Article pubs.acs.org/EF
Spreading of Multi-component Oils on Water Makoto Kunieda,† Yunfeng Liang,*,† Yasuhiro Fukunaka,† Toshifumi Matsuoka,† Koichi Takamura,‡ Nina Loahardjo,‡ Winoto Winoto,‡ and Norman R. Morrow‡ †
Department of Urban Management, Kyoto University, Kyoto 615-8540, Japan Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071, United States
‡
ABSTRACT: The spreading of multi-component oils on water has been investigated by direct observations and predicted from measurements of the interfacial tensions and surface tensions of decane, toluene, heptane, and their mixtures. Pure decane does not naturally spread at ambient conditions, as indicated by its negative spreading coefficient. However, when decane is mixed with toluene and heptane, the mixture spreads on water over a wide range of compositions. The spreading coefficients are highly nonlinear with respect to concentration and feature a maximum. The spreading is ascribed to preferential accumulation of toluene at the oil/water interface and heptane at the oil/vapor interface. Molecular dynamics simulations corroborate the hypothesis of preferential accumulation. The accumulation of lighter alkanes at the oil/vapor interface reduces the surface tension, and the accumulation of aromatics at the oil/water interface decreases the interfacial tension. As a consequence, the oil mixture spreads over water.
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INTRODUCTION Wetting and spreading phenomena are ubiquitous in natural and technological processes and concern areas where chemistry, physics, and engineering intersect.1−5 Wettability of oil/water/ rock is a key factor in the flow of fluids through the pore spaces in a reservoir and, hence, plays a primary role in oil recovery.6−10 Laboratory studies have shown that the wetting behavior of oil on water in the presence of gas also strongly impacts oil recovery by gas injection.11−13 Ellipsometric studies of the wetting behavior and its transition have been reported for alkanes on water and aqueous solutions.14−20 The state of spreading of a liquid on another liquid can be estimated from the Neumann triangle of forces for the three tensions to obtain the spreading coefficient, S.1,2 In the case of spreading of oil on water, the three tensions are the surface tensions (SFTs) of water and oil against a gas phase and the interfacial tension (IFT) of oil against water. The spreading coefficients, S, for oil-on-water can be calculated from1,2 S = γwv − (γov + γwo)
Wetting phenomena are governed by interfacial interactions as well as the interactions at the contact line of the three phases, acting usually over a few molecular distances. Unlike the bulk liquid, the interface, surface, and contact line are noncentrosymmetric. These length scales are being probed with experimental techniques, such as atomic force microscopy and surface force measurement, or theoretical tools, such as molecular dynamics (MD), to obtain new insights into surface phenomena.22 MD simulations have been performed to study static and dynamic interfacial phenomena and the wetting behavior of liquids on solid surfaces.23−25 A multi-component light oil MD model was used to investigate the structural properties of a multi-component oil/water interface. It was found that the aromatic component (toluene) preferentially accumulates at the oil−water interface, while the other hydrocarbons are more uniformly distributed throughout the oil phase.25 It is of interest to test the light oil model against experiments to gain further insight into the conditions under which oil spreads on water. Measurements of SFTs and IFTs were performed for ternary mixtures of toluene, heptane, and decane. Next, the spreading coefficients were calculated from the SFT and IFT data and compared to the results of direct observations of the spreading behavior of oil mixtures on water. The investigation was complemented by MD simulations of the molecular distribution of a multi-component oil drop on water.
(1)
where γwo is the IFT between the liquid phases w and o and the subscripts w, v, and o correspond to water, vapor, and oil, respectively. It can be readily understood by the difference of the work of the adhesion for the oil−water interface and that of the cohesion of the oil.1 Because the spreading coefficient describes the force balance at a three-phase contact line, if the spreading coefficient is positive, then oil will spread on water. Oil with a negative spreading coefficient cannot spread and forms a liquid lens. The change from non-spreading to spreading occurs at the boundary according to whether the coefficient is positive or negative.4,5,18 For multi-component oils, values of SFTs (oil/vapor) and IFTs (oil/water) imply preferential adsorption of components.21 For example, the wetting and spreading of crude oil on water will be influenced by composition even in the absence of surfactant. © 2012 American Chemical Society
Special Issue: 12th International Conference on Petroleum Phase Behavior and Fouling Received: October 8, 2011 Revised: January 22, 2012 Published: January 25, 2012 2736
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Table 1. Experimental Results Measured at 20 °C and Calculated Spreading Coefficients mole fraction toluene
a
heptane
decane
density (g/cm3)
1 0 0
0 1 0
0 0 1
0.867 0.684 0.730
0.79 0.60 0.40 0.20 0.80 0.60 0.40 0.20 0.11 0 0 0 0 0 0
0.21 0.40 0.60 0.80 0 0 0 0 0 0.80 0.60 0.50 0.40 0.20 0.10
0 0 0 0 0.20 0.40 0.60 0.80 0.89 0.20 0.40 0.50 0.60 0.80 0.90
0.819 0.779 0.742 0.712 0.821 0.789 0.764 0.745 0.738 0.696 0.706 0.711 0.715 0.723 0.726
0.60 0.40 0.20 0.40 0.20 0.20
0.20 0.40 0.60 0.20 0.40 0.20
0.20 0.20 0.20 0.40 0.40 0.60
0.784 0.751 0.721 0.758 0.731 0.738
SFT (mN/m) Single Component 28.2 20.0 23.6 Binary Mixture 25.2 23.4 22.0 21.0 26.0 25.0 24.4 24.0 23.7 21.3 22.0 22.3 22.5 23.1 23.3 Ternary Mixture 24.2 23.0 22.0 23.6 22.7 23.3
IFT (mN/m)
Sa (mN/m)
spreading behaviorb
35.2 50.1 51.0
9.1 2.5 −2.1
S S NS
37.1 38.9 41.1 44.8 37.5 40.0 42.9 46.2 48.5 50.2 50.7 50.6 50.0 50.0 50.7
10.2 10.3 9.5 6.7 9.0 7.6 5.3 2.4 0.3 1.0 −0.1 −0.4 −0.1 −0.5 −1.5
S S S S S S S S int S int int NS NS NS
39.0 42.1 45.5 42.7 45.1 46.0
9.4 7.5 5.0 6.2 4.7 3.2
S S S S S S
Spreading ceofficient derived from measurements with water SFT of 72.5 mN/m. bS, spreading; NS, non-spreading; int, intermediate/autophobic.
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the x and z plane, and (2) the length along the y direction can be a relatively small value; thus, less numbers of molecules are needed to obtain the representative results and reduce the computational time. The thin water layer was calculated from NPT (constant number of molecules, pressure, and temperature, respectively) simulation. Then, the cylindrical droplet was put next to the thin water layer to give continuity in the y direction and, hence, a 2D profile in the x and z plane. Finally, the molecular distribution was calculated from NVT simulation. In all NVT and NPT calculations, the temperature was controlled by the Nose−Hoover thermostat31 to 298 K. In NPT calculations, the pressure was controlled by the Parrinello−Rahman method32 to 0.1 MPa. The particle mesh Ewald summation33 was used for the electrostatic interactions, and a selectively chosen cutoff of 14 Å was used for calculation of the van der Waals interactions, as needed in two-phase calculation. A 1.0 fs time step was used, and output coordinates were obtained every 1.0 ps. Calculations run for 5.0 ns showed a sufficiently close approach to equilibrium states. Snapshots of molecular distributions were prepared by visual molecular dynamics (VMD) software.34 Calculation of IFT. MD calculations of heptane−toluene binary mixture interfaces against vapor and water were carried out in NVT ensemble for the mixture/vapor system and NPT ensemble for the mixture/water system. The IFT was calculated from the MD results. The IFT, γ, is defined,23 when the z axis is perpendicular to the interface, as
EXPERIMENTAL AND COMPUTATIONAL METHODS
Experimental Procedures. Toluene, heptane, and decane used in this study were commercial products with a minimum of 99.0% purity. Polar contaminants were removed by flow through silica gel and alumina columns up to 6 times until the IFTs were constant. The binary and ternary mixtures, listed in Table 1, were prepared gravimetrically prior to the measurement. An Anton Paar density meter DMA48 was used to measure the density of the mixtures. Distilled water was used in the IFT measurements. A KRÜ SS K100 tensiometer was operated in the Wilhelmy plate mode26 using a platinum plate. For all measurements, approximately 80 cm3 of a liquid was placed in a glass vessel of 5 cm in diameter, set inside the chamber of the tensiometer. Before each run, the platinum plate and glass vessel were flamed and then equilibrated with the sample in a closed compartment, with the experimental temperature held at 20 °C by an external thermostat (well below the boiling points of toluene, heptane, and decane, which are 102, 90, and 164 °C, respectively, at the experimental condition of ∼0.8 bar at Laramie, WY, 2200 m above sea level). Qualitative spreading states of the oil mixtures on water were observed by placing one drop of oil on water in a glass beaker of 5 cm in diameter at the ambient temperature (20−22 °C). Details of MD. The droplet model used in this study was a ternary mixture of toluene, heptane, and decane, with 250, 250, and 500 molecules, respectively. The MD simulation was performed using the GROMACS package.27 The water molecules were modeled by the SPC/E model.28,29 A revised version of the CHARMM 27 force field30 was used to model all hydrocarbons. First, the droplet model was calculated from NVT (constant number of molecules, volume, and temperature, respectively) simulation to form a cylindrical shape. This particular cylindrical droplet has at least two advantages in the MD simulations: (1) it is easy to obtain a two-dimensional (2D) profile in
γ=−
∫ [PL(z) − PB(z)]dz
(2)
where PB(z) and PL(z) are the bulk pressure and lateral pressure, respectively. The integral is defined over the boundary layer and can 2737
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Figure 1. Calculated spreading coefficients of binary oil mixtures on water. Values for ternary mixtures with constant mole fractions of (a) decane and (b) heptane are also included. The presence of the maximum values in S can be seen for the binary mixture at 0.4 heptane and 0.6 toluene, and the ternary mixture of 0.2 decane, 0.2 heptane, and 0.6 toluene. be extended to infinity. With two interfaces perpendicular to the z axis, this gives
⎞ 1 ⎛ Px + Py γ=− ⎜ − Pz⎟Lz 2⎝ 2 ⎠
and tests of spreading behavior. Results of density, SFT, and IFT are presented in Table 1. Satisfactory agreement between experimental and literature values was obtained for all measurements. The literature values of SFT ranged from 28.50 to 28.53 mN/m for toluene, from 20.05 to 20.15 mN/m for heptane, and from 23.7 to 23.9 mN/m for decane. Experimental values were 28.2 mN/m for toluene, 20.0 mN/m for heptane, and 23.6 mN/m for decane. Literature values of IFT against water were from 36.0 to 36.4 mN/m for toluene, from 50.2 to 51.9 mN/m for heptane, and from 51.7 to 51.9 mN/m for decane.36−41 Experimental values were 35.2 mN/m for toluene, 50.1 mN/m for heptane, and 51.0 mN/m for decane. The measured SFT of water, 72.5 mN/m, was in close agreement with a literature value of 72.9 mN/m.42 For each mixture, data independency of time was tested by another SFT measurement performed ∼30 min after the first measurement. The results fell within 0.15 mN/m of the first measured data reported in Table 1. Data for binary and ternary mixtures showed consistent trends with respect to concentration, indicating that the mixtures were free of contaminants and constant in composition. The spreading coefficients for each system were calculated from eq 1 and compared against the observed spreading behavior in the last two columns in Table 1. Examples of initial slow spreading on water followed by retraction into a lens after a few seconds are identified as intermediate (int) in the last column. This autophobic behavior might be related to the time required for the toluene and water to become mutually saturated, which likely results in a reduced SFT of water similar to that reported for benzene.2 The last two columns in Table 1 confirm that oil is either non-spreading (NS) or intermediate (int) at S < 0.3. There is excellent agreement between the calculated and observed spreading behavior of these binary and ternary mixtures.
(3)
in which Pα = pαα(α = x,y,z) are the diagonal elements of the pressure tensor and LZ is the box length in the z direction used in the calculations.
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RESULTS AND DISCUSSION Prediction of SFTs and IFTs. The SPC/E model underpredicted the SFT of water (63.7 mN/m versus a literature value of 72.9 mN/m). Two additional models, namely, the TIP4P2005 and SPC/E flexible models, were tested. Both of these models were able to reproduce the SFT of water more closely than the SPC/E model.35 However, the decane/water IFT calculated using the SPC/E flexible model was much too high (71 mN/m versus a literature value of 51.8 mN/m), and the TIP4P2005 model gave a high value for the toluene/water IFT (45 mN/m versus a literature value of 36.4 mN/m).25 Agreement between the MD-predicted IFTs for each hydrocarbon against water supported use of the SPC/E model.25 This indicates that results of the MD simulations successfully predict the interaction between water and different hydrocarbons. However, agreement was not close enough for the prediction of spreading coefficients from the SFTs and IFTs obtained from MD simulations. In detail, the calculated spreading coefficients of toluene, heptane, and decane on top of the water from the SPC/E model are −4.6, −8.8, and −9.7 mN/m, respectively. These values are systematically smaller than the experimental values of 9.06, 2.45, and −2.06 mN/m for toluene, heptane, and decane (see Table 1), respectively. Effect of Composition on Spreading: Experimental Observations. A total of 23 oil mixtures were prepared for physical property, oil/vapor and oil/water IFT measurements, 2738
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and e of Figure 3, respectively. They correspond to Figure 2b. The density maps of toluene, heptane, and decane calculated
Calculated spreading coefficients from Table 1 are plotted in panels a and b of Figure 1 as functions of the mole fraction of toluene. Here, the upper and lower solid lines in both panels represent, respectively, the spreading coefficient of the heptane/toluene and decane/toluene binary mixtures. Points at the mole fraction of toluene equal to 0 represent the S values for the heptane/decane binary mixtures. Figure 1 clearly shows that the calculated spreading coefficients for the binary mixtures are highly nonlinear with respect to composition, especially at above 0.4 mole fraction of toluene. It is especially interesting to note that the maximum spreading coefficient was obtained for a toluene/heptane mixture of 0.6:0.4 by mole fraction, where S is 10.3 mN/m. This value is 1.3 and 7.9 mN/m higher than S for pure toluene and heptane, respectively. Thus, the binary mixture spreads more readily than either of the pure liquid components. The spreading coefficient of pure toluene is 9.1 mN/m. The addition of as much as 0.2% mole fraction of decane has no significant effect on the spreading behavior because S = 9.0 mN/m for this mixture. This is especially remarkable because S = −2.1 for the pure decane. Spreading coefficients for the ternary mixtures of heptane/ toluene/decane are also included in panels a and b of Figure 1, for constant mole fractions of decane and heptane, respectively. From Figure 1a, the maximum in S versus mole fraction of toluene was obtained for 0.2 mole fraction of decane. S = 9.4 mN/m for the ternary mixture of 0.2:0.2:0.6 mole fraction of decane/heptane/toluene, respectively, which is 0.3 mN/m higher than S for pure toluene. These observed complexities of the spreading behavior of the binary and ternary mixtures are believed to result from specific adsorption of the toluene and heptane at the oil/water and oil/ vapor interfaces, respectively. Changes in oil composition at interfaces were investigated through MD simulations. Molecular Distribution Inside an Oil Droplet: MD Simulation. Snapshots of a unit cell for MD calculations at the beginning and after 5.0 ns of simulation are shown in panels a and b of Figure 2, respectively. At the beginning, the minimum
Figure 3. Snapshots of (a) toluene, (c) heptane, and (e) decane after 5.0 ns simulation and density maps of (b) toluene, (d) heptane, and (f) decane averaged over 2.0−5.0 ns.
from each trajectory and averaged over 2.0−5.0 ns of simulation time are shown in panels b, d, and f of Figure 3, respectively. In panels a and b of Figure 3, accumulation of toluene at the oil/water interface was clearly observed. On the other hand, the decane concentration is higher away from both the oil/water and oil/vapor interfaces. To investigate the details of these phenomena, mole fraction profiles and partial density profiles of each component were obtained along the z axis averaged at the range of 9 < x < 11 nm, over 2.0−5.0 ns of simulation. Results are shown in panels a and b of Figure 4, respectively. Three regions can be identified along the z axis of the droplet: the water region, the bulk oil region, and the vapor region, as labeled in Figure 4. As shown in Figure 4b, toluene accumulates at the oil/water interface. The maximum density of toluene at the oil/water interface is about 3 times its density in the bulk region. Accumulation of toluene at the oil/water interface (z ∼ 5.0 nm) was accompanied by weak accumulation of heptane. At the oil/vapor interface (z ∼ 8.5 nm), all density profiles change gradually (Figure 4b). The mole fraction profiles in Figure 4a show weak accumulation of heptane at the oil/vapor interface. If the vapor phase was in equilibrium with
Figure 2. Snapshots of the oil droplet on water (a) at the beginning and (b) after 5.0 ns of simulation (orange, toluene; red, heptane; black, decane; and blue, water).
distance between the oil and water surfaces was set to less than 0.5 nm. Once the simulation started, the oil approached the water surface with contact at 30 ps because of attractive interactions, such as van der Waals forces. Then, the oil started to spread on the water surface. Finally, the oil formed a lens shape at around 1.0 ns of simulation time. After that, the oil droplet kept its shape apart from the effect of thermal fluctuations. Snapshots of toluene, heptane, and decane molecules after 5.0 ns of simulation are shown in panels a, c, 2739
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MD simulation, toluene molecules in the oil spread as a thin film over the water surface in about 1 ns, followed by spreading of the bulk oil. The film thickness of about 0.5 nm is comparable to the molecular size of toluene. In addition to toluene, heptane showed preferential accumulation at the threephase contact line relative to decane (Figure 3d). This indicates that results of the MD simulations successfully predict positive values of S for toluene and heptane. Values of S from eq 1 are negative, but this is because of limitations of the SPC/E model with respect to predicting values of the SFT of water, which are too low. MD calculations of the details of the molecular distributions across interfaces were also made for heptane/toluene binary mixtures against vapor and water. A snapshot of a unit cell of the oil/vapor interface for an equimolar heptane/toluene mixture is shown in Figure 5a. The partial density profiles are shown in Figure 5c. Weak accumulations of heptane occurred at the mixture/vapor interfaces. The final mole fraction of heptane in the bulk phase of the mixture (4 < z < 6 nm in Figure 5c) is around 0.4. This is lower than the initial value of 0.5 because of accumulation of heptane at the interface. MD calculations of heptane/toluene binary oil/water interfacial systems at different mole fractions in the oil phase were performed with initial mole fractions of toluene in the binary mixtures set at 0.1 increments from 0 (pure heptane) to 1 (pure toluene). A snapshot of a unit cell of an equimolar oil/water system is shown in Figure 5b. Partial density profiles are shown in Figure 5d. Accumulation of toluene was observed at the oil/ water interface. Although the simulation has shown that toluene accumulates at the oil/water interface and heptane accumulates at the oil/vapor interface, the accumulation of toluene plays a dominant role, as shown in Figure 3b. This can be explained by the larger difference of the IFTs for toluene− and decane/ heptane−water interface systems (35.2 versus 51.0/50.1 mN/ m, respectively). Furthermore, a relatively large difference (around 8.2 mN/m) of the SFT of toluene and heptane is
Figure 4. (a) Mole fraction profiles and (b) partial density profiles along the z axis averaged at the range of 9 < x < 11 nm, over 2.0−5.0 ns of simulation. The color code in panel b also applies to panel a. A small amount of oil molecules at z = 0 nm are resulting from the adsorption of the vaporized molecules from the oil drop. As shown in Figure 2, the simulation box is constructed by imposing periodic boundary conditions. When a molecule vaporizes and leaves the box in one direction, a periodic molecule will enter through the opposite direction and be eventually adsorbed in the liquid water surface of the opposite side.
the liquid phase as in the simulation, the vapor phase would be rich in heptane, as illustrated in Figure 4a. In general, components of an oil mixture that move preferentially to the three-phase contact line will spread on the water surface ahead of the bulk oil. For example, there is a thin film of toluene at the water surface (Figure 3b). During the
Figure 5. Snapshots of a 1:1 heptane/toluene binary mixture: (a) vapor and (b) water after 5.0 ns of simulation (orange, toluene; red, heptane; and blue, water) and corresponding partial density profiles along the z axis averaged over 2.0−5.0 ns of simulation for (c) vapor and (d) water. 2740
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(12) Kalaydjian, F. J.-M.; Moulu, J.-C.; Vizika, O.; Munkerud, P. K. Proceedings of the 68th Annual Techincal Conference and Exhibition of the Society of Petroleum Engineers; Houston, TX, Oct 3−6, 1993, SPE 26671. (13) Kantzas, A.; Chatzis, I.; Dullien, F. A. L. Proceedings of the Society of Petroleum Engineers (SPE) Enhanced Oil Recovery Symposium; Tulsa, OK, April 16−21, 1988; SPE 17379. (14) Ragil, K.; Meunier, J.; Broseta, D.; Indekeu, J. O.; Bonn, D. Phys. Rev. Lett. 1996, 77, 1532−1535. (15) Shahidzadeh, N.; Bonn, D.; Ragil, K.; Broseta, D.; Meunier, J. Phys. Rev. Lett. 1998, 80, 3992−3995. (16) Pfohl, T.; Riegler, H. Phys. Rev. Lett. 1999, 82, 783−786. (17) Bertrand, E.; Dobbs, H.; Broseta, D.; Indekeu, J.; Bonn, D.; Meunier, J. Phys. Rev. Lett. 2000, 85, 1282−1285. (18) Bertrand, E.; Bonn, D.; Meunier, J.; Segal, D. Phys. Rev. Lett. 2001, 86, 3208. (19) Ross, D.; Bonn, D.; Posazhennikova, A. I.; Indekeu, J. O.; Meunier, J. Phys. Rev. Lett. 2001, 87, 176103. (20) Rafai, S.; Bonn, D.; Bertrand, E.; Meunier, J.; Weiss, V. C.; Indekeu, J. O. Phys. Rev. Lett. 2004, 92, 245701. (21) Takamura, K.; Loahardjo, N.; Winoto, W.; Buckley, J. S.; Morrow, N. R.; Kunieda, M.; Liang, Y.; Matsuoka, T. In Crude Oil/ Book 2; Romero-Zerón, L., Ed.; InTech: Rijeka, Croatia, 2012. (22) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed.; Academic Press: Amsterdam, The Netherlands, 2011. (23) van Buuren, A. R.; Marrink, S.-J.; Berendsen, H. J. C. J. Phys. Chem. 1993, 97, 9206−9212. (24) De Coninck, J.; Blake, T. D. Annu. Rev. Mater. Res. 2008, 38, 1− 22. (25) Kunieda, M.; Nakaoka, K.; Liang, Y.; Miranda, C. R.; Ueda, A.; Takahashi, S.; Okabe, H.; Matsuoka, T. J. Am. Chem. Soc. 2010, 132, 18281−18286. (26) Mennella, A.; Morrow, N. R. J. Colloid Interface Sci. 1995, 172, 48−55. (27) Hess, B.; Kutzner, C.; van der Spoel, D.; Lindahl, E. J. Chem. Theory Comput. 2008, 4, 435−447. (28) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. J. Phys. Chem. 1987, 91, 6269−6271. (29) Alejandre, J.; Tildesley, D. J.; Chapela, G. A. J. Chem. Phys. 1995, 102, 4574−4583. (30) Klauda, J. B.; Brooks, B. R.; MacKerell, A. D. Jr.; Venable, R. M.; Pastor, R. W. J. Phys. Chem. B 2005, 109, 5300−5311. (31) Nose, S. Mol. Phys. 1984, 52, 255−268. (32) Parrinello, M.; Rahman, A. J. Chem. Phys. 1982, 76, 2662−2666. (33) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen, L. G. J. Chem. Phys. 1995, 103, 8577−8593. (34) Humphrey, W.; Dalke, A.; Schulten, K. J. Mol. Graphics 1996, 14, 33−38. (35) Vega, C.; de Miguel, E. J. Chem. Phys. 2007, 126, 154707. (36) Goebel, A.; Lunkenheimer, K. Langmuir 1997, 13, 369−372. (37) Donahue, D. J.; Bartell, F. E. J. Phys. Chem. 1952, 56, 480−484. (38) Yeung, A.; Dabros, T.; Masliyah, J. J. Colloid Interface Sci. 1998, 208, 241−247. (39) Moran, K.; Yeung, A.; Masliyah, J. Langmuir 1999, 15, 8497− 8504. (40) Rehfeld, S. J. J. Phys. Chem. 1967, 71, 738−745. (41) Aveyard, R.; Haydon, D. A. Trans. Faraday Soc. 1965, 61, 2255− 2261. (42) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley and Sons: New York, 1997.
responsible for the accumulation of heptane at the oil/vapor interface. Therefore, it is reasonable to ascribe the nonlinearity of the spreading coefficient of heptane and toluene binary solutions as a function of the toluene mole fraction (as shown in Figure 1) to both the accumulation of toluene at the oil/ water interface and that of heptane at the oil/vapor interface. No clear effect of heptane and decane mixtures in terms of the spreading coefficient was observed. This is likely due to similarities between heptane and decane. More discussions on the accumulation of lighter alkanes at the oil/vapor interface have been given.21
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CONCLUSION The spreading behavior of decane/toluene/heptane mixtures has been investigated by experiment and molecular simulation. Direct observations and predictions from IFT and SFT measurements for decane, toluene, heptane, and mixtures of these oils show that a drop of decane, when mixed with toluene and heptane, spreads on water. The spreading coefficients are highly nonlinear with respect to chemical composition of the oil. The MD simulations show accumulation of lighter alkanes at the oil/vapor interface and accumulation of aromatics at the oil/water interface. Accumulation of light alkanes at the oil/ vapor interface leads to lower SFT for the oil. Accumulation of aromatic and polar components at the oil/water interface explains why oil/water IFTs are lower than predicted by averaging values for individual components of the oil. From the composition at the line of contact of a bulk oil droplet, toluene is predicted to spread preferentially over the water surface ahead of the bulk oil.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Japan Oil, Gas and Metals National Corporation (JOGMEC) and Japan Petroleum Exploration Company Limited (JAPEX) for financial support and the valuable discussion with A. Ueda. Makoto Kunieda is grateful for a Japan Society of the Promotion of Science Research Fellowship through KAKENHI 226752.
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