Adsorption of Nitrobenzene on the Surface of Ice: A Grand Canonical

Article pubs.acs.org/JPCC

Adsorption of Nitrobenzene on the Surface of Ice: A Grand Canonical Monte Carlo Simulation Study Zihao Fu,† Ning He,‡ Putian Zhou,§ Jiaxu Liu,‡ Hong-Bin Xie,*,† Qi Yu,† Fangfang Ma,† Zhiqiang Fu,† Zhongyu Wang,† and Jingwen Chen† †

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China ‡ State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China § Department of Physics, University of Helsinki, P.O. Box 64, FIN-00014 Helsinki, Finland S Supporting Information *

ABSTRACT: The adsorption of nitrobenzene at the surface of hexagonal ice was studied by grand canonical Monte Carlo (GCMC) simulations at 200 K by employing TIP5P water model and our modified force field for nitrobenzene. We found that the number of adsorbed nitrobenzene molecules gradually increases with relative pressure before the condensation point and the condensation precedes the monolayer adsorption saturation. The adsorption follows the Langmuir shape only up to a very low coverage. At this low coverage, the adsorption of the molecules occurs independently from each other to adsorption sites (called α sites), where adsorbed nitrobenzene molecules lie almost in parallel with the ice surface to facilitate strong electrostatic interactions with ice surface. More importantly, in the α-type adsorption, a typical O−H···π bond for the adsorption of aromatics on the ice surface is not preferable for nitrobenzene. With increasing surface coverage, additional adsorbed molecules do not take unoccupied α sites due to attractive interactions among adsorbates, inducing a deviation of the adsorption isotherm from the Langmuir shape. In addition, the calculated adsorption energy (−75.98 kJ/mol) for nitrobenzene agrees well with the value (−71.35 kJ/mol) from our validating quantum chemistry calculations, implying the reliability of the results from GCMC simulations. dangling O−H bonds of the ice surface. Petitjean et al.28 first studied the effect of polar group −CHO on the adsorption of aromatic compound by taking benzaldehyde as a case. They found that the adsorption energy of benzaldehyde is more negative than that of benzene due to the formation of O···H−O hydrogen bonds between O atom of −CHO and H atom of H2O, indicating an enhancing effect of the group −CHO. Besides benzaldehyde, there are many other aromatic pollutants containing polar groups and H-bond donors or acceptors in the atmosphere. Therefore, these compounds could also be effectively adsorbed on the ice surface due to the additional interactions between the introduced polar group and the ice surface besides O−H···π bonds. Nitroaromatic compounds (NACs)29 are typical aromatic pollutants containing polar groups and H-bond acceptors. Nitrobenzene is the simplest nitroaromatic compound. It has been listed as a priority pollutant by many countries due to its mutagenicity, carcinogenic effects, and tendency to accumulate in the environment.30−33 The concentration of nitrobenzene

1. INTRODUCTION Ice and snow are critically important environmental components of cold ecosystems at high altitudes and latitudes.1−4 Ice can also efficiently adsorb and scavenge pollutants from the atmosphere via physical or chemical interactions due to its low temperature and high surface-area-to-volume ratio.2−10 These physical/chemical interactions at the air−ice interface can alter the chemical activity of adsorbates via changing the electronic structure characteristics, which further affect the fate of atmospheric pollutants on the local, regional, and global scales.2,9−12 Therefore, understanding the adsorption behavior of atmospheric pollutants on ice surface is important for their environmental risk assessment. Up to now, numerous studies have been performed on the adsorption of volatile organic compounds (VOCs) on the ice surface, such as alcohols,13−15 aldehydes,15−18 carboxylic acids,19−21 ketones,15,22−25 and chlorofluorocarbons.26 However, little attention has been paid to the adsorption of aromatic chemicals. Més zár et al. 27 found that the adsorption mechanisms of benzene, naphthalene, phenanthrene, and anthracene on air−ice interface are similar to those of polar compounds. Typically, O−H···π bonds are formed between the delocalized π bonds of the aromatic compounds and the © XXXX American Chemical Society

Received: April 14, 2017 Revised: July 6, 2017 Published: July 6, 2017 A

DOI: 10.1021/acs.jpcc.7b03531 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (A) Simulation system for free energy profile: a nitrobenzene molecule, a liquid slab with 970 water molecules, and a dummy atom K. (B) Potential of mean force (PMF) for moving nitrobenzene with different charge parameters through a water slab. “PMF 1” represents the PMF of nitrobenzene with original OPLS charge; “PMF 2” represents the PMF of nitrobenzene with RESP charge; “PMF 3” represents the PMF of nitrobenzene with RESP charge × scaling factor, 1.05; and “PMF 4” represents the PMF of nitrobenzene with RESP charge × scaling factor, 1.10. The horizontal short dotted line shows the average value of three hydration energies obtained from corresponding experimental Henry’s law constants.53−55,60

was found to be up to a few ppbv in the urban atmosphere.34,35 In view of electronic structure, nitrobenzene includes a polar group −NO2, which can serve as an H-bond acceptor. Thus, similarly to benzaldehyde, nitrobenzene could be adsorbed onto the ice surface more strongly than benzene.27 However, its adsorption behavior on the ice surface is still unknown. Among the methods to study the adsorption behavior, the classical force field based grand canonical Monte Carlo (GCMC) method is particularly suitable.8,19,26−28,36,37 In GCMC simulation, the average number of adsorbate molecules is determined when the chemical potential of adsorbate is fixed. The adsorption isotherm can be computed directly by systematically varying the chemical potential in a set of GCMC simulations. The GCMC method has been applied successfully for studying various adsorption processes.8,18,19,26,37 In principle, the success of GCMC method for studying adsorption processes also depends on the reliable force field. Therefore, a reliable force field has to be developed for studying the adsorption of adsorbates on ice surface, especially for the force field of adsorbates since the force field for ice (water) has been well developed.38−41 In this study, we employed the GCMC method to investigate the adsorption behavior of nitrobenzene on ice surface. To obtain reliable results, we first rationalized the force field of nitrobenzene by modifying the OPLS-AA force field of nitrobenzene. With the modified force field and TIP5P model for ice water, we calculated and analyzed the adsorption isotherm of nitrobenzene. We also characterized the properties of the adsorption layer in details at various coverages, in terms of the orientation of the nitrobenzene molecules and the energy of their interactions with the ice surface as well as with the adsorption layer. The results are important for understanding not only the adsorption behavior of nitrobenzene on ice surface but also the other N-containing group substituted aromatics.

simulation box edges are 100.0, 38.891, and 35.926 Å for the X, Y, and Z, respectively. Along the surface normal axis, X, the ice phase is arranged in 18 molecular layers, consists of 2880 water molecules, and is located in the center of the simulation box with two adsorption surfaces. The three-dimensional periodic boundary condition has been applied. The chemical potential (μ) of nitrobenzene has been systematically varied in the ranges from −69.06 to −58.09 kJ/mol, which cover the simulation cases from those with no molecule adsorbed to those with the simulation box completely filled. In this way, the adsorption isotherm has simply been obtained as the average number ⟨N⟩ of nitrobenzene molecules adsorbed in the system as a function of its μ values. The GCMC simulations have been performed using the MMC code of Mezei (MMC program).45−47 All the potential models in the system are rigid. Water molecules are modeled by the five-site TIP5P potential model since the melting point calculated by this model is known to be rather close to that of real water.38 The nitrobenzene molecules are modeled by our modified OPLS-AA force field, which is presented in detail in the following section. The geometry of the nitrobenzene used was listed in the Supporting Information (SI). The interaction energy of the molecular pair is calculated as the sum of charge− charge Coulomb and the site−site Lennard-Jones (LJ) energy. The total potential energy of the system is calculated as the sum of the interaction energies of all molecular pairs. An interaction cutoff of 12.5 Å is employed. In the insertion/deletion step, the nitrobenzene molecules are tentatively (with 50%−50% probabilities) inserted to or deleted from the system using Mezei’s cavity biased algorithm.45,47 In a particle transfer step, a randomly chosen water or nitrobenzene molecule is randomly translated to a maximum distance of 0.25 Å, and then randomly rotated by no more than 15° around a randomly chosen spacefixed axis. In the MMC program, the particle displacement and insertion/deletion attempts are performed in an alternating order. Moreover, the insertions are only attempted into the centers of empty cavities of radii with at least 2.9 Å.47 Cavities are searched along a 100 × 100 × 100 grid domain, which is regenerated after every 1 × 106 Monte Carlo steps. 4 × 108 trial moves are used in our simulation, first 2 × 108 Monte Carlo steps are used to obtain equilibration, and the other 2 × 108 Monte Carlo steps are used as a production run. Finally, for every 5 × 105 Monte Carlo steps in the production run, one

2. COMPUTATIONAL DETAILS 2.1. Ice Surface Model and GCMC Simulation. A series of Monte Carlo simulations has been performed on grand canonical ensemble (μ, V, T) at the tropospheric temperature T = 200 K to investigate the adsorption of nitrobenzene on the ice surface. Here, we selected 0001 surface of hexagonal (Ih) ice as the ice surface model, which has been widely used to study the adsorption of VOCs on the ice surface.8,16,42−44 The basic B


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state (aqueous concentration c01 = 1 mol/L, gas pressure p0 = 1 atm, and T = 298.15 K) by transferring a target molecule from the gas to the liquid phase, is related to Henry’s law constant by the equation

sample configuration is for further analysis at selected chemical potential values. 2.2. Force Field Parameter Modification for Nitrobenzene. The scheme of “keeping LJ parameters and changing atomic charges” proposed by Vácha et al.48 was used to modify the force field parameters of nitrobenzene. Here, the force field parameters for nitrobenzene consist of the modified atomic charges and the LJ parameters of its OPLS-AA force field.49 The modified atomic charges for nitrobenzene were obtained by adjusting (multiplying a scaling factor) the initial atomic charges until the simulations with new force field parameters for nitrobenzene and TIP5P water model can reproduce the experimental hydration free energies at 298.15 K. The initial atomic charges of nitrobenzene used for the force field parameter modification were obtained by fitting the electrostatic potential calculated at the HF/6-31G* level with the restrained electrostatic potential (RESP) method.48 The hydration free energy (ΔG*) of nitrobenzene was determined from the potential of mean force (PMF)50,51 for the transfer process of single nitrobenzene molecule from the gas phase across the bulk liquid water and back to the gas phase at 298.15 K. Classical molecular dynamics (MD) simulations with the umbrella sampling method were used to calculate the PMF for the hydration process of nitrobenzene within package GROMACS 4.5.5.52 The method has been successfully used to calculate the hydration free energy values for various compounds.48,50,51 The MD simulation system (see Figure 1A) for the hydration free energy calculation consists of one nitrobenzene molecule, 970 TIP5P water molecules (water slab), and a dummy atom K (for controlling the distance of the nitrobenzene molecule from the water surface). The pre-equilibrated water slab was placed in the middle of a rectangular simulation box, with the x-, y-, and z-dimensions of 3.09, 3.09, and 20.0 nm, respectively. Periodic boundary conditions were applied in all three dimensions. A nitrobenzene molecule was placed approximately 1.5 nm away from the water surface. The dummy atom K was placed 1.0 nm away from the nitrobenzene molecule. A 1.4 ns constant volume and temperature (NVT) simulation was performed in which the center of mass (CoM) of the nitrobenzene molecule was pulled along the z-direction (normal to the interface) toward the center of the water slab and then across the opposite interface into the gas phase using a harmonic restraining potential with the force constant of 1000 kJ·mol−1·nm−2 and a pull rate of 0.005 nm/ps.51 After this simulation, 280 configurations (“windows”) with different positions of nitrobenzene along the z-axis were selected. The width of window is 0.025 nm. Then, for every window, a 3 ns NVT simulation was run with a biasing harmonic potential (the force constant of 1000 kJ·mol−1 nm−2) imposed on the zcoordinate between the nitrobenzene CoM and dummy atom K. The points from the simulations were saved every 2 ps. A total number of 1000 points from 0.5 to 2.5 ns were collected for each window to construct the umbrella sampling population histograms. Finally, the free energy profile was calculated by the weighted histogram analysis method (WHAM).56,57 All the WHAM calculations were performed by the statistical utility “g_wham”58 in GROMACS 4.5.5 package. Since there is no experimental hydration (Gibbs) energy of nitrobenzene, we used the experimental Henry’s law constant to derive the benchmark value for calculated hydration free energy of nitrobenzene.59,60 The standard hydration free energy, ΔG0, defined as the free energy change at the standard

⎛ p0 ⎞ ΔG° = −RT ln⎜ 0 KH0⎟ ⎝ c1 ⎠

(1)

K0H

where represents the standard Henry’s law constant and R is the universal gas constant. Since our MD simulation calculates the hydration free energy of nitrobenzene (ΔG*) in the limit of infinite dilution. Here, eq 2 was used to convert ΔG0 to ΔG*.60−62 ⎛ p0 ⎞ ΔG* = ΔG° + RT ln⎜ 0 ⎟ = ΔG° − 7.9 kJ/mol ⎝ c1 RT ⎠

(2)

3. RESULTS AND DISCUSSION 3.1. Force Field Parameter Modification for Nitrobenzene. The free energy profiles for a nitrobenzene molecule across a TIP5P liquid water slab are presented in Figure 1B. The free energy profiles shown in Figure 1B were computed with TIP5P water force field and various nitrobenzene force fields including original OPLS (PMF 1), OPLS LJ parameters and RESP charge (PMF 2), OPLS LJ parameters and RESP charge × scaling factor, 1.05 (PMF 3), and OPLS LJ parameters and RESP charge × scaling factor, 1.10 (PMF 4). The free energy profiles change greatly with various nitrobenzene force fields, indicating the charge or scaling factor has great effect on the calculated free energy profiles. In addition, regardless of the nitrobenzene force fields, all of the free energy curves have the same general character (Figure 2), which is similar to those for

Figure 2. General free energy profile of a nitrobenzene molecule along the z-coordinate through a water slab. ΔG* represents the hydration free energy of nitrobenzene.

benzene, naphthalene, anthracene, and other polycyclic aromatic hydrocarbons.48,51 When the nitrobenzene molecule is gradually moved toward the water surface (left-hand side of Figure 2), the free energy gradually decreases to reach a minimum, corresponding to the nitrobenzene adsorption at the liquid−vapor interface, which is followed by an increase of free energy as the nitrobenzene molecule enters the aqueous phase. Then the free energy keeps a nearly constant value in the bulk liquid region before they reach another minimum, corresponding to the nitrobenzene adsorption at the opposite liquid− C


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Table 1. Lennard-Jones (LJ) Parameters for Nitrobenzene Adopted from the OPLS-AA Force Field and Modified Atomic Charges of Nitrobenzene

Figure 3. Adsorption isotherms of nitrobenzene (NB) in the form of average number of adsorbate ⟨N⟩ on ice surface vs its μ values (A) and in the form of surface density vs its relative pressure (B). The annotations (NBs I−III) by red arrows were used in further analysis.

of OPLS-AA LJ parameters and RESP charge × 1.05, agrees well with the experimental value. In contrast, other nitrobenzene force fields either overestimate or underestimate the ΔG* values. It deserves to be mentioned that the original OPLS-AA force field of nitrobenzene gives a higher ΔG* value than the experiment. Therefore, use of OPLS-AA LJ parameters and RESP charge × 1.05 is our choice for modified force field of nitrobenzene, which is detailed in Table 1. In order to further test the rationality of our modified force field, we calculated the dipole moment, density, enthalpy of evaporation, and heat capacity of the nitrobenzene (see computational details in SI) based on the modified force field. The calculated dipole moment of nitrobenzene is 5.29 D, which is slightly higher than the experimental value (4.22 ± 0.08 D).64 We noted that calculated dipole moment of nitrobenzene with original OPLS-AA is 3.30 D,64 which is lower than the experimental value. In addition, the calculated density (1.202 g/mL), enthalpy of evaporation (13.63 kcal/ mol), and heat capacity (41.46 ± 2.04 cal mol−1 K−1) agree well with the corresponding experimental values (density is 1.198 g/ mL,65 enthalpy of evaporation is 13.15 kcal/mol,65 and heat capacity is 44.4 cal mol−1 K−149) at 298.15 K. We noted that the calculated density, enthalpy of evaporation, and heat capacity based on the original OPLS-AA force field are 1.158 g/mL,65 12.65 kcal/mol,65 and 46.1 ± 2.0 cal mol−1 K−1,49 respectively.

vapor interface. The lower free energy in the interface than that in the interior of the liquid indicates the preference of the nitrobenzene adsorption at the liquid−vapor interface instead of full solvation in the bulk liquid.51 In addition, the calculated ΔG* value is the free energy difference between the interior of the slab and the gas phase in Figure 2. We noted that calculated free energy profiles (Figure 1B) present a left−right asymmetry with respect to the middle of the water slab. The possible reason is that the nitrobenzene molecule could bring one or several water molecules from the liquid water into the gas phase when it is pulled out from the water slab (right-hand side in Figure 1A), which leads to an unequal interaction between the water surface and nitrobenzene molecule in entering and exiting the water interface. The same phenomenon was also reported in previous studies on the free energy profiles of naphthalene51,63 and 1-methylnaphthalene.51 To avoid possible error in ΔG* values caused by the pulling out effect, all ΔG* values in this study were calculated from the left-hand side free energy profiles. The ΔG* values calculated by eq 2 with three different experimental Henry’s law constants are calculated to be −17.22, −17.11, and −17.44 kJ/mol, respectively. The horizontal dotted line in Figure 1B shows the average value of these three ΔG* values. As can be seen in Figure 1B, the ΔG* value from PMF 3, based on the nitrobenzene force field consisting D


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The Journal of Physical Chemistry C Therefore, the calculated density and enthalpy of evaporation based on our modified force filed are a little more consistent with the experimental values than those based on the original OPLS-AA force field. The consistence of the calculated heat capacity from the simulation based on the original OPLS-AA force field and our modified force field with experiment is similar. The good agreement in the density, enthalpy of evaporation, and heat capacity between the calculated and experimental values indicates that our modified force field of nitrobenzene can describe well the intermolecular interaction among nitrobenzene molecules. Considering the results of a good description for the interaction between nitrobenzene and TIP5P water (deduced from predicted consistent ΔG* values with experiment), we can conclude that the combination of the modified nitrobenzene force field and TIP5P water model could predict well the adsorption behavior of nitrobenzene on the ice surface. 3.2. Adsorption Isotherm. The adsorption isotherm of nitrobenzene, expressed by the average number of adsorbed molecules ⟨N⟩ in the basic simulation box as a function of its chemical potential μ, is plotted in Figure 3A, and the corresponding data are shown in Table 2. As can be seen from Figure 3A, the number of adsorbed nitrobenzene molecules gradually increases with increasing chemical potential up to μ = −63.49 kJ/mol, whereafter the isotherm has a sudden jump to a constant plateau, in which the simulation box is fully filled with nitrobenzene molecules. The filled nitrobenzene

molecules could be in the solid state as discussed in the SI. Therefore, the point of condensation, i.e., where the vapor and solid phases have the same μ value, should be some point between the sudden jump point and the first point of constant plateau. To further probe the adsorption behavior of nitrobenzene on the surface of ice, we have converted the isotherm from ⟨N⟩−μ to the Γ−prel form, where Γ is the surface density of the nitrobenzene molecules and prel is the relative pressure, i.e., the pressure of the vapor phase normalized by p0 of the condensation point. Γ and prel are related to ⟨N⟩ and μ through the following two equations,8 respectively. Γ=

−69.06 −68.90 −68.48 −68.07 −67.65 −67.24a −66.82 −66.40 −65.99 −65.57 −65.16 −64.74 −64.33 −63.91 −63.49b −63.24 −63.08c −62.66 −62.25 −61.83 −61.42 −61.00 −60.58 −60.17 −59.75 −59.34 −58.92 −58.51 −58.09 a

⟨N⟩

prel =

1.004 1.238 1.674 1.804 2.185 2.232 2.772 3.662 4.227 5.121 5.400 6.536 7.197 9.838 10.769 280.229 278.219 276.076 280.406 279.512 279.801 275.720 277.771 276.099 280.000 276.292 277.116 275.170 277.269

−2

3.26 × 10 3.60 × 10−2 4.62 × 10−2 5.93 × 10−2 7.62 × 10−2 9.78 × 10−2 0.126 0.161 0.207 0.266 0.342 0.439 0.563 0.723 0.928

p exp(μ/kBT ) = p0 exp(μ0 /kBT )

(4)

In eq 3, YZ is the cross section area of ice surface, and the factor of 2 represents the two ice surfaces contained in the basic simulation box. In eq 4, μ0 is the chemical potential value at the condensation point, which is approximated as −63.37 kJ/mol. kB is Boltzmann constant. Therefore, the conversion of ⟨N⟩−μ to Γ−prel form of the isotherm can only be extended to the condensation point, beyond which prel would lose its physical meaning. The converted Γ−prel adsorption isotherm for nitrobenzene is shown in Figure 3B, and the corresponding data are listed in Table 2. As shown in Figure 3B, the slope of the isotherm decreases after a steeply increasing region at low prel; however, there is no saturation value before the condensation point. We try to fit the adsorption isotherm of nitrobenzene with Langmuir formalism,66 i.e., the function

Γ (μmol/m2)

p/p0

(3)

and

Table 2. Data of the Adsorption Isotherm of Nitrobenzene on Ice Surface Obtained from GCMC Simulations μ (kJ/mol)

⟨N ⟩ 2YZ

0.060 0.074 0.099 0.107 0.130 0.133 0.165 0.218 0.251 0.304 0.321 0.389 0.428 0.585 0.640

Γ = Γmax

prel KL prel KL + 1

(5)

where KL is the partition coefficient and Γmax is the surface density of the saturated monomolecular system, to probe the adsorption character of nitrobenzene. However, as can be seen in Figure 3B, the adsorption isotherm of nitrobenzene does not demonstrate well the Langmuir character in the whole considered pressure range due to some fluctuations. These fluctuations around the fitted curve are probably attributed to the violation of at least one of two assumptions of the Langmuir isotherm, which are (i) the saturated adsorption layer is strictly monomolecular; (ii) lateral interactions between the adsorbed molecules are negligible. Further analysis (in the energetics of adsorption section) confirms the fluctuations result from the existence of the weak lateral interactions between the adsorbed nitrobenzene molecules. Nevertheless, the isotherm has Langmuir-like character below 0.6 of prel (see the inset of Figure 3B). Within the range of Langmuir-like adsorption isotherm, the fitting value of the Langmuir partitioning coefficient KL is 2.80 ± 0.29 and Γmax is 0.69 ± 0.04 μmol/ m2. In addition, the adsorption of the molecules occurs independently from each other to adsorption sites where they can be strongly bound to the ice surface. These adsorption sites are called α-type adsorption sites in the following discussion for convenience. 3.3. Characterization of Adsorption Layer. Associated with the behavior of the adsorption isotherm, we have selected

System NB I. bSystem NB II. cSystem NB III. E


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Figure 4. Instantaneous equilibrium snapshots (top and side view) of the adsorption layer of nitrobenzene on the ice surface. C, N, H, and O atoms are shown by cyan, blue, white, and red color, respectively.

Figure 5. (A) Density profiles of the ipso carbon atom of the adsorbed nitrobenzene molecules in selected systems (NBs I−III). All profiles shown are averaged over two surfaces of ice in the basic GCMC simulation box. The vertical short dotted line indicates the boundary of the adsorption monolayer. (B) Distribution of cosine γ, formed by the axis perpendicular to the benzene ring of nitrobenzene and the surface normal axis of the ice surface.

three representative μ values for nitrobenzene at which the properties of the adsorbed molecules are analyzed in detail. These selected μ values, denoted as NB I (μ = −67.24 kJ/mol), NB II (μ = −63.49 kJ/mol), and NB III (μ = −63.08 kJ/mol), are marked in Figure 3 and also indicated in Table 2. The top and side views of their corresponding equilibrium snapshots are shown in Figure 4. 3.3.1. Density Profiles. To characterize the adsorption layer of nitrobenzene in detail, we have calculated the density profiles of its carbon atom numbers (Figure 5A) at those collected sample configurations (NBs I−III). As can be seen in Figure 5A, the profile curves for both systems NBs I and II are unimodal. The position of density peak of NB II is almost the same as that of NB I. Therefore, the adsorbed nitrobenzene molecules in NB II are aligned at similar orientations relative to the ice surface as those in NB I. In addition, the density peak for system NB II, just below the point of condensation, is much smaller than the first peak of condensed system NB III, indicating the condensation in fact precedes the saturation of even the monolayer adsorption. A similar case was also found in

the adsorption of naphthalene and phenanthrene on the ice surface.27 To get a deeper insight into the mechanism of the adsorption for nitrobenzene, we further analyzed the orientation of adsorbed nitrobenzene molecules relative to the ice surface and binding energy in the first adsorption layer. Similar to previous studies,67,68 the first minimum position (see the vertical short dotted line (X = 38.5 Å) in Figure 5A) in the density profile of the condensed system (NB III) is defined as the boundary of the first adsorption layer. The adsorbed nitrobenzenes, far away from 38.5 Å, are excluded in such analysis. 3.3.2. Orientation of Adsorbed Molecules. The distribution of cosine γ, i.e., the angle between the vector perpendicular to the benzene ring of a nitrobenzene molecule and the surface normal vector of the ice phase, is used to present the orientation variation of the adsorbed molecules relative to the ice surface at the first adsorption layer. The P(cos γ) distributions are shown in Figure 5B for the noncondensed systems NB I and II. As is seen in Figure 5B, the distributions for NBs I and II present only one peak at a cos γ value between F


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NB I with low surface coverage, the P(Ublat) distribution exhibits a sharp and high peak at 0 kJ/mol, indicating the adsorbed nitrobenzene molecules on the ice surface are isolated. The dominant contribution to Ub comes from the interaction of the adsorbed molecules with the ice phase Ubice. The mean value of Ubice for this distribution turns out to be −75.98 ± 0.77 kJ/mol (error corresponding to the 95% level of confidence). The electrostatic and van der Waals energies are −53.61 ± 0.76 and −22.31 ± 0.97 kJ/mol, respectively. Therefore, the electrostatic interactions contribute the most to the interaction between nitrobenzene molecule and the ice surface. The dominant electrostatic interactions could be the driving force for the orientation of adsorbed nitrobenzene on the ice surface, i.e., H atoms with positive charge of nitrobenzene point to O atoms with negative charge of H2O, and O atom with negative charge of −NO2 points to H atom with positive charge of H2O. In the system NB II (which is in the vicinity of the condensation point), the peaks of P(Ublat) and P(Ub) slightly shift to low energy, while the peaks of P(Ubice) shift to high energy, compared to corresponding peaks in NB I. The lower Ublat in NB II indicates that the driving force from the adsorbed molecules makes a part of nitrobenzene molecules change their adsorbed sites to non-α-type sites, leading to the decreased interactions between adsorbed nitrobenzene molecules and the ice surface. This can also explain the Langmuir deviation at high prel (Figure 3B). For NB III, Ublat is about twice lower than its Ubice. Therefore, the total binding energy mainly comes from the interactions of the adsorbed molecules. 3.4. Quantum Chemistry Calculations. To further confirm the reliability of the modified force field of nitrobenzene and adsorption results, we compared the interaction energy between nitrobenzene and ice surface from GCMC simulations with that from quantum chemistry calculations. We used 48 H2O molecules cut from hexagonal (Ih) ice as the ice surface model in the quantum chemistry calculations. The 48 H2O cluster consists of two bilayers and was used to describe the (0001) face of hexagonal (Ih) ice, consistent with that used in our GCMC simulations. This 48 H2O molecule ice surface model has been used in previous studies.42−44 The geometry optimizations were performed at M062X/6-31+G(d,p) level followed by M062X/6-311++G(3df,2pd) single-point energy calculations. The M062X functional was selected since it has shown good performance for nonbonded interactions.69 The Gaussian 09 software package70 was employed to perform the

0.9 and 1. This indicates all adsorbed nitrobenzene molecules in NBs I and II lie almost parallel with the ice surface. However, the peak of NB II is lightly broader than that of NB I. This means that the adsorbed nitrobenzene molecules in NB II are a little less parallel with the ice surface than those in NB I. In addition, the broader peak in NB II could mean the interaction of a nitrobenzene molecule with the ice surface or another nitrobenzene molecule in NB II is not completely the same as that in NB I, consistent with its identity as an outlier in the fitting Langmuir adsorption isotherm. As can be seen in the snapshots from system NB I, most H atoms of all adsorbed nitrobenzene molecules point toward O atom of the ice surface and O atom of nitrobenzene form H-bonds with the ice surface, corresponding to the α-type adsorption. In addition, in the αtype adsorption, either O atom or H atom of H2O points toward the phenyl group plane of nitrobenzene. This is different from the case of the adsorption of benzene on an ice surface, where only H atom points toward the benzene plane to form an OH···π bond.27 3.3.3. Energetics of Adsorption. In order to investigate the energetic background of the adsorption, we have calculated the total binding energies of adsorbed molecules on the ice surface Ub, i.e., the sum of interaction energies of the adsorbed molecules with the ice phase Ubice and with the other adsorbed molecules Ublat. The distributions of Ubice, Ublat, and Ub energies are shown in Figure 6 for all collected systems (NBs I−III). For

Figure 6. Distribution of the total binding energy (Ub) of adsorbed molecule on the ice surface and the interactions with the other adsorbed molecules (Ublat) and with the ice phase (Ubice) for the adsorption of nitrobenzene at selected systems.

Figure 7. Most stable adsorption configuration of nitrobenzene on ice surface by quantum chemistry calculations. G


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low coverage range, nitrobenzene molecules are particularly strongly bound to special sites, called α-sites, where adsorbed nitrobenzene molecules lie almost in parallel with the ice surface to facilitate strong electrostatic interactions with the ice surface. More importantly, in α-type adsorption, a typical O− H···π bond for the adsorption of aromatics on the ice surface is not preferable for nitrobenzene. With increasing surface coverage, some adsorbed nitrobenzene molecules do not take unoccupied α-sites due to attractive interactions among adsorbates, inducing a deviation of the adsorption isotherm from the Langmuir shape. The calculated adsorption energies at α-sites for nitrobenzene are more negative than that of benzene and benzaldehyde, confirming the important role of substituted group −NO2 in the process of its adsorption on the ice surface. In addition, the adsorption energies for nitrobenzene agree well with values from our validating quantum chemical calculations, further confirming the rationality of the modified force field for nitrobenzene and reliability of the adsorption results.

quantum chemistry calculations. The most stable adsorption configuration of nitrobenzene on the ice surface was determined by identifying the lowest Gibbs free energy from several different adsorption configurations. The most stable adsorption configuration of nitrobenzene is presented in Figure 7. As can be seen in Figure 7, the nitrobenzene molecule lies nearly in parallel with the ice surface with H atoms of nitrobenzene pointing toward the O atom of ice surface and O atom of nitrobenzene forming hydrogen bonds with ice surface. The adsorption configuration of nitrobenzene surface identified by quantum chemistry calculations is consistent with those at αtype adsorption sites from GCMC simulations. In addition, the interaction energy between nitrobenzene and ice surface from quantum chemistry calculation (−71.35 kJ/mol) is comparable to that from GCMC simulations, −75.98 ± 0.77 kJ/mol. The consistent results between GCMC and quantum chemistry calculation further confirm the feasibility of our modified force field for nitrobenzene and the reliability of adsorption results. 3.5. Comparison with Adsorption of Other Aromatics on Ice Surface. It is interesting to compare the adsorption behavior of nitrobenzene with its parent compound benzene and polar group −CHO containing compound benzaldehyde on the ice surface. Previous study27 found that benzene follows a simple adsorption isotherm, that is, the number of adsorbed benzene molecules gradually increases with relative pressure prel before condensation point, similar to that of nitrobenzene. However, the adsorption isotherm of benzaldehyde28 is quite complicated, presenting a “double plateaus” mode with a first steep increase, then plateau, and condensation plateau at last, which is different from that of nitrobenzene. Obviously, the substituted benzenes present distinguishing adsorption mechanisms on the ice surface depending on the substituents. By comparing the Ubice and Ublat before multilayer adsorption among three adsorbates, we can conclude that less negative Ublat for benzene and nitrobenzene could be the reason for their different adsorption mechanisms from that of benzaldehyde. In addition, this study provides the first case that polar group containing aromatics (nitrobenzene) follow a similar adsorption isotherm to their parent compound benzene. In view of adsorption energy, nitrobenzene is more negative than that of benzene and benzaldehyde. The more negative adsorption energy of nitrobenzene should result from the strong electron withdrawing ability of −NO2, which causes nitrobenzene to have much higher dipole moment, and thus leads to an enhanced or additional dipole−dipole interaction between the adsorbates and ice surface, compared with benzaldehyde and benzene.

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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.jpcc.7b03531. Geometry of nitrobenzene, computational details of density, enthalpy of vaporization, and heat capacity of the nitrobenzene and the discussion on the phase state of nitrobenzene in NB III (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +86-411-84707844. E-mail: [email protected]. ORCID

Hong-Bin Xie: 0000-0002-9119-9785 Jingwen Chen: 0000-0002-5756-3336 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Mihaly Mezei (Icahn School of Medicine at Mount Sinai) for providing the Monte Carlo Program MMC and instructions on the calculation. We thank Dr. Jicun Li for the help of our umbrella sampling simulation. This study was supported by the National Natural Science Foundation of China (21477015, 21677028, and 21325729), the Major International (Regional) Jo in t Resear ch Pr oject (21661142001), the Program for Changjiang Scholars and Innovative Research Team in University (IRT_13R05), and the Programme of Introducing Talents of Discipline to Universities (B13012).

4. CONCLUSIONS In this study, we first investigated the adsorption of Ncontaining group substituted aromatics (nitrobenzene) on the ice surface under tropospheric conditions, an issue that is of great relevance in the field of atmospheric chemistry and environmental risk assessment. The employed tool is GCMC simulation with TIP5P water model and our modified force field parameters for nitrobenzene. We found the number of adsorbed nitrobenzenes gradually increases with relative pressure prel before the condensation point, and the condensation precedes the monolayer adsorption saturation, which is similar to the case for the adsorption of its parent compound benzene, however, quite different from the case of benzaldehyde. Adsorption of nitrobenzene follows the Langmuir shape only up to a very low coverage. Within this

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REFERENCES

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Adsorption of Nitrobenzene on the Surface of Ice: A Grand Canonical

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