The energetics and structure of ionic liquid aqueous solutions

Some ionic liquids are considerably soluble in water, and their aqueous solutions therefore became the subject of a variety of potential applications [35-38]. This also fostered various theoretical and experimental studies on the structure and energetics of those solutions. Most of them were focused on ILs of the 1-3-dialkylimidazolium family. From these studies a general picture emerged where in the limit of very low concentrations of water, the H2O molecules tend to be isolated from each other or exist as small independent clusters dispersed in the continuous IL polar network [39-44]. At least for 1-3-dialkylimidazolium-based ionic liquids, water interacts preferably with the anionic moiety, where hydrogen bonding tends to be more efficient. This is not so, however, for species with less pronounced protic character, such as methanol or acetonitrile [45]. As the water concentration increases, continuous H2O networks start to appear, which dramatically change the properties of the mixture. At this concentration range the IL and H2O domains co-exist and form a nano-segregated biphasic system. For solutions richer in water, the IL networks start to break apart generating IL clusters dispersed in the continuous water phase [39,46]. On further dilution, the average size of the IL clusters decreases until solvated isolated ions are obtained.

Of particular interest to us was the relationship between the structure and energetics of aqueous IL solutions. This motivated the study of 1-ethyl-3-methylimidazolium ethylsulfate, [C2mim][EtSO4] (Figure 1) aqueous solutions based on: (i) enthalpies of solution, ΔslnH, and dilution, ΔdilH, obtained by solution and flow calorimetry, respectively; (ii) the interpretation of the observed experimental trends using molecular dynamics simulations [47,48]. The variation of with the concentration of IL exhibited a minimum at xH2O = 0.99 that could be approximately captured by our MD simulation results but not through extrapolations based on previously reported experimental or simulation results (Figure 2).

fig1

Figure 1. Molecular structure a of the 1-ethyl-3-methylimidazolium ethylsulfate, C2mim][EtSO4], ionic liquid.

 

fig 2

Figure 2. Molar enthalpies of solution of [C2mim][EtSO4] in water (per mol of IL) at 298.15 K as a function of the ionic liquid molar fraction. Magenta solid line, our work experimental data. Red solid line, our work molecular dynamics simulation data. Blue solid line and Green solid line, previous experimental and molecular dynamics data.

 

The separation of our values obtained by MD simulations into contributions from IL-IL, H2O-H2O, and IL-H2O interactions revealed that the minimum essentially results from two opposing effects (Figure 3): (i) the fact that the differences between the IL-IL and H2O-H2O interactions in the solution and in the pure liquids are both positive and increase with the dilution of the IL and (ii) the simultaneously more negative character of the IL-H2O contribution (which is only present in the solution). It was finally concluded that the observed trends in are dominated by electrostatic rather than dispersion interactions. This fact is in line with the previously reported sensitivity of MD simulation results for this system to the effective atomic point charges assigned to the water molecules.

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Figure 3. Configurational internal energy of solution, eq24, calculated from MD simulations (red line) as a sum of contributions from different types of interaction. All internal energy quantities are expressed in kJ per mol of ionic liquid.

 

The analysis of the MD simulations results using different in-house developed analysis tools to describe the size and connectivity of different water and ion aggregates as a function of the solutions concentration also allowed the identification of four concentration ranges where four distinct structural regimes are present (Figures 4 and 5): isolated water molecules (x H2O < 0.5); chain-like water aggregates (0.5 < xH2O < 0.8); bicontinuous system (0.8 < xH2O < 0.95); and isolated ions or small ion clusters, respectively (xH2O > 0.95). These include two different percolation limits: that of water in the ionic liquid network (xH2O ~ 0.8) and that of the ionic liquid in water (xH2O ~ 0.95).

fig3

Figure 4. Snapshot images of the water clusters in simulation boxes with a) xH2O=0.5, b) xH2O=0.8 and c) xH2O=0.92. The pictures show that when small amounts of H2O molecules are present they are either isolated or form chains around the polar network of the ionic liquid (not represented). When the water percolation limit is attained (between b) and c)), the water molecules form a continuous branched network surrounding the ionic liquid.

 

fig4

Figure 5. Snapshots of the ionic liquid aggregates in simulation boxes with a) xH2O = 0.95, b) xH2O = 0.97 and c) xH2O = 0.996. Ions of the same color belong to the same aggregate. The pictures show that at xH2O = 0.95 the IL network starts to break down and multiple aggregates are formed. Finally, when just a few IL pairs are present at xH2O = 0.996, only isolated ions and dimmers are found in solution.

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