Water Phase Diagram Is Significantly Altered by ... - ACS Publications

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Water Phase Diagram Is Significantly Altered by Imidazolium Ionic Liquid Vitaly V. Chaban†,‡,* and Oleg V. Prezhdo‡ †

MEMPHYS - Center for Biomembrane Physics, Syddansk Universitet, Campusvej 55, Odense M 5230, Kingdom of Denmark Department of Chemistry, University of Rochester, 120 Trustee Road, Rochester, New York 14627, United States

‡

ABSTRACT: We report unusually large changes in the boiling temperature, saturated vapor pressure, and structure of the liquid−vapor interface for a range of 1-butyl-3methyl tetrafluoroborate, [C4C1IM][BF4]−water mixtures. Even modest molar fractions of [C4C1IM][BF4] significantly affect the phase behavior of water, as represented, for instance, by strong negative deviations from Raoult’s law, extending far beyond the standard descriptions. The investigation was carried out using classical molecular dynamics employing a specifically refined force field. The changes in the liquid−vapor interface and saturated vapor pressures are discussed at the atomistic resolution. The reported results guide the search for novel scientific and technological applications of ion−molecular systems. SECTION: Liquids; Chemical and Dynamical Processes in Solution difference is likely due to fluorination of the first anion. The difference between ion sizes likely plays a role as well. In the present work, we report atomistic resolution molecular dynamics (MD) simulations of several 1-butyl-3-methylimidazolium tetrafluoroborate, [C4C1IM][BF4]−water mixtures, and center the discussion on how the RTIL alters evaporation behavior of water. According to the implemented molecular models, [C4C1IM][BF4] introduces major changes to the liquid−gas phase transition of water. The observed changes cannot be described by the classical Raoult’s law. As such, the [C4C1IM][BF4]−water systems must be considered highly nonideal mixtures. Figure 1 depicts the constructed atomistic model for the liquid−vapor interface of the [C4C1IM][BF4]−water mixtures.

I

onic and molecular liquids work together for a number of fascinating applications.1−6 Over past decades, the field of room-temperature ionic liquids (RTILs) has been in the spotlight of the scientific and industrial community. RTILs are viewed as a promising alternative to traditional organic solvents. Cations of RTILs are bulky organic molecules with manifold substituents. The aromatic ring contains either positively charged nitrogen, sulfur, or phosphorus atoms (e.g., N,N′dialkylimidazolium, N-alkylpyridinium, alkylammonium, alkylphosphonium, alkylsulphonium, tiazolium, etc.). Asymmetry of cation is an important prerequisite for the ionic compound to remain liquid at room temperature. In turn, anions are inorganic or organic species, such as halides, tetrafluoroborate, hexafluorophosphate, bis(trifluoromethylsulfonyl)imide, acetate, dicyanamide, and so on. Many of these solvents are nonflammable, and they exhibit negligible vapor pressure and excellent thermal stability.6−11 In combination with certain molecular liquids, including water, RTILs are used for separation applications,7 as electrolyte solutions,12 and even for nuclear fuel reprocessing.13−15 The mixtures of imidazolium RTILs and water have been investigated in detail.16−25 Recent works have been devoted to the liquid−vapor equilibria of imidazolium RTILs and organic solvents, such as methanol, ethanol, tetrahydrofuran, acetone, and so on.26−29 Still, experimental data on liquid−vapor equilibria in systems containing ionic liquids are scarce because the majority of conventional equilibrium cells are not adequate for this kind of systems. In the two month old publication, Passos et al. suggested that boiling point elevation strongly depends on the anion. For instance, [C4C1IM][CF3SO3] elevates the boiling point of water by 6.2 K, while [C4C1IM][C1SO3] elevates the boiling point by 27.2 K. The molar fraction of water in both cases equals to 72%. The observed © 2014 American Chemical Society

Figure 1. Liquid−vapor equilibrium simulated for the equimolar mixture of [C4C1IM][BF4] and water. [C4C1IM] cations, [BF4] anions, and water molecules are shown using red, yellow, and white balls, respectively. The coordinate axes show the orientation of the simulated system in the MD simulation cell. Received: March 19, 2014 Accepted: April 22, 2014 Published: April 22, 2014 1623

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Each atom of the ionic liquid and water corresponds to one interaction site. Therefore, the water model contains 3sites, the 1-butyl-3-methylimidazolium cation contains 25 sites, and the tetrafluoroborate anion contains 5 sites. Saturated vapor pressures in all systems plotted as a function of temperature (Figure 2) obey an exponential dependence.

Figure 3. Water boiling point elevation versus molar fraction of the ionic liquid in the mixtures. (a) Values obtained from atomistic simulations (inset: boiling point elevation versus molality of solution). (b) Values obtained from Raoult’s law using the tabulated ebullioscopic constant for water, 0.512 (red solid line), and considering the van’t Hoff factor for [C4C1IM][BF4] (green dashed line). Note that although mixtures are simulated, the vapor pressure is generated exclusively by water molecules (Figure 1); therefore, only the water normal boiling point is changed. Figure 2. Saturated vapor pressure as a function of temperature for the equilibrated liquid−vapor interfaces. The percentages on the plots show molar fractions of [C4C1IM][BF4] in its mixtures with water. Horizontal dotted lines correspond to the normal atmospheric pressure.

[C 4 C 1IM][BF 4 ]−water mixture increases by 25 K. In comparison, Raoult’s law prediction is just 3.26 K, whereas correction for the van’t Hoff factor increases it to 6.52 K. Both predictions are several times smaller than the actual rise in the boiling point. This negative deviation from Raoult’s law reflects strongly favorable interactions in the liquid phase of the simulated mixtures. It is known from experiments that imidazolium RTILs mix with water in virtually any proportion. Water and imidazolium RTILs possess very different phase diagrams. Indeed, imidazolium RTILs exhibit a very large liquid range, up to several hundreds of degrees Celsius. Many RTILs decompose before evaporation. The combination of the vast differences in the phase diagrams of water and the RTIL, with strongly favorable interactions, allows [C4C1IM][BF4] to heavily adjust the temperature-dependent properties of water. There are many molecular liquids, which mix with water in any or nearly any proportion (for instance, dimethyl sulfoxide, acetonitrile, low molecular alcohols), but none of them alters the water phase diagram so significantly. The mentioned liquids evaporate at comparable or lower temperatures than water. Therefore, the discussed effect cannot be observed. The density of liquids at the liquid−vapor interface (Figure 4) provides important information regarding their phase behavior. The liquid−vapor interfaces appear well-defined for all [C4C1IM][BF4]−water systems at 360 K. The liquid densities of the mixture components evolve in agreement with the composition of the simulated systems. Interestingly, the cation density is larger than the anion density, even though anions contain heavier atoms (fluorines and no hydrogens). In turn, cations contain a ring, which always exhibits a higher mass

The point of intersection of these curves with the horizontal line, standing for the normal atmospheric pressure, defines the normal boiling point for each mixture. The TIP3P water model somewhat underestimates the normal boiling point (362 K). However, as compared with many other water models, the underestimation by 11 K is not critical for the current investigation. Although it is quite easy to adjust the boiling point for small molecules (for instance, by an increase in the dipole moment), this step would also imply a reparameterization of the RTIL−water interactions in the liquid phase. Such reparameterization requires additional, time-consuming testing and likely further adjustments of the interaction parameters. This work reports only boiling point differences between pure water and water containing certain fraction of RTIL. Therefore, the absolute boiling point of water is not of primary importance. Noteworthy, even modest molar fractions of [C4C1IM][BF4] significantly alter saturated vapor pressures of water and, consequently, increase the normal boiling point. RTIL ions were not detected in the vapor phase of any of the simulated system. While water evaporates, RTIL remains liquid. Hence, the simulated liquid phase is more RTIL-rich than it is implied by the initial mixture composition. Figure 3 summarizes the boiling points as a function of the RTIL content. Notably, the boiling point of the equimolar 1624

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Figure 6. Vapor pressure and vapor density as functions of simulation time for the equimolar [C4C1IM][BF4]−water mixture (the most viscous mixture). The vertical dotted lines show simulation times, when the aqueous vapor becomes saturated; therefore, the liquid− vapor interface becomes equilibrated. The simulation was conducted at 360 K. Figure 4. Mass densities of particles along the normal direction to the film of liquid. The densities of water, [C4C1IM] cation, and [BF4] anion are shown by the red solid, green dashed line, and blue dasheddotted lines, respectively. The computation was done at 360 K.

simulated systems. While vapor density equilibrates within 5 ns indeed, vapor pressure requires 30 ns to reach the equilibrium value. (See Figure 6.) Until that time, the vapor pressure continues to grow instead of fluctuating around an equilibrium value. A fluctuation would indicate insufficient sampling but fast equilibration. Other simulated systems require smaller sampling, as they are less viscous, and the dynamics of the individual particles in those systems is faster. Further analysis indicates that the liquid−vapor interface in the RTIL-rich systems is more structured than in the RTIL-poor systems and in pure water. The density of water vapor near the interface depends on the system composition and is larger for RTILpoor mixtures. To recapitulate, the conducted MD simulations showed that [C4C1IM][BF4] greatly alters the liquid−vapor diagram of water as well as its normal-pressure at the phase transition point. This result is particularly interesting from the fundamental point of view and should be general to most RTILs because they differ significantly from molecular liquids in the phase transitions and volatility. The unusual phase behavior stems from the ability of many RTILs to mix freely with water over a wide temperature range. At the same time, one should not expect the presented trends to hold for amphiphilic RTILs and RTILs exhibiting limited solubility in water. The ability to create aqueous systems that boil and produce purely aqueous vapor at 125 instead of 100 °C is attractive for technological applications.

Figure 5. Vapor density as a function of molar fraction of [C4C1IM][BF4] and temperature of the mixtures. Recall that only water molecules were found in the simulated vapor phase (Figure 1).

density than the aliphatic compound of the same composition. No RTIL ions were detected in the vapor phase (logarithm of zero was plotted as minus infinity, Figure 4). Vapor densities are summarized in Figure 5 versus molar fraction and temperature. The higher is the RTIL content, the smaller is the water vapor density. None of the densities exceeds 1 kg m−3. The vapor densities in the equimolar [C4C1IM][BF4]−water mixtures are over two times smaller than vapor densities in the 10% [C4C1IM][BF4]−water mixture at the same temperatures. Appropriate sampling is a central issue in all molecular simulation methods based on the ergodic hypothesis. It is commonly believed that liquid−vapor interface simulations require relatively short simulation times because MD of the gaseous phase is fast. Figure 6 plots vapor pressures and vapor densities versus simulated time for the most viscous of the

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METHODOLOGY The presented analysis is based on MD simulation of 25 liquid−vapor interfaces and five different [C4C1IM][BF4]− water mixture compositions (Table 1). Each system was simulated during 100 000 ps with an integration time-step of 0.002 ps. The first 30 ns were disregarded as equilibration (see Figure 6), while the equilibrium properties were obtained based on the 70 ns trajectory part. The coordinates and pressure tensor components were saved every 10 ps, that is, every 5000 time steps. MD trajectories were propagated using the GROMACS simulation suite.30−32 The analysis was carried 1625

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Table 1. List of Simulated Systems and Certain Simulation Parameters no.

no. [C4C1IM] cations

1−4 5−8 9−12 13−16 17−20 21−25

100 200 300 400 500

no. [BF4] anions

no. water molecules

no. interaction centers

100 200 300 400 500

1000 900 800 700 600 500

3000 5700 8400 11 100 13 800 16 500

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]; vvchaban@ gmail.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by grant CHE-1300118 from the US National Science Foundation. MEMPHYS is the Danish National Center of Excellence for Biomembrane Physics. The Center is supported by the Danish National Research Foundation.

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340; 340; 340; 340; 340; 340;

360; 360; 360; 360; 360; 360;

380; 380; 380; 380; 380; 380; 400

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out using in-home tools developed by V.V.C. and GROMACS standard analysis tools, where possible.30−32 Vapor pressure was directly recomputed from the virial.32 The force field for [C4C1IM][BF4], suggested previously by one of us,33−35 was used. The Coulomb and Lennard-Jones parameters for the OPLS/AA implementation of TIP3P36 are compatible with the parameters of our RTIL force field;33−35 therefore, additional parametrization was not required. The combination of TIP3P with the RTIL force field describes very well the miscibility of the RTIL and water. All simulations were conducted in the constant temperature, constant volume ensemble (NVT). The film composed of [C4C1IM][BF4] and water (Table 1) was located at the center of the simulation box. We added 25 nm of vacuum to create an interface. All systems were maintained at the requested temperature (see Table 1 for details) using velocity rescaling Bussi−Parrinello thermostat.37 The time constant of 1.0 ps was applied for weak temperature coupling. All velocities were coupled to the external heat bath simultaneously, without division into liquid and vapor particles. No restrictions were imposed to particle exchange between liquid and saturated vapor phases. The cutoff distance of 1.2 nm for the LennardJones potential was employed in conjunction with shifted force modification between 1.1 and 1.2 nm. The electrostatic interactions were computed using the direct pairwise Coulomb potential at separations smaller than 1.4 nm and using the reaction-field-zero scheme for all distances beyond the cutoff. The neighbor list was updated every 0.02 ps within 1.4 nm. Periodic boundary conditions in the three Cartesian directions were simulated.

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temperatures (K) 320; 320; 320; 320; 320; 320;

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