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An All-Atom Molecular Dynamics Study of WaterDodecane Interface in the Presence of Octanol Baofu Qiao, and Wei Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10997 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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The Journal of Physical Chemistry

An All-Atom Molecular Dynamics Study of Water-Dodecane Interface in the Presence of Octanol Baofu Qiao,†,* Wei Jiang‡,* † Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA. ‡ Leadership Computing Facility, Argonne National Laboratory, Argonne, Illinois 60439, USA.

ABSTRACT In this work, classical atomistic molecular dynamics (MD) simulations were performed to investigate the structural properties of octanol at water/dodecane interface. Multiple combinations of simulation conditions and force fields were employed in order to obtain high fidelity interfacial structure. The analysis of noncovalent interactions shows that the convergence of interfacial octanol distribution requires a simulation time scale larger than 100 ns. Finite size effect diminishes with increasing dimension of simulation box both laterally and vertically. In the largest simulated systems with vanishing finite size effect, the additive CHARMM force field shows similar interfacial aggregation behavior with the DRUDE polarizable model. Self-assembly of octanol molecules at water/oil interface is observed, expected to reduce the free energy barrier between water phase and oil phase. This work provides an MD simulation benchmark for structural study of complex liquid/liquid interface with cosurfactant reducing interfacial tension between two immiscible phases.

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INTRODUCTION Liquid-liquid interfaces are of paramount interest due to their abundance and significant roles in fundamental research and technical applications, e.g., assembly of nanoparticles,1 functionalized carbon materials,2 colloidal self-assembly,3 ion transport,4-5 to name a few. Due to the sharp change in the dielectric permittivities of two immiscible liquids across the interface, the structures are greatly different from those in bulk solutions.6-8 Experimentally X-ray and neutron scattering techniques9 have been extensively employed in investigating the assembly of amphiphilic molecules at liquid-liquid interfaces, along with some other experimental techniques including sum frequency generation spectroscopies,10-12 nuclear magnetic resonance,5 and so on. However the molecular structures are still in debate due to the limited resolutions of experimental instruments. Complementary to experimental techniques, all-atom computer simulations have been proven valuable in a broad spectrum of aqueous13 and organic systems.14-15 Nevertheless the liquid-liquid interfaces are only limitedly investigated with computer simulations,6-7, 16-24 among which the interfacial tension25-27 was focused on. Furthermore, due to the high computing cost, systematic convergence analysis of allatom simulation was generally avoided. Alternatively, coarse-grained model has been employed to avoid the demanding need of high performance computing resources, at the expense of loss of quantitative or fine structure information. 28 In the present work, we aim to study structural properties of liquid-liquid interface with full capability of all-atom brutal-force molecular dynamics (MD) simulations, namely running long single-trajectory without sampling enhancement approaches. In specific, ternary solution octanol/water/dodecane is employed for its generality in microemulsions, for which short-chain alcohol molecules are frequently added as co-surfactants for fine tuning of microemulsions.5 Increasing number of studies on alcohol molecules have been reported recently on the novel structures of the so-called “surfactant-free” microemulsions in bulk solutions.

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Nevertheless our quantitative knowledge in terms of their

influences at the liquid-liquid interfaces is still limited. The present work is thus poised to be a benchmark of atomistic simulation in studying the weak self-assembly of alcohol at liquid-liquid interface. We expect that the results will pave the way towards more complex multi-phase liquid-liquid interface simulations for various engineering applications. From point of view of MD simulation, finite size effect and induced polarization due to heterogeneous interfacial structure have been long-standing issues for liquid-liquid interface simulations. In this report, a variety of systems were studied to calibrate the effects of finite size simulation box in liquid-liquid interface simulations, as well as the influences of atomistic (additive CHARMM and polarizable DRUDE) force fields. In the last two ACS Paragon Plus Environment

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decades, polarizable models have demonstrated advantage over the traditional additive force fields in simulating interface systems.32-33 A simulation with DRUDE model is thus performed to catch significant polarization effects that might be missed in additive force fields. It needs to be emphasized that this report focuses on interfacial structure investigation. A systematic kinetic study of interfacial self-assembly process, which demands sophisticated sampling algorithm such as local sampling enhancement, 34 transition path sampling, 35 etc., is our ongoing work.

SIMULATION METHODOLOGY Two sets of simulations were carried out to investigate the effects of finite size simulation box and force fields in the water/dodecane interface simulations. For the former, we simulated three systems, which are named as systems CHARMM-s1, CHARMM-s2 and CHARMM-s3 in Table 1. The composition ratio of octanol/water/dodecane molecules is fixed, with the simulation box dimension varied. Each simulation box suffixed with s1, s2 or s3 contains around 0.05 million atoms. In these three simulations, the CHARMM General Force Field (CGenFF),36 which has already been implemented in the MD package GROMACS,37 was employed for all the molecules with no modifications. The CHARMM TIP3P water model was used with the structures constrained using the SETTLE algorithm.

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GROMACS version 4.5.539 was employed for all the finite size effect

investigations. The force field parameters are also presented in the Supporting Information for completion. Table 1. Compositions and Dimensions of the Simulated Systems a system

#octanol

#dodecane

#water

LXY /nm b

LZ /nm b

CHARMM-s1

300 (1.0)

460 (1.6)

8000 (27.6)

4.7

~21.8

CHARMM-s2

300 (1.0)

460 (1.6)

8000 (27.6)

6.1

~13.0

CHARMM-s3

300 (1.0)

460 (1.6)

8000 (27.6)

7.2

~9.3

CHARMM-L

2400 (1.4)

3600 (2.0)

32000 (18.1)

10

~29.4

DRUDE-L

2400 (1.4)

3600 (2.0)

32000 (18.1)

10

~29.4

a) The overall concentrations of the components (M) are listed in the parentheses b) Equilibrium simulation box edge lengths in XY plane (LXY) and Z dimension (LZ).


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The initial structures were generated with the package Packmol.40 The octanol molecules were randomly distributed in the whole simulation box to eliminate any possible biased effect on the selfassembly process. Initially water and dodecane molecules formed separate water phase and oil phase, respectively (Figure 1a). The energy of the initial structure was first minimized using the steepest descent algorithm, followed by 1 ns pre-equilibration under the condition of constant pressure and temperature (NPT). Subsequent production runs were performed under NPT condition, with the temperature (298K) of octanol, water and oil separately coupled with the Nosé-Hoover algorithm41. The semiisotropic Parrinello-Rahman algorithm42 was applied to couple the pressure in the Z dimension (reference pressure PZ = 1 bar, compressibility of 4.5×10-5 bar-1 and relaxation time of 4 ps) while keeping the XY plane fixed with a compressibility of 0. Three-dimensional periodic boundary conditions (PBC) were used. The short-range Coulomb interactions were calculated up to 1.2 nm with the long-range interactions calculated using the Particle Mesh Ewald (PME) method.43-44 The shortrange van der Waals (vdW) interactions using the Lennard-Jones 12-6 potential were calculated up to 1.2 nm, along with the long-range dispersion correction for energy and pressure. All chemical covalent bonds were constrained by means of the LINCS algorithm,45-46 so that a leap-frog integration time of 2 fs was stably supported. Each simulation was performed for 200 ns with a saving frequency of 10 ps per frame to collect the simulation trajectory. Influences of atomistic force fields were investigated with another two simulations with significantly larger simulation box and around 0.3 million atoms (CHARMM-L and DRUDE-L in Table 1). Specifically the polarizable DRUDE force field47-49 was compared with the additive CHARMM potential.36 The DRUDE force field parameters of alkane and octanol molecule have been reported by Ansimov et al.50 and those of the SWM4-NDP water model by Lamoureux et al.51 Given the large sizes of the two „L‟ simulations, the greatly scalable package NAMD (version 2.10)52 was employed in double precision on Mira of Argonne Leadership Computing Facility. For each of the two systems, 4096 CPU cores were employed in performing the simulation. In the simulations of CHARMM-L and DRUDE-L, the non-bonded pair lists were searched up to the distance of 13.5 Å. The nonbonded switching function was turned on between 10 Å and 12 Å. The short-range Coulomb interactions were calculated up to a distance of 12 Å, with the long-range interactions calculated using the PME algorithm43-44 under the three-dimensional PBC conditions. The six-order interpolation was utilized with the tolerance of 10-6 and the grid spacing of 1 Å for the Ewald algorithm. The isothermal-isobaric ensemble was employed. The Langevin dynamics was used at the reference temperature of 298.15 K and the damping coefficient of 5 ps-1. The integration time step was ACS Paragon Plus Environment

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2 fs with all the covalent bond lengths constrained. The impulse multiple time step algorithm53 was utilized, where the non-bonded interactions were calculated every one time step along with every two time steps for the long-range electrostatic interactions. The Nosé-Hoover Langevin piston semiisotropic pressure coupling54-55 was employed to reach the optimal system density with PZ = 1 atm (damping time scale 50 fs and oscillation period 100 fs) while fixing the simulation box edge length in X and Y dimensions. This simulation was performed for 200 ns. Due to the large storage requirement, this simulation trajectory was saved at a frequency of 1 frame per ns. In the simulation on the system DRUDE-L, an integration time step of 1 fs was employed, and the non-bonded interactions were calculated every time step. The other simulation control parameters were the same as those in the CHARMM-L simulation.

RESULTS AND DISCUSSION 1. Finite Size Effects

Figure 1. (a) Snapshots of the octanol (highlighted) self-assembly at different simulation times, and (b) noncovalent interactions (Coulomb and vdW) between octanol and different molecules in the system CHARMM-s1.

In the small size simulations, as illustrated in Figure 1a with the system CHARMM-s1, the octanol molecules rapidly aggregated from random distribution into large micelles in aqueous phase in a few ns timescale. As expected, the weak polar headgroups of octanol molecules extend outwards and are stabilized by H-bonds with surrounding water environment. These micelles sojourn in water phase for 100 ns or so before they diffuse to the organic phase. These findings are consistent with the low


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solubility of octanol in water (540 mg/L at 298 K

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). In the oil phase, the octanol molecules form

polar-nonpolar self-assembly through H-bond interactions with neighboring octanol and water molecules and nonpolar interactions with dodecane molecule (Figure S1).57 Figure 1b exhibits the short-range noncovalent interactions between octanol and different molecules as a function of simulation time. In these calculations all the Coulomb and vdW interactions are calculated up to the atom-atom distance of 1.2 nm. The energy profiles are consistent with the snapshots in Figure 1a. Within the first 10 ns simulation all the energy profiles changed sharply, and gradually converged later on. Impressively, observable changes occurred at simulation time of ~110 - 120 ns, due to the „translocation‟ of those octanol-bearing micelles from the water phase to the oil (Figure 1a). Moreover, these calculations indicate that the vdW interactions dominate over the Coulomb interactions between octanol molecules, and between octanol and dodecane, whereas the Coulomb interactions dominate between octanol and water. These simulated results are in accordance with the fact that the Coulomb interactions play a more crucial role when polar molecules (water) are involved, whereas the vdW interactions are more significant for the non-polar (dodecane) and weak polar (octanol) molecules.

Figure 2. The last snapshots (200 ns) in systems (a) CHARMM-s1, (b) CHARMM-s2 and (c) CHARMM-s3. (d) The density profiles of the octanol oxygen atoms as a function of their coordinates along the Z dimension. z = 0 denotes the center of the oil phase on the basis of the dodecane carbon atoms. The density profiles of water and dodecane are provided in Figure S2.

To investigate finite size effect, the systematically increased lateral areas (X×Y) for the octanol disbribution at the water/oil interface are adopted, as shown in Table 1 and Figure 2a-c. To quantify the finite size effect, the density profile of octanol oxygens in the Z dimension is calcualted on the basis ACS Paragon Plus Environment

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of the simulation trajectories of 150 - 200 ns (Figure 2d). It can be seen that varying box lengths in XY plane greatly affects the density profiles. Firstly the peak height gradually decreases with increasing box length in XY plane. A striking finding is that in the system CHARMM-s1, a second aggregation layer at |z| ≈ 3.4 nm is stably formed beneath the primary water/dodecane interfacial layer at |z| ≈ 5.5 nm. This secondary peak becomes weaker, however still observable, in the system CHARMM-s2, and disppears in the system CHARMM-s3 with the largest lateral area in XY plane. 2. Effects of Atomistic Force Fields

Figure 3. Snapshots of (a) the initial and (b) the final (200 ns) structures of the system CHARMM-L. All octanol molecules are dissolved in the dodecane phase. (c) Convergence of interaction energies between octanol and different types of molecules in the system CHARMM-L.

The above solid evidence of how finite size effect induces misguiding interfacial behavior endorses the necessity of constructing large simulation box. In addition to the elimination of finite size effect, we ran two parallel simulations using the pairwise additive CHARMM and the polarizable DRUDE potentials. The initial and final structures are illustrated in Figure 3a and b, respectively, for the CHARMM-L system. To quantify the convergence of the simulations, we calculated the intermolecular interactions between octanol and different types of molecules in the CHARMM simulation. Figure 3c shows that the simulation reached equilibration at a timescale of 100 ns. Similar to Figure 1b, the electrostatic interactions play a predominant role in the interactions between water molecules and octanol, while the vdW interactions dominate the interactions between octanol and octanol, and between octanol and dodecane. In addition to the noncovalent interactions in Figure 3c, we calculated the average lateral area per alcohol as a function of simulation time (Figure S3), which also evidences the convergence of the simulation at around 100 ns. ACS Paragon Plus Environment

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The last simulation snapshots for the CHARMM and DRUDE force fields are presented in Figure 4a and b, respectively. The number density profiles of octanol oxygen atoms with different force fields were calculated and provided in the insets of Figure 4a and b. These calculations were performed with the trajectory generated during the simulation period of 160 – 200 ns. As shown in Figure 4, the calculated number density profiles resemble each other with no observable distribution difference. Indeed, as octanol molecule only has a weak polar headgroup interacting with water phase, it is natural that polarization effect plays a negligible role in the current study.

Figure 4. The last snapshots of (a) system CAHRMM-L and (b) system DRUDE-L. The number density profiles of the octanol oxygen atoms are inserted as blue and yellow curves, respectively. The standard deviations are presented as the error bars of the density profiles.

3. Structures at Liquid/Liquid Interface All the simulations evidence that the convergence of the interfacial adsorption of octanol molecules occurs at a timescale of around 100 ns. Compared with many biomolecule simulations of order of microseconds on GPUs, 100 ns timescale on CPU seems not long enough. However, it needs to be stressed that the current simulation involves over three hundreds of thousands atoms with full range of interactions. For such systems current serial computation on GPU doesn‟t show any time-to-solution advantage over massively parallel computing on large CPU cluster. Double-precision and large pairwise cutoff distance on CPU, which guarantees energy accuracy and conservation, is also an important factor to consider. On the other side, the simulated problem here is dominated by weak electrostatic interactions and vdW interactions. Compared to complex biophysical processes such as protein folding/unfolding, this problem doesn‟t possess any comparable complicated conformation ACS Paragon Plus Environment

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sampling and kinetics and therefore shows no necessity of microseconds or longer simulation. Note that to speed up the aggregation behavior, simulated annealing technique has been suggested recently by us5,

14, 57-60

and others.61 With the simple sampling acceleration technique, simulated co-

surfactant/surfactant self-assembly is observed occurring at timescale of even smaller than 100 ns.5 Retrieved from the trajectory of 160 – 200 ns, the strengths of the interactions in the whole systems roughly follow the order: vdW (octanol-dodecane) > vdW (octanol-octanol) ≈ Coulomb (octanolwater) > vdW (octanol-water) > Coulomb (octanol-dodecane) > Coulomb (octanol-octanol). The vdW interactions dominate between molecules of octanols, and between octanol and dodecane, which are ascribed to the weak polar feature of octanol and the nonpolar feature of dodecane molecules. With a high solubility of octanol in oil phase, strong vdW interactions are presented between octanol and dodecane and independent of the system size. As expected, the electrostatic interactions play a critical role in the interactions between octanol and water.

Figure 5. (a) Second order orientational parameter of octanol molecule and number density profile of octanol oxygen in the system CHARMM-L. Error bars (standard deviations) are included for the orientational parameters. (b) Definition of the angle

in Eq. (1), where Z represents the normal vector

of the water/dodecane interface.

From the point of view of free energy, the octanol layer plays a role of reducing free energy barrier between water and oil phases, driven by electrostatic interactions with water and vdW interactions with dodecane. The two interactions tend to align octanols at the interface. As a result of such an ordered orientation, the presence of positive Coulomb interactions between octanol molecules is observed, demonstrated in Figure 1b and Figure 3c. The ordered octanol layer indicates decrease of entropy, which is effectively compensated by the more negative interaction energy with water phase and oil phase to reduce the overall free energy. At any rate, the ordering of octanols is the critical factor for surface tension reduction. In this regard, we calculated the second order orientational parameters ACS Paragon Plus Environment

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( )

( )

∑[

( )

],

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(1)

where ( ) denotes the angle between the Z direction and the vector pointing from octanol oxygen to the terminal carbon on the same molecule (Figure 5b),

( ) is the number of octanol oxygen atoms

distributed in a slab between (z - 0.05) nm and (z + 0.05) nm with a bin width of 0.1 nm. Within this context a value of unit of O(z) stands for that octanol chains exactly parallel the Z direction, O(z) = -0.5 for perfect vertical orientation to Z direction, and O(z) = 0 for random orientation. The calculated result is shown in Figure 5a for the system CHARMM-L. Strong orientation of the octanol molecules was demonstrated at the interfacial regimes of 9 < |z| < 11 nm, where the two primary orientation peaks of octanol molecules can be clearly observed. The two orientation peaks indicate a tilt angle of approximately 35 °of the octanol molecules at the interface. It needs to be noted that due to the limited amount of octanols in the simulation box and self-assembly of octanols at water/oil interface, the overall concentration of octanol in oil is different from the „effective‟ concentration of octanols in the bulk dodecane phase. The amount of octanol molecules at water/dodecane interface is in equilibration with that in the bulk oil phase. That is, the distribution of interfacial octanol molecules is affected by both the lateral area and the “effective” concentration of octanol molecules in the oil phase. The effective bulk concentration of octanol in the simulations is estimated from the number of octanols distributed in the central region of the simulation box and the corresponding volume of the central region. Along z direction, the central region is defined as a slab between the upper position and the lower position of the minimum density (right adjacent to the primary distribution peaks) of octanol oxygen. In this context, the thickness of the central region is 18 nm in the CHARMM-L system (-9 < z < 9 nm, inset of Figure 4). The effective bulk concentrations are thus estimated as 1.1 M, 0.3 M, 0.03 M, 1.6 M and 1.7 M for systems CHARMM-s1, CHARMMs2, CHARMM-s3, CHARMM-L and DURDE-L systems, respectively. The interfacial area per octanol molecule is calculated as 0.30 and 0.35 nm2 for CHARMM-L and DRUDE-L systems, respectively (0.27, 0.28 and 0.35 nm2 for CHARMM-s1, CHARMM-s2 and CHARMM-s3, respectively). These values are in reasonable agreement with the experimental value of 0.24 nm2 per octanol at the dodecane/water interface.62 They also agree well with the reported values of 0.18 – 0.27 nm2 per octanol at water-air interfaces by a variety of experimental approaches63 (around 0.3 nm2 per hexadecanol at water-air interfaces64). From the density profiles of octanol oxygen atoms, the calculated full width at half maximum are ~ 0.6 nm, indicating a sharp interface in the systems investigated. 65


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CONCLUSIONS By systematically increasing the atomistic simulation box size and comparing the additive CHARMM force field and the polarizable DRUDE potential, we have obtained converged simulation results for octanol adsorption at water/oil interface. It is shown that well converged interfacial structures require simulation timescale of 100 ns or so with all-atom MD simulation. Effective elimination of finite size effect demands large system dimension and long MD trajectory with massively parallel computing, a challenging task for the majority of computational researchers. The simulated interfacial aggregation behavior with additive CHARMM potential and more accurate DRUDE polarizable model doesn‟t show considerable difference, due to the weak polar nature of octanol molecules. The calculated interfacial area of octanol at water-dodecane interface, ~ 0.30 nm2 per octanol molecule, is in reasonable agreement with the experiment value of 0.24 nm2 at water/dodecane interface. As a summary, by rigorously quantifying the convergence of the liquid-liquid simulations with high fidelity all-atom force fields and vanishing finite size effect, we expect that this work will provide an MD simulation benchmark for complex liquid/liquid interface with co-surfactants reducing interfacial tension between two immiscible phases.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Supporting figure on the octanol aggregates in oil phase, density profiles in the small systems, the interfacial area per octanol as a function of the simulation time and CHARMM and DRUDE force field parameters (PDF)

AUTHOR INFORMATION Corresponding Authors * Email: [email protected]; [email protected] ORCID B.Q.: 0000-0001-8870-5985


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Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences, under Contract DE-AC0206CH11357. The computing resources, provided on Blues, a high-performance computing cluster, operated by the Laboratory Computing Resource Center at Argonne National Laboratory, are gratefully acknowledged. This research used resources of the Argonne Leadership Computing Facility, which is a DOE Office of Science User Facility supported under Contract DE-AC02-06CH11357.

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