J . Phys. Chem. 1990, 94, 531-536 a procedure described in ref 10. The mono- and diperchlorates of the 2,6-disubstituted Mannich bases were prepared by addition of equimolar amounts of 0.1 mol/dm3 ethanol solutions of HCIOl to a solution of the free base in 100% ethanol. The solvent was removed under reduced pressure and the residue was dissolved in CH2C12(0.2 mol/dm3). The IR spectra were taken with an FTIR spectrophotometer, IFS 1 13 V (Bruker, Karlsruhe, FRG), using a cell with Si windows (layer thickness 0.4 mm, detector DTGS, resolution 2, and NSS-250). The spectra in the region 500-10 cm-' were obtained by using a He-cooled bolometer as detector. Acknowledgment. Our thanks are due to the Deutsche Forschungsgemeinschaft and due to the Fonds der Chemischen Industrie for providing the facilities for this work. Registry No. 2-[ (Diethylamino)methyl]-4-butoxyphenol, 78525-98-1; 2-[(diethylamino)methyl]-4-fluorophenol,60460-58-4; 2-[(diethylamino)methyl]-4-phenylphenol, 66840-00-4; 4-chloro-2-[(diethylamino)methyl]phenol, 20484-33-7; 3,4-dichloro-2-[(diethylamino)(IO) Brzezinski, B.; Maciejewska, H. Pol. J . Chem., in press.
531
methyllphenol, 123380-96-1; methyl 3-[(diethylamino)methyI]-4hydroxybenzoate, 92198-14-6; ethyl 3-[(diethylamino)methyl]-4hydroxybenzoate, 78329-97-2;2- [(diethylamino)methyl]-4-nitrophenol, 65538-54-7;2,6-bis[(diethylamino)methyl]-4-butoxyphenol,123380-97-2; 2,6-bis[(diethylamino)methyl]-4-fluorophenol, 123380-98-3; 2,6-bis[ (diethylamino)methyl]-4-phenylphenol,123380-99-4;4-chloro-2,6-bis[(diethylamino)methyl]phenol, 61 195-52-6; 3,4-dichloro-2,6-bis[(diethylamino)methyl]phenol, 123381-00-0; methyl 3,5-bis[(diethylamino)methyl]-4-hydroxybenzoate, 123381-01-1; 4-nitro-2,6-bis[(diethylamino)methyl]phenol, 61 150-99-0; ethyl 3,5-bis[(diethylamino)methyl]-4-hydroxybenzoate, 123381-02-2; 2,6-bis[(diethylamino)methyl]-3,4-dinitrophenol, 123381-03-3; 2,6-bis[(diethylamino)methyl]-4-butoxyphenol monoperchlorate, 123381-04-4; 2,6-bis[(diethylamino)methyl]-4-fluorophenolmonoperchlorate, 123381-05-5;2,6bis [ (diethylamino)methyl]-4-phenylphenol monoperchlorate, 12338106-6; 4-chloro-2,6-bis[(diethylamino)methyl]phenolmonoperchlorate, 1 2338 1-07-7; 3,4-dichloro-2,6-bis[ (diethylamino)methyl]phenol monoperchlorate, 123381-08-8; methyl 3,5-bis[(diethylamino)methyl1-4hydroxybenzoate monoperchlorate, 123381-09-9;ethyl 3,5-bis[(diethylamino)methyl]-4-hydroxybenzoatemonoperchlorate, 123381-10-2; 2,6bis[(diethylamino)methyl]-4-nitrophenol monoperchlorate, 123381-1 1-3; 2,6-bis[(diethylamino)methyl]-3,4-dinitrophenol monoperchlorate, 12338 1-12-4.
A Molecular Dynamics Study of the Hexanelwater Interface Ilene Locker Carpenter and Warren J. Hehre* Department of Chemistry, University of California, Irvine, California 9271 7 (Received: January 24, 1989; In Final Form: August 4, 1989)
A molecular dynamics simulation of the interface between liquid hexane and liquid water has been performed. The sample consisted of a two-phase system comprising 187 hexanes and 1200 waters, confined to a box measuring 83.19 X 30.1 X 30.1 AS.A total of 94.4 ps was simulated (60 ps of equilibration and 34.4 ps of analysis). The interface was found to be about 10 8, wide. It does not appear to be molecularly sharp. A small number of hexane molecules are detached, Le., completely surrounded by water. This is inconsistent with the known solubility of hexane in water and is believed to be an artifact of the intermolecular potentials used. Differences between the interfacial and bulk liquids were observed. Water is more structured in the interfacial region than in bulk, and hexane molecules in the interfacial region were found to exhibit an excess of trans conformations relative to bulk. Neither component displays any preferential orientation with respect to the lab frame.
1. Introduction
Interfaces are common features in solution chemistry. Liquid/vapor and liquid/solid interfaces have been studied relatively extensively.' A large number of computer simulation studies of liquid/vapor interfaces have been reported.2 In recent years, a number of computer simulation studies of aqueous interfaces have been reported. Among these are studies by J o n ~ s o nMarchesi? ,~ and Lee et al.s of water between different types of walls and studies by Wilson et aL6 and Townsend et al.' of the aqueous liquid/vapor ( I ) (a) Croxton, C. A. Staristical Mechanics ofrhe Liquid Surface; Wiley: Chichester, 1980. (b) Nicholson, D.; Parsonage, N. G. Computer Simulation and the Statistical Mechanics of Adsorption; Academic Press: London, 1982. (2) Including, for example: (a) Lee, J. K.; Barker, J. A.; Pound, G. M. J . Chem. Phys. 1974,60,1976. (b) Liu, K. S. J . Chem. Phys. 1974,60,4226. (c) Abraham, F. F.; Schreiber, D. E.; Barker, J. A. J. Chem. Phys. 1975,62, 1958. (d) Chapela, G. A.; Savillie, G.; Rowlinson, J. S . Faraday Discuss. Chem. Soc. 1975, 59, 22. (e) Rao, M.; Levesque, D. J . Chem. Phys. 1976, 65, 3233. (f) Chapela, G.A,; Savillie, G.;Thompson,S . M.; Rowlinson, J. S. J . Chem. Soc., Faraday Trans. 2 1977, 73, 1133. (8) Miyazaki, J.; Barker, J. A.; Pound, G.M. J . Chem. Phys 1976,64, 3364. (h) Kalos, M. H.; Percus, J. K.; Rao, M. J . Srat. Phys. 1977, 17, 11 1. (i) Walton, J. P.R. B.; Tildesley, D. J.; Rowlinson, J. S.Mol. Phys. 1983, 48, 1357. G ) Lee, D. J.; Telo da Gama, M. M.; Gubbins, K. E. Mol. Phys. 1984,53, 11 13. (k) Lee, D. J.; Telo da Gama, M. M.; Gubbins, K. E. J . Phys. Chem. 1985, 89, 1514. (I) Thomson, S.M. Faraday Discuss. Chem. Soc. 1978,68, 107. (m)Thomson, S. M.: Gubbins. K . E. J . Chem. Phvs. 1981. 74. 6467. ( n l Weber. T. A,: Helfand, E J Chem Phys 1980, 72, 4014 ( 0 ) Matsumoto, M ; Kataoka, Y J Chem Phvs 1989. 90 2390 (3) Jonsson, B . Chem. Phys. Lett. 1980, 82, 520. (4) Marchesi, M. Chem Phys. Lett. 1983, 97, 224 (5) Lee, C. Y . ;McCammon, J. A.; Rossky, P.J. J . Chem. Phys. 1984,80, 4448.
0022-3654/90/2094-053 1$02.50/0
interface. Much less work has been done on the interfaces separating immiscible liquids, and while many important phenomena owe their existence to interactions occurring a t liquid/liquid interfaces (among them, phase-transfer catalysis and detergency), there is at present little detailed understanding of even the simplest systems. Experimental work probing microscopic detail is very limited, and only a few theoretical studies of liquid/liquid interfaces have appeared. Among the latter is a simulation of the benzene/water interface? a study of the interface between model atomic liquids: and simulation of model interfaces between water and hexanol.I0 In this work we focus attention on the detailed structure of the interface between hexane and water. The most important specific issue to be addressed is the inherent width of the interface between the two immiscible liquids. Comparisons need to be made both with previous studies, e.g., the benzene/water interface,s which found a molecularly sharp interface slightly broadened by capillary waves, and with the results obtained from application of the same simulation techniques to the study of two liquids that are known to be miscible. Closely related to the question of interfacial width are the roughness of the interface and the possible mixing of one ( 6 ) Wilson, M. A.; Pohorille, A.; Pratt, L. R. J . Phys. Chem. 1987, 91, 4873. (7) Townsend, R. M.; Gryko, J.; Rice, S. A. J . Chem. Phys. 1985, 82, 4391. (8) Linse, P. J . Chem. Phys. 1987, 86, 4177. (9) Meyer, M.; Mareschal, M.; Hayoun, M. J . Chem. Phys. 1988, 89, 1067. (10) Gao, J.; Jorgensen, W. L. J . Phys. Chem. 1988, 92, 5813.
0 1990 American Chemical Society
532 The Journal of Physical Chemistry, Vol. 94, No. 2, 1990
Carpenter and Hehre
TABLE I: Molecular Geometries"
molecule hexane water
geometrical parameter value r(C-C) L(C-c-C) r(0-H) L(H-0-H)
geometrical molecule Darameter value methanol r(C-0) 1.43
1.53 1 12.0 1.00 109.5
r(O-H) L(C-0-H)
0.945 108.5
2T
'Bond lengths in angstroms; bond angles in degrees. TABLE 11: Parameters for Intermolecular Potentials'
CH, CHZ 0 H
Aii'I2 2966.7 2436.1 793.3 0.0
Ci,l/2 49.82 40.9 1 25.01 0.00
9i O.OOb
0.00 -0.82' 0.4Ib
" A,
in kcal.AI2/mol, C, in kcal.A6/mol, q in electrons. nol, qCH, = 0.265, qo = -0.700, and qH = 0.435.
TABLE III: Details of Simulations of Pure Hexane, Pure Water, a Dilute Solution of Hexane in Water, and MetLnol/Water
methanol/ metha-
component into the other within or near the interfacial region. Other topics of interest concern the properties of molecules in the interfacial region relative to those in the bulk. Of specific interest in the hexane/water system are changes in the numbers and strength of hydrogen bonds in the water component and in the distribution of conformers in the hexane component. We emphasize that the hexane/water system detailed in this paper is a basis for future studies from our laboratory. Other systems of immiscible liquids need to be simulated, so that trends can be examined for systems with different relative solubilities and interfacial tensions. In the absence of experimental data on liquid-liquid structure, a series of simulations would be useful for determining the reliability of the simulation results. 11. Computational Methods A molecular dynamics simulation was performed on a sample comprising 187 hexane and 1200 water molecules, using the GROMOS' I molecular dynamics program running on the CRAY XMP-48 at the San Diego Supercomputer Center and a modified version of GROMOS running on a Convex C120 computer at U.C. Irvine. The 0 P L S l 2 and SPCI3 potentials were used to describe the hexane and water interactions, respectively. Both of these potentials treat nonbonded interactions between sites i and j in terms of the simple functional form
+ Aij/rij12- C . . / r . 6
Eij = qiqj/4atorij
V
V
(1)
A and C are van der Waals parameters, qi is the charge on site i, and rij is the distance between sites. The usual combining rules'2 are employed, namely, Aij = (AiiAj)l/*and C, = (CiiCjj)k/2. The
SPC model for water has three sites, one on each nucleus. In the OPLS model, hexane is represented by six sites, one on each carbon nucleus. Intramolecular nonbonded interactions have been included for sites separated by three or more atoms. The OPLS potential includes an intramolecular potential to describe the torsional motions, which has the form of a truncated Fourier series
V(4) = XVl(1
+ cos 4) + Y2V2(1- cos 24) + Y2V3(I + cos 34)
(2)
with VI = 1.41 1 kcal/mol, V2= -0.271 kcal/mol, and V3 = 3.145 kcal/mol. Finally, angle-bending motions in hexane have been described in terms of a potential of the form
v(e) = y2c(e- e,)2
-7
Figure 1 . Initial configuration. Hexane is on the left side of the box, and water is on the right side. The x axis is chosen to be perpendicular to the interface.
(3)
with C = 110.0 kcal/mol. The SHAKE'^ procedure was used to constrain all bond lengths as well as the bond cngle in water. The ( I I ) van Gunsteren, W. F.; Berendsen, H. J . C. Biomos b.v., Groningen. (12) Jorgensen, W. L.; Madura, J. D.; Swenson, C. J. J . Am. Chem. SOC.
1984, 106, 6638.
(13) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; Hermans, J . Intermolecular Forces; Pullman, B., Ed.; Reidel: Dordrecht, Holland, 1981; p 331. (14) Ryckaert, J. P.; Ciccotti. G.;Berendsen, H. J. C. J . Compur. Phys. 1977, 23, 327.
DroDertY
no. of hexanes no. of waters no. of methanols cutoff distance, 8, equilibration time, ps simulation time, ps L,,b 8, Ly,b A L,,b A temperature, K pressure,c atm
water
hexane 187
216 9.0 5.0" 8.5 18.95 18.95 18.95 299 f 6 784 f 761
solution
209 12.5 43.0 8.5 45.09 30.10 30.10 298 f 3 115 f 152
water
1
8.5 30.0 470 18.62 18.62 18.62 298 f 6 938 f 754
575 256 8.5 60.0 35.0 81.94 20.49 20.49 297 f 3 644 f 261
"Started from a previously equilibrated sample of water. b B o dimen~ sions. CError bars are rms fluctuation in pressure. model geometries and the parameters used in the potentials are given in Tables I and 11, respectively. The initial configuration consisted of two phases, with hexane on the left side of a box of dimensions 83.19 X 30.1 X 30.1 As and water on the right side (Figure 1). (We demonstrate below that this box size is large enough to allow the central parts of the two liquids to attain bulk properties.) The hexane molecules were initially in the all-trans configuration, at a density of 0.678 g/cm3. The initial water density was 1.O g/cm3. Spherical cutoffs based on molecule center-of-geometry separations were used to truncate the intermolecular interactions at 12.5 A. Periodic boundary conditions were applied in all three directions; this generates two interfaces perpendicular to the x axis. A time step of 1.7 fs was used, and 60 ps of equilibration was performed. Another 34.4 ps was simulated, during which properties were analyzed. Every fifth configuration was used, which gave a total of 4020 configurations for analysis. Constant-temperature dynamics was performed, with a reference temperature of 298 K. The calculated average temperature during the simulation was 299 K with root-mean-square (rms) fluctuation of 2 K. Ideally, one would have liked to perform the simulation at constant pressure as well, in order to account for the partial molar volumes of mixing. The expected volume changes for immiscible liquids are, however, very small in comparison to the volume fluctuations which we have found to result from the constantpressure algorithm implemented in the GROMOS program.I5 Alternative constant-pressure algorithms would perhaps alleviate this problem but were not explored. For the hexane/water simulation the calculated average pressure was -16.5 atm with rms fluctuation of 165 atm. A number of additional simulations were carried out to aid in the analysis of the hexane/water system. These include simulations of pure water, pure hexane, and a single hexane molecule in water. To address the question of whether the simulation of the hexane/water system is long enough to preclude eventual mixing, a simulation was carried out on a system of two miscible liquids (specifically methanol and water) starting from the twophase system. A three-site potential of form (1) was used to describe methanol.I6 Our results clearly show that this simulation ( 1 5) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; Dinola, A,; Haak, J. R. J . Chem. Phys. 1984, 81, 3684.
A Molecular Dynamics Study of the Hexane/Water Interface
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 533
I
positlon
conflyuratlon number Figure 2. Fluctuation in the x coordination of the center of mass of the water, XW, in the external frame.
1
b
behaves quite differently as a function of time from the hexane/water system. These differences, which are discussed below, provide additional evidence that the interfacial width in the hexane/water case has stabilized in the time period simulated. Details of these simulations are given in Table 111. 111. Results A. Nomenclature. To facilitate discussion of the properties of the hexane/water system, we refer to the two Gibbs dividing surfaces xGI and xG2, as well as XH and xw, the centers of mass of the hexane and water molecules, respectively. The dividing surfaces are defined according to xGI I xw
+ N+/D
(4a)
- N-/D
(4b) where N+ is the average number of water molecules in the volume x > xw, N- is the average number of water molecules in the volume x C xw, and D = N b / L , where Nb is the number of water molecules in a slab of thickness L centered around xw. L was chosen to be I O A. The definitions of the dividing surfaces are analogous to those proposed by Linse in his study of the benzene/water interface.8 Because the center-of-mass positions and the positions of the dividing surfaces fluctuate relative to the external frame, it is desirable to calculate properties of the system with respect to an internal frame. In the analyses below, the origin of this frame is taken to be either xG1,xc2, xH, or xw. Figure 2 shows the fluctuation in the position of xw in the external frame of reference. We refer to the regions around xH and xw as “bulk” regions. To simplify the analysis of water interfacial structure, we have considered only the middle interface. This has been divided into four regions, labeled A through D, moving from the pure water region to the pure hexane region. The boundaries of these regions are ( X G+~ 6, XGZ + 12), (XGZ+ 3, X G ~+ 61, (XGZ, XGZ,+ 3), and (xG2- 3, x,,), where all distances are in angstroms. The conformer distribution in hexane requires more averaging than the structural properties calculated for water; consequently, both interfaces were analyzed, and each interface was divided into only two regions, termed “inner“ and “outer”. The inner regions extended from XH - 25 to XH - 15 and from XH + 10 to XH 20, and the outer regions were defined by xH - 35 to xH- 25 and xH 20 to xH 30. The conformer distributions reported are averages over both interfaces. B. Density Profiles and Interfacial Width. Density profiles were obtained by calculating the density in slabs 2-&thick parallel to the y z plane. Density profiles for the hexane/water system, obtained as averages during the first and last quarter of the 3 4 9 simulation period, are shown in Figure 3, and b, respectively. These plots indicate that the interface is quite stable over the time period of the simulation. In particular, the water profile is nearly unchanged. Unambiguous characterization of the width of the interface separating two immiscible components is not possible. x G ~1 XW
+
+
+
(16) Jorgensen, W.
L.J . Phys. Chem. 1986, 90,
1276.
positlon Figure 3. Density profiles for hexane/water system over the first (a) and last (b) quarters of the 34-ps simulation period. The origin is taken to be the x coordinate of the center of mass of the water. Density is measured in amu/A3.
-5! s
2204 1
3
time (ps) Figure 4. Width of the interface, in angstroms, for hexane/water (circles) and methanol/water (triangles)systems, as a function of time. T = 0 correspondsto the initial (preequilibration)configuration. For both systems, the widths of the two interfaces have been averaged.
The definition used here is the distance required for the water density to drop from 90% to 10%of its bulk value. On this basis, the width is calculated to be 10 f 3 A. This is significantly larger than the water liquid/vapor6 and the benzene/waters interfacial widths, which, according to the same definition, are each about 5 A. A plot of the interfacial width as a function of time (including the equilibration period) (solid line plot in Figure 4) indicates that the interface is established within roughly the first 40 ps of the simulation. Although fluctuations in width after this time are substantial, and it is possible that the interfacial widths might increase if a longer period of time were simulated, it appears that the average width does not increase after the equilibration period. Density profiles for the methanol/water system calculated over the same time periods are shown in Figure 5a,b. In contrast to the hexane/water system, the density profiles for this system
534 The Journal of Physical Chemistry, Vol. 94, No. 2, 1990
I-
Carpenter and Hehre
., :? ,-.-
.60
2.
poslt lon I
F-----l -60
-40
-20
0
20
poslt lon Figure 5. Density profiles for methanol/water system, as in Figure 3.
continue to change with time. The width of the methanol/water "interface" (dotted line plot in Figure 4) does not level off during the time of the simulation but continues to increase rapidly. This is not unexpected: because the two components are miscible, after a long enough simulation period, both methanol and water should be present throughout the box and the density of each should be nearly uniform. The simulation reported here is long enough for the first condition to have been met; a much longer simulation time is clearly required for the overall composition to become uniform throughout the box. Nonetheless, comparison of the results for the two systems is useful. Because the conditions and lengths of the two simulations are the same, we feel this provides evidence that the hexane/water simulation has been carried out for a sufficiently long time to allow the liquids to mix if this was ever going to occur. The density profiles of hexane and water are dissimilar; we see in Figure 3 that the hexane density drops to zero much more slowly than the water density. In fact, a small number of hexane molecules are completely surrounded by water molecules. These hexane molecules, which continue to diffuse farther into the water region as the simulation progresses, account at least in part for the small changes in the hexane density profile over the course of the simulation. In contrast to the hexane, the water density drops rapidly to zero, and no isolated water molecules are found in the hexane region. Even after accounting for those hexane molecules which have dissolved in the water, the interface between the two components still appears to be quite rough. A representative snapshot is shown in Figure 6. Unlike the benzene/water interface,* the hexane/water interface does not appear to be molecularly sharp. The relatively large apparent solubility of hexane in water, reflected in the observation of hexanes that "break off" from the interface to be completely surrounded by water, does not agree with experimental observations. The solubility of hexane in water is 0.001 23 wt %,I7 which corresponds to approximately one hexane ( 1 7) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. Organic Solvents: Physical Properties and Methods of Purification;Wiley: New York, 1986.
Figure 6. Snapshot of the middle hexane/water interface (30 Ix I58 in the lab frame). For clarity, the hydrogen atoms on hexane have been eliminated. Produced using MacModel (E. M. Berdahl, S. D. Kahn, L. Wiedeman, and W. J. Hehre, University of Illinois/University of California), using a Tektronix 4693D printer.
molecule in lo6 water molecules. The solubility of water in hexane is nearly a factor of 10 greater than the solubility of hexane in water,17 although this is still a very small number. The observation that no water molecules dissolve in hexane is therefore consistent with experiment. The small amount of mixing of hexane into water is clearly inconsistent with observation and is likely an artifact of the particular intermolecular potentials employed. It is possible that either the water-water potential or the hexanehexane potential is at fault. Liquid/vapor studies by Wilson et aL6 indicate that a potential which gives a good account of the properties of bulk water may not yield surface properties such as the surface tension in agreement with experiment. However, we feel that the hexane-water potential is perhaps more suspect. While a large amount of work has been done developing potentials for pure liquids, potentials describing the interactions between unlike molecules in multicomponent systems have received relatively little attention. In part, this is because of a lack of experimental data with which to compare. In this work, the hexane-water potential Lennard-Jones terms have been determined by combining rules from the hexane-hexane and water-water potentials (vide supra). Because there is no charge on the sites in hexane, there is no Coulombic term in the hexane-water potential. Further research is needed, and is in progress in our laboratory, to develop and assess critically potentials for all the interactions involved in the simulation as well as to assess the accuracy of the combining rules. C. BuZk Structure. The hydrogen-bonding characteristics for water in the region xw f 4 A were analyzed by using an energetic criterion. Two molecules are considered hydrogen bonded if they have an interaction energy of less than -2.25 kcal/mol. The fraction of water molecules participating in n hydrogen bonds has been calculated and is displayed in histogram form in Figure 7. Also displayed is the corresponding histogram for pure water. The average number of hydrogen bonds per molecule is found to be 3.4 in the bulk, compared to 3.5 in our simulation of pure water. The difference is believed to be the result of imposed small density differences. Further evidence for similarity of the properties of bulk and pure water follows from comparison of radial distribution functions (RDFs). A comparison of 0-0RDFs for bulk and pure water is provided in Figure 8 (solid and long dashed lines, respectively).
A Molecular Dynamics Study of the Hexane/Water Interface ."
1
I
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 535 TABLE V Hydrogen-Bonding Characteristics in the Interfacial Area region bulk A B C
0.4
0.3
D f(n)
3.4 3.4 3.7 2.8 2.1
~
N
N
~
nHB/nNN
5.1 4.8 4.8 3.5
2.4
a Average number of hydrogen bonds per molecule. ber of nearest neighbors per molecule.
02
0.1
123456 123456 123456 123456 123436 123456
pure bulk A 6 C D Figure 7. Distribution of hydrogen bonds for pure, bulk, and interfacial water. Average is over 1000 configurations.
3.0
~HB'
1
S
2.0
4.0
6.0
Figure 8. Oxygen-oxygen radial distribution functions for bulk (solid line), pure (long dashed line), and interfacial (short dashed line) water. Average is over 1000 configurations. TABLE IV: Conformer Distribution in Hexane region % gauche % ' trans region '% gauche outer 11 89 pure 17 inner 10 90 solution' 28 bulk 20 80
7% trans 83 72
" A single hexane in water.
RDFs for 0-H and H-H show the same degree of similarity and are not displayed. By this measure, the structure of the bulk water is nearly identical with that of pure water obtained by using the same potential, indicating that water in the bulk is not affected by the presence of an interface of a few angstroms away. The most interesting structural property of a liquid such as hexane is the relative abundance of various conformers. The percentages of gauche and trans dihedral angles in bulk and pure hexane are given in Table IV. The three dihedral angles in each molecule are not distinguished. The data for the two-component system were obtained by sampling hexane molecules in the region xHf 4 A, while the latter data derive from the simulation of the pure liquid. Because the time for "conformational flips" in alkanes is long,Is long equilibration and sampling periods are needed to obtain accurate values of these functions. The small differences between bulk and pure hexane are, we suspect, largely a consequence of poor statistics rather than the presence of the interfaces. The similarity of the bulk and pure li uids for both components suggests that the box length of 83.19 chosen in this study is sufficient. This is consistent with previous work on the benzene/water interfaces where a box length of approximately 62 A was found to be sufficient to produce bulk liquids largely unaffected by the presence of the interfaces. The presence of bulk
1
(18) Jorgensen, W.L. J . Phys. Chem. 1983, 87, 5304.
0.65 0.70 0.16 0.8 1 0.88
Average num-
liquid between the two interfaces is essential for the properties of the two interfaces to be independent. D. Structure of Interfacial Area. An analysis of hydrogen bonding for water in the interfacial area has been performed. The average number of hydrogen bonds per molecule, nHB, was determined by using the same criteria as previously employed for bulk and pure water ( E I -2.25 kcal/mol). The average number of nearest neighbors, nm, was also calculated, assigning molecules with oxygen atoms within 3.5 8, of each other as nearest neighbors. The fraction of water molecules participating in n hydrogen bonds for interfacial regions A-D is shown graphically in Figure 7. The corresponding values of nHB, n", and nHB/&N are given in Table V. As one might anticipate, the hydrogen-bonding characteristics of the water in section A are the closest to those in bulk water. Note that while the number of water molecules as well as the number of nearest neighbors decreases in going from region A to region D, the fraction of nearest-neighbor molecules that participates in hydrogen bonds actually increases. This indicates that water molecules on the edge of the interfacial area, where the number of neighboring water molecules is smaller than in the bulk, orient themselves to take better advantage of the existing hydrogen-bonding possibilities. Such behavior has also been noted in the simulation of the benzene/water interface,s the aqueous liquid/vapor interface,6 and water near a hydrophobic ~ u r f a c e . ~ Radial distribution functions for water were calculated in the interfacial region. The 0-0 R D F in region A is representative of the RDFs in the interfacial region and is shown in the small dashed line plot in Figure 8. The 0-0RDFs show increased water structuring in the interfacial region. Specifically, the peaks in the radial distribution functions are significantlysharper than those in bulk water. Both the 0-H and H-H RDFs show a similar effect. The noted differences in bulk and interfacial RDFs are consistent with the results of the hydrogen-bonding analysis and with the study of water at a planar hydrophobic s ~ r f a c e .As ~ in other cases of hydrophic hydration, the increased structure allows the number of hydrogen bonds per water molecule to remain large, even though the number of nearest-neighbor water molecules is reduced by the presence of hexane. We find no evidence for preferential orientation of water molecules in the interfacial area with respect to the external frame. This result differs from that found in the simulations of the benzene/water interface: and water next to a planar hydrophobic s ~ r f a c eand , ~ is likely caused by the large degree of roughness present in the hexane surface. The hexane/water interface simulated here is very nonplanar on a microscopic scale. The dihedral angle distribution for hexane in the interfacial region (Table IV) is found to be slightly different from that in bulk hexane. Specifically,there is an increased percentage of trans dihedral angles relative to bulk (or pure) hexane. In contrast, in dilute aqueous solution there is an increased gauche population. An increased percentage of trans conformers was observed at the hexanol/water interfacelo (for those angles at the alkane end of the molecule). This was explained as a packing effect, reflecting the more ordered environment in the interface. Our calculations indicate that there is no preferential orientation of hexane molecules so the increased trans population remains unexplained, although it is possible that additional averaging would remove this unusual conformer distribution. IV. Conclusion Several conclusions may be drawn from the molecular dynamics simulation of the liquid hexane/liquid water interface described
536
J . Phys. Chem. 1990, 94, 536-540 to that observed in dilute aqueous solutions of hexane. Both the validity and generality of these conclusions need to be examined in detail. Specifically, simulations with other potentials, and with other pairs of immiscible liquids, are required in preface to more ambitious studies, such as those involving the partitioning of small molecules into two phases, the transport of molecules across the interface, and the disposition of surfactants at the interface. Work in these directions is currently in progress in our laboratory.
in this paper. First, the width of the interface (approximately IO A) is significantly greater than the dimensions of a single molecule. The interface appears not to be “molecularly sharp”. A small number of hexane molecules are found to be detached from the bulk and to be completely surrounded by water molecules. This is not consistent with the known solubility of hexane in water and is, therefore, an artifact of the intermolecular potentials used. While both the hexane OPLS and water SPC potentials reproduce the properties of the bulk liquids well, it is possible that they do not, in combination, describe the properties of a hexane/water mixture correctly. Both interfacial water and hexane show differences from the bulk. Water near the interface is significantly more structured than bulk water; this structuring maximizes the number of hydrogen bonds per molecule. Hexane near the interface exhibits an increase in trans conformers relative to the pure liquid, opposite
Acknowledgment. This research was supported in part by the donors of the Petroleum Research Fund, administered by the American Chemical Society, a Control Data Corporation PACER Fellowship to I.L.C., and a grant of computer time from the San Diego Supercomputer Center. Registry No. Hexane, 110-54-3; water, 7732-18-5.
3P Hg, Cd, and Zn Photosenskized Chemistry of Vinyl Halides in Krypton Matrix Harry E. Cartland**+and George C. Pimentelt Chemical Biodynamics Division, Lawrence Berkeley Laboratory, University of California. Berkeley, California 94720 (Received: February 6, 1989; In Final Form: July 31, 1989)
The reaction of group IIB metals in the 3P state with vinyl fluoride, chloride, and bromide is studied in krypton matrix. The primary process in all cases is hydrogen halide elimination to form a hydrogen halide/acetylene hydrogen-bondedcomplex. Insertion of metal atoms into C-CI and C-Br bonds, but not into C-H and C-F bonds, is also observed. The insertion photochemistry can be explained by a mechanism which requires that the process occur on a triplet surface with the vinyl halide in the planar ground-state conformation.
Introduction Reaction of group IIB metals in their lowest 3Pstates in solid Kr at 12 K with several halogenated ethenes has recently been demonstrated. In one study, excited metal atoms were found to insert into the C-CI bond of 2-chloro- 1,l -difluoroethylene to give difluorovinyl metal chlorides as the sole reaction product.’ Molecular C12 and HCI elimination competes effectively with insertion chemistry in experiments with the dichloroethylenes, a result which contrasts with the observation of only HCl elimination on photolysis (A > 200 nm) in the absence of metal atomsS2 (Hereafter, ref 1 and 2 are respectively I and 11.) We present here a summary of additional similar group IIB metal atom chemistry in cryogenic matrices. Isolated vinyl halides, abbreviated VX (X = F, C1, Br), were the reactants with which initial studies in this area commenced. These species were chosen for a number of reasons. Their I R spectra are relatively simple and complete assignments have been made.3-5 Products derived from these species are expected to be readily identifiable. The HgCP) photosensitized chemistry of a number of these molecules in the gas phase has been reported.&* Finally, the concerted elimination of hydrogen fluoride from vinyl fluoride has recently been subjected to theoretical inve~tigation.~ Thus the wide range of available background material makes the vinyl halides a logical starting point for investigations of this nature. In addition to the vinyl halides, results with a number of other reactants and the conditions under which the experiments were performed are also presented. These include studies with chloroethylenes other than those found in 11, and ethylene. Experimental Section The cryogenic apparatus, deposition technique, and Fourier transformation infrared spectroscopy have previously been de-
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Present address: Science Research Laboratory, United States Military Academy, West Point, NY 10996-5000. 3 Deceased.
0022-3654/90/2094-0536$02.50/0
scribed in I. Typically, 1-2 mmol of a premixed matrix gas/ reactant sample at M / R = 100 was deposited on a CsI substrate with a flow rate of 0.5-1 .O mmol h-I. Krypton, which was used as the host in most experiments, was condensed at 20 K. The Ar, N2,and Xe mixtures used in several experiments were deposited at 12, 12, and 25 K, respectively. Spectra (0.5 or 0.25 cm-’ resolution) were recorded and photolyses were conducted at 12 K. Atomic Hg, Cd, and Zn were added to the matrix as described in I. Matrices were normally photolyzed with the 1OOO-W Hg-Xe high-pressure arc lamp equipped as in I, either “broadband”, in which case the atmospheric cutoff near 200 nm determined the short-wavelength limit, or through one of the interference filters described in Table I of I. In this paper, photolysis bandwidth refers to the full width at half-maximum transmission of a particular filter. In some experiments the high-pressure arc source was replaced with a GE-AH4 medium-pressure Hg lamp (outer envelope removed) or a quartz Phillips-Osram resonance lamp, both of which were mounted in the nitrogen-purged spectrometer cavity. The same filters were used in these experiments. Usually it was found that the higher power and spectrally broadened output afforded (1) Cartland, H. E.; Pimentel, G. C. J . fhys. Chem. 1986, 90, 1822 (referred to in text as I). (2) Cartland, H. E.; Pimentel, G. C. J . fhys. Chem. 1986,90, 5485 (referred to in text as 11). (3) Shimanouchi, T. Natl. Stand. Ref. Data Ser., ( U S . )Natl. Bur. Stand. 1972, No. 39. (4) Gullikson, C. W.; Nielson, J. R. J. Mol. Spectrosc. 1957, 1 , 158. (5) McKean, D. C. Spectrochim. Acta 1975, 31A. 1167. (6) Tsunashima, S.; Gunning, H. E.; Strausz, 0. P. J. Am. Chem. Soc.
1976, 98, 1690.
(7) Strausz, 0 .P.; Norstrom, R. J.; Salahub, D.; Gcsavi, R. K.: Gunning, H. E.: Csizmadia, I. G. J. Am. Chem. Soc. 1970, 92, 6395. (8) Bellas, M. G.; Wan, J. K. S.; Allen, W. F.; Strausz, 0. P.; Gunning, H . E. J . f h y s . Chem. 1964, 68, 2170. ( 9 ) Kato, S.: Morokuma, K. J . Chem. fhys. 1981, 74(11), 6285.
0 1990 American Chemical Society
