Molecular Dynamics Study of the Bulk and Interface Properties of

Article pubs.acs.org/JPCB

Molecular Dynamics Study of the Bulk and Interface Properties of Frother and Oil with Saltwater and Air Leebyn Chong,*,† Yungchieh Lai,† McMahan Gray,† Yee Soong,† Fan Shi,†,‡ and Yuhua Duan*,† †

National Energy Technology Laboratory, United States Department of Energy, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania 15236, United States ‡ AECOM, P.O. Box 618, South Park, Pennsylvania 15219, United States S Supporting Information *

ABSTRACT: For water treatment purposes, the separation processes involving surfactants and crude oil at seawater−air interfaces are of importance for the chemical and energy industries. Little progress has been made in understanding the nanoscale phenomena of surfactants on oily saltwater−air interfaces. This work focuses on using molecular dynamics with a united-atom force field to simulate the interface of linear alkane oil, saltwater, and air with three surfactant frothers: methyl isobutyl carbinol (MIBC), terpineol, and ethyl glycol butyl ether. For each frother, although the calculated diffusivities and viscosities are lower than the expected experimental values, our results show that diffusivity trends between each frother agree with experiments but the method cannot be applied for viscosity. Binary combinations of liquid (frother or saltwater)−air and liquid−liquid interfaces are equilibrated to study the density profiles and interfacial tensions. The calculated surface tensions of the frother−air interfaces are like that of oil−air, but lower than that of saltwater−air. Only the MIBC−air and terpineol−air interfaces agreed with our experimental measurements. For the frother−saltwater interfaces, the calculated results showed that terpineol has interfacial tensions higher than those of MIBC−saltwater. The simulated results indicate that the frother−oil systems underwent mixing such that the density profiles depicted large interfacial thicknesses. paper recycling.11−14 Research has been focused on various aspects of this technology, including surfactant selection and concentration,15 rising bubble behavior,16 surface froth stability,17 flotation unit designs,18 and particle properties and novel applications.6 As one of the newer applications, surfactant-induced froth can separate oil from water in a similar manner to how hydrophobic particles are separated in mineral recovery. Experiments have already moved forward to prove the viability of froth flotation in removing oil from water under a variety of conditions. Recovery of oil from sludge has reached up to 55 wt % in stirred batch flotation cells, with froth generated by sodium dodecyl benzene sulfonic acid, an anionic surfactant.19 For emulsified oil, where the oil droplets in water are 3−20 μm in diameter, there are qualitative observations of thin film spreading when oil droplets come into contact with fine bubbles of approximately 50 μm.20 Flotation columns operating in a continuous mode result in noticeable oil separation, but Watcharasing et al. encountered difficulties in achieving low interfacial tension and good foaming, reporting decreased separation when the surfactant concentration

1. INTRODUCTION The separation of oil from water is of interest for the chemical and energy industries from an environmental standpoint and arises in situations such as wastewater treatment and spills. The hydrophobic immiscibility of oil and water, plus the difference in density, allow for gravimetric separators, currently approved by the American Petroleum Institute, to remove oil from water by coalescing rising oil droplets via parallel plates and skimming the bulk oil from the water surface.1 Although these units are able to remove a majority of the oil from water, there are conditions where a homogenous mixture can form, such as in the cases of low oil concentrations, high temperatures, and the presence of stabilizing agents.2−4 These oil-in-water emulsions can be stable over long periods of time and may not passively separate by gravity. Current treatment technologies for these mixtures include dissolved air flotation, where air is dissolved in the water at high pressure and then released at atmospheric pressure to cause air bubbles to form and adsorb to the hydrophobic oil.5−7 The oily bubbles rise to the surface and are skimmed off. A similar mechanism is utilized in froth flotation. Surfactant-like molecules are added to the feed in dilute concentrations to promote the formation of a skimmable froth, which enhances the collection of hydrophobic species on the bubbles.8−10 Such froth flotation methods have been employed and well-studied in processes such as mineral beneficiation and © 2017 American Chemical Society

Received: December 28, 2016 Revised: March 13, 2017 Published: March 15, 2017 2788

DOI: 10.1021/acs.jpcb.6b13040 J. Phys. Chem. B 2017, 121, 2788−2796

Article

The Journal of Physical Chemistry B

Figure 1. Procedure for creating liquid−air and liquid−liquid interfaces from bulk liquid phases by using water−frother as an example.

exceeded the critical micelle concentration.21,22 To effectively separate oil from water using rising bubbles and foams, a strong understanding of the liquid−gas interface is beneficial in selecting an optimum surfactant and concentration. According to experimental studies, properties such as surface tensions and energies can be calculated as indicators of bubble formation, stability, and size.23−27 Multiscale numerical methods are capable of simulating the thin membranes of foams to extrapolate macroscopic behavior.28 Microscale approaches, such as the lattice Boltzmann method,29 are a way to capture shape deformation phenomena at complex liquid−liquid interfaces, whereas nanoscale techniques are able to utilize molecular features such as bonding and charges to explore surfactant interactions at interfaces.30−32 Liquid−air interface modeling has branched out into droplet surface tension studies, such as in the study of brine−air interfaces with organic matter performed by Salameh et al.33 Xu et al. conducted a molecular dynamics (MD) investigation into the head-groups of anionic surfactants at oil−water interfaces,31 and Nguyen et al. used MD in studying the air−saltwater interface with two alcohol frothers used in mineral flotation, methyl isobutyl carbinol (MIBC), and 1-hexanol.32 Both groups report that polarity and ionic states play important roles in the interfacial tension, energy, and thickness.31,32 Although previous works have proven the feasibility in applying MD models to ternary interfaces, there is a lack of reporting of nonionic surfactant frothers at binary interfaces. To investigate this area, we put forth research that profiles three frothers’/surfactants’ interactions with air, saltwater, and oil interfaces. The three surfactants possess different functional groups that are characteristic of various classes of frothers: MIBC, an alcohol; α terpineol, a cyclic alcohol that constitutes pine oil; and ethyl glycol butyl ether (EGBE), a simple polyglycol ether. To further investigate how the frother molecules interact with oil and seawater and to select good frother candidates from the vast materials for best separation performance for oil spills in seawater, in this study, we aim to determine the accuracy of a united-atom force field (FF) in modeling these frothers by comparing the simulated viscosities and liquid−air surface tensions with experiments. Of the frothers that are consistent with experimental data, we then measure the interfacial tensions and density profiles of the frother−oil and frother−saltwater binary systems.

vapor−liquid data from experiments. Their FF was intended to be transferable and thus can be applied to organic molecules of similar size and structure via combining rules. The TraPPE FF is a united-atom model similar to optimized potentials for liquid simulations (OPLS), where the CHx of organic liquids is represented by one moiety, thereby implicitly modeling the hydrogens.39 By uniting the hydrogens and carbon into one interaction site, the total number of sites and calculations is reduced, leading to the ability to model larger length and time scales without becoming more computationally intensive. The FF’s applicability to interfaces can be found in other studies, in which it has been implemented to model oil molecules.40,41 The simulated liquids are MIBC, terpineol, EGBE, C17 (17carbon linear alkane), 3.5 wt % NaCl water, and air. MIBC, terpineol, and EGBE are examples of known flotation frothers in mineral and wastewater treatment.42 C17 and saltwater serve as simplified representations of oil and seawater, respectively. Air is not modeled as an explicit liquid but rather a vacuum as done by other MD studies for liquid−air surfaces.43,44 To simulate water, the single point charge extended (SPC/E) model is implemented via SHAKE algorithm, which constrains the O−H bond lengths and H−O−H angles as 0.1 nm and 109.47°, respectively.45 For the Na+ and Cl− ions in SPC/E, LJ parameters were optimized by Horinek et al. with respect to solvation free energy, solvation entropy, and radial distribution functions.46 All remaining LJ parameters for pairs of unlike atoms are determined by Lorentz−Berthelot combining rules,47 as practiced during TraPPE FF development.35−38 A detailed description of our simulation method and the FF parameters are summarized in the Supporting Information. In addition to all of the liquid−air interfaces modeled, we carried out simulations of each frother near oil or saltwater. The binary liquid interfaces consist of a thin film arrangement with dimensions of approximately 12.4 nm × 9.3 nm × 9.3 nm with periodic boundaries for all three dimensions. A 6.2 nm thick region of a liquid is located at the center of the x axis and parallel to the yz plane. The remaining space of equal volume is occupied by a second liquid. The number of water/oil/ surfactant molecules in each volume was adjusted to correspond with their respective densities. For the MIBC− water interface, this is 2562 MIBC molecules and 18 000 H2O molecules. In the case of the terpineol−water and EGBE−water systems, the number of surfactants are 1954 and 2470, respectively. In addition to simulating the interfaces, bulk systems of each pure liquid were simulated to verify diffusion coefficients and viscosities with experiments. The mean-squared displacement across time is proportional to diffusion.48 Under a shear field, the resulting velocity profile is proportional to the flux, and this proportionality constant is the viscosity.49 Figure 1 displays the steps in constructing the liquid−air and liquid−liquid interfaces. The setup of the interfaces starts with bulk water, oil, or surfactant molecules in a box of 6.2 nm × 9.3

2. COMPUTATIONAL AND EXPERIMENTAL DETAILS 2.1. Simulation Methods. In this study, the classical MD simulations were conducted using a Large-scale Atomic/ Molecular Massively Parallel Simulator’s (LAMMPS)34 velocity Verlet time integration. The transferable potentials for phase equilibria (TraPPE) FF was applied to the oil and frothers. Developed by Siepmann et al.,35−38 this FF was parameterized by fitting bonds, angles, dihedrals, and nonbonded potentials to critical temperatures, saturated liquid densities, and other 2789


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Table 1. Simulated Diffusion Coefficients and Shear Viscosities of Bulk Liquids Compared to that of Experimental Measured Data this model liquid

T (°C)

ρ (g/cm3)

H2O oil MIBC terpineol EGBE

25 25 20 25 25

0.997 0.894 0.808 0.930 0.900

2.693 5.963 2.526 6.755 2.807

× × × × ×

10−9 10−11 10−10 10−11 10−10

nm × 9.3 nm. For water, the y and z dimensions are approximate to ensure the densities are 1−0.994 g/cm3 for temperatures 5−35 °C, respectively. The box is then gradually expanded over 10 nm along the x axis without rescaling the atom coordinates until the length along the x axis is 12.4 nm. This results in air−liquid interfaces that we analyzed in 1 ns production runs after 5 ns of equilibration. Time steps were 1 fs, and the equilibration and production runs were repeated five times with different random seeds. The setup for the liquid− liquid interfaces are a continuation from the liquid−air interfaces such that surfactant molecules are added to both sides of the water or oil film until the desired density is reached. These target densities are listed in Table 1 and originate from the literature values. Except for water, the densities are assumed to be constant for all temperatures. In both the liquid−air and liquid−liquid interfaces, each phase is assumed to be at its bulk density. A canonical ensemble was utilized for all systems keeping the number of atoms, volume, and temperature constant. A Langevin thermostat with a damping parameter of 100 fs was applied to maintain temperatures of 5, 15, 25, and 35 °C for each interface. Like the liquid−air interfaces, each liquid−liquid system was equilibrated for at least 5 ns followed by 1 ns production runs with time steps of 1 fs. The diffusion coefficients are proportional to the change in the mean-square distance traveled by the molecule’s center of mass over time by the following equation for three-dimensional systems50 ∂⟨r 2(t )⟩ = 6D ∂t

γ=

1 2

∫0

η (cP)

D (m2/s)

η (cP)

0.687 0.920 1.318 5.997 0.629

2.299 × 10−9

0.896 8.28 3.46 40.34 3.00

Lx

⎛ Pyy + Pzz ⎞ ⎜Pxx − ⎟ dx 2 ⎝ ⎠

(3)

Lx is the length of the simulation box normal to the interface plane and Pxx, Pyy, and Pzz are the diagonal components of the pressure tensor.32 The components are calculated from the total kinetic energy tensor and virial tensor that include force contributions from bonded and nonbonded potentials. Equation S7 from the Supporting Information summarizes the tensor calculation.51 In addition, the tail corrections for the surface tension were omitted to allow comparisons with existing MD results that did not utilize tail corrections when modeling saltwater−air interfaces.32 In other studies that modeled pure water−air interfaces, tail correction calculations were applied to account for the cutoffs to the LJ potential. Such corrections can add approximately 4 mN/m to the abovementioned surface tension calculations.52 2.2. Experimental Methods. The viscosity was measured using a MCR 302 rheometer (Anton Paar). The rheometer is equipped with a Julabo F32 heating system. The temperature was controlled at 20 °C for MIBC and 25 °C for H2O, North Slope crude oil, EGBE, and terpineol. Surface tension measurements were carried out with a Theta Optical Tensiometer. Briefly, the Tensiometer consists of a sample compartment, light source, lens, and image capture camera. A pendant drop of a sample was formed within the sample compartment using a syringe, and the drop was recorded by the image capture camera. The surface tension was then obtained by analyzing the droplet through application of the Young−Laplace equation.53,54 Measurements for each sample were repeated and then averaged. Measurements for samples at 25 °C or above were conducted using the Theta temperature control unit, which includes a heating plate in the sample compartment, and the measurements for samples at 5 °C were conducted by precooling the liquid to the temperature.

(1)

In eq 1, D is the diffusion coefficient and ⟨r2(t)⟩ is the meansquare displacement. To derive shear viscosity, a nonequilibrium MD method was used where the Müller−Plathe algorithm exchanged the momenta of an atom in the center region of the box with those of an atom at the edge.49 Regions are slab divisions of the box along the same axis, and thus the momentum swapping results in a velocity profile along one dimension. As per the following relation Px ∂v = −η x 2tA ∂z

experiment

D (m2/s)

3. RESULTS AND DISCUSSION 3.1. Diffusivities and Viscosities. To verify the applicability of the TraPPE FF for the frothers, the selfdiffusion coefficients and viscosities were calculated. To determine diffusivity and viscosity, simulations of pure liquids were equilibrated at their experimental liquid densities in cubed systems of 6.2 nm edges to model the bulk. Temperatures for bulk water, oil, terpineol, and EGBE were maintained at 25 °C, and MIBC was maintained at 20 °C. The difference in utilizing a lower temperature thermostat for MIBC was due to the available literature values of MIBC density. To confirm that equilibration had been reached, their diffusion coefficients were sampled across 1 ns time intervals. Figure 2 displays the diffusivities across 10 ns after equilibration. As one can see, the diffusion coefficients of terpineol and oil in the figure are similar and stable over time, which is characteristic for large molecules

(2)

the shear viscosity η is the ratio of the momentum flux to the velocity profile’s slope. In eq 2, A is the yz cross-sectional area and Px is the total momentum transferred over time t. Interfacial tensions and density profiles make up most of this work’s results and are of importance because they can provide insight into how frothers affect saltwater, oil, and air interface behavior. Surface or interfacial tension was calculated from the pressure tensor 2790


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increase of molecular activity, as suggested by the Eötvös rule57−59 γV 2/3 = k(Tc − T )

(4)

where γ is the surface tension, V is the molar volume, Tc is the critical temperature, and k is a constant valid for almost all substances. The simulated surface tensions for saltwater, oil, MIBC, terpineol, and EGBE are compared with those from experiments, as shown in Figure 3, and the exact numbers are

Figure 2. Diffusivity across simulated time for bulk liquids at 25 °C (20 °C for MIBC).

due to shorter displacements of their center of mass. For water, MIBC, and EGBE, the diffusivities exhibit higher diffusivities than those of terpineol and oil due to larger displacements of smaller molecules from their initial positions. The calculated diffusivities and viscosities for the equilibrated bulk systems are summarized in Table 1 along with values from our experiments and the literature. The table shows that our calculated viscosities are underestimations of the experimental values. Although the SPC/E water diffusivity and viscosity point to faster dynamics than that of experimental values, the results are more in line with measurements from other computational works. An example of this is SPC/E water, where the simulated diffusivity and viscosity were found in the literature to be 2.7 × 10−9 m2/s and 0.66 cP, respectively.55,56 These values are less than the experimental values in Table 1 but are consistent with our MD calculated results. The disagreement between theoretical and experimental viscosities signifies the imperfect fit of the TraPPE FF for simulating transport phenomena. However, the viscosity trends in experiments and MD are consistent such that terpineol > MIBC > EGBE. Additionally, the substantially higher experimental viscosity of terpineol relative to MIBC and EGBE is reflected in our calculated viscosities. The density chosen for C17 is equal to the density of the light crude North Slope oil used in experiments, which is 0.8268 g/cm3. Because of the complex mixture of crude oil, the viscosity of our C17 alkane is markedly lower than the viscosity range of light crude. During the equilibration of EGBE, the frequency of high potential overlaps rendered trajectory sampling unobtainable. Despite several relaxation strategies, bulk EGBE failed to equilibrate when its charges were not reduced. For a glycol ether, such as EGBE, the TraPPE FF normally includes partial charges on the oxygens and neighboring carbons. After observing the Coulombic potential acting as the dominant contributor to the total energy, we halved the partial charges for atoms of EGBE to resolve the equilibration issues. This adjustment was applied to both bulk and interface systems. 3.2. Liquid Interface with Air. The experimental results show that the viscosity decreases with increasing temperature for all liquids. This is because cohesive forces decrease with an

Figure 3. Liquid−air surface tensions from simulation and experiments.

Table 2. Calculated Interfacial Tensions of Binary Systems (mN/m)a

a

liquid A

liquid B

saltwater

air

oil

air

MIBC

air

terpineol

air

EGBE

air

oil

saltwater

MIBC terpineol EGBE MIBC terpineol EGBE

saltwater saltwater saltwater oil oil oil

5 °C 61.16 (73.99) 21.1 (20.79) 17.77 (23.37) 26.48 (25.07) 15.49 (27.28) 60.87 (52.48)c 8.05 20.66 57.14 −6.16 2.51 6.04

15 °C 62.15 (73.5)b 14.94 19.49 26.85 16.72 65.15 (51.86)c 16.5 33.02 64.53 −9.29 −20.05 12.95

25 °C

35 °C

59.96 (72.7) 18.46 (20.27) 16.39 (22.64) 17.63 (24.5) 10.95 (26.88) 66.2 (51.24)c 15.16 19.39 61.94 −2.9 3.72 1.93

59.89 (71.39) 14.01 (19.63) 20.03 (22.14) 17.68 (23.85) 14.14 (26.53) 57.69 13.96 10.34 48.21 4.31 −5.34 −2.59

Values in parentheses are from experiments. bRef 62. cRef 63.

included in Table 2. On the basis of this chart, oil and frother surface tensions do not display a clear pattern with respect to temperature even though the experimental measurements indicate that a higher temperature leads to a lower surface tension. Saltwater, MIBC, and EGBE surface tensions were calculated and found to be lower than experimental values. However, the graph displays a noticeably higher surface tension 2791


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Figure 4. Equilibrated interfaces at 25 °C of (a) oil−saltwater and (b) MIBC−saltwater plus their (c, d) density profiles, respectively. Systems contained water (red and white), NaCl (purple and green), MIBC (blue and yellow), and oil (brown).

calculated surface tensions appear to fluctuate, whereas the experimental values decrease by less than 1 mN/m. Such a surface tension change may be too small to be accurately resolved by MD simulations. 3.3. Oil and Frother Interface with Saltwater. Figure 4 illustrates the trajectories and density profiles of the oil− saltwater and MIBC−saltwater interfaces. The thin film in the center is water with dissociated sodium chloride ions. Flanking the water are oil molecules (Figure 4a) or frother molecules (Figure 4b). In both cases, an interface develops with a width that is wider for the frother−saltwater systems than the oil− water system. The corresponding density profiles of Figure 4c,d reinforce this observation. They show the interface of oil−water to be clearly defined with a water density gradient that is 11.081 Å wide on average when measured from the minimum density to the maximum density. In contrast, the interface with water widens to approximately 23.745 Å if the oil is replaced with a frother like MIBC. This average interfacial thickness becomes 18.996 Å for terpineol−saltwater and decreased further to 12.664 Å for EGBE−saltwater. These differences in density profiles for terpineol and EGBE are viewable in Figure S1. For all interfaces, the sodium chloride ions did not diffuse out of the water phase. Additionally, pair correlation functions or radial distribution functions provide insight into the packing behavior at the

for saltwater over the frothers in both simulations and experiments. For saltwater−air, our calculated surface tensions were approximately 10 mN/m less than the anticipated 72 mN/m. However, our values are consistent with other literature reports that used MD to calculate the saltwater−air surface tension as 60.5 mN/m.32,52,60,61 Additionally, we observe a small increase in surface tension of 2.26 mN/m from 35 to 15 °C, which is close to the increase of 3.09 mN/m found in standard tables.62 The frother surface tensions are of greater interest because there is a lack of existing MD data on MIBC, terpineol, and EGBE. When compared to values from our simulations, the MIBC’s experimental surface tension is within our theoretical surface tensions’ maximum standard error of 4.78 mN/m. Our calculated surface tensions for terpineol also show good agreement with experimental values, especially at the low temperatures 5 and 15 °C. However, EGBE’s calculated surface tension is particularly lower than the experimental surface tension resulting in a disagreement with the experimental trend: EGBE surface tension should be slightly higher than the terpineol surface tension. A possible reason is due to the reduced charges we applied to EGBE molecules. Thus, the discrepancy of EGBE’s surface tensions should also be expected. With regards to the frothers’ surface tension relation to temperature, our calculated results do not fully follow the experimental trend. From 35 to 5 °C, the frothers’ 2792


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The Journal of Physical Chemistry B interface. The pair correlation function of the oil and water interface in Figure 5 focuses on the C−C pairs for oil molecules

Figure 5. Pair correlation function of the oxygens of water and carbons of oil for the saltwater−oil interface at 25 °C.

Figure 6. Saltwater−liquid interfacial tensions for various temperatures.

and the O−O pairs for H2O molecules. The pair correlation function for O−O in water follows the profile of a typical LJ liquid of spherical particles with a large peak at 3.166 Å representing how most of the nearest neighbor water molecules are located at the LJ distance parameter for the SPC/E water model. At longer pair distances, the distribution converged to a value of two because the function is normalized with respect to water’s number density. With half the box containing water and the other half containing oil, the convergence of the function to two is expected. The lack of distribution peaks beyond the nearest water neighbor is expected for LJ liquids with little or no order to the structure. For the oil pair correlation function, the C−C distribution shows the effect of linear polymer chains. Because of the rigid bond lengths of 1.54 Å, bonded carbons are not included in the distribution. We observe the first distribution at approximately 3.75 Å, which is consistent with the LJ parameters; it has a low count likely caused by the volume exclusion of the neighboring bonded carbons sterically hindering additional carbon neighbors. Subsequent peaks may be attributed to intramolecular carbons that are two or more bonds away in separation from the reference carbon. Using the pressure tensor method, the interfacial tensions between saltwater and the frothers were calculated and are depicted in Figure 6 and listed in Table 2. On the basis of this chart, the oil−saltwater interfacial tensions are relatively equal to the saltwater−air surface tensions in Figure 3, which is to be expected for immiscible or separated species. Furthermore, frother interfacial tensions appear to follow a trend where MIBC < terpineol < EGBE. The MIBC−saltwater interfacial tensions are 50 mN/m, like the oil−saltwater and saltwater−air interfaces. Thus, EGBE is anticipated to maintain a smaller interface contact with saltwater, MIBC, and terpineol. This is supported by density profiles where a noticeably thicker width of interfacial mixing is observed for MIBC−saltwater than that of EGBE−saltwater. As mentioned earlier, the average width of the water interface follows EGBE < terpineol < MIBC, which is the same trend as interfacial tension. A wider interfacial thickness correlates with a lower interfacial tension. In addition, the effect of temperature on frother interfacial tensions appears to be mostly consistent with experiments where the interfacial tensions trend higher for

lower temperatures. However, the interfacial tensions at 5 °C do not maintain this pattern. Although unexpected, the cause may be attributed to the limitation of TraPPE because the FF was parameterized with respect to vapor−liquid equilibrium data at temperatures above 15 °C. 3.4. Frother Interface with Oil. The frother−oil interface was also simulated to account for all possible interfaces that can occur during froth flotation. However, we observed significant miscibility for all frothers with oil such that negative interfacial tensions, as shown in Table 2, were calculated. Negative interfacial tensions should not be feasible, and their presence indicates the lack of a well-defined interface. Of the frother−oil interfacial tensions calculated, EGBE−oil had one negative interfacial tension, terpineol−oil had two negative interfacial tensions, and MIBC−oil had three negative interfacial tensions. These values demonstrate that there was no clear effect caused by temperature. Figure 7 illustrates the mixing that occurred for MIBC and EGBE in the form of snapshots and density profiles. As Figure 7a shows, there was significant diffusion of each phase into one another. The density profile in Figure 7c depicts a gradient that is almost half the width of the oil film. Interestingly, EGBE−oil systems exhibited less mixing than MIBC−oil systems. In comparing Figures 7a,b, EGBE does not appear to be diffusing into the oil phase as clusters or lone molecules, as seen in the MIBC−oil system. The density profile of EGBE−oil in Figure 7d reinforces this observation with a gradient that is approximately 40 Å and does not encompass half the width of the oil phase. As shown in Figure S2, the interfacial thickness for terpineol appeared to be slightly wider than that of EGBE−oil but thinner than that of MIBC−oil. This would suggest that terpineol is more miscible in oil than EGBE but less miscible than MIBC. For a hydrophobic thin film, such as oil, these differences between EGBE and MIBC show the possible role of electrostatics in the stability of an interface. Unlike MIBC, the more even distribution of partial charges on EGBE molecules likely diminished the hydrophobicity of the molecule and resulted in less mixing in oil than MIBC. In the case of EGBE−saltwater, the hydrophilic nature of EGBE is not sufficient to lead to mixing, as indicated by its high interfacial tension in Figure 6. It is probable that the difference in the charge density between water and EGBE is 2793


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Figure 7. Equilibrated configurations at 25 °C of (a) MIBC−oil and (b) EGBE−oil with their respective (c, d) density profiles. Systems contained frother (blue and yellow) and oil (brown).

diffusivity results are of the same order of magnitude as experiments and reflect the trends for each frother. Although complete agreement was not achieved, the results serve to reflect the limitations of the TraPPE FF for viscosity. Instead, we found the TraPPE FF was more suitable for modeling interfaces based on our surface tension measurements. To validate our surface tension calculations, we next modeled interfaces of each liquid with air and compared the results to our experimentally measured surface tensions. Although our EGBE surface tensions were lower than what was measured, we achieved good agreement with experimental results for oil, MIBC, and terpineol. Because of variations in the surface tension across temperatures, the MD simulated results did not explicitly show the decreasing surface tension with increasing temperature that was seen in experiments. Next, we simulated saltwater interfaces with oil and frothers as part of the liquid− liquid interfacial tension calculations. In this configuration, we found that EGBE−saltwater had higher surface tensions and smaller interface thicknesses than terpineol−saltwater and MIBC−saltwater. Finally, we performed frother−oil modeling to better understand the interface stability. Density profiles reveal significant mixing for all frothers with oil such that practical interfacial tension calculations could not adequately characterize the interface. The differences in miscibility of the

large enough to maintain hydrophobic−hydrophilic interactions. Terpineol’s distribution of partial charges makes it more like MIBC than EGBE. However, terpineol’s observed degree of mixing in oil is not as pronounced as MIBC in oil. Thus, other aspects such as sterics, may explain the difference. As we have found earlier, MIBC and terpineol differed in viscosity and diffusivity by an order of magnitude.

4. CONCLUSIONS In this work, we simulated various interface configurations for saltwater, oil, MIBC, terpineol, and EGBE using classical MD with the TraPPE FF. The TraPPE united-atom model affords us sampling of longer time scales on larger length scales by coarse-graining the hydrogens of aliphatic carbons. This allows us to apply bonded and nonbonded potential parameters to the oil and the frothers. We adopted a simple structure of oil as a 17-carbon alkane to act as representative of light crude due to the similarity in molecular weight. MIBC, terpineol, and EGBE were selected as frothers because they are simple molecules that represent the alcohol, cyclic alcohol, and glycol ether chemical classes of common flotation frothers. We first simulated bulk systems to calculate the diffusivities and viscosities of each pure liquid. These results revealed faster dynamics than what was measured in experiments. Our 2794



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frothers with oil and the frothers with saltwater reveal the overall hydrophobic nature of the frothers. Of the three frothers tested, only MIBC and terpineol were shown to have comparable liquid−air surface tensions with experiments, which supports the applicability of the TraPPE FF for these two frothers. As the MIBC−saltwater interfacial tensions are lower than the terpineol−saltwater interfacial tensions, an emulsion of MIBC in saltwater is theoretically more stable than that of terpineol in saltwater. Further conclusions regarding oil adsorption to air bubbles and froth will require modeling saltwater−air interfaces with varying surface concentrations of frother and oil. So far, we have only explored the binary interfaces of saltwater−frother, oil−frother, and frother−air. During froth flotation, the presence of three phases, frother− oil−saltwater, will be common and should be the next point of interest to develop our understanding of froth stability and improve oil separation.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b13040. Details on MD method and FF parameters; additional interface figures and density profiles (PDF)

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 412-386-7524 (L.C.). *E-mail: [email protected]. Tel: 412-386-5771 (Y.D.). ORCID

Leebyn Chong: 0000-0003-4881-9210 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Bureau of Safety and Environmental Enforcement (BSEE) Acquisition Operations Branch on Oil Spill Response and Research program with funding number E15PG00032. L.C. would like to thank the support from the National Energy Technology Laboratory Research Participation Program, sponsored by the U.S. Department of Energy and administered by the Oak Ridge Institute for Science and Education. He would also like to thank Dr. Dan Sorescu for his advice and suggestions regarding the simulations. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. 2795


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Molecular Dynamics Study of the Bulk and Interface Properties of

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