Adsorption of Aromatic Hydrocarbon Molecules at the Surface of Ice

Article pubs.acs.org/JPCC

Adsorption of Aromatic Hydrocarbon Molecules at the Surface of Ice, As Seen by Grand Canonical Monte Carlo Simulation Zsuzsanna E. Mészár,† György Hantal,‡ Sylvain Picaud,*,§ and Pál Jedlovszky*,∥,⊥,▽ †

Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Szt. Gellért tér 4, H-1111 Budapest, Hungary ‡ Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States § Institut UTINAMUMR CNRS 6213, Faculté des Sciences et Techniques, Université de Franche-Comté, F-25030 Besançon Cedex, France ∥ Laboratory of Interfaces and Nanosized Systems, Institute of Chemistry, Eötvös Loránd University, Pázmány Péter stny, 1/a, H-1117 Budapest, Hungary ⊥ Research Group of Technical Analytical Chemistry of the Hungarian Academy of Sciences, Szt. Gellért tér 4, H-1111 Budapest, Hungary ▽ EKF Department of Chemistry, Leányka utca 6, H-3300 Eger, Hungary ABSTRACT: The adsorption of four aromatic hydrocarbon compounds, benzene, naphthalene, anthracene, and phenanthrene, at the surface of Ih ice is investigated by grand canonical Monte Carlo (GCMC) computer simulation under tropospheric conditions at 200 K. By systematic variation of the value of adsorbate chemical potential in the simulations, the adsorption isotherms are determined. It is found that adsorption follows the Langmuir mechanism only up to a rather low relative pressure value in every case. In this range specific surface sites, called α sites, to which adsorbate molecules can be bound particularly strongly in specific orientation, are occupied. In these α sites, presumably the dangling OH bonds of the ice surface form O−H....π-type hydrogen bonds with the delocalized π electrons of the adsorbed aromatic molecule lying parallel with the ice surface. Once these α sites are saturated, lateral interactions become increasingly important, leading to large fluctuations of the lateral density of the adsorption layer and an increasing deviation of the adsorption isotherm from the Langmuir shape. The adsorption layer is found to be strictly monomolecular and even unsaturated in every case, as condensation well precedes the saturation of this monolayer for all four aromatic adsorbates considered in this study.

1. INTRODUCTION Aromatic hydrocarbon (AH) molecules are ubiquitous atmospheric pollutants arising typically from incomplete combustion of carbonaceous materials of both natural and anthropogenic origin.1,2 These molecules are not only known to have carcinogenic and mutagenic effects,3,4 but, in the troposphere, they can also undergo photochemical and/or oxidation reactions, the products of which are even more toxic.5 However, the kinetics of these reactions as well as the products that are formed may be strongly modified when these AH molecules are trapped at atmospheric air/water (water droplets, aerosols, fog) or air/ice interfaces (ice particles, snowflakes, snowpack).6 In addition, such trapping mechanisms that involve surface adsorption, incorporation into bulk ice, or dissolution in bulk water participate in scavenging AH molecules by precipitation, resulting in a cleaning effect of the atmosphere. A thorough characterization of the interactions between AH molecules and water or ice surfaces is thus required to better © 2013 American Chemical Society

quantify the effect of the corresponding trapping mechanisms. From the experimental point of view, the uptake of some gaseous AHs has been studied at the surface of ice7−9 and snow9−11 and also during vapor depositional crystal growth of ice.12 These studies have shown that AHs are reversibly adsorbed at the ice surface rather than incorporated into the ice lattice, the AH molecules being preferentially adsorbed parallel to the ice surface. Apart from these experimental approaches, computer simulation methods provide an excellent tool to obtain a molecular-level insight into the mechanism of AH adsorption on ice, since in computer simulations the appropriately chosen model of the system of interest is seen at atomic resolution.13 Thus, classical molecular dynamics simulations10,11,14−17 and potential energy calculations based on quantum approaches10 have been recently used to Received: February 12, 2013 Revised: March 11, 2013 Published: March 12, 2013 6719

dx.doi.org/10.1021/jp401532x | J. Phys. Chem. C 2013, 117, 6719−6729

The Journal of Physical Chemistry C

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characterize the adsorption and incorporation processes of AH molecules at the ice surface. The results of these studies agreed quite well with the experimental findings, showing rather weak interactions between AH and water molecules at the ice surface. However, these theoretical studies were performed at low coverage only and for a limited set of molecules (benzene, naphthalene, methylnaphthalene, and phenanthrene). As a consequence, some questions still remain open on the adsorption of AH molecules on ice, especially concerning the details of the competition between AH−ice and AH−AH interactions upon coverage increase. Here, we make use of the grand canonical Monte Carlo (GCMC) method,13,18 which, among the various computer simulation techniques, is particularly suitable for studying adsorption. In a GCMC simulation the chemical potential of a given compound (i.e., the adsorbate) is fixed, and the average number of these molecules is determined in the calculation.13 When the chemical potential is systematically varied in a set of GCMC simulations, the adsorption isotherm (i.e., the number of adsorbed molecules as a function of their chemical potential) can be computed directly. The GCMC method has been applied successfully for studying various adsorption processes, such as the adsorption of water and other small molecules at various solid surfaces, such as the surface of metal oxides,19−24 ice,25−30 covalent organic frameworks,31−33 carbonaceous materials,34−42 and self-assembled monolayers,43,44 as well as in protein crystals,45 clay minerals,46 and zeolites.47−53 The aim of the present paper is to investigate the adsorption of the smallest aromatic hydrocarbon molecules:, one-ring benzene, two-ring naphthalene, and three-ring anthracene and phenanthrene at the (0001) surface of Ih ice by GCMC simulation. Besides calculating and analyzing the adsorption isotherms themselves, we also characterize the properties of the adsorption layer in detail at various coverages, in terms of the orientation of the adsorbed molecules and of the energy of their interaction with the ice phase as well as with the adsorption layer and with the entire system. The paper is organized as follows. In section 2, details of the computer simulations performed are given. The obtained adsorption isotherms are presented and discussed in detail in section 3, whereas detailed characterization of the adsorption layer is presented in section 4. Finally, in section 5, the main conclusions of this study are summarized.

Figure 1. Structures of the four aromatic adsorbate molecules considered, together with the numbering scheme of their atoms used in Table 1. (Note that the C and H atoms linked with a chemical bond are marked with the same number.)

in the basic box to the condensed phase of the aromatic molecule. Water molecules have been described by the rigid five-site TIP5P model,54 the melting point of which is known to be rather close to that of real Ih ice.55 AH molecules have been modeled by the potential proposed by Vácha et al.56,57 According to these models, the total energy of the system has been calculated as the sum of the pair interaction energies, and the interaction energy of a molecule pair has been calculated as the sum of the Lennard-Jones and charge−charge Coulomb energy terms of all pairs of their interaction sites. All AH molecules have been rigid in the simulations; the C−C and C−H bonds have been 1.387 and 1.087 Å long, respectively, whereas the C−C−C and C−C−H bond angles have been 120°. The Lennard-Jones parameters of the C and H atoms of the aromatic molecules have been σC = 3.40 Å and εC = 0.36 kJ/mol and σH = 2.60 Å and εH = 0.063 kJ/mol, respectively.56 The fractional charges corresponding to the different atoms of the AH molecules are collected in Table 1. It should be noted that although the potential models of Vácha et al. were originally parametrized in combination with the SPC/E water model,58 it was later demonstrated by Liyana-Arachchi et al.16 on the example of naphthalene that these models are able to well reproduce the experimental free energy of hydration also in combination with the TIP5P model of water. All interactions have been truncated to 0 beyond the center− center cutoff distance of 15 Å. In accordance with the original parametrization of the water model used,54 no long-range correction has been employed. The simulations have been performed with the program MMC.59 In the simulations, particle transfer and insertion/deletion steps have been done in an alternating order. In a particle transfer step, a randomly chosen molecule (water or hydrocarbon) has been randomly translated to a maximum distance of 0.25 Å and randomly rotated around a randomly chosen space-fixed axis by no more than 15°. In an insertion/deletion step, either, by 50% probability a new aromatic molecule has been added to the system or, by 50% probability, a randomly chosen aromatic molecule has been removed from the system. Particle transfer steps have been accepted or rejected according to the standard Metropolis criterion,13 while the insertion/deletion steps have been done by use of the cavity-biased algorithm of Mezei.60,61

2. COMPUTER SIMULATIONS The adsorption of four AH molecules, benzene, naphthalene, anthracene, and phenanthrene, at the (0001) surface of a proton-disordered hexagonal Ih ice crystal has been simulated by the Monte Carlo method on the grand canonical (μ, V, T) ensemble at the tropospheric temperature of T = 200 K. The structures of the four aromatic molecules considered, along with the numbering scheme of their atoms used throughout this paper, are shown in Figure 1. The X, Y, and Z edges of the rectangular basic simulation box have been 100, 35.926, and 38.891 Å long, respectively, where X is the surface normal axis. Standard periodic boundary conditions have been applied. The basic box has contained 2880 water molecules, originally arranged in 18 layers according to the ice Ih crystal geometry in the middle of the basic box along its X axis. The chemical potential of the aromatic compound, μ, has been kept fixed in the simulations, but has been systematically varied from simulation to simulation covering the chemical potential range corresponding from systems of nearly zero molecules 6720



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Table 1. Fractional Charges Located on Different Atoms of Aromatic Hydrocarbon Molecules in the Potential Models Used57 benzene

naphthalene

anthracene

q, e

atom

q, e

atom

q, e

atom

q, e

C H

−0.16 0.16

C1 C2 H2 C3 H3

0.2212 −0.3234 0.2037 −0.1946 0.2037

C1 H1 C2 C3 H3 C4 H4

−0.454 80 0.253 84 0.184 15 −0.283 55 0.197 35 −0.196 32 0.198 85

C1 C2 H2 C3 H3 C4 H4 C5 H5 C6 C7 H7

0.087 93 −0.347 35 0.227 62 −0.171 36 0.193 36 −0.195 23 0.200 56 −0.323 76 0.210 60 0.239 44 −0.356 25 0.234 44

Thus, new particles have only been tried to be inserted into centers of empty cavities of the radius of at least 2.6 Å. Suitable cavities have been searched for along a 100 × 100 × 100 grid in the basic box. This grid has been regenerated after every 106 Monte Carlo steps. The probability of finding a cavity, Pcav, needed to correct the acceptance criteria60,61 has simply been calculated as the ratio of the number of cavities found to grid points tested. In the simulations, about 10% of the particle transfers and at least 0.2−0.5% of the insertion/deletion attempts have been successful. To calculate the adsorption isotherms, the average number of hydrocarbon molecules has been calculated in the simulations corresponding to different chemical potentials. The chemical potential values used in the simulations and average number of AH molecules found in the systems are collected in Tables 2−4.

Table 3. Data for the Adsorption Isotherm of Naphthalene on Ice, as Obtained from Simulations μ, kJ·mol−1 15.78 16.61 17.44 18.28 19.10 19.94a 20.35 20.72 21.18 21.60b 22.01 22.43c 22.85d 24.92 28.25

Table 2. Data for the Adsorption Isotherm of Benzene on Ice, as Obtained from Simulations

a

phenanthrene

atom

μ, kJ·mol−1

⟨N ⟩

30.32 31.99 33.65 34.48 35.31 36.14 36.98 37.39a 37.81 38.22 38.64 39.05b 39.47 39.89 40.30 40.72 41.13 41.55c 41.96d 43.63 45.29 46.95 48.62

0.14 0.37 0.99 1.81 2.92 4.44 7.16 8.86 11.44 13.91 17.25 22.85 27.11 38.06 46.48 52.84 62.38 67.87 358.7 355.0 354.1 358.1 355.5

p/p0 1.03 × 2.81 × 7.64 × 1.26 × 2.08 × 3.42 × 5.64 × 7.24 × 9.30 × 0.119 0.153 0.197 0.253 0.325 0.417 0.535 0.687 0.883

10−3 10−3 10−3 10−2 10−2 10−2 10−2 10−2 10−2

a

Γ, μmol·m−2

⟨N ⟩ 1.44 1.97 3.06 4.71 6.72 9.33 10.35 10.72 13.81 15.64 24.90 31.59 235.9 233.3 230.8

Γ, μmol·m−2

p/p0 1.62 × 2.66 × 4.39 × 7.24 × 0.119 0.197 0.253 0.325 0.417 0.535 0.687 0.883

−2

10 10−2 10−2 10−2

0.086 0.118 0.183 0.281 0.401 0.556 0.617 0.639 0.824 0.933 1.485 1.884

System N1. bSystem N2. cSystem N3. dSystem N4.

The systems have been equilibrated by performing 108−109 Monte Carlo steps long runs. The average number of aromatic molecules in the system, ⟨N⟩ , has then been calculated in 2 × 108 Monte Carlo steps long production runs. Finally, at selected chemical potential values, 2000 sample configurations, separated by 2.5 × 105 Monte Carlo steps each, have been saved for further evaluation. These systems (in order of increasing chemical potential values) are marked here as B1− B4 for benzene, N1−N4 for naphthalene, A1−A4 for anthracene, and P1−P4 for phenanthrene (see Tables 2−4). In the analyses, the two ice surfaces present in the basic box have been treated separately; thus, all the results have been averaged not only over the 2000 sample configurations but also over these two ice surfaces per system. For illustration, an equilibrium snapshot of the four systems considered are shown in Figure 2 as obtained at the chemical potential value that precedes condensation.

0.008 0.022 0.059 0.108 0.174 0.265 0.427 0.528 0.682 0.830 1.029 1.363 1.617 2.270 2.772 3.152 3.721 4.048

3. ADSORPTION ISOTHERMS The adsorption isotherms in their ⟨N⟩ versus μ form have simply been calculated by determining the average number of hydrocarbon molecules, ⟨N⟩ , as a function of their chemical potential, μ, the value of which has been set in each simulation. The obtained ⟨N⟩−μ isotherms are plotted in Figure 3 and the corresponding data are summarized in Tables 2−4. As is seen,

System B1. bSystem B2. cSystem B3. dSystem B4. 6721



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Table 4. Data for the Adsorption Isotherms of Anthracene and Phenanthrene on Ice, as Obtained from Simulations anthracene μ, kJ·mol−1 −5.28 −3.62 −2.79 −1.96 −1.54 −1.12 −0.71 −0.29 0.12 0.54a 1.37 2.20 2.61 3.03 3.86b,c 4.28 4.70 5.11 5.53 5.94d 6.35e 7.19f 7.61 8.02 8.85 9.68 10.52 11.35g 11.76 13.01h 16.34 18.00 19.66 a

⟨N ⟩

p/p0

phenanthrene Γ, μmol·m−2

0.13

2.96 × 10−4

0.008

0.94

8.05 × 10−4

0.056

1.45 2.06

2.19 × 10−3 3.61 × 10−3

0.086 0.123

3.43 4.84

5.95 × 10−3 9.80 × 10−3

0.205 0.289

5.72

1.62 × 10−2

0.341

7.55

2.67 × 10−2

0.450

9.14 11.13

4.39 × 10−2 7.24 × 10−2

0.545 0.664

0.119 0.197 0.325 0.535 0.883

0.763 0.961 1.210 1.371 1.458

⟨N ⟩ 0.88 1.27 1.90 2.19 2.50 3.24 3.32 3.66 4.22 4.93 5.54 6.20 7.00 7.78 10.27 10.92 13.53 14.63 17.55 21.30 179.5

p/p0 1.03 × 2.81 × 4.63 × 7.64 × 9.80 × 1.26 × 1.62 × 2.08 × 2.67 × 3.42 × 5.64 × 9.30 × 0.119 0.153 0.253 0.325 0.417 0.535 0.687 0.883

10−3 10−3 10−3 10−3 10−3 10−2 10−2 10−2 10−2 10−2 10−2 10−2

Γ, μmol·m−2 0.052 0.076 0.113 0.131 0.149 0.193 0.198 0.218 0.251 0.294 0.330 0.370 0.418 0.464 0.613 0.651 0.807 0.873 1.047 1.270

178.0 12.79 16.11 20.29 22.99 24.44 165.3 161.9 162.7 159.0 161.0

176.0 176.8

System P1. bSystem A1. cSystem P2. dSystem P3. eSystem P4. fSystem A2. gSystem A3. hSystem A4.

all these isotherms exhibit a continuous rise up to the point of condensation, apart from the very slight plateau, covering only about 1−2 kJ/mol chemical potential range right below the point of condensation of anthracene. Also, the steepness of the isotherm slightly decreases before the point of condensation in the case of benzene, whereas for naphthalene and phenanthrene an exponential-like isotherm, having a gradually increasing slope in the entire range of the vapor phase, is obtained. It is also seen that condensation of smaller AHs occurs at higher chemical potential values. Among the two components of equal molecular mass, anthracene condenses at considerably (i.e., by about 5.5 kJ/mol) higher chemical potential value than phenanthrene. To further analyze the adsorption isotherms, we have converted the ⟨N⟩−μ data to the more conventional Γ versus prel form, where Γ is the surface density of the adsorbed molecules Γ=

N 2YZ

prel =

p exp(βμ) = p0 exp(βμ0 )

(2)

In eq 1 the factor of 2 in the denominator accounts for the two ice surfaces present in the basic box; in eq 2 β = 1/kBT, where kB is the Boltzmann constant, and μ0 is the chemical potential value corresponding to the point of condensation. The value of μ0 is estimated to be 41.76, 22.64, 11.55, and 6.15 kJ/mol for benzene, naphthalene, anthracene, and phenanthrene, respectively, from the corresponding ⟨N⟩−μ isotherms. According to eq 2, the ⟨N⟩−μ isotherms can only be converted to the Γ versus prel form up to the point of condensation, above which prel loses its physical meaning. The Γ and prel values corresponding to the different systems simulated are also included in Tables 2−4. The adsorption isotherms in their Γ−prel form are shown in Figure 4 as obtained for the four AH molecules considered. As is seen, after a steeply increasing part at low prel values, the slope of the isotherms decreases gradually; however, with the exception of anthracene, they do not approach a saturation value. Furthermore, all four data sets deviate considerably from the Langmuir isotherm.62,63 To demonstrate this, we have fitted the function,

(1)

and prel is the relative pressure (i.e., pressure normalized by that of the saturated vapor, p0), calculated as21 6722



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Figure 3. Adsorption isotherms, shown in the form of average number of molecules of benzene (black squares), naphthalene (red circles), anthracene (green up triangles), and phenanthrene (blue down triangles) in the basic simulation box as a function of their chemical potential. The lines connecting the points are just guides to the eye. Arrows indicate the systems that have been considered for detailed analyses.

Figure 4. Adsorption isotherms in the surface density vs relative pressure form, as obtained for benzene (black squares), naphthalene (red circles), anthracene (green up triangles), and phenanthrene (blue down triangles). Dashed lines show the best Langmuir fit to the simulated data. (Inset) Low-pressure part of these isotherms (symbols), together with the Langmuir fit to these low-pressure data only (solid lines). Arrows indicate the systems that have been considered for detailed analyses.

Figure 2. Instantaneous equilibrium snapshot of the systems containing (a) benzene, (b) naphthalene, (c) anthracene, and (d) phenanthrene molecules, simulated at the chemical potential value that just precedes condensation (i.e., systems B3, N3, A3, and P3, respectively), shown in both top view (left column) and side view (right column). Water and adsorbate molecules are represented by blue and black sticks, respectively.

Γ = Γmax

Although the shape of the isotherms obtained for naphthalene and phenanthrene might also be compatible with multilayer adsorption (as they reach a saturation plateau around the p0 value of 0.3−0.4 and exhibit another rise at larger pressures), the equilibrium snapshots shown in Figure 2 suggest that the adsorption of these compounds is limited to solely one molecular layer. This suggests that the deviation of the obtained isotherms from the Langmuir shape can probably be attributed to the non-negligible lateral interaction between the adsorbed aromatic molecules. To investigate this point in detail, we have calculated the number density profiles of the C atoms across the various systems simulated. The resulting density profiles are shown in Figure 5. As is clear, the profiles obtained in the systems below the point of condensation are unimodal, with the exception of systems B3 and A3, in which a shoulder and a small secondary peak, respectively, are seen at the outer side of the main density peak. However, the position of this shoulder and secondary

prel K prel K + 1

(3)

corresponding to the Langmuir isotherm to the obtained Γ−prel data for all the four aromatic compounds. (In eq 3, Γmax is the surface density of the saturated system and K is the Langmuir partition coefficient.) As is seen from Figure 4, the simulated data fluctuate around the fitted function in every case. Since the Langmuir description of adsorption is based on two assumptions(i) the saturated adsorption layer is strictly monomolecular, and (ii) there are no lateral interactions between the adsorbed molecules63the failure of the Langmuir fit indicates that at least one of these two conditions is not fulfilled in the systems studied here. 6723



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distribution exhibits a long, exponentially decaying tail at its large area side.64 The VP area distributions obtained in systems B2, N2, A2, and P2 are shown in Figure 6. As is seen, at the intermediate

Figure 6. Voronoi polygon area distribution of the adsorbed molecules in the ice surface plane, YZ, as obtained in the intermediate surface density systems B2 (black squares), N2 (red circles), A2 (green up triangles), and P2 (blue down triangles). For the definition of VP of the molecules, see text. The vertical axis is shown on a logarithmic scale, in order to emphasize the exponential character of the decaying tail of the peak at large area values.

Figure 5. Carbon atom number density profiles of the adsorbed benzene (top panel), naphthalene (second panel), anthracene (third panel), and phenanthrene (bottom panel) molecules in systems 1−4. The number density profile of the surface waters is also indicated (dotted line). All profiles shown are averaged over the two ice surfaces present in the basic simulation box.

pressures corresponding to these systems, the P(A) distributions exhibit an exponentially decaying tail at high area values, indicating considerable lateral density fluctuations in the adsorption layer. This finding reveals that the adsorption of these aromatic compounds is facilitated by the presence of other, already adsorbed molecules. In other words, AH molecules prefer to be adsorbed in the vicinity of each other at the surface of ice. The fact that the attraction between the aromatic molecules contributes non-negligibly to the thermodynamic driving force of their adsorption can thus account for the non-Langmuir character of the adsorption isotherm. It should finally be noted that an adsorption isotherm of rather similar shape was previously observed for formic acid at the surface of ice.27 Thus, up to a certain, low p0 value, the formic acid isotherm exhibited a Langmuir shape, whereas at higher pressures it turned again to a sharp increase. This shape indicated (i) the presence of a few sites of particularly strong adsorption (called α sites) and (ii) that after the saturation of these α sites (which well precedes the saturation of the first molecular layer) lateral interactions strongly contribute to the driving force of the adsorption.27 The similar shape of the isotherms obtained here suggests a similar adsorption mechanism in the present case. Thus, here again, all the isotherms are perfectly Langmuir-like up to a certain, rather low p0 value (see the inset of Figure 4). Up to this point, the adsorption of the molecules occurs independently from each other to adsorption sites at which they can be particularly strongly bound to the ice phase. These α sites are likely to be the dangling O−H bonds at the ice surface. (In the case of molecules of more than one aromatic ring, the arrangement of these dangling OH groups at the α site is likely to correspond to the arrangement of the aromatic rings within the adsorbed molecule.) At such α sites a weak hydrogen bond can be formed between the dangling OH group and the aromatic π electrons of the adsorbed molecule. Similar O−

peak does not coincide with that of the second peak in the corresponding condensed systems, indicating that, in accordance with what is seen in the equilibrium snapshots (Figure 2), these shoulders reflect the presence of a considerable fraction of adsorbed molecules that are aligned strongly tilted or perpendicular rather than nearly parallel relative to the ice surface, and not the formation of a second molecular layer. This point is further investigated in the following section. It is also seen that the density peak obtained in systems just below the point of condensation is always considerably smaller than the first peak of the corresponding condensed system. This difference, which is particularly large in the case of naphthalene and phenanthrene, indicates, in accordance with the snapshots seen in Figure 2, that condensation in fact precedes saturation of even the monomolecular adsorption layer. This finding can also explain the rising shape of the obtained isotherms even upon approaching the point of condensation. To demonstrate that the deviation of the obtained isotherms from the Langmuir form is indeed due to the non-negligible lateral interaction of the adsorbed molecules, we have performed Voronoi analysis of the adsorption layer. For this purpose, we have projected the centers-of-mass of the adsorbed molecules to the surface plane of the ice phase, YZ, and calculated the distribution of the area A of their Voronoi polygons (VP). In a set of planar seeds, the VP of a given seed covers all the points in the plane that are closer to this seed than to any other one. If the seeds are randomly distributed, their VP area distribution follows a Gaussian shape. However, in the case of large density fluctuations of the seeds, this 6724



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formation of O−H···π-type hydrogen bonds between the dangling OH groups of the ice phase and delocalized π electrons of the adsorbed aromatic molecules is a strong driving force of the adsorption, and these adsorption sites become saturated at rather low pressures. Parallel adsorption on the surface is also in agreement with previous simulation results.10,11,15−17 It is also seen that in the A3 system the P(cos γ) distribution exhibits a small second peak around 0.65, indicating that in this system the molecules have also a secondary orientational preference for tilted alignments. Integrating the P(cos γ) distribution up to the cos γ value of 0.85 (i.e., the minimum position between these two peaks) reveals that about 5% of the adsorbed anthracene molecules contribute to this secondary peak. This value agrees well with the fraction of anthracene molecules contributing to the second peak of the anthracene density profile in this system (see Figure 5). To demonstrate that the preference for the tilted orientation can be attributed to these relatively distant anthracene molecules, we have calculated the P(cos γ) distribution in the A3 system separately for those anthracene molecules the center-of-mass of which is located at |X| ≤ 35 Å (i.e., that contribute to the main peak of the anthracene density profile), and for those having their center-of-mass at |X| > 35 Å (i.e., contributing to the second peak of this density profile). The obtained P(cos γ) distributions are shown by open and solid circles, respectively, in the inset of Figure 7. This figure clearly shows that the secondary peak of the P(cos γ) distribution at 0.65, corresponding to tilted orientations, is given by those anthracene molecules that are located farther from the ice surface than the main density peak. This finding also confirms our previous assumption that the presence of a second, small density peak in the A3 system, well below the X value corresponding to the second peak in condensed anthracene (i.e., to the position of the second molecular layer above the ice surface), reflects the appearance of another orientational preference rather than the building up of a second molecular layer at the ice surface. 4.2. Energetics of Adsorption. In order to characterize the energetic background of the adsorption, we have calculated the distribution of binding energy of the adsorbed molecules in the 12 noncondensed systems simulated. The Ub binding energy is simply defined as the total interaction energy of a given adsorbed molecule with the rest of the system (i.e., the energy required to bring this molecule to infinite distance). Besides the distribution of the full Ub binding energy, we have also calculated the distribution of the contributions to Ub due to interaction of the adsorbed aromatic molecule with the ice phase, Ubice, and due to interaction with other adsorbed molecules, Ublat. Clearly, Ub is simply the sum of Ubice and Ublat. The P(Ub), P(Ubice), and P(Ublat) distributions obtained in the three noncondensed systems of the four adsorbates considered are shown in Figure 8. As is seen, the total binding energy distribution looks rather similar in the first two systems (i.e., B1 and B2, N1 and N2, A1 and A2, P1 and P2), far from the point of condensation in every case. It is also seen that the lateral contribution to Ub is almost negligible in these systems, as the P(Ublat) distribution exhibits a very sharp and high peak at zero energy. These systems correspond to the more or less Langmuir-like part of the adsorption isotherms. In the vicinity of the point of condensation, however, the P(Ubice) distribution is always shifted to higher energy values and develops a second peak at the high-energy side of the main

H···π-type hydrogen bonding was also found to play an important role in the adsorption of benzaldehyde at the ice surface.29 Once the α sites are saturated, lateral interaction, presumably between the aromatic π electrons of two parallelaligned neighboring adsorbed molecules (again, in accordance with previous findings for the adsorption of benzaldehyde)29 becomes also an important factor of the adsorption, and hence the isotherms start to deviate from the Langmuir shape.

4. CHARACTERIZATION OF THE ADSORPTION LAYER 4.1. Orientation of Adsorbed Molecules. To characterize the orientation of the adsorbed molecules relative to the ice surface, we have calculated the cosine distribution of the angle γ, formed by the surface normal axis of the ice phase, X, and the axis perpendicular to the plane of the adsorbed molecule. Thus, γ is the angle between the molecular plane and plane of the ice surface. The P(cos γ) distributions obtained in the three noncondensed systems of the four adsorbates considered are shown in Figure 7. As is seen, the adsorbed aromatic molecules

Figure 7. Cosine distribution of the angle γ, formed by the ice surface plane YZ and plane of an adsorbed benzene (top panel), naphthalene (second panel), anthracene (third panel), and phenanthrene (bottom panel) molecule in systems 1−3. (Inset) Separate distributions of the anthracene molecules that are located (○) closer to and (●) farther from the center of the basic box along the surface normal axis X than 35 Å, i.e., the minimum between the two density peaks, in system A3.

strongly prefer parallel alignment with the ice surface in every case, as the obtained distributions always exhibit a sharp and high peak at the cos γ value of 1. This preference for the parallel alignment is stronger at lower surface coverages, in accordance with the fact that at lower surface coverages the competition of the adsorbed molecules is less strong, and hence their arrangement can be closer to the optimal one than at higher coverages. It should also be noted that the observed strong preference of the adsorbed molecules for parallel alignment with the ice surface, in particular at low coverages, is in full accordance with our previous finding that the possibility of 6725



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Figure 8. Distribution of the total binding energy of an adsorbed (a) benzene, (b) naphthalene, (c) anthracene, and (d) phenanthrene molecule (i.e., the energy of its interaction with the rest of the system, bottom panels), and its contributions coming from the interaction with the other adsorbed molecules (middle panels) and with the ice phase (top panels) in systems 1 (solid curves), 2 (dashed lines), and 3 (dash-dotted lines). (Inset) Separate distributions of the anthracene molecules that are located (○) closer to and (●) farther from the center of the basic box along the surface normal axis X than 35 Å, i.e., the minimum between the two density peaks, in system A3.

peak. On the other hand, the P(Ublat) distribution broadens considerably to low (attractive) energies. Thus, in the N3, A3, and P3 systems it exhibits a shoulder between about −25 and −50 kJ/mol; in A3 and P3 it exhibits also a small peak at about −30 and −20 kJ/mol, respectively, whereas in B3 the peak at zero energy completely disappears and the P(Ublat) distribution has a single peak at about −20 kJ/mol. This result is again in clear accordance with what has already been found in the analysis of the adsorption isotherms, namely, that at high relative pressures the non-Langmuir character of the isotherms originates from the non-negligible lateral interactions. Indeed, in the B3 system the P(Ubice) and P(Ublat) distributions have their peaks at −34 and −22 kJ/mol, respectively, whereas in the N3, A3, and P3 systems the low energy tail of the P(Ublat) distribution extends roughly to the position of the main, lowest energy peak of P(Ubice). The strong overlap of the P(Ubice) and P(Ublat) distributions in these systems confirms that lateral

interactions contribute comparably with the adsorbate−ice interaction to the total binding energy, and hence lateral interactions represent an important part of the thermodynamic driving force of adsorption in these systems.

5. SUMMARY AND CONCLUSIONS In this paper we presented a detailed analysis of the adsorption of several small aromatic hydrocarbon compounds at the surface of ice under tropospheric conditions by computer simulation methods. The results revealed that the mechanism of this adsorption is not at all as simple as might be expected for apolar adsorbates and demonstrated the delicate interplay of the ice−adsorbate and lateral interactions in determining the thermodynamic driving force of the adsorption. Interestingly, the observed adsorption mechanism turned out to be rather similar to that of a strongly polar compound, formic acid.27 Thus, up to a certain point at low relative pressure values, the 6726



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(2) Zhang, Y.; Tao, S. Global Atmospheric Emission Inventory of Polycyclic Aromatic Hydrocarbons (PAHs) for 2004. Atmos. Environ. 2009, 43, 812−819. (3) Agency for Toxic Substances and Disease Registry. Toxicological Profile for Polycyclic Aromatic Hydrocarbons (PAHs); U.S. Department of Health and Human Service, Public Health Service, Atlanta, GA, 1995; www.atsdr.cdc.gov/ToxProfile/tp69.pdf. (4) Ramirez, N.; Cuadras, A.; Rovira, E.; Marcé, R. M.; Borrull, F. Risk Assessment Related to Atmospheric Polycyclic Aromatic Hydrocarbons in Gas and Particle Phases near Industrial Sites. Environ. Health Perspect. 2011, 119, 1110−1116. (5) Finlaysson-Pitts, B.; Pitts, J. N. Chemistry of the Upper and Lower Atmosphere; Academic Press: New York, 2000. (6) Donaldson, D. J.; Valsaraj, K. T. Adsorption and Reaction of Trace Gas-Phase Organic Compounds on Atmospheric Water Film Surfaces: A Critical Review. Environ. Sci. Technol. 2010, 44, 865−873. (7) Fries, E.; Haunold, W.; Jaeschke, W.; Hoog, I.; Mitra, S. K.; Borrmann, S. Uptake of Gaseous Aromatic Hydrocarbons by NonGrowing Ice Crystals. Atmos. Environ. 2006, 40, 5476−5485. (8) Kahan, T. F.; Donaldson, D. J. Photolysis of Polycyclic Aromatic Hydrocarbons on Water and Ice Surfaces. J. Phys. Chem. A 2007, 111, 1277−1285. (9) Abbatt, J. P. D.; Bartels-Rausch, T.; Ullerstam, M.; Ye, T. J. Uptake of Acetone, Ethanol and Benzene to Snow and Ice: Effects of Surface Area and Temperature. Environ. Res. Lett. 2008, 3, No. 045008. (10) Heger, D.; Nachtigallová, D.; Surman, F.; Krausko, J.; Magyarová, B.; Brumovský, M.; Rubeš, M.; Gladich, I.; Klán, P. SelfOrganization of 1-Methylnaphthalene on the Surface of Artificial Snow Grains: A Combined Experimental−Computational Approach. J. Phys. Chem. A 2011, 115, 11412−11422. (11) Dominé, F.; Cincinelli, A.; Bonnaud, E.; Martellini, T.; Picaud, S. Adsorption of Phenanthrene on Natural Snow. Environ. Sci. Technol. 2007, 41, 6033−6038. (12) Fries, E.; Starokozhev, E.; Haunold, W.; Jaeschke, W.; Mitra, S. K.; Borrmann, S.; Schmidt, M. U. Laboratory Studies on the Uptake of Aromatic Hydrocarbons by Icy Crystals during Vapor Depositional Crystal Growth. Atmos. Environ. 2007, 41, 6156−6166. (13) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Clarendon: Oxford, U.K., 1987. (14) Ardura, D.; Kahan, T. F.; Donaldson, D. J. Self-Association of Naphthalene at the Air−Ice Interface. J. Phys. Chem. A 2009, 113, 7353−7359. (15) Chen, J.; Ehrenhauser, F.; Liyana-Arachchi, T. P.; Hung, F. R.; Wornat, M. J.; Valsaraj, K. T. Adsorption of Gas-Phase Phenanthrene on Atmospheric Water and Ice Films. Polycyclic Aromat. Compd. 2011, 31, 201−226. (16) Liyana-Arachchi, T. P.; Valsaraj, K. T.; Hung, F. R. Molecular Simulation Study of the Adsorption of Naphthalene and Ozone on Atmospheric Air/Ice Interfaces. J. Phys. Chem. A 2011, 115, 9226− 9236. (17) Liyana-Arachchi, T. P.; Valsaraj, K. T.; Hung, F. R. Ice Growth from Supercooled Aqueous Solutions of Benzene, Naphthalene and Phenanthrene. J. Phys. Chem. A 2012, 116, 8539−8546. (18) Adams, D. J. Grand Canonical Ensemble Monte Carlo for a Lennard-Jones Fluid. Mol. Phys. 1975, 29, 307−311. (19) Puibasset, J.; Pellenq, R. J. M. Water Adsorption on Hydrophilic Mesoporous and Place Silica Substrates: A Grand Canonical Monte Carlo Simulation Study. J. Chem. Phys. 2003, 118, 5613−5622. (20) Puibasset, J.; Pellenq, R. J. M. Water Adsorption in Disordered Mesoporous Silica (Vycor) at 300 and 650 K: A Grand Canonical Monte Carlo Simulation Study of Hysteresis. J. Chem. Phys. 2005, 122, 094704−1−10. (21) Daub, C. D.; Patey, G. N.; Jack, D. B.; Sallabi, A. K. Monte Carlo Simulations of the Adsorption of CO2 on the MgO(100) Surface. J. Chem. Phys. 2006, 124, 114706−1−9. (22) Croteau, T.; Bertram, A. K.; Patey, G. N. Adsorption and Structure of Water on Kaolinite Surfaces: Possible Insight into Ice Nucleation from Grand Canonical Monte Carlo Calculations. J. Phys. Chem. A 2008, 112, 10708−10712.

adsorption is still perfectly Langmuir-like. In this pressure range the aromatic compounds occupy specific surface positions (the α sites) to which they can be bound particularly strongly in certain orientations. At these α sites, similarly to the adsorption of benzaldehyde,29 the dangling OH bonds of the ice surface can form an O−H···π-type hydrogen bond with the delocalized π electrons of the aromatic molecule, which lies parallel to the ice surface. Up to the saturation of these α sites, lateral interactions are negligible. At higher pressures, however, the newly arriving aromatic molecules can be bound much less strongly to the ice surface than those occupying the α positions, while on the other hand they interact much more strongly with the other adsorbed molecules. In other words, at higher pressures the importance of the lateral interactions increases strongly. Simultaneously, the adsorption layer exhibits strong lateral density fluctuations, and the adsorption isotherm increasingly deviates from the Langmuir shape. However, the adsorption layer remains strictly monomolecular up to the point of condensation. Furthermore, condensation precedes even the saturation of this monomolecular layer in every case, in agreement with experimental findings for the benzene molecule.9 It should be finally noted that lateral interactions were also found to play a key role in the opposite adsorption process, that is, adsorption of water at the surface of larger graphene-based carbonaceous particles, such as soot grains.40,41 This fact emphasizes the importance of the studied adsorption processes in atmospheric chemistry. Namely, in spite of the relatively low mutual affinity of water and graphene-like aromatic compounds, they can facilitate the formation of larger grains of the other particles at the surface of each other by providing adsorption sites for them. Once a few unlike molecules are already adsorbed at the surface of a particle formed by the other molecules (i.e., either a few water molecules in suitable pockets at the surface of soot41 or a few aromatic molecules at the α sites of the ice surface), they can attract more molecules via their strong lateral interactions. These aggregates of the adsorbed molecules can then provide new surfaces for the opposite adsorption process. This mechanism can then lead to the formation of mixed aqueous−aromatic grains in the troposphere.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (S.P.), [email protected]. hu (P.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the Hungarian OTKA Foundation under Project 104234, and by the HungarianFrench Intergovernmental Science and Technology Program (BALATON) under Project TÉT_09-1-2010-0067. We are grateful to Dr. Mária Darvas (SISSA, Trieste) for useful discussions and to Dr. Pavel Jungwirth (UOCHB, Prague) for providing the potential models of the hydrocarbons.

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REFERENCES

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Adsorption of Aromatic Hydrocarbon Molecules at the Surface of Ice

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