions **B** – **D**. Snapshots after 500 ns, with either all components (left), or with NaCl hidden (right). Color codes: IL ions green, water cyan, Cl⁻ orange, Na⁺ yellow.</figcaption></figure>
<table><caption>Figure 5: IL<sup>±0.9</sup>/MCl<sub>n</sub> solutions: densities along the z-axis (in Å) of the box, averaged over the last 5 ns. Color codes: IL green, salt red, water blue.</caption><tbody><tr><td rowspan="4">148 IL</td><td><img src="image_1.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td><td><img src="image_2.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td><td><img src="image_3.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td></tr><tr><td>A : no salt<br>Exp mono / d<sub>wat</sub>=0.8</td><td>B : 148 NaCl<br>Exp mono / d<sub>wat</sub>=0.6-0.9</td><td>C : 296 NaCl<br>Exp mono / d<sub>wat</sub>=0.5-0.9</td></tr><tr><td><img src="image_4.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td><td><img src="image_5.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td><td><img src="image_6.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td></tr><tr><td>D : 444 NaCl<br>Exp cloud-bi / d<sub>wat</sub>=0.4-0.9</td><td>G : 296 CsCl<br>Exp mono / d<sub>wat</sub>=0.5-0.9</td><td>H : 444 CsCl<br>Exp mono / d<sub>wat</sub>=0.5-0.9</td></tr><tr><td rowspan="2">296 IL</td><td><img src="image_7.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td><td><img src="image_8.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td><td><img src="image_9.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td></tr><tr><td>J : 148 SrCl<sub>2</sub><br>Exp mono / d<sub>wat</sub>=0.5-0.9</td><td>K : 296 SrCl<sub>2</sub><br>Exp bi / d<sub>wat</sub>=0.4-0.9</td><td>L : 296 EuCl<sub>3</sub><br>d<sub>wat</sub>=0.2-0.9</td></tr><tr><td rowspan="3">592 IL</td><td><img src="image_10.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td><td><img src="image_11.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td><td><img src="image_12.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td></tr><tr><td>E : 296 NaCl<br>Exp cloud-mono / d<sub>wat</sub>=0.4-0.9</td><td>F : 444 NaCl<br>Exp bi / d<sub>wat</sub>=0.3-0.9</td><td>I : 700 CsCl<br>Exp cloud-bi / d<sub>wat</sub>=0.4-0.8</td></tr><tr><td><img src="image_13.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td><td><img src="image_14.png" alt="Volume/rachel/1/dermat-0.9-5ms.png"></td><td></td></tr><tr><td></td><td>A2 : no salt<br>Exp mono / d<sub>wat</sub>=0.8</td><td>D2 : 1776 NaCl<br>Exp cloud-bi / d<sub>wat</sub>=0.4-0.9</td><td></td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>Figure 6: 148 IL<sup>±0.9</sup>/MCl<sub>n</sub> solutions at two CsCl or SrCl<sub>2</sub> salt concentrations, and EuCl<sub>3</sub> solution L. Snapshots after 500 ns, with either all components (left), or with salt hidden (right). Color codes: IL ions green, water cyan, Cl<sup>-</sup> orange, Cs<sup>+</sup> brown, Sr<sup>2+</sup> dark blue, Eu<sup>3+</sup> blue.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 7: 296 IL$^{±0.9}$ solutions with NaCl or CsCl. Snapshots after 500 ns with either all species (**left**) or with the salt hidden (**right**). Color codes: IL green, water cyan, Cl⁻ orange, Na⁺ yellow, Cs⁺ brown.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 8: RDFs of IL ions in 148 IL$^{±0.9}$/ MCl$_n$ solutions A (no salt: yellow), B (148 NaCl: orange), C (296 NaCl: red), D (444 NaCl: purple), J (148 SrCl$_2$: cyan), K (296 SrCl$_2$: green) L (296 EuCl$_3$: black) and P (25 EuCl$_3$: blue). The corresponding integration numbers are given in Table 3.</figcaption></figure>
<table><caption>Figure 9: 148 IL$^{±0.9}$/ 296 MCl$_n$ solutions. Integration number of RDFs up to 5 and 20 Å as a function of time (in ns). Color codes: NaCl blue, CsCl, black, SrCl$_2$ cyan, EuCl$_3$ brown. An extended version is given in Figure S22.</caption><thead><tr><th></th><th>5 Å</th><th>20 Å</th></tr></thead><tbody><tr><td>M<sup>n+</sup>···O<sub>w</sub></td><td><img src="image_0_0_0.png" alt="Volume 0: Rachel's AMBER Rectangular RDF integrals, 0-10 NaCl, Na-Ow_5.png"></td><td><img src="image_0_0_1.png" alt="Volume 0: Rachel's AMBER Rectangular RDF integrals, 0-10 NaCl, Na-Ow_20.png"></td></tr><tr><td>BMI···BMI</td><td><img src="image_0_0_2.png" alt="Volume 0: Rachel's AMBER Rectangular RDF integrals, 0-10 NaCl, BHS-BHS_5.png"></td><td><img src="image_0_0_3.png" alt="Volume 0: Rachel's AMBER Rectangular RDF integrals, 0-10 NaCl, BHS-BHS_20.png"></td></tr><tr><td>BMI···TfO</td><td><img src="image_0_0_4.png" alt="Volume 0: Rachel's AMBER Rectangular RDF integrals, 0-10 NaCl, BHS-TF_5.png"></td><td><img src="image_0_0_5.png" alt="Volume 0: Rachel's AMBER Rectangular RDF integrals, 0-10 NaCl, BHS-TF_20.png"></td></tr><tr><td>TfO···TfO</td><td><img src="image_0_0_6.png" alt="Volume 0: Rachel's AMBER Rectangular RDF integrals, 0-10 NaCl, TF_5.png"></td><td><img src="image_0_0_7.png" alt="Volume 0: Rachel's AMBER Rectangular RDF integrals, 0-10 NaCl, TF_20.png"></td></tr><tr><td>BMI···O<sub>w</sub></td><td><img src="image_0_0_8.png" alt="Volume 0: Rachel's AMBER Rectangular RDF integrals, 0-10 NaCl, BHS-Ow_5.png"></td><td><img src="image_0_0_9.png" alt="Volume 0: Rachel's AMBER Rectangular RDF integrals, 0-10 NaCl, BHS-Ow_20.png"></td></tr><tr><td>TfO···O<sub>w</sub></td><td><img src="image_0_0_10.png" alt="Volume 0: Rachel's AMBER Rectangular RDF integrals, 0-10 NaCl, TF-Ow_5.png"></td><td><img src="image_0_0_11.png" alt="Volume 0: Rachel's AMBER Rectangular RDF integrals, 0-10 NaCl, TF-Ow_20.png"></td></tr></tbody></table>
<figure><img src="image_10.png" /><figcaption>Figure 10: "Big box" simulations on IL<sup>±0.9</sup> solutions with no salt (<b>A2</b>) and with NaCl (<b>D2</b>) after 200 ns (<b>A2</b>) and 500 ns (<b>D2</b>).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Figure 11: 148 IL<sup>±0.9</sup> / 296 MCl<sub>n</sub> solutions C and K. Energy component analysis (in 10<sup>3</sup> kcal.mol<sup>-1</sup>) as a function of time. Total Potential Energy E<sub>pot</sub> during the first 10 ns. Internal IL energy, IL···wat and salt···wat interactions during the first 100 ns. An extended version is given in Figures S23 to S25.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Figure 12: Typical solvated IL ions arrangements in the IL phase of the biphasic EuCl₃ solution L (left) and at the aqueous IL/vacuum interface (right)</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Figure 13: Air/liquid interface of an aqueous solution of 296 [BMI][TfO]⁺⁰.⁹. Line 1: snapshots highlighting interfacial TfO⁻ (left) and BMI⁺ ions (right): CF₃ in blue, SO₃⁻ in red, butyl chain in green, imidazolium ring in black. Line 2: IL and water densities, between 20 and 40 Å of the z-axis (in Å), averaged over 50 ns. Color codes: water blue; IL green; BMI⁺ dotted black; TfO⁻ dotted orange. An extended version is given in Figure S27.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Figure 14 : Snapshots of ions at IL/salt interfaces of biphasic solutions **D**, **K**, **L** (slices of 3.5 nm width; water hidden for clarity). Color codes: Cl⁻ orange, Na⁺ yellow, Sr²⁺ dark blue, Eu³⁺ cyan.</figcaption></figure>
# TOC Table of Content.

<figure><img src="image_15.png" /></figure>