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Electrostatic potential gap at the interface between triethylamine and water phases studied by molecular dynamics simulation
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Chemical Physics Letters 448 (2007) 70–74 www.elsevier.com/locate/cplett
Electrostatic potential gap at the interface between triethylamine and water phases studied by molecular dynamics simulation Shinji Kajimoto a, Noriyuki Yoshii b, Jonathan Hobley c, Hiroshi Fukumura Susumu Okazaki b
a,*
,
a
c
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan b Institute for Molecular Science, Myodoji, Okazaki 444-8585, Japan Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602, Singapore Received 5 May 2007; in final form 25 September 2007 Available online 29 September 2007
Abstract Molecular dynamics calculations were carried out in order to investigate the interfacial properties of the two-phase coexistence state of the triethylamine (TEA) and water mixture, which is known to have a lower critical soluble temperature. Two kinds of initial configuration were adopted. One was a two-phase coexistence state and the other was a random mixed state of TEA and water molecules. After an equilibration calculation of several nanoseconds, the density profiles converged to the same equilibrated two-phase coexistence state. In the equilibrated state, anisotropic orientations were observed for both molecules, which makes an electrostatic potential gap between these phases. 2007 Elsevier B.V. All rights reserved.
1. Introduction Some binary liquid mixtures split into two phases from single phase upon changing temperature [1,2]. Some of these mixtures, such as triethylamine (TEA)/water or 2-buthoxyethanol/water, have lower critical solution temperatures (LCSTs) and split into two phases with a temperature increase. It is considered that hydrogen bonding between unlike molecules, which is in competition with that between the same molecules plays an important role in phase separation [3]. Below the LCST, the hydrogen bonding between unlike molecules makes the mixture miscible. With increasing temperature, the hydrogen bonding between unlike molecules breaks because it becomes unfavorable in terms of orientational entropy. Then the hydrogen bonding between molecules of the same type becomes dominant and the mixture splits into two phases.
*
Corresponding author. Fax: +81 22 795 6567. E-mail address: [email protected] (H. Fukumura).
0009-2614/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.09.077
Recently, we reported the evolution of hydrogen bonding after the initiation of phase separation using a nanosecond laser temperature jump (T-jump) method [4,5]. In the case of TEA/water mixtures, it was revealed that the evolution of hydrogen bonding and the compositional changes of phases are completed within 1–2 ls after the T-jump. After that the domains of the new phases continue to grow from tens of nanometer to micrometer in size keeping their chemical composition the same as their equilibrated states. In this stage, the dynamic phase separation medium has high availability of the interface between interconnected nano-phases so that the dynamic medium is expected to have different properties from that of the equilibrated state and hence can provide a unique reaction field. However there are not so many studies on the interface between the partially mixed phases and the nature of the interface of such mixtures is still unknown. In recent years, many molecular dynamics (MD) studies on liquid–liquid interfaces have been done in order to understand their properties such as the interfacial structure [6,7]. These calculated results have good agreement with
S. Kajimoto et al. / Chemical Physics Letters 448 (2007) 70–74
2. Calculations 2.1. Systems All of the systems calculated in this study contain 240 molecules of TEA and 1320 molecules of water. Two different initial configurations were allowed to equilibrate. The first one was a random mixed configuration consisting of a single mixed phase of TEA and water. The other was a two-phase coexistence state with a TEA phase having a TEA mole fraction of 0.5 and a water phase having a TEA mole fraction of 0.034. With this composition, the water phase contains 40 TEA molecules and 1120 water molecules and the TEA phase contains the rest.
1.1 Density / g ·cm-3
experimental results, such as X-ray or neutron scattering and the MD calculation is now recognized to be a powerful tool to investigate the liquid–liquid interfacial microscopic structure [8]. However, in these works, only immiscible binary mixtures were studied. In mixtures which have a LCST, hydrogen bonding between unlike molecules is expected to give additional complexity to the interfacial properties of the two-phase coexistence system. In this study, we performed MD calculations on a TEA and water mixture in order to study the interfacial structure of the two-phase coexistence system. The calculations were done with two different initial configurations to examine the sensitivity of the result on the starting condition. From the equilibrated two-phase state, the interfacial properties were obtained, including electrostatic properties and these are discussed in terms of hydrogen bonding.
71
1.0 0.9 0.8 0.7
0
5
10
15
Time / ns Fig. 1. Time evolution of the density of the water phase (solid line) and TEA phase (dotted line) obtained from the MD calculation started from the two-phase state (gray) and from the mixed state (black).
and the calculated state for both calculations are shown in Fig. 2a. The axis along the long sides of the rectangular cell is the Z-axis. The water and TEA phases are identified as the two areas, in which the density is uniform in Fig. 2a. The equilibrated states are independent of the initial configuration. We therefore conclude that the systems have reached their true thermally equilibrated states. Compared to the water phase the TEA phase needs a longer calculation time for the density to achieve its equilibrated value,
2.2. Molecular dynamics calculations [9,10] The MD calculations were performed in NPT ensemble using an isotropic fluctuation cell and a Nose-Hoover chain thermostat. For TEA and Water molecules, OPLSAA [11] and TIP4P model [12] were adopted, respectively. During the MD simulation, bond lengths were constrained with a SHAKE/ROLL, RATTLE/ROLL algorithm [13,14]. The equation of motion was integrated using the r-RESPA algorithm with a 1 fs time step. The rectangular ˚ ) contained 9240 atoms in 3D-periodic cell (35 · 35 · 70 A ˚ . The boundary conditions. The potential cut-off was 10 A long range coulombic interactions were calculated using the particle mesh Ewald (PME) method [9]. The pressure and temperature were set to 1 atm and 283.15 K, respectively. The calculation was done for longer than 20 ns for both initial configurations. 3. Results and discussion 3.1. Equilibration The compositional changes of the TEA and water phases as a function of the calculation time are shown in Fig. 1. The density profiles of the two initial configurations
Fig. 2. (a) Equilibrated density (solid line) profile calculated from the twophase state (black) and from the mixed state (gray). The corresponding density profile of the initial states is also shown with dotted lines. (b) q(z) Æ Æcoshæ for TEA molecule (gray) and for water (black) as a function of the z-coordinate. Those with respect to the water molecule, hydrogen bonded (dotted) and non-hydrogen bonded molecules (thin line) to the nitrogen atom of TEA molecule are shown. Definition of hGN and hMO are also shown schematically (see text). (c) Density profile for water molecules.
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suggesting that water clusters in the TEA phase could be a metastable state. With the mixed system as the initial state, a calculation time of 12 ns was necessary before the equilibrated state was achieved. The interface between the two phases is oriented perpendicular to the Z-axis as expected in order to minimize the interfacial area and the interfacial energy. When the system was initiated from the two-phase state, equilibration occurred within 5 ns of calculation time. After equilibration, the water phase contained a few TEA molecules and the TEA phase contained a few water molecules. Further, there was molecular exchange between the two phases across the interface even after the densities of each phase reached the equilibrated value. This exchange indicates that the two phases coexist in thermal equilibrium. The mole fraction of TEA in the water phase and the TEA phase were 0.002, 0.99, respectively meaning that two water molecules dissolve in the TEA phase and 2–3 TEA molecules dissolve in the water phase. It should be noted that the equilibrated composition calculated in this study is different from that of the real TEA–water mixture since real TEA and water are miscible at this temperature [15]. This disagreement could come from the potential parameter used in this study, especially the term for coulombic interactions, which is related to hydrogen bonding. Actually the present interaction between TEA and water molecules is weaker than the interaction expected from the density functional calculation [16]. The effect of intermolecular interaction on phase separation needs to be studied further and will be published elsewhere [17]. However, even with the potential parameter used here, the composition of each phase becomes more similar at lower temperatures. This implies that there is a LCST for TEA and water represented by the potential parameter. The LCST is considered to be lower than 243.15 K, since the mixture does not become miscible even at this temperature. Under such low temperature, the diffusion of molecules is so slow that it is difficult to achieve equilibrium using an MD calculation.
unique orientation in the vicinity of the interface. The TEA molecules locate at the interface with their nitrogen atoms facing toward the water phase. This unique orientation implies that there is hydrogen bonding between TEA and water at the interface which determines the TEA orientation at the interface. To estimate the extent of hydrogen bonding at the interface, we show the pair distribution function of hydrogen on the water molecule around the nitrogen, qN–H(r), and the running coordination number, NN–H(r), in Fig. 3. In this ˚ , where the figure, NN–H(r) becomes unity around 2.7 A radial distribution function takes its first minimum value. These results indicate that almost all of the TEA molecules are hydrogen bonded to water molecules at the interface. As a result of the hydrogen bonding, the water molecules are also expected to have a unique orientation at the interface. To investigate the orientation of the water molecules, we introduce the MO vector. The point M represents the center of mass of the two hydrogen atoms, and the MO vector is the vector from the point M to the oxygen atom. With regard to the angle hMO, which is formed by the MO vector and the Z-axis, the product of the number density of oxygen, qO(z), and the cosine function, coshMO, is shown in Fig. 2b. In the water phase, the cosine of the angle is zero meaning there is no specified orientation. However, at the interface, the direction indicated by the cosine function is opposite from the expected orientation. To form hydrogen bonds with TEA, the water molecule should point their hydrogen atoms toward the TEA phase, but the result appears to indicate that the water molecules point their oxygen atoms toward the TEA phase at the interface, which is initially counter intuitive. However this is the averaged orientation of all of the water at the interface and closer examination of the interface reveals a more complex structure involving H-bonded and non H-bonded water as described below. To investigate the influence of hydrogen bond on the water orientation at the interface, the product of the number density and the cosine function, q(z) Æ Æcosh(z)æ, is
3.2. Orientation at the interface 80x10
-3
3
60 ρN-H(r)
2 NN-H(r)
˚ thick, which corresponds to three The interface is 10 A water molecules and 1–2 TEA molecules. To investigate the interfacial structure, the orientation of the molecules at the interface was determined. Here we introduce a GN vector to determine the orientation of the TEA molecules. The point G represents the center of mass of three methylene carbon atoms, which are bonded to the nitrogen atom, and the GN vector is the vector from G to the nitrogen atom. In order to take account of the partial density of TEA, the product of number density of nitrogen, qN(z), and the cosine of the angle, hGN(z), which is formed by the GN vector and the Z-axis, is shown in Fig. 2b. The TEA molecules do not have any specific orientation in the TEA phase, however there is a
40 1 20 0
0
2
4
6
8
10
12
14
0
r/Å Fig. 3. Pair distribution function qN–H(r) (solid line) and its running coordination number, NN–H(r) (dotted line), at the vicinity of the interface ˚. with a width of 10 A
S. Kajimoto et al. / Chemical Physics Letters 448 (2007) 70–74
shown in Fig. 2b for the water molecules hydrogen bonded with TEA molecules (H-bonded water) and water molecules non-hydrogen bonded with TEA molecules (non Hbonded water), respectively. The hydrogen bond distance between the nitrogen and hydrogen was determined to be ˚ which is the first minimum point of qN–H(r). This 2.7 A result indicates that the H-bonded water molecule orients the hydrogen atoms toward TEA phase as expected. On the other hand, the non H-bonded water molecules orient their oxygen atoms toward the interface. The number of non H-bonded water molecules is larger than the number of H-bonded ones even in the vicinity of the interface as shown in Fig. 2c. It should be noted that the peak value of Æcosh(z)æ for the non H-bonded water molecules (0.2) is smaller than that for TEA molecules (0.5). This implies that the non H-bonded water molecules at the interface orient their dipole moment in parallel with the interface rather than perpendicular to the interface. 3.3. Electrostatic property at the interface The electrostatic potential profile of the interface was calculated based upon Poisson’s equation. Z Z 0 1 z z /ðzÞ /ð0Þ ¼ qðz00 Þdz00 dz0 e0 0 0
Charge density / C ·m-3
where /(z) and q(z) are respectively the electrostatic potential and the charge density as a function of the z-coordinate [18]. The charge density, q(z), and the electrostatic potential, /(z), are shown in Fig. 4a and b, respectively. The contribution from each molecule is also shown in Fig. 4. According to this figure, there is a positive potential of 0.65 V in the vicinity of the interface with respect to the water phase.
40
a
73
The potential gap originates mainly from the charge distribution of the water molecules. The orientation of the water molecules, in which one or both of their hydrogen atoms points toward TEA phase, leads the large potential gap of 0.55 V. This value is a little higher than the potential gap studied for water at a hydrophobic interface [19]. This could be due to the existence of hydrogen bonds between the water and the TEA molecules. On the other hand, the contribution of the TEA molecules to the potential gap is small. The orientation of the TEA molecules leads to a potential gap of 0.1 V with respect to the water phase with the same direction as the water. This result indicates that the direction of the dipole moment is the same, which results in hydrogen bonding. Additionally, the potential caused by the TEA molecules has a minimum value in the vicinity of the interface. The depth of the potential drop is 0.15 V and this should result from the orientation of the TEA molecules. As discussed previously, the TEA molecules extend their nitrogen atom, which has a partially negative charge, toward the water phase rather than the methylene group, which has a partial positive charge. The result of this charge distribution is expected to cause a simple positive potential with regard to water phase. The more complicated potential arises from the position of the terminated methyl group. The methyl group, especially the terminated hydrogen, is directed into the water phase. The hydrogen atom is charged partially positive so the distribution of the charge arising from TEA molecules, takes a positive value near the water phase. In summary, the interface can be separated into three layers with respect to the TEA molecules as shown in Fig. 5. The first layer is the closest to the water phase and mainly contains the hydrogen atoms of the methyl group. The second one contains the nitrogen and carbon atom of the methyl group and is charged partially negative. The last one is the closest to the TEA phase and contains methylene group, which has a partial positive charge. After
0 6
-40x10
φ/ V
0.8
b-30
-20
-1 0
0
10
20
30
20
30
total water TEA
0.4 0.0 -0.4 -30
-20
-10
0 10 Z-axis / Å
Fig. 4. Charge density (a) and electrostatic potential profile (b) as a function of the z-coordinate. Solid line; the total system, dotted line; TEA molecule contribution and broken line: water molecule contribution.
Fig. 5. Schematic image of water and TEA molecules at the interface.
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all, the charge distribution caused by TEA molecules changes from positive to negative and again positive near to the interface. This distribution of charge causes the minimum in the electrostatic potential. This electrostatic potential indicates that not only the molecules, which have a high polarizability, but also dissolved cations would tend to move to the interface. 4. Conclusion We have performed molecular dynamics calculations for the two-phase coexistence state of the TEA and water mixture. At the interface between the two phases, both molecules have unique orientations, which results from hydrogen bonding between TEA and water molecules. These anisotropic orientations lead to a large potential gap of 0.65 V between two phases. Additionally, resulting from the orientation of the TEA molecules, the electric potential has the minimum value in the vicinity of the interface. Acknowledgements This work has been done as a part of the Next Generation Super Computing Project, Nanoscience Program, MEXT, Japan. This work has been partly supported by the MEXT (16072203, 17105001 and 18066020). The Authors also thank Okazaki Research Center for Computational Science, National Institute of Natural Sciences for the use of supercomputers.
References [1] N. Goldenfeld, Lectures on phase transitions and the renormalization group, Frontiers in Physics, vol. 85, Addison-Wesley Publishing Company, Illinois, 1992. [2] Y. Maeda, Langmuir 17 (2001) 1737. [3] J.S. Walker, C.A. Vause, Sci. Am. 256 (1987) 92. [4] J. Hobley, S. Kajimoto, A. Takamizawa, K. Ohta, Q. Tran-Cong, H. Fukumura, J. Phys. Chem. B 107 (2003) 11411. [5] A. Takamizawa, S. Kajimoto, J. Hobley, H. Fukumura, Phys. Rev. E 68 (2003) 020501(R). [6] P.A. Fernandrino, M.N.D.S. Cordeoiro, J.A.N.F. Gomes, J. Phys. Chem. B 103 (1999) 6290. [7] I. Benjamin, Chem. Phys. Lett. 393 (2004) 453. [8] G. Luo et al., Electrochem. Commun. 7 (2005) 627. [9] U. Essmann, L. Perera, M.L. Berkowitz, T. Darden, H. Lee, L.G. Pedersen, J. Chem. Phys. 103 (1995) 8577. [10] M.P. Allen, D.J. Tidesley, Computer Simulation of Liquids, Oxford University Press, Oxford, 1989. [11] R.C. Rizzo, W.L. Jorgensen, J. Am. Chem. Soc. 121 (1999) 4827. [12] W.L. Jorgensen, J. Chandrasekhar, J.D. Madura, R.W. Impey, M.L. Klein, J. Phys. Chem. 79 (1983) 926. [13] G.J. Martyna, M.E. Tukerman, D.J. Tobias, M.L. Klein, Mol. Phys. 87 (1996) 1117. [14] G.J. Martyna, D.J. Tobias, M.L. Klein, J. Chem. Phys. 101 (1994) 4177. [15] R. Gstaud, D. Beysens, G. Zalczer, J. Chem. Phys. 93 (1990) 3432. [16] N.U. Zhampeisov, K. Ohta, S. Kajimoto, J. Hobley, K. Hatanaka, H. Fukumura, Int. J. Quantum Chem. 105 (2005) 376. [17] S. Kajimoto, N. Yoshii, J. Hobley, H. Fukumura, S. Okazaki, to be submitted for publication. [18] W. Shinoda, M. Shimizu, S. Okazaki, J. Phys. Chem. 102 (1998) 6647. [19] S.I. Mamatkulov, P.K. Khabibullaev, R.R. Netz, Langmuir 20 (2004) 4756.
Chemical Physics Letters 448 (2007) 70–74 www.elsevier.com/locate/cplett
Electrostatic potential gap at the interface between triethylamine and water phases studied by molecular dynamics simulation Shinji Kajimoto a, Noriyuki Yoshii b, Jonathan Hobley c, Hiroshi Fukumura Susumu Okazaki b
a,*
,
a
c
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan b Institute for Molecular Science, Myodoji, Okazaki 444-8585, Japan Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602, Singapore Received 5 May 2007; in final form 25 September 2007 Available online 29 September 2007
Abstract Molecular dynamics calculations were carried out in order to investigate the interfacial properties of the two-phase coexistence state of the triethylamine (TEA) and water mixture, which is known to have a lower critical soluble temperature. Two kinds of initial configuration were adopted. One was a two-phase coexistence state and the other was a random mixed state of TEA and water molecules. After an equilibration calculation of several nanoseconds, the density profiles converged to the same equilibrated two-phase coexistence state. In the equilibrated state, anisotropic orientations were observed for both molecules, which makes an electrostatic potential gap between these phases. 2007 Elsevier B.V. All rights reserved.
1. Introduction Some binary liquid mixtures split into two phases from single phase upon changing temperature [1,2]. Some of these mixtures, such as triethylamine (TEA)/water or 2-buthoxyethanol/water, have lower critical solution temperatures (LCSTs) and split into two phases with a temperature increase. It is considered that hydrogen bonding between unlike molecules, which is in competition with that between the same molecules plays an important role in phase separation [3]. Below the LCST, the hydrogen bonding between unlike molecules makes the mixture miscible. With increasing temperature, the hydrogen bonding between unlike molecules breaks because it becomes unfavorable in terms of orientational entropy. Then the hydrogen bonding between molecules of the same type becomes dominant and the mixture splits into two phases.
*
Corresponding author. Fax: +81 22 795 6567. E-mail address: [email protected] (H. Fukumura).
0009-2614/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.09.077
Recently, we reported the evolution of hydrogen bonding after the initiation of phase separation using a nanosecond laser temperature jump (T-jump) method [4,5]. In the case of TEA/water mixtures, it was revealed that the evolution of hydrogen bonding and the compositional changes of phases are completed within 1–2 ls after the T-jump. After that the domains of the new phases continue to grow from tens of nanometer to micrometer in size keeping their chemical composition the same as their equilibrated states. In this stage, the dynamic phase separation medium has high availability of the interface between interconnected nano-phases so that the dynamic medium is expected to have different properties from that of the equilibrated state and hence can provide a unique reaction field. However there are not so many studies on the interface between the partially mixed phases and the nature of the interface of such mixtures is still unknown. In recent years, many molecular dynamics (MD) studies on liquid–liquid interfaces have been done in order to understand their properties such as the interfacial structure [6,7]. These calculated results have good agreement with
S. Kajimoto et al. / Chemical Physics Letters 448 (2007) 70–74
2. Calculations 2.1. Systems All of the systems calculated in this study contain 240 molecules of TEA and 1320 molecules of water. Two different initial configurations were allowed to equilibrate. The first one was a random mixed configuration consisting of a single mixed phase of TEA and water. The other was a two-phase coexistence state with a TEA phase having a TEA mole fraction of 0.5 and a water phase having a TEA mole fraction of 0.034. With this composition, the water phase contains 40 TEA molecules and 1120 water molecules and the TEA phase contains the rest.
1.1 Density / g ·cm-3
experimental results, such as X-ray or neutron scattering and the MD calculation is now recognized to be a powerful tool to investigate the liquid–liquid interfacial microscopic structure [8]. However, in these works, only immiscible binary mixtures were studied. In mixtures which have a LCST, hydrogen bonding between unlike molecules is expected to give additional complexity to the interfacial properties of the two-phase coexistence system. In this study, we performed MD calculations on a TEA and water mixture in order to study the interfacial structure of the two-phase coexistence system. The calculations were done with two different initial configurations to examine the sensitivity of the result on the starting condition. From the equilibrated two-phase state, the interfacial properties were obtained, including electrostatic properties and these are discussed in terms of hydrogen bonding.
71
1.0 0.9 0.8 0.7
0
5
10
15
Time / ns Fig. 1. Time evolution of the density of the water phase (solid line) and TEA phase (dotted line) obtained from the MD calculation started from the two-phase state (gray) and from the mixed state (black).
and the calculated state for both calculations are shown in Fig. 2a. The axis along the long sides of the rectangular cell is the Z-axis. The water and TEA phases are identified as the two areas, in which the density is uniform in Fig. 2a. The equilibrated states are independent of the initial configuration. We therefore conclude that the systems have reached their true thermally equilibrated states. Compared to the water phase the TEA phase needs a longer calculation time for the density to achieve its equilibrated value,
2.2. Molecular dynamics calculations [9,10] The MD calculations were performed in NPT ensemble using an isotropic fluctuation cell and a Nose-Hoover chain thermostat. For TEA and Water molecules, OPLSAA [11] and TIP4P model [12] were adopted, respectively. During the MD simulation, bond lengths were constrained with a SHAKE/ROLL, RATTLE/ROLL algorithm [13,14]. The equation of motion was integrated using the r-RESPA algorithm with a 1 fs time step. The rectangular ˚ ) contained 9240 atoms in 3D-periodic cell (35 · 35 · 70 A ˚ . The boundary conditions. The potential cut-off was 10 A long range coulombic interactions were calculated using the particle mesh Ewald (PME) method [9]. The pressure and temperature were set to 1 atm and 283.15 K, respectively. The calculation was done for longer than 20 ns for both initial configurations. 3. Results and discussion 3.1. Equilibration The compositional changes of the TEA and water phases as a function of the calculation time are shown in Fig. 1. The density profiles of the two initial configurations
Fig. 2. (a) Equilibrated density (solid line) profile calculated from the twophase state (black) and from the mixed state (gray). The corresponding density profile of the initial states is also shown with dotted lines. (b) q(z) Æ Æcoshæ for TEA molecule (gray) and for water (black) as a function of the z-coordinate. Those with respect to the water molecule, hydrogen bonded (dotted) and non-hydrogen bonded molecules (thin line) to the nitrogen atom of TEA molecule are shown. Definition of hGN and hMO are also shown schematically (see text). (c) Density profile for water molecules.
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suggesting that water clusters in the TEA phase could be a metastable state. With the mixed system as the initial state, a calculation time of 12 ns was necessary before the equilibrated state was achieved. The interface between the two phases is oriented perpendicular to the Z-axis as expected in order to minimize the interfacial area and the interfacial energy. When the system was initiated from the two-phase state, equilibration occurred within 5 ns of calculation time. After equilibration, the water phase contained a few TEA molecules and the TEA phase contained a few water molecules. Further, there was molecular exchange between the two phases across the interface even after the densities of each phase reached the equilibrated value. This exchange indicates that the two phases coexist in thermal equilibrium. The mole fraction of TEA in the water phase and the TEA phase were 0.002, 0.99, respectively meaning that two water molecules dissolve in the TEA phase and 2–3 TEA molecules dissolve in the water phase. It should be noted that the equilibrated composition calculated in this study is different from that of the real TEA–water mixture since real TEA and water are miscible at this temperature [15]. This disagreement could come from the potential parameter used in this study, especially the term for coulombic interactions, which is related to hydrogen bonding. Actually the present interaction between TEA and water molecules is weaker than the interaction expected from the density functional calculation [16]. The effect of intermolecular interaction on phase separation needs to be studied further and will be published elsewhere [17]. However, even with the potential parameter used here, the composition of each phase becomes more similar at lower temperatures. This implies that there is a LCST for TEA and water represented by the potential parameter. The LCST is considered to be lower than 243.15 K, since the mixture does not become miscible even at this temperature. Under such low temperature, the diffusion of molecules is so slow that it is difficult to achieve equilibrium using an MD calculation.
unique orientation in the vicinity of the interface. The TEA molecules locate at the interface with their nitrogen atoms facing toward the water phase. This unique orientation implies that there is hydrogen bonding between TEA and water at the interface which determines the TEA orientation at the interface. To estimate the extent of hydrogen bonding at the interface, we show the pair distribution function of hydrogen on the water molecule around the nitrogen, qN–H(r), and the running coordination number, NN–H(r), in Fig. 3. In this ˚ , where the figure, NN–H(r) becomes unity around 2.7 A radial distribution function takes its first minimum value. These results indicate that almost all of the TEA molecules are hydrogen bonded to water molecules at the interface. As a result of the hydrogen bonding, the water molecules are also expected to have a unique orientation at the interface. To investigate the orientation of the water molecules, we introduce the MO vector. The point M represents the center of mass of the two hydrogen atoms, and the MO vector is the vector from the point M to the oxygen atom. With regard to the angle hMO, which is formed by the MO vector and the Z-axis, the product of the number density of oxygen, qO(z), and the cosine function, coshMO, is shown in Fig. 2b. In the water phase, the cosine of the angle is zero meaning there is no specified orientation. However, at the interface, the direction indicated by the cosine function is opposite from the expected orientation. To form hydrogen bonds with TEA, the water molecule should point their hydrogen atoms toward the TEA phase, but the result appears to indicate that the water molecules point their oxygen atoms toward the TEA phase at the interface, which is initially counter intuitive. However this is the averaged orientation of all of the water at the interface and closer examination of the interface reveals a more complex structure involving H-bonded and non H-bonded water as described below. To investigate the influence of hydrogen bond on the water orientation at the interface, the product of the number density and the cosine function, q(z) Æ Æcosh(z)æ, is
3.2. Orientation at the interface 80x10
-3
3
60 ρN-H(r)
2 NN-H(r)
˚ thick, which corresponds to three The interface is 10 A water molecules and 1–2 TEA molecules. To investigate the interfacial structure, the orientation of the molecules at the interface was determined. Here we introduce a GN vector to determine the orientation of the TEA molecules. The point G represents the center of mass of three methylene carbon atoms, which are bonded to the nitrogen atom, and the GN vector is the vector from G to the nitrogen atom. In order to take account of the partial density of TEA, the product of number density of nitrogen, qN(z), and the cosine of the angle, hGN(z), which is formed by the GN vector and the Z-axis, is shown in Fig. 2b. The TEA molecules do not have any specific orientation in the TEA phase, however there is a
40 1 20 0
0
2
4
6
8
10
12
14
0
r/Å Fig. 3. Pair distribution function qN–H(r) (solid line) and its running coordination number, NN–H(r) (dotted line), at the vicinity of the interface ˚. with a width of 10 A
S. Kajimoto et al. / Chemical Physics Letters 448 (2007) 70–74
shown in Fig. 2b for the water molecules hydrogen bonded with TEA molecules (H-bonded water) and water molecules non-hydrogen bonded with TEA molecules (non Hbonded water), respectively. The hydrogen bond distance between the nitrogen and hydrogen was determined to be ˚ which is the first minimum point of qN–H(r). This 2.7 A result indicates that the H-bonded water molecule orients the hydrogen atoms toward TEA phase as expected. On the other hand, the non H-bonded water molecules orient their oxygen atoms toward the interface. The number of non H-bonded water molecules is larger than the number of H-bonded ones even in the vicinity of the interface as shown in Fig. 2c. It should be noted that the peak value of Æcosh(z)æ for the non H-bonded water molecules (0.2) is smaller than that for TEA molecules (0.5). This implies that the non H-bonded water molecules at the interface orient their dipole moment in parallel with the interface rather than perpendicular to the interface. 3.3. Electrostatic property at the interface The electrostatic potential profile of the interface was calculated based upon Poisson’s equation. Z Z 0 1 z z /ðzÞ /ð0Þ ¼ qðz00 Þdz00 dz0 e0 0 0
Charge density / C ·m-3
where /(z) and q(z) are respectively the electrostatic potential and the charge density as a function of the z-coordinate [18]. The charge density, q(z), and the electrostatic potential, /(z), are shown in Fig. 4a and b, respectively. The contribution from each molecule is also shown in Fig. 4. According to this figure, there is a positive potential of 0.65 V in the vicinity of the interface with respect to the water phase.
40
a
73
The potential gap originates mainly from the charge distribution of the water molecules. The orientation of the water molecules, in which one or both of their hydrogen atoms points toward TEA phase, leads the large potential gap of 0.55 V. This value is a little higher than the potential gap studied for water at a hydrophobic interface [19]. This could be due to the existence of hydrogen bonds between the water and the TEA molecules. On the other hand, the contribution of the TEA molecules to the potential gap is small. The orientation of the TEA molecules leads to a potential gap of 0.1 V with respect to the water phase with the same direction as the water. This result indicates that the direction of the dipole moment is the same, which results in hydrogen bonding. Additionally, the potential caused by the TEA molecules has a minimum value in the vicinity of the interface. The depth of the potential drop is 0.15 V and this should result from the orientation of the TEA molecules. As discussed previously, the TEA molecules extend their nitrogen atom, which has a partially negative charge, toward the water phase rather than the methylene group, which has a partial positive charge. The result of this charge distribution is expected to cause a simple positive potential with regard to water phase. The more complicated potential arises from the position of the terminated methyl group. The methyl group, especially the terminated hydrogen, is directed into the water phase. The hydrogen atom is charged partially positive so the distribution of the charge arising from TEA molecules, takes a positive value near the water phase. In summary, the interface can be separated into three layers with respect to the TEA molecules as shown in Fig. 5. The first layer is the closest to the water phase and mainly contains the hydrogen atoms of the methyl group. The second one contains the nitrogen and carbon atom of the methyl group and is charged partially negative. The last one is the closest to the TEA phase and contains methylene group, which has a partial positive charge. After
0 6
-40x10
φ/ V
0.8
b-30
-20
-1 0
0
10
20
30
20
30
total water TEA
0.4 0.0 -0.4 -30
-20
-10
0 10 Z-axis / Å
Fig. 4. Charge density (a) and electrostatic potential profile (b) as a function of the z-coordinate. Solid line; the total system, dotted line; TEA molecule contribution and broken line: water molecule contribution.
Fig. 5. Schematic image of water and TEA molecules at the interface.
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all, the charge distribution caused by TEA molecules changes from positive to negative and again positive near to the interface. This distribution of charge causes the minimum in the electrostatic potential. This electrostatic potential indicates that not only the molecules, which have a high polarizability, but also dissolved cations would tend to move to the interface. 4. Conclusion We have performed molecular dynamics calculations for the two-phase coexistence state of the TEA and water mixture. At the interface between the two phases, both molecules have unique orientations, which results from hydrogen bonding between TEA and water molecules. These anisotropic orientations lead to a large potential gap of 0.65 V between two phases. Additionally, resulting from the orientation of the TEA molecules, the electric potential has the minimum value in the vicinity of the interface. Acknowledgements This work has been done as a part of the Next Generation Super Computing Project, Nanoscience Program, MEXT, Japan. This work has been partly supported by the MEXT (16072203, 17105001 and 18066020). The Authors also thank Okazaki Research Center for Computational Science, National Institute of Natural Sciences for the use of supercomputers.
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