Adsorption and Orientation of Ionic Liquids and Ionic Surfactants at

Article pubs.acs.org/JPCC

Adsorption and Orientation of Ionic Liquids and Ionic Surfactants at Heptane/Water Interface Amin Reza Zolghadr,* Mohammad Hadi Ghatee, and Ali Zolghadr Department of Chemistry, Shiraz University, Shiraz 71946-84795, Iran S Supporting Information *

ABSTRACT: Molecular dynamics simulation of heptane/ionic-liquid/water system was performed to study the effect of hydrophobic and hydrophilic ionic liquids (ILs) on the interfacial structure of heptane/water as a model for oil/water systems. The results are compared with the simulated water/sodium-dodecyl-sulfate (SDS)/heptane interface. Also, the self-assembly and orientation of ILs and SDS molecules at heptane/vapor interface are studied. We observed that the behavior of these surfactants at heptane/water and heptane/ vapor interfaces is very different. The computed density profiles provide a detailed view of the interfacial structure and a route to discuss quantitatively how the oil and water phases organize the surfactant molecules. The effect of ILs [Cnmim]Cl and [Cnmim]PF6 (with n = 4, 8, and 12) and SDS on the interfacial tension of heptane/water was simulated and compared at T = 343.15 K. The results indicate that ILs with long alkyl chain could behave similar to a conventional surfactant.

1. INTRODUCTION Interfaces, at which different phases are in close contact, are of special interest because the properties of the molecules at the interface can differ decidedly from those in the bulk. Interfacial tension as an equilibrium thermodynamic property is significantly important in various areas of chemistry, biology, and technology. For instance, in enhanced oil recovery techniques, the residual oil will be mobilized if the capillary forces are reduced because of interfacial tension reduction during surface-active agents flooding into the oil reservoir.1,2 The structure of liquid/liquid interface plays an important role in many applications, such as solubility, liquid−liquid extraction process, and multiphasic catalytic reactions. Room-temperature ionic liquids (ILs) are receiving considerable attention because of their unique properties, such as negligibly low vapor pressure, potential application as environment-friendly replacements for the more toxic solvents, wide electrochemical windows, and reaction media used today in the chemical industry. The growing interest in a sustainable, “green” chemistry has led to an amazing interest in these organic salts.3,4 ILs also exhibit excellent surface/interface activity and micelle formation because they possess amphiphilic structure with two hydrophilic and hydrophobic portions. Such molecules have an affinity for both the oil and the water phase and are therefore highly surface active. A rapidly developing application area for ILs is that of twophase homogeneous catalytic reaction, where one phase containing the catalyst is immiscible with the second phase containing the reactant and products. Such a catalytic reaction is believed to occur at the interface between the IL and the organic phase and should be dependent on the access of catalyst to the surface and the transfer of material across the interface.5 A clear understanding of the mechanisms behind © XXXX American Chemical Society

these processes requires a more detailed examination of the liquid−liquid interfaces. Molecular dynamics (MD) has been a useful tool to study surface between two fluids allowing atomic-level insight into the structure−property relationship. For example, using fully atomistic MD simulations, we have studied bulk and surface properties of polar liquids6 and ILs.7 The air−liquid and liquid−liquid interfacial structure of ILs,8,9 normal alkanes,10 and ILs/alkane systems11 are relatively rich in the literature, but a few recently published experimental works have studied the effect of ILs as a surface-active agent at alkane/water interface. Recently, Ayatollahi and coworkers have proposed 1-dodecyl-3-methylimidazolium chloride ([C12mim]Cl) as an effective interfacial tension reducing agent.12 Hemmateenejad et al. have studied and compared the aggregation behavior of this IL and a surfactant having the same aliphatic chain, dodecyl trimethylammonium chloride (DTAC), in methanol−water binary solvents (with different mole fractions) using conductometry.13 They found that similar to the surfactant DTAC, the IL [C12mim]Cl forms micellar aggregates in pure water and methanol as well as in their binary mixtures. The anionic surfactant sodium dodecyl sulfate (SDS) is one of the most widely used detergents, with many applications in industry, science, and technology. SDS has been studied with sum frequency generation (SFG) at the air/water interface,14,15 CCl4/water interface,16 and nanoscopic oil droplets in water.17 In a common planar oil/water system such as bulk nhexadecane in contact with bulk water, the surfactant SDS Received: June 17, 2014 Revised: August 1, 2014

A

dx.doi.org/10.1021/jp506012x | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Optimized structure of 1-alkyl-3-methyl-imidazolium chloride ([Cnmim]Cl) with n = 4, 8, and 12 including the atom’s label. The optimized structure of [Cnmim]PF6 and SDS molecules is shown in the Supporting Information (Figure S1).

will reduce the interfacial tension from 52 to 10 mN·m−1 by populating the interface.18 Molecular modeling approaches such as MD and Monte Carlo (MC) simulations have been extensively carried out to investigate SDS both in solution and at interfaces.19,20 Accordingly, the present study was conducted, for the first time, to study the water/IL/heptane interface from a molecular point of view and compare it with the well-known water/SDS/ heptane system. In addition, detailed studies on the structures and dynamics of the ILs and SDS monolayer at the heptane/ vapor surface were performed by MD simulations. The aim of the present work is to ascertain in detail how pure hydrophilic and hydrophobic ILs are distributed between aqueous and normal alkane (heptane) phases and how the distribution depends on alkyl chain length of ILs. The distribution behavior is related to the occurrence of interfacial tension minima, and the orientation of surfactant molecules is analyzed by using the bivariate orientational analysis approach. Three hydrophilic ILs, 1-alkyl-3-methylimidazolium chloride, [Cnmim]Cl, and three hydrophobic ILs, 1-alkyl-3-methylimidazolium hexafluorophosphate with n = 4, 8, and 12 representing butyl, octyl, and dodecyl alkyl groups, respectively (see Figure 1 and Figure S1 in the Supporting Information), were simulated to determine the molecular basis of microscopic and macroscopic behaviors. Additionally, because of the great dependence of interfacial behavior on the anion type as well, we aim to study detailed structure and orientational pattern of molecules at liquid−vapor interface of these ILs. To our knowledge, this work is the first systematic study of ILs as surface-active agents at the alkane/water interface from a molecular point of view. In Section 2, computational methods containing density functional theory calculations, force field parameters, and details of MD simulation are presented. In Section 3, the results of interfacial tensions, dynamical, structural, and orientational properties are presented and discussed. The concluding remarks are presented in Section 4.

for all molecules are shown in Table S1 in the Supporting Information. In the case of [Cnmim]Cl, the charge of chloride anion is roughly the same for ILs with butyl, octyl, and dodecyl groups and is equal to −0.850e, −0.848e, and −0.849e, respectively. For [Cnmim]PF6, the charges on the P (2.544e) and each F (−0.587e) atoms of PF6− anion are exactly the same for all ILs. The results of NBO calculations indicate that carbon and nitrogen atoms of the imidazolium ring are negatively charged, with the exception of CR. In addition, all hydrogen atoms are positively charged. Force-Field Parameters. For the cationic part of ILs, the force field is in the form of explicit fully flexible all-atom force field developed by Canongia Lopes et al.25 The electrostatic charges for dialkylimidazolium salts have been taken from our DFT calculations described in the previous section and Table S1 in the Supporting Information. The intramolecular interaction potential has the form V intramolecular = Vbond + Vbend + Vtorsion

(1)

The bond stretching is represented by harmonic potentials

∑

Vbond =

bonds

kb (ri − req)2 2

(2)

where ri is the bond distance, req is the equilibrium bond length, and kb is the force constant of the bond harmonic oscillation. The bending potentials are represented by harmonic functions of the form

∑

Vbend =

angles

kθ (θ − θeq)2 2

(3)

where kθ is the force constant and θeq is the equilibrium value of the angle. The torsional motion is described by cosine-type potential function of the form 3

Vtorsion =

∑ ∑ dihedrals i = 1

2. COMPUTATIONAL METHODS Density Functional Theory Calculations. The structure of heptane, ILs, and SDS molecules was optimized by density functional theory (DFT) at the B3LYP/6-311++G(d,p) level of theory using the Gaussian 03 program.21 The optimized structures were checked by harmonic vibrational frequencies22 to ensure the optimized structures are minima on the potential energy surface.23 We allocated a new set of partial charges for heptane, ILs, and SDS by population analysis carried out using the natural bond orbital (NBO) method24 implemented in Gaussian 03. The net charge for heptane, a single IL ion pair, and SDS molecule is zero in all cases. Values of partial charges

Vi [1 + ( − 1)i − 1 cos(i Φ)] 2

(4)

Intermolecular interactions, including the atom−atom (12− 6) Lennard-Jones (LJ) potential for the van der Waals interactions and the Columbic electrostatic interactions between point charges centered on the atoms, are described as ⎧ ⎡⎛ ⎞12 ⎛ ⎞6 ⎤ ⎫ qiqj ⎪ σij ⎥ ⎪ ⎢ σij ⎜ ⎟ ⎜ ⎟ ⎬ ⎨ − + 4 ε ∑ ij⎢⎜ ⎟ ⎜ ⎟ ⎥ r ⎝ rij ⎠ ⎦ 4πε◦rij ⎪ j>1 ⎪ ⎩ ⎣⎝ ij ⎠ ⎭

N−1 N

V

intermolecular

=

∑ i=1

(5)

The values of the parameters of eqs 1−5 were taken from refs 25 and 26 for ILs. (See Table S2 in the Supporting B



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Information.) εij is the energy minima for the LJ interactions of atoms i and j, and σij is their interatomic separation at zero potential. qi and qj are the charges on atoms i and j, respectively, and εo is the dielectric permittivity constant of the vacuum. The cross-term parameters εij and σij were given by combining and mixing rules, εij = (εiiεjj)1/2 and σij = 1/2(σii + σjj), respectively. Parameters employed to model the intramolecular and intermolecular interactions of heptane were obtained from the all-atom OPLS force field of Jorgensen and coworkers.27 Force-field parameters for the SDS surfactant headgroup (SO4−) atoms were taken from Schweighofer et al., 28 and the hydrocarbon chain was described by the OPLS force field.27 The Lennard-Jones parameters for Na+ cation was taken from Dang model.29 (See Table S3 in the Supporting Information.) Water was modeled using the SPC/E force field.30 Simulation Details. We simulated 14 different systems to investigate the interfacial structure of three hydrophilic ILs ([C4mim]Cl, [C8mim]Cl, and [C12mim]Cl), three hydrophobic ILs ([C4mim]PF6, [C8mim]PF6, and [C12mim]PF6) and an ionic surfactant (SDS) adsorbed at the heptane/water interfaces and heptane/vapor surfaces. The simulations were performed at ambient pressure (1.01325 × 105 Pa) using the DL_POLY program version 2.17.31 The equations of motion were solved using Verlet-Leapfrog integration algorithm under the periodic boundary conditions. The Columbic long-range interactions were calculated using Ewald’s method with a precision of 1 × 10−5. The potential cutoff distance value of 15 Å was used for each simulations. First, a single heptane molecule with a geometrical structure optimized via DFT calculation was replicated to obtain a simulation box containing 250 heptane molecule to construct each liquid system. The simulation had to begin with short time step between 1 × 10−4 and 5 × 10−4 ps for 200 ps. Then, it was followed by time step of 1 × 10−3 ps. After the initial equilibration with short time step, the temperature of the system was increased at intervals of 20 K up to 400 K. At each temperature, the simulation was performed for 500 ps under constant NPT conditions using the Nosé−Hoover thermostat/barostat algorithm32,33 and the modification of Melchionna et al.,34 as implemented in the DL_POLY program. Then, the temperature of the system was decreased to 300 K at steps of 50 K. The equilibrated heptane ensemble was extended in the z direction to 100 Å to create a heptane slab with the final thickness of ∼56 Å. After this preliminary equilibration run, 24 surfactant molecules were added over both slab surfaces, and the free space of the box was filled with 1000 water molecules. The heating/cooling procedure simulations were continued at different temperatures, from 300 to 400 K. At each temperature, 500 ps was allocated for system equilibration under constant NVT conditions with the Nosé−Hoover thermostat, followed by 500 ps under constant NPT condition. The relaxation times for the thermostat and barostat were 0.1 and 2.0 ps, respectively. The final temperature of the systems was kept at 343.15 K, and an additional 5 ns equilibration simulation was performed for each system. Finally, MD simulations were extended for additional 6 ns collecting statistical data under NVT condition. Then, the water molecules were removed from heptane/ surfactant/water systems and the simulation was continued up to 6 ns for each systems to simulate the heptane/surfactant monolayer at liquid/vapor surface.

3. RESULTS AND DISCUSSION Interfacial Tension. Perhaps the most important macroscopic property defining an interfacial system is the interfacial tension. Prediction of surface tension would be of great interest in surfactant research. Interfacial tension can be calculated directly in MD simulations using the so-called virial route, which relies on relating interfacial tension, γ, to the integral of the components of the pressure tensor over the interfacial plane.35 The inhomogeneities in pressure due to the interface produce a finite value of tension. Using this method, the interfacial tension is proportional to the integral of the difference between the diagonal components of the pressure tensor36,37 γ = −bz(Pxx + Pyy − 2Pzz)/4

(6)

where bz is the length of the simulation box along the z axis, for example, along the principle slab axis perpendicular to liquid/ liquid interface, and Pxx, Pyy, and Pzz are the principal components of the pressure tensor. The interfacial tension is a good test of the accuracy of the model for liquid−liquid interfaces, as this property has been well studied by experiments. It appears that the model is able to reproduce accurately the heptane/water interface, as demonstrated by the good agreement between the calculated interfacial tension (45.8 mN·m−1 at 343 K) and the experimental (46.82 mN·m−1 at 343 K,38 51.9 mN·m−1 at 295 K,39 48.55 mN·m−1 at 323 K40). The critical micelle concentration (CMC) is the concentration at which the surfactant molecules form a uniform monolayer at the oil/water interface. Further increase in the surfactant concentration above the CMC would result in a less pronounced decline in the oil/water interfacial tension because the oil/water interface is already occupied by a complete monolayer of surfactant molecules. Above the CMC the surfactant aggregation occurs either in the oil phase or in the aqueous phase, depending on conditions. The force field that we used for ILs is that of Canongia Lopes and Pádua (CLaP),25,26 which is parametrized for ILs and used vastly for study of their liquid-state properties in the literature.41 If experimental data are made available, force fields can be parametrized specifically for the simulation of IL at heptane/water interface to reproduce accurate surface tension reduction consistent with the experimental data, but this situation is generally rare in this case. To remedy the lack of experimental interfacial tension data for heptane/IL/water system, in the meantime we performed classical MD simulation of the bulk density of [C4mim]Cl as a test case using the previously mentioned force field. In this simulation, an ensemble comprising 216 ion pairs was equilibrated. The computed densities (1.047 g·cm3) using this force field at 343.15 K were found to be in good agreement with experimental results (1.072 g·cm3).42 The complete results on temperature-dependent bulk properties including the effect of various partial atomic charges obtained by using different quantum mechanical methods will be published elsewhere. Values of interfacial tension obtained from MD simulation of heptane/water interface in the presence of different kind of surfactant are shown in Table 1. The trend of surface tension results shows good agreement with experimental data reported by Ayatollahi and coworkers.12 They indicated that [C12mim] Cl leads to the reduction of interfacial tension from 39.98 to 6.87 mN·m−1 for a system containing 5000 ppm [C12mim]Cl aqueous solution and crude oil. Because the paraffin chain plays C



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Table 1. Interfacial Tension of Different Surfactants in Heptane/Water System surfactant

interfacial tension (mN m−1)

none [C4mim]Cl [C4mim]PF6 [C8mim]Cl [C8mim]PF6 [C12mim]Cl [C12mim]PF6 SDS

45.80 21.65 27.20 12.18 16.89 7.24 10.68 8.31

Figure 2. Mean-squared displacements of alkyl chain of surfactant molecules present at the heptane/water interface.

the main role in the interfacial tension of a kind of multicomponent system, our heptane/water system could be used to model the crude oil/water system well. Our results show a reduction of interfacial tension from 45.8 to 7.24 mN· m−1 as a result of addition of [C12mim]Cl molecules to the heptane/water system. Furthermore, Saien and Asadabadi observed that [C7mim]Cl and [C8mim]Cl reduce the interfacial tension of toluene/water system by 76 and 80%, respectively.43 Interestingly, our results are in the same range of experiments ((45.8) − 7.24/(45.8) × 100 = 84.2%). The effectiveness of a surfactant can be quantified by the magnitude of the reduction in oil/water interfacial tension. The oil/water interfacial tension will decrease progressively in proportion to the increase in the concentration of surfactants adsorbing at the oil/water interface. The effectiveness of a surfactant can be defined by the magnitude of the decrease in oil/water interfacial tension to the CMC. The efficiency of a surfactant can be defined as the concentration or amount of surfactant required to reach the CMC. A favorable surfactant will have high degrees of both effectiveness and efficiency. However, effectiveness and efficiency are completely different concepts that do not necessarily follow similar trends. The lowest calculated heptane/water interfacial tension in this study is 7.24 mN/m and was obtained for the [C12mim]Cl. Therefore, [C12mim]Cl that dissolves readily in water has the best surface activity from both effectiveness and efficiency points of view. Dynamic Profiles. Transport properties basically represent the fluid fundamental feature with good accuracy. These properties not only determine the engineering aspects of the fluids but also provide a good insight into different fluid types of various classes of fluids on the molecular basis. The mean square displacements (MSDs) can be calculated from the knowledge of dynamic particles coordinates

obtained from the linear regime of MSD curves at long simulation time using the Einstein relation 1 d lim ⟨|ric(t ) − ric(0)|2 ⟩ (7) 6 t →∞ dt A linear fit to the MSD from 4000 to 6000 ps and using the Einstein relation gives the diffusion coefficients 2.23 × 10−10, 2.10 × 10−10, and 2.17 × 10−10 m2·s−1 for alkyl chain in [C12mim]Cl, [C12mim]PF6, and SDS, respectively. Interestingly, the values of diffusion coefficients for [C12mim]Cl and [C12mim]PF6 are very close to that of SDS, suggesting the similar behavior of these molecules at the heptane/water interface. Diffusion coefficients for [C8mim]Cl, [C8mim]PF6, [C4mim]Cl, and [C4mim]PF6 ILs are 3.27 × 10−10, 4.5 × 10−10, 3.57 × 10−10, and 7.85 × 10−10 m2·s−1, respectively. These values are higher than the diffusion coefficient obtained for the traditional surfactant SDS. Interestingly, the MSD for [C12mim]PF6 shows two different linear regimes at short and long times. This may be attributed to the strong interaction between the [C12mim]+ cation ring and the PF6− anion and the repulsion between this hydrophobic ion and water molecules. Density Profiles. The atom density profiles obtained from MD simulations were used to determine spatial positioning and average orientational ordering of the molecules at the liquid/ liquid interface, qualitatively. The quantitative approach for the determination of orientational distribution will be considered in the next section. Simulations of a liquid/liquid interface provide a good means to estimate the bulk density of a liquid. The equilibrium density estimated for bulk heptane between z = −23 and +23 Å (of this liquid/liquid equilibrated system, not shown) is 0.657 g·cm−3, which is quite comparable to the experimental value of 0.641 g· cm−3 for pure heptane at 343 K and 1 atm.44 The averaged density of water is 0.989 g·cm−3, which is very close to the experimental value of 0.977 g·cm−3 for pure water. The density profile of heptane/[C4mim]Cl/water system obtained by all-atom simulation is shown in Figure 3A (left). The average density of heptane increases to 0.668 g·cm−3 because of repulsive interactions between IL and heptane, but the density of water decreases to the average value of 0.961 g· cm−3 due to the presence of IL in this phase. The peaks of [C4mim]Cl density are observed at the interface of heptane/ water, which can be defined as region I. The density of [C4mim]Cl in water is ∼0.15 g·cm−3, but it is zero in heptane. These results indicate that the [C4mim]Cl IL is mainly at the interface of heptane/water interface and to a lesser extent in bulk water. Di =

N

1 MSD = ⟨∑ |ric(t ) − ric(0)|2 ⟩ = Δ |r(t )|2 N i=1

(6)

rci (t)

where is the location of the center of mass of ion i at time t. Figure 2 shows the MSD of ILs alkyl chain and hydrophobic chain of SDS up to 6 ns. Remarkably, the slope of the MSD of the ILs with short alkyl chains is larger than that of the ILs with long alkyl chains. By comparing the MSDs in Figure 2 at long time, it is clear that the MSDs of surfactants with different alkyl chain decrease in the order of [C4mim]PF6 > [C4mim]Cl > [C8mim]PF6 > [C8mim] Cl > [C12mim]PF6 > [C12mim]Cl > SDS. The small MSD is consistent with relatively low dynamics of SDS molecules compared with that of ILs. Diffusion coefficients, Di, were D



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Figure 3. Total density profiles (left) and atom density profiles (right) of (A) heptane/[C4mim]Cl/water and (B) heptane/[C4mim]PF6/water systems.

Figure 4. Snapshot of the simulation systems: (A) heptane/[C4mim]Cl/water and (B) heptane/[C4mim]PF6/water.

simulations.49 Atom density profiles presented in right panel of Figure 3A indicate that cations and anions coexist at the interface and in aqueous phase. Clearly, the butyl group is directed toward the heptane phase, while the hydrophilic atoms of the imidazolium ring are pointed into the water phase. The simulation results suggest that the CH3 group of [C4mim]+

It is believed that the IL cation and anion are located adjacent to one another as manifested in the X-ray results.45 Armstrong et al. have found (using mass-spectrometric methods) that the vapor phase of ionic liquids contains ion pairs.46 Ion pairs were also discovered to exist in dilute solutions of ionic liquids both in experiments47,48 and in MD E



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Figure 5. Total density profiles (left) and atom’s density profiles (right) of: (A) heptane/[C8mim]Cl/water and (B) heptane/[C8mim]PF6/water systems.


increase in the density of water phase (1.07 g·cm−3) in the presence of this hydrophobic IL. We have recently concluded an ineffective H···F interaction between H atoms of pure water and F atoms of PF6− by Car−Parrinello MD of [C4mim]PF6/ water system because of the difference in their calculated partial atomic charges.50 High density peaks of [C4mim]PF6 at the heptane/water interface support this idea. This sharp peak indicates that IL [C4mim]PF6 is held together by Coulomb

cation is also pointing away from the heptane phase, as in the case of chloride ion. Whereas [C4mim]Cl mixes readily with water, the hydrophobic IL [C4mim]PF6 makes a distinct boundary with both water and heptane phases. The density profiles of heptane/ [C4mim]PF6/water system were calculated from the sum of densities of each atom weighted by its atomic number and are shown in left panel of Figure 3B. The results indicates an F



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Figure 7. Total density profiles (left) and atom density profiles (right) of: (A) heptane/[C12mim]Cl/water and (B) heptane/[C12mim]PF6/water systems.

heptane phase is 0.652 g·cm−3, and the average density of water is 1.05 g·cm−3. A decrease in the density of heptane phase compared with that of heptane/[C4mim]PF6/water system (0.662 g·cm−3) is due to the presence of long alkyl chain of [C8mim]+ cation. Two dense interfacial layers of [C8mim]PF6 appear as clearly as the peaks that were observed for [C4mim]PF6. The right panel of Figure 5B shows that the P atoms of the anion are found to some extent in the same region as the N1 atoms of the imidazolium ring (with profile peak distance up to 0.3 Å). The CA atoms of the methylene group also point toward the water phase. The relevant snapshots of these systems are shown in Figure 6. The resulting density profiles, shown in Figure 7A, are the illustration of the heptane/[C12mim]Cl/water system. The CMC could be defined as the concentration of surfactant, which corresponds to the onset of aggregation in either the aqueous or the oil phase.52 Therefore, some remarkable features are observed for the [C12mim]Cl IL: (i) The local density of this IL in the water phase is much higher than the other ILs. (ii) This higher density may be due to the lower CMC of this [C12mim]Cl. (iii) Some heptane molecules are dissolved in water because of the presence of small micelle-like structures of [C12mim]Cl ILs. These findings are in line with lowest interfacial tensions that were obtained for the heptane/ water systems containing [C12mim]Cl molecules. The right panel of Figure 7A shows the number density profile of individual atoms of [C12mim]+. It can be seen that C12 atom of alkyl chain is present in the water phase as well as the heptane phase. Figure 7B shows that [C12mim]PF6 forms any aggregates in aqueous phase. The average density of heptane phase reduces

forces.51 The atoms density profile for heptane/[C4mim]PF6/ water system is shown in Figure 3B (right). In the heptane side of the interfacial region, hydrophobic atoms including butyl chain have distinct profiles showing an excess number density over the ring atoms, methyl group, and PF6− anion. The presence of alkyl chain favorably in the heptane side pushes the methyl group into the water side of the interface. The profiles of the P atom of the anion and N1 atom of the cation overlap to some extent (with profile peaks distance of up to 0.9 Å) at the interfacial region, showing significant imidazolium cation−PF6− anion contact. The corresponding snapshots of heptane/ [C4mim]Cl/water, and heptane/[C4mim]PF6/water are given in Figure 4A,B, respectively. Figure 5A (left) depicts the total density profiles of heptane/ [C8mim]Cl/water system. The average density of heptane phase is 0.634 g·cm−3, and of the water phase is 1.02 g·cm−3. The results indicate a decrease in the density of heptane phase and an increase in the density of water phase in heptane/ [C8mim]Cl/water compared with the heptane/[C4mim]Cl/ water system. This can be attributed to the higher density of [C8mim]Cl at the interface, which is clearly seen by two dense peaks at ∼0.5 g·cm−3. Therefore, [C8mim]Cl dissolves in water to a lesser extent with an average density of 0.070 g·cm−3. The atom’s density profile of [C8mim]Cl is shown in the right panel of Figure 5A. The alkyl side chains of the cations tend to protrude toward the heptane phase, in accordance with common surfactant molecules. N1 atoms of the imidazolium ring, CA atom of methylene group, and chloride anion point toward the water phase. The probability of finding chloride anion in the water phase is higher than that in the cation atoms. The left panel of Figure 5B depicts the total density profiles of heptane/[C8mim]PF6/water systems. The average density of G



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Figure 9. (A) Total density profiles (left) and atom density profiles (right) of heptane/SDS/water. (B) Corresponding snapshot of the systems.

to 0.646 g·cm−3, which indicates that the alkyl chain of [C12mim]+ tends to protrude to the heptane phase. Therefore, because of higher density of this long alkyl chain in heptane phase, a shoulder can be seen clearly in the density profile of systems containing ILs with [C12mim]+ cations. The simulation snapshots of these systems are shown in Figure 8. Figure 9A shows the density profiles of heptane/SDS/water system along the z-axis direction of the simulation box. From the density profile, it is obvious that consistent with findings for ILs the system consists of two phases with two well-defined interfaces. It should be noted here that the densities of each phase in this system (0.645 g·cm−3 for heptane and 1.07 g·cm−3 for water) agree well with those of the pure bulk molecules. The atom density of this system, shown in the right panel of Figure 9A, indicates that Na+ cations along with the head groups atoms oriented mainly toward the water phase, while the alkyl chain tails are extended toward the heptane phase.

Therefore, a shoulder is seen in the density profile of heptane at each interface. The corresponding snapshot of this system is shown in Figure 9B. Orientation of Cation. The molecular environment at interfaces is different from that of the bulk material due to the differences in the forces experienced by the molecules in the bulk and at the surface.53 The molecules at the surface layer are subjected to unbalanced forces because they are in contact with fewer molecules. Because surface molecules in liquid are free to move, they can orient and arrange themselves in such a way as to keep the surface energy of the system at its minimum.54 According to Langmuir’s principle, the measured surface energy or surface tension corresponds to the part of the molecule that is present at the interface.55 The simulated ensembles were divided into different layers, based on the average simulated densities of imidazolium ring atoms, to study the imidazolium-ring orientation at interface. H



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projection of Z onto the plane made by the two vectors, the normal of imidazolium-ring plane surface (n1), and the normal of the rbisector−n1 plane (n2). In general, the choice of such parameters is not a trivial task.58 The maps give a contour of ϕ versus cos(θ). Orientational maps of [C 4 mim]Cl and [C 4 mim]PF 6 molecules located at two different regions are shown in Figure 11. For [C4mim]Cl, at interfacial region (I), the most probable region is centered at 0.02 < cos θ < 0.12 with 0 < ϕ < 9°. In this orientation, the plane of the imidazolium ring of IL is perpendicular to the plane of the interface, and N1−C1 bonds protrude toward the nonpolar phase. This orientation corresponds to the alignment of N3−CA bonds, located toward the aqueous phase. In the bulk of water (region (II)), it is clearly seen that the orientational ordering decreased compared with the interfacial region, but an orientation with a limited range of 0.12 < cos θ < 0.14 with 54 < ϕ < 58° is still probable. For [C4mim]PF6 the interfacial region are divided into two layers (I) and (II). In layer (I), the hydrophobic side of interface, three different orientational preferences can be identified for the imidazolium ring plane cation. The most probable regions are centered at −0.12 < cos θ < −0.02 with 0 < ϕ < 8°, −0.13 < cos θ < 0.01 with 35 < ϕ < 49°, and −0.14 < cos θ < 0.12 with 65 < ϕ < 90°. Thus, while the bisector takes maximum angle of 98° with respect to the Z axis, ring plane tumbles sideways and takes three different orientations. In layer (II), the hydrophilic side of interface, the most probable orientation is at −0.12 < cos θ < 0.01 with 48 < ϕ < 76°. As a result, if a [C4mim]+ cation enters from region (I) to region (II), the bisector vector of imidazolium-ring plane keeps roughly a right angle orientation with respect to Z axis, while its sideways tumbling becomes much restricted. The bivariate maps of [C 8 mim]Cl and [C 8 mim]PF 6 molecules are shown in Figure 12. It can be seen that the [C8mim]+ cations that are in close contact with heptane phase are oriented with their plane parallel to the Z axis and the bisector vector perpendicular to the Z axis, that is, with one of the nitrogens pointing into the heptane phase and the other one toward the water phase (0.01 < cos θ < 0.11 with 0 < ϕ < 9° and 0.06 < cos θ < 0.11 with 0 < ϕ < 5° for [C8mim]Cl and [C8mim]PF6, respectively). The bisector vector shows a deviation from the right angle and tilt in a manner that their octyl groups protrude the heptane phase. As we move away from the heptane phase, the orientational anisotropy in layer (II) decreases compared with the layer (I) due to the decrease in hydrophobic interaction between heptane and the alkyl chain of ILs. Interestingly, the orientational ordering in the case of [C12mim]Cl is to some extent different from the cases of ILs with shorter alkyl chain length and also from [Cnmim]PF6 ILs. As shown in Figure 13, at the heptane-rich side of the interface (region I) the main orientational preference of the [C12mim]+ appears at 0.05 < cos θ < 0.14 with 0 < ϕ < 6°. It is also seen that this peak is slightly extended toward smaller cos θ values in region II. Beyond this layer, the distributions resemble the isotropic orientation typical of the bulk liquid with the exception of a preferred orientation characterized by −0.16 < cos θ < 0.01 with 83 < ϕ < 90°. For [C12mim]PF6 molecules in region (I), the most probable orientation is located in a limited range: 0.05 < cos θ < 0.12 with 51 < ϕ < 58°. (See Figure 13.) A molecule in this layer has the ring directed toward the heptane surface with the bisector vector angled by ∼84°, while the ring plane tilted sideways by

For [C4mim]Cl, two regions, (I) and (II), are defined as the domain 27.0 ≤ |Z| ≤ 36.5 Å containing the molecules that are presented at the interfacial region and 36.5 ≤ |Z| ≤ 46.0 Å containing the [C4mim]Cl molecules that are dissolved in bulk water. The density of [C4mim]Cl in the bulk water is about one-third of its value at the interface. For [C4mim]PF6, interfacial layer (I), located in the domain 23 ≤ |Z| ≤ 29.1 Å, is the heptane side of the interface. At the boundary of this layer, where the interfacial region is started, the density of heptane reaches its bulk value. The layer (II) located at 29.1 ≤ |Z| ≤ 36.6 Å is the aqueous side of the interface. At the boundary of this layer, the water density reaches its bulk value. In the same manner, we have divided the density profile of [C8mim]Cl, [C8mim]PF6, and [C12mim]PF6 into two different regions. Interestingly, for the heptane/[C12mim]Cl/water system, the dissolution of this long-chain IL in the aqueous phase leads to a hydrophobic region that can dissolve the heptane phase. As is seen from Figure 7A (left), region II defined as 36.0 ≤ |Z| ≤ 47.0 Å containing the IL and heptane molecules penetrated into the polar phase. In this layer, the IL is more dense than heptane. Finally, a dense layer appears as a particular layer in the aqueous phase with distinguish density profile, where a maximum density is seen for heptane and the density of IL passes through a minimum. The location of this dense region (III), which contains both IL and heptane molecules is at about 47 ≤ |Z| ≤ 54 Å. This feature implies that increasing the alkyl chain length of hydrophilic IL leads to an increase in the orientational ordering at the interface due to enhancement in van der Waals interactions between the alkyl groups and the decrease in the critical micelle concentration. Full description of molecular orientation by the determination of bivariate orientational distribution56,57 is applied here to these ILs presented at the heptane/water interface for the first time. We have previously used the orientational distribution maps to explain the trend of experimental surface tension and surface entropy for pyridine and its alkyl derivatives6 and also for the [Cnmim]I/vapor interface (with n = 4, 6, and 8).7 For this purposes, the bivariate orientational method involves two independent spherical polar angles (ϕ and θ) assigned as shown in Figure 10. θ is the angle between the Z axis normal to the interface, the principle axis of the simulation box, and the bisector vector of the imidazolium-ring, rbisector. ϕ is the angle between imidazolium ring plane normal vector and the

Figure 10. Molecular fixed coordinates (rbisector, n1, and n2) defined for bivariate orientational analysis of [Cnmim]Cl. Spherical polar angles (ϕ and θ) are shown. The definition also applies to the ring plane of [Cnmim]PF6 where n = 4, 8, and 12. I



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Figure 11. Bivariate orientation distribution of IL molecules for the heptane/[C4mim]Cl/water and heptane/[C4mim]PF6/water system. Red corresponds to high normalized probability and blue corresponds to low probability.

[C4mim]PF6 ILs are 1.84 × 10−9, 1.44 × 10−9, 7.24 × 10−10, and 2.21 × 10−9 m2·s−1, respectively. The self-diffusion coefficients of alkyl chains are remarkably higher than that of heptane/water, reflecting that the mobility of surfactant molecules are generally restricted at interfaces due to the formation of electric double layers. Comparison of the results obtained for heptane/air and heptane/water interfaces indicates that the removal of water has a strong impact to the mobility of surfactant molecules, as the self-diffusion coefficient for [C12mim]Cl alkyl chain is more than 21 times at heptane/ vapor surface compared with the heptane/water interface. We observed that the surfactant molecules diffuse rapidly from the surface to the bulk of heptane and form micellar systems that are stable over the duration of the simulation. The surfactant structure in the bulk of heptane is analyzed in terms of the density profiles of the molecules. These profiles can give us information on how the molecules array along the interface. Therefore, density profiles are calculated in the z direction, that is, normal to the liquid/vapor interface. For instance, in Figure 16A, the z-dependent density profiles for the [C12mim]Cl molecules, the headgroups, anions, and the hydrocarbon tails are shown. The interesting feature depicted

∼51 to 58°. In layer (II), which is the aqueous side of interface, the orientational preferences are not as dominant as the heptane side. However, the preferential orientation characterized by the values of −0.01 < cos θ < 0.12 with 0 < ϕ < 20° is found to be the case. In this orientation, the plane of the imidazolium ring is perpendicular to the plane of the interface and tilted sideways. On the basis of the analysis of our results, we can draw a picture of the interactions taking place at the heptane/IL/water interface. Such a scheme is depicted in Figure 14. Self-Aggregation of the Surfactants at Heptane Interface. To study the interface of surfactant molecules with heptane bulk, we just removed the water molecules from the well-equilibrated heptane/surfactant/water systems and continued the simulations for more 6 ns. Figure 15 shows the MSD of alkyl chain of surfactant molecules present at the heptane/air interface as a function of simulation time. A linear fit to the MSD from 1500 to 3500 ps and using the Einstein relation gives the diffusion coefficients 4.82 × 10−9, 2.75 × 10−9, and 1.01 × 10−9 m2·s−1 for alkyl chain in [C12mim]Cl, [C12mim]PF6, and SDS, respectively. Diffusion coefficients for [C8mim]Cl, [C8mim]PF6, [C4mim]Cl, and J



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Figure 12. Bivariate orientation distribution of IL molecules for the heptane/[C8mim]Cl/water and heptane/[C8mim]PF6/water system. Red corresponds to high normalized probability and blue corresponds to low probability.

interfacial tension at the heptane/water interface of systems containing definite number of hydrophobic or hydrophilic ILs with different alkyl chain lengths and also common surfactant molecules was calculated. We considered [Cnmim]Cl and [Cnmim]PF6 (n = 4, 8, and 12) and also a traditional surfactant, SDS, and studied the effect of these surface active agents on the heptane/water interface. We have found good agreement between the interfacial tensions calculated from MD simulation results and the experimentally measured ones for heptane/ water interface and systems containing [C12mim]Cl and SDS molecules for which some experimental data were available. Distribution of ILs at the interface of heptane/water systems was clarified by the results of interfacial tension, self-diffusion coefficient, density profile, and bivariate orientational analysis. As previously explained, aggregation occurs for [C12mim]Cl in the aqueous phase, which indicates that CMC is reached for this IL molecules, while the other surface-active agents did not form any aggregate in either the oil or aqueous phase at this definite concentration. The small chloride anion prefers both surface and the bulk, while large hexafluorophosphate anion likes to stay just at the interface. This can be attributed to the

in these profiles is that the headgroup atoms and anions forms two well-defined peaks in the heptane bulk, while the tails are distributed more uniformly in the bulk. The same distribution are seen for [C12mim]PF6 and SDS molecules, which are shown in Figures 16B and 17A, respectively. The snapshots of the mentioned systems at the final simulation stage are shown in Figures 16C,D and also 17B. Recently, Zolghadr et al. explained that paraffin groups have an important effect on multicomponent interfacial tension behavior.44,59 They indicated that the paraffins are mostly positioned at the oil/CO2 and oil/N2 interface, where the oil is a multicomponent mixture. Accordingly, they concluded that the heavier components of diesel fuel (including hexadecane) tend to expose to the interface. This can make an initial picture of how ILs replacing surfactants affect a mixture of paraffins/ water interface that can be examined later.

4. CONCLUSIONS Detailed results on the structures, dynamics, and surface properties of ILs as surface active molecules at the heptane/ water interface were obtained by MD simulations. The K



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Figure 13. Bivariate orientation distribution of IL molecules for the heptane/[C12mim]Cl/water and heptane/[C12mim]PF6/water systems. Red corresponds to high normalized probability and blue corresponds to low probability.

Figure 15. Mean-squared displacements of alkyl chain of surfactant molecules present at the heptane/air interface.

Diffusion coefficients for different surfactant molecules present at heptane/water interface were calculated. The obtained values for [C12mim]Cl and [C12mim]PF6 are very close to that of SDS, suggesting the similar behavior of these molecules at the heptane/water interface. The diffusion coefficients of ILs with shorter alkyl chains are higher than those of ILs with longer alkyl chain. This trend is reversed for the surfactant molecules at heptane/vapor interface. In this

Figure 14. Schematic representation of the structures of [Cnmim]Cl and [Cnmim]PF6 with n = 4, 8, 12 at the heptane/water interface.

repulsive forces that exist between charge densities of PF6− anions and water molecules. L



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Figure 16. Atom density profiles of (A) heptane/[C12mim]Cl and (B) heptane/[C12mim]PF6 systems. The corresponding snapshots are also shown in panels C and D, respectively.

The ILs need to have a certain orientation to be adsorbed from bulk phase to the interface. At the heptane side of the interface, the cation was found to be oriented with the imidazolium ring nearly perpendicular to the surface plane with a tilt angle of 0 < ϕ < 10° for [Cnmim]Cl, while the ring plane tumbled sideways for [Cnmim]PF6. At the aqueous side of the interface, the cations distributed with a more isotropic orientation.

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ASSOCIATED CONTENT

S Supporting Information *

Nonbonded parameters of the force fields including of Lennard-Jones parameters and the atomic charges and also the optimized structures of ([C12mim]PF6 and SDS molecules with atoms labeling. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +98 711 646 0788. Tel: +98 711 613 7157. Figure 17. Atom density profiles of (A) heptane/SDS system and (B) the corresponding snapshot.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are indebted to the research council of the Shiraz University for the financial support.

case, the diffusion of surfactant molecules from surface to the heptane bulk occurs with a higher rate, and the surfactant molecules form aggregates in heptane bulk. M



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Adsorption and Orientation of Ionic Liquids and Ionic Surfactants at

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