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2008, 112, 647-649 Published on Web 10/20/2007
Molecular Dynamics Study of Ion Transfer and Distribution at the Interface of Water and 1,2-Dichlorethane Collin D. Wick† and Liem X. Dang*,‡ Department of Chemistry, Louisiana Technical UniVersity, Ruston, Louisiana 71270, and Pacific Northwest National Laboratory, Richland, Washington 93352 ReceiVed: August 16, 2007; In Final Form: September 26, 2007
Molecular dynamics simulations were carried out to study the transfer of cesium and chloride ions across the H2O-1,2-dichloroethane (DCE) interface. The free energy of transfer for the ions agreed very well with experiment. In addition, cesium had a free energy minimum near the H2O-DCE interface, showing a propensity for the aqueous region near the H2O-DCE interface. However, the density and free energy profile for the chloride ion showed it was somewhat repelled from the H2O-DCE interface. We found that DCE had an average interfacial orientation that resulted in unfavorable interactions with anions but favorable ones with cations, causing the observed ion interfacial behavior. This study shows that DCE has favorable interfacial properties for cation extraction, but less favorable ones for chloride ion extraction.
I. Introduction
II. Potential Models and Computational Methods
Water and its interfacing solvent play a decisive role in the transport mechanism of ions and molecules across liquid interfaces. Understanding the factors that contribute to the propensity of ions for the interface and its transfer across interfaces of water with immiscible solvents is of fundamental importance for electrochemical applications, phase transfer catalysis, cell membrane transfer, and drug delivery.1-3 The molecular structure of aqueous-organic interfaces has recently been studied using both experimental and theoretical studies. Vibrational sum-frequency spectroscopy and molecular dynamics simulations of the H2O-CCl4 and the H2O-1,2-dichloroethane (DCE) interfaces reported by Richmond and co-workers have contributed significantly to our understanding of the structural details and spectroscopic properties, such as the hydrogen bonding and depth profile of water molecules, at these interfaces. For instance, a recent experimental study has found that DCE forms a diffuse interface with water when compared with CCl4.4,5 Benjamin and co-workers pioneered studies of ion transport and the molecular structure of liquid-liquid interfaces.6-8 What these studies have not addressed is the effect of this interfacial structure on the distribution and transfer of polarizable ions at the liquid-liquid interface. To bring an understanding of this, we carried out molecular dynamics simulations with polarizable potentials to study the transfer of Cs+ and Cl- across the H2O-DCE interface, and the distribution of Na+ and Cl- at the H2O-DCE interface for a 1 M NaCl aqueous solution. We found that Cs+ had a free energy minimum near the H2O-DCE interface, while Cl- was generally repelled from the interface.
A polarizable DCE model was developed for this work and was parametrized to reproduce the experimental liquid density, heat of vaporization, percentage of liquid gauche defects, and surface tension of DCE with water (gwat). The DCE model was fully atomistic with Lennard-Jones (LJ), Coulombic, and polarizable interactions located at every site. The Coulombic interactions were taken from MP2 ab initio calculations with an aug-cc-pvtz basis set using CHELPG charges with the NWChem computational package.9,10 The point polarizabilities were taken from a previous paper.11 The bond lengths and bondbending parameters were taken from the Amber force field,12 and the dihedral potential was parametrized to give a similar Cl-C-C-Cl dihedral energy profile as previous work following the equation13
* To whom correspondence should be addressed. E-mail: liem.dang@ pnl.gov. † Louisiana Technical University. ‡ Pacific Northwest National Laboratory.
10.1021/jp076608c CCC: $40.75
Udihedral )
Vn
∑n 2 (1 + cos[nφ - γn])
(1)
The values we used are -1.65, 1, and 0.8 kcal/mol for n ) 1, 2, and 3, respectively. The value for γ was 0 except for n ) 2 in which it was φ. The nonbonded parameters used for this force field are given in Table 1. The four site Dang-Chang water model14 was used along with ion interaction potentials described in previous work,15,16 and the CCl4 interaction potential was taken from previous work also.17 The calculated values for a variety of thermodynamic properties are given in Table 2, showing good agreement with experiment. Molecular dynamics simulations were carried out for pure DCE in the NpT ensemble to calculate the density, heat of vaporization, and average gauche defects at ambient conditions. The heat of vaporization was calculated by taking the average energy per mol at 1 atm pressure for the liquid, subtracting it from average energy per mol of the vapor at its ideal gas volume, © 2008 American Chemical Society
648 J. Phys. Chem. C, Vol. 112, No. 3, 2008
Letters
TABLE 1: Parameters for the DCE Force Field atom
σLJ (Å)
LJ (kcal/mol)
q (e)
R (Å3)
C Cl H
3.41 2.40 3.53
0.13 0.04 0.13
0.115 0.055 -0.225
0.878 0.135 1.91
TABLE 2: Comparison of a Variety of Calculated Properties with Experiment
a
property
model
expt
Fliquid (g/mL) ∆Hvap (kcal/mol) %gaucheliquid γwat (dyn/cm)
1.246 ( 0.001 8.2 ( 0.2 64 ( 1% 26 ( 3
1.2454a 8.40b 65%c 24d
Ref 26. b Ref 27. c Ref 28. d Ref 4
and adding R to account for P∆V of the gas phase (the liquid phase was negligible). A total of 216 DCE molecules were included in that system. Also, neat H2O-DCE, a 1 M NaCl H2ODCE system, and a 1 M NaCl H2O-CCl4 system were simulated, which included 2669 water molecules and 712 DCE or 500 CCl4 molecules in an elongated periodic box, forming two H2O-DCE interfaces. For the 1 M NaCl systems, 48 of each ion were included. The length of the boxes in z-coordinates (the interfaces were normal to the z-coordinate) was around 23-25 Å for both H2O and DCE, and the x and y coordinates were 40 Å. The systems were equilibrated at constant pressure, while the production runs were carried out at constant volume. For these systems, data was taken from 500 ps of simulations with a 1 fs time step, following extensive equilibration. Finally, free energy profiles for Cs+ and Cl- across the H2O-DCE interface were calculated using the potential of mean force (PMF) technique.15 The systems used for the PMF calculations included 550 H2O molecules, 160 DCE molecules, and a single ion in an elongated box with one H2O-DCE interface, and two liquid-vapor interfaces (one with H2O and one with DCE). The PMF calculation mapped out the free energy profile in 1 Å increments in which 300 ps of data collection followed 100 ps of equilibration for each point in the profile. All simulations had a LJ potential truncation of 9 Å with analytic tail corrections, and the particle mesh Ewald summation technique was used to handle long-ranged electrostatics. The Berendsen thermostat and barostat were used to control the temperature and pressures when applicable.18
Figure 1. Density profiles from the H2O-DCE system (top), and Na+ and Cl- density profiles for the H2O-DCE and H2O-CCl4 systems (bottom).
Figure 2. Free energy profile across the H2O-DCE interface for Cs+ and Cl-.
III. Results and Discussion
Figure 3. Potential drop for the neat H2O-vapor, H2O-DCE, and H2OCCl4 systems.
The density profile for the pure H2O-DCE system is given in Figure 1 with respect to the Gibbs dividing surface (GDS) of the H2O interface. What is apparent is the diffuse nature of the interface, giving a fairly wide interfacial region. By fitting a hyperbolic tangent to the H2O density profile, an interfacial width can be calculated.19 The values obtained for the interfacial width of H2O-DCE was 5.79 Å, which is much larger than for either the H2O-vapor (3.64 Å) or the H2O-CCl4 interfaces (3.55 Å) using the same potential models. This is likely brought on by a combination of more favorable interactions between H2O and DCE than between H2O and CCl4, and an overall lower surface tension for H2O-DCE than the other two.4 The larger H2O-DCE interfacial width has been indirectly observed previously by experiment.4 The NaCl density profiles for the H2ODCE and the H2O-CCl4 systems are given in Figure 1. What is apparent from these is that while Cl- has a propensity for the H2O-CCl4 interface, which is consistent with the H2O-vapor interface,20 Cl- is somewhat repelled from the H2O-DCE interface. Na+, on the other hand is repelled from both interfaces,
as is the case for H2O-vapor. However, an anomalously highdensity peak is observed for Na+ between -5 and -10 Å from the GDS for the H2O-DCE system. This shows that while Na+ does not have a propensity for the H2O-DCE interface itself, something is attracting it to the aqueous region near the interface (more on this below). The PMF for Cl- and Cs+ is given in Figure 2. The calculated values for the free energy of transfer from H2O to DCE for Cland Cs+ are 12.8 ( 0.5 and 6.1 ( 0.3 kcal/mol, respectively, which compares to the experimental values of 12.4 and 5.7 kcal/ mol,21 respectively, giving excellent agreement. An interesting feature is present in the Cs+ free energy profile between -5 and -10 Å from the GDS in that there is a clear free energy minimum of around -1 kcal/mol. This is consistent with what was observed for the 1 M NaCl in the H2O-DCE system in that for both cases, cations have an attraction to the region between -5 and -10 Å from the GDS. Because this has not been observed for the H2O-CCl4 and H2O-vapor interfaces,22 there
Letters
Figure 4. Snapshot of the interface with Cs+ near the H2O-DCE interface.
is apparently something special about the interfacial DCE molecules that cause this. Figure 3 gives the potential drop for the H2O-vapor system taken from previous work23 and the H2O-DCE and H2O-CCl4 systems from this work. The potential drop is the integral of the negative of the electric field, and the method for its calculation is described elsewhere.24,25 The potential drop calculated for the H2O-DCE is significantly lower than that for either the H2O-vapor and H2O-CCl4 systems. This points to more oriented H2O molecules at the H2O-DCE interface than the other two interfaces. Because the potential drop is negative, it points to a positive electric field at the interface, which corresponds to H2O hydrogens pointing away from the H2O center of mass (or toward the vapor/organic phase). As a result, H2O hydrogens are pointing toward the other phase to a greater degree at the H2O-DCE interface than the others. A cation is known to interact with H2O oxygens, and the greater degree of H2O orientation at the H2O-DCE interface will make it more likely that H2O molecules near the interface will have their oxygens pointing toward the region just within the interface. A cation in this region will find more H2O oxygens to interact with, which will increase its propensity for this region. In addition, the minimum dimer energies between a cation and DCE have the cation near the DCE chloride atoms when DCE is in its gauche conformation. For anions, they interact with the hydrogens of a gauche DCE molecule. We calculated the average C-Cl vector for DCE molecules with respect to the z-axis (normal to the interface) in the region within 5 Å of the H2O-DCE GDS for the pure system. The average value for the angle with the z-axis (with positive values corresponding with Cl pointing toward the water phase) was found to be 〈cos θz〉 ) 0.050 ( 0.005, which has been observed qualitatively for the H2O-DCE interface previously.7 Because of this, Cl- generally has unfavorable interactions with interfacial DCE, which will result in the Cl- interfacial concentration to be reduced in comparison to other interfaces. For cations, this will result in favorable interactions with interfacial DCE but also more favorable interactions with interfacial H2O molecules because the average dipole in the interfacial region is strongly pointing toward the DCE phase (shown in the potential drop). As a result, cation interfacial concentration is enhanced for H2O-DCE when compared to H2O-vapor and H2O-CCl4. This fact is reinforced by the snapshot given in Figure 4, which shows a H2O-DCE interface with the DCE chloride atoms somewhat oriented toward the H2O bulk and near interfacial Cs+. IV. Conclusion We have performed molecular dynamics simulations with polarizable interactions to study ion distributions and transfer
J. Phys. Chem. C, Vol. 112, No. 3, 2008 649 at the H2O-DCE interface and compared it with H2O-vapor and H2O-CCl4. Our results showed that cations, specifically Na+ and Cs+, have enhanced concentrations and lowered free energies only near the H2O-DCE interface, while Cl- is repelled from the H2O-DCE interface while enhanced at the H2O-vapor and H2O-CCl4 interfaces. The reasons for the uniqueness of the H2O-DCE interface draws from the average interfacial DCE orientation, which results in favorable cation interactions but unfavorable Cl- interactions. This study suggests that DCE has very beneficial interfacial properties for cation extraction, but for Cl- the interfacial properties of DCE may hinder their extraction. Acknowledgment. This work was supported by the Office of Basic Energy Sciences of the Department of Energy, in part by the Chemical Sciences program, and in part by the Engineering and Geosciences Division. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy. Part of the computational work was carried out using the resources of the Louisiana Optical Network Initiative. References and Notes (1) Biophysics of Water; Franks, Mathias, S., Eds.; Wiley-Interscience: New York, 1982. (2) McLaughlin, S. Annu. ReV. Biophys. Biophys. Chem. 1989, 18, 113-136. (3) Honig, B. H.; Hubbell, W. L.; Flewelling, R. F. Annu. ReV. Biophys. Biophys. Chem. 1986, 15, 163-193. (4) Walker, D. S.; Brown, M.; McFearin, C. L.; Richmond, G. L. J. Phys. Chem. B 2004, 108, 2111-2114. (5) Walker, D. S.; Moore, F. G.; Richmond, G. L. J. Phys. Chem. C 2007, 111, 6103-6112. (6) Benjamin, I. Science 1993, 261, 1558-1560. (7) Benjamin, I. J. Chem. Phys. 1992, 97, 1432-1445. (8) Schweighofer, K. J.; Benjamin, I. J. Phys. Chem. 1995, 99, 99749985. (9) Bylaska, E. J.; de Jong, W. A.; Kowalski, K.; Straatsma, T. P.; Valiev, M.; Wang, D.; Apra`, E.; Windus, T. L.; Hirata, S.; Hackler, M. T.; et al. NWChem version 5.0, 2006. (10) Kendall, R. A.; Apra, E.; Bernholdt, D. E.; Bylaska, E. J.; Dupuis, M.; Fann, G. I.; Harrison, R. J.; Ju, J. L.; Nichols, J. A.; Nieplocha, J.; Straatsma, T. P.; Windus, T. L.; Wong, A. T. Comput. Phys. Commun. 2000, 128, 260-283. (11) Applequist, J.; Carl, J. R.; Fung, K. K. J. Am. Chem. Soc. 1972, 94, 2952-&. (12) Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M.; Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman, P. A. J. Am. Chem. Soc. 1995, 117, 5179-5197. (13) Jorgensen, W. L.; Binning, R. C.; Bigot, B. J. Am. Chem. Soc. 1981, 103, 4393-4399. (14) Dang, L. X.; Chang, T. M. J. Chem. Phys. 1997, 106, 8149-8159. (15) Dang, L. X. J. Phys. Chem. B 2002, 106, 10388-10394. (16) Smith, D. E.; Dang, L. X. J. Chem. Phys. 1994, 101, 7873-7881. (17) Chang, T. M.; Dang, L. X. J. Chem. Phys. 1996, 104, 6772-6783. (18) Berendsen, H. J. C.; Postma, J. P. M.; Vangunsteren, W. F.; Dinola, A.; Haak, J. R. J. Chem. Phys. 1984, 81, 3684-3690. (19) Matsumoto, M.; Kataoka, Y. J. Chem. Phys. 1988, 88, 3233-3245. (20) Mucha, M.; Frigato, T.; Levering, L. M.; Allen, H. C.; Tobias, D. J.; Dang, L. X.; Jungwirth, P. J. Phys. Chem. B 2005, 109, 7617-7623. (21) Marcus, Y. Ion SolVation; John Wiley and Sons Ltd.: New York, 1985. (22) Wick, C. D.; Dang, L. X. J. Phys. Chem. B 2006, 110, 68246831. (23) Wick, C. D.; Dang, L. X.; Jungwirth, P. J. Chem. Phys. 2006, 125. (24) Wilson, M. A.; Pohorille, A.; Pratt, L. R. J. Chem. Phys. 1988, 88, 3281-3285. (25) Dang, L. X.; Chang, T. M. J. Phys. Chem. B 2002, 106, 235-238. (26) Handbook of Chemistry and Physics, 87th ed.; CRC Press: Cleveland, OH, 2007. (27) Yaws, C. L. Chemical Properties Handbook; McGraw-Hill: New York, 1999. (28) Tanabe, K. Spectrochim. Acta, Part A 1972, 28A, 407.
