Adsorption of Acetaldehyde on Ice As Seen from Computer Simulation

Article pubs.acs.org/Langmuir

Adsorption of Acetaldehyde on Ice As Seen from Computer Simulation and Infrared Spectroscopy Measurements Mária Darvas,†,‡ Jérôme Lasne,§ Carine Laffon,§ Philippe Parent,§ Sylvain Picaud,† and Pál Jedlovszky*,‡,∥,⊥ †

Institut UTINAMUMR CNRS 6213, Faculté des Sciences, 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 § Laboratoire de Chimie Physique - Matière et Rayonnement, Université Pierre et Marie Curie (UPMC − Univ. Paris 06) and CNRS (UMR 7614), 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France ∥ HAS Research Group of Technical Analytical Chemistry, Szt. Gellért tér 4, H-1111 Budapest, Hungary ⊥ EKF Department of Chemistry, Leányka u. 6, H-3300 Eger, Hungary ‡

ABSTRACT: Detailed investigation of the adsorption of acetaldehyde on Ih ice is performed under tropospheric conditions by means of grand canonical Monte Carlo computer simulations and compared to infrared spectroscopy measurements. The experimental and simulation results are in a clear accordance with each other. The simulations indicate that the adsorption process follows Langmuir behavior in the entire pressure range of the vapor phase of acetaldehyde. Further, it was found that the adsorption layer is strictly monomolecular, and the adsorbed acetaldehyde molecules are bound to the ice surface by only one hydrogen bond, typically formed with the dangling H atoms at the ice surface, in agreement with the experimental results. Besides this hydrogen bonding, at high surface coverages dipolar attraction between neighboring acetaldehyde molecules also contributes considerably to the energy gain of the adsorption. The acetaldehyde molecules adopt strongly tilted orientations relative to the ice surface, the tilt angle being scattered between 50° and 90° (i.e., perpendicular orientation). The range of the preferred tilt angles narrows, and the preference for perpendicular orientation becomes stronger upon saturation of the adsorption layer. The CH3 group of the acetaldehyde molecules points as straight away from the ice surface within the constraint imposed by the tilt angle adopted by the molecule as possible. The heat of adsorption at infinitely low coverage is found to be −36 ± 2 kJ/mol from the infrared spectroscopy measurement, which is in excellent agreement with the computer simulation value of −34.1 kJ/mol.

1. INTRODUCTION Oxygenated hydrocarbons are abundant in the upper troposphere (UT),1−3 where their sources include direct biogenic emissions and biomass burning, while their destruction is dominated by reactions with OH radicals and by photolysis.4 The destruction of oxygenated hydrocarbons results in the formation of HOx (OH + HO2) and hence leads to an enhanced abundance of strong oxidants in the atmosphere. These oxidants can affect the concentration of many other trace gases and likely modify ozone cycles in the UT. In addition to having a strong impact on the HOx production, oxygenated hydrocarbons may also impact on the formation of the peroxyacetyl-nitryl radical (PAN), which is a long-lived NOy reservoir being able to transport nitrogen oxides from one region of the atmosphere to another one.5 At the same time, UT is characterized by low temperatures ranging from about 188 to 233 K and by the presence of ice clouds (cirrus), which can promote heterogeneous processes, whose role cannot be ignored to better understand the physicochemical behavior of this region of the atmosphere.6 © 2012 American Chemical Society

Because of their atmospherical importance, a growing number of experimental and theoretical studies have been recently devoted to the characterization of the interactions between oxygenated hydrocarbons and ice.4,7−22 They have shown that the interaction between such molecules and ice corresponds to physical adsorption (i.e., without any chemical bonding), and the corresponding adsorption enthalpies have been measured within the range between −70 and −40 kJ/mol, with the exception of formaldehyde for which the adsorption energy has been calculated to be around −30 kJ/mol.15 This high value (with respect to other oxygenated hydrocarbons) has thus been invoked to explain the low affinity of formaldehyde to the ice surface. In a similar way, experimental investigations at low temperatures (i.e., below 200 K) showed that the uptake of acetaldehyde by pure ice surfaces should be also quite weak.4 However, because acetaldehyde is relatively abundant in the Received: November 14, 2011 Revised: January 12, 2012 Published: February 9, 2012 4198

dx.doi.org/10.1021/la204472k | Langmuir 2012, 28, 4198−4207

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UT (between 30 and 100 pptv) and its atmospheric fate is of great interest, its uptake on ice has been recently reinvestigated at tropospheric temperatures, using coated wall flow tube experiments between 203 and 253 K.20 Adsorption of acetaldehyde on ice was thus showed to be totally reversible, with measured values of the adsorption enthalpy falling between 41.6 ± 2.3 kJ/mol (at 223 K) and −36.4 ± 2.0 kJ/ mol (at 203 K),20 significantly lower than the values calculated for formaldehyde.15 The goal of the present paper is thus to examine the interaction of acetaldehyde with the ice surface at 200 K (i.e., a temperature which is relevant for the upper troposphere), on the basis of grand canonical ensemble Monte Carlo (GCMC) simulations,23,24 similarly to our previous study on formaldehyde.15 The main advantage of using the GCMC method is that it not only allows the analysis of the energy and surface orientation of the adsorbed molecules at various surface coverages, but it is capable of determining the adsorption isotherm from extremely low pressures up to and above the point of condensation.14−17,21 Further, the information obtained from the simulations is supported, for the first time, by a direct comparison with experimental results based on infrared spectroscopy measurements. This approach, which thus combines computer simulations and infrared spectroscopy in the investigation of the acetaldehyde/ice system, allows us to get more detailed and relevant information than what could be reached either by experiment or theory alone. The good agreement obtained here between simulated and experimental results highlights the relevance of the calculations, and reinforces also the conclusions reached earlier for the formaldehyde/ice system. The paper is organized as follows. In section 2, details of both the calculations and the infrared spectroscopy measurements performed are given. In section 3, the results obtained from the simulations, concerning the adsorption isotherm, the energetic background of the adsorption, and the orientation of the adsorbed acetaldehyde molecules at various surface coverages are presented. The experimental results are presented, discussed, and compared with the results obtained from the simulations in section 4. Finally, in section 5 the main conclusions of this study are summarized.

Table 1. Data of the Adsorption Isotherm of Acetaldehyde on Ice As Obtained from our Simulations μ/kJ mol−1 −47.11 −45.42 −43.73 −42.87 −42.04 −41.20a −40.35 −39.51 −38.67 −37.82b −36.98 −36.14 −35.29 −34.45c −34.28 −34.11 −33.72 −33.60 −31.92 −30.23 a

⟨N⟩ 1.57 2.48 7.62 10.40 15.17 22.31 34.32 48.00 64.33 86.50 102.0 114.9 125.4 137.0 141.6 609.0 605.0 608.9 614.3 629.4

Γ/μmol m−2

p/p0 4.76 × 1.29 × 3.52 × 5.80 × 9.56 × 1.58 × 2.60 × 4.29 × 7.07 × 0.116 0.192 0.317 0.522 0.861 0.951

−4

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

0.09 0.15 0.45 0.62 0.90 1.33 2.04 2.85 3.82 5.14 6.06 6.82 7.45 8.14 8.41

System I. bSystem II. cSystem III.

used potential models are summarized in Table 2. All interactions have been truncated to zero beyond the molecule-based cutoff distance of

Table 2. Interaction Parameters of the Water and Acetaldehyde Potential Models Used in the Simulations molecule water

acetaldehyde

a

interaction site

σ/Å

(ε/kB)/K

q/e

O H La CH3 CH O

3.12 0.00 0.00 3.75 3.72 3.05

80.6 0.0 0.0 98.0 54.0 79.0

0.000 0.241 −0.241 −0.043 0.525 −0.482

Nonatomic interaction site.

12.5 Å. In accordance with the original parametrization of the water potential models used,25 no long-range correction has been applied for the electrostatic interactions. The simulations have been performed using the program MMC.28 In the simulations particle displacement and particle insertion/deletion steps have been made in alternating order. In a particle displacement attempt a randomly chosen molecule has been randomly translated by no more than 0.25 Å and randomly rotated around a randomly chosen space-fixed axis by no more than 15°. Water and acetaldehyde molecules have been chosen for particle displacement steps by 50%− 50% probabilities. In a particle insertion/deletion step either a randomly chosen acetaldehyde molecule has been attempted to be removed from the system or a new acetaldehyde molecule has been attempted to be added to the system. Particle insertion and deletion attempts have been made by 50%−50% probabilities. The particle insertion/deletion steps have been done using the cavity-biased scheme of Mezei;29,30 i.e., particles have only been attempted to be inserted into empty cavities of the radius of at least 2.5 Å. Suitable cavities have been searched for along a 100 × 100 × 100 grid. The systems have been equilibrated by performing 108 Monte Carlo steps. After equilibrium has been reached the number of acetaldehyde molecules in the system has been averaged over 2 × 108 sample configurations. Finally, at selected chemical potential values (see Table 1) 2500 sample configurations, separated by 2 × 105 Monte Carlo steps long trajectories each, have been saved for detailed analysis.

2. METHODS 2.1. Computer Simulations. Adsorption of acetaldehyde on Ih ice has been simulated by the Monte Carlo method on the grand canonical (μ,V,T) ensemble at 200 K. The X, Y, and Z edges of the rectangular basic simulation box have been set to 100.0, 35.926, and 38.891 Å long, respectively, the X-axis being perpendicular to the ice surface. Standard periodic boundary conditions have been applied. 2880 water molecules, arranged in 18 molecular layers of protondisordered Ih ice, have been placed in the middle of the simulation box along the X-axis. The number of the acetaldehyde molecules has been left to fluctuate by fixing their chemical potential instead. To determine the full adsorption isotherm, a set of 20 simulations have been performed, in which the chemical potential of acetaldehyde, μ, has been systematically varied from −47.1 to −30.2 kJ/mol. The chemical potential values used in the simulations as well as the resulting average number of acetaldehyde molecules in the basic box, ⟨N⟩, are collected in Table 1. Water molecules have been described by the rigid, five-site TIP5P model25 because this model is known to reproduce the melting point of Ih ice quite well.26 For describing acetaldehyde, the parameters of the transferable potential for phase equilibria (TraPPE) force field27 have been used. According to this force field, the CH3 and CH groups have been treated as united atoms. The interaction parameters of the 4199


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respectively), and Γ is the surface density of acetaldehyde. The values of Γ and prel can simply be obtained as

2.2. Experiments. The ice films were grown under ultrahigh vacuum (base pressure 1 × 10−10 Torr) on a polycrystalline gold plate cooled with a liquid helium cryostat. Ultrapure water and acetaldehyde (99%, Aldrich) were degassed with several freeze−pump−thaw cycles prior to the dosing. Water and acetaldehyde were introduced through independent gas lines. The pressure, measured by an ionization gauge was corrected from the gauge ionization efficiency of 1.95 for acetaldehyde31 and given in langmuirs (1 langmuir = 1 × 10−6 Torr s). The Fourier transform infrared spectra were recorded in absorption− reflection mode (FT-RAIR) at the reflection angle of 5° with a JASCO 6300 spectrometer under vacuum, using a resolution of 4 cm−1. About 4 monolayers (i.e., roughly 11 Å) thick amorphous water ice films were prepared at 120 K by exposing the gold plate to a water background pressure of 1 × 10−7 Torr for 40 s. Once the water films were condensed, they were exposed to acetaldehyde at 90 K, below the temperature of sublimation of solid acetaldehyde (100 K). The low temperature of the experiments ensured that the acetaldehyde molecules impinging on ice definitely stick on the surface, making the infrared features related to the interaction of the acetaldehyde layer in contact with water well visible. However, since the experimental temperature of 90 K is below the sublimation temperature, obviously an acetaldehyde multilayer could grow on the top of this adsorbed layer (and, at low exposures, islands of acetaldehyde multilayer could also grow at the side of an incomplete monolayer), giving rise to features related to interactions between the acetaldehyde molecules themselves.

Γ=

⟨N ⟩ 2YZ

(1)

and32 prel =

p exp(μ) = p0 exp(μ0)

(2)

The factor of 2 in the denominator of eq 1 reflects the fact that, due to the periodic boundary conditions, the basic simulation box contains two ice surfaces, whereas μ0 in eq 2 is the chemical potential value corresponding to the point of condensation, i.e., μ0 = −34.2 kJ/mol. Obviously, the conversion of the ⟨N⟩ (μ) isotherm to the Γ(prel) form can only be done up to the point of condensation. The Γ and prel values corresponding to the calculated points of the isotherm are also listed in Table 1, whereas the adsorption isotherm in its Γ(prel) form is shown in

3. SIMULATION RESULTS 3.1. Adsorption Isotherm. The isotherm resulting from the set of GCMC simulations is shown in Figure 1. The

Figure 2. Adsorption isotherm of acetaldehyde on ice, as obtained from our simulation (circles), together with the curve fitted by the Langmuir equation (see eq 3) to these data (solid line). The arrows indicate the systems used in the detailed analyses.

Figure 2. The shape of the isotherm can be well described by the Langmuir formalism, i.e. Γ = Γmax

Figure 1. Average number of acetaldehyde molecules in the basic simulation box as a function of their chemical potential. The arrows indicate the systems used in the detailed analyses.

prel K prel K + 1

(3)

where the parameters Γmax and K stand for the Langmuir partitioning coefficient and saturated surface density, respectively. To demonstrate the Langmuir character of the isotherm, we have fitted eq 3 to the simulated data points (see Figure 2). The values of Γmax and K resulted in 8.98 μmol/m2 and 97.8, respectively. The Langmuir character of the isotherm suggests that the saturated adsorption layer should be monomolecular and that the lateral interactions between the adsorbed acetaldehyde molecules should not play a major role in the adsorption process. These points will be addressed in more detail in the following subsections. 3.2. Characterization of the Adsorption Layer. In order to get a deeper insight into the molecular level details of the adsorption process, we have collected sample configurations and analyzed them in various respects at three selected chemical potential values, i.e., at −41.2, −37.8, and −34.4 kJ/

number of adsorbed molecules increases with increasing chemical potential up to the point of condensation, although its slope of exponential increase at low μ values gradually decreases at higher chemical potentials. Contrary to benzaldehyde,21 no plateau of the isotherm is observed before the point of condensation. The point of condensation, i.e., where the vapor and liquid phases of acetaldehyde have the same chemical potential value, corresponds to the sudden jump of the isotherm at μ = −34.2 kJ/mol; above this value the simulation box contains liquid acetaldehyde. To get a deeper insight into the adsorption process it is worth converting the isotherm to the more conventional Γ(prel) form, where prel = p/p0 is the relative pressure (p and p0 being the pressure of the system and that of the saturated vapor, 4200


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Figure 3. Instantaneous equilibrium snapshot of systems I (top), II (middle), and III (bottom), as taken out from the simulations, shown both in side (left) and top views (right). Water molecules are shown by blue sticks; acetaldehyde molecules are shown by balls and sticks. The O and C atoms of the acetaldehyde molecules are shown by gray and red colors, respectively, while the H atoms are omitted from the figure for clarity.

mol. The systems corresponding to these μ values are referred to here from now on as systems I, II, and III, respectively. As is seen from Figure 2, system I corresponds to the linearly rising part of the Langmuir-like isotherm, i.e., to the situation when adsorbed molecules are largely isolated from each other. System II corresponds to that part of the isotherm where it turns from a linear rise to a nearly constant part. Here the adsorption layer is not yet saturated; however, a large part of the adsorption sites is already occupied, and hence the adsorption of the acetaldehyde molecules is no longer independent from each other. Finally, system III is located at the nearly constant part of

the Γ(prel) isotherm, and hence it corresponds to the saturated adsorption layer of acetaldehyde. Equilibrium snapshots of systems I−III are shown in Figure 3 both from top and side views. 3.2.1. Density Profiles. The density profiles of the acetaldehyde O atoms and CH3 groups along the interface normal X-axis are presented in Figure 4 as obtained in systems I−III. All the profiles shown are averaged over the two interfaces present in the basic box. As is seen, all the profiles are unimodal, and the increasing coverage of the ice surface only leads to the increase of the peak heights but not to their shift to 4201


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Figure 4. Number density profile of the O atom (filled symbols, blue) and CH3 group (open symbols, red) of the acetaldehyde molecules in systems I (squares), II (circles), and III (triangles), as obtained from the simulations. For reference, the outer tail of the number density profile of the water O atoms in system I is also shown (dashed line). All profiles shown are symmetrized over the two surfaces present in the basic simulation box.

Figure 5. Distribution of the binding energy of an adsorbed acetaldehyde molecule (i.e., the energy of interaction between the adsorbed molecule and the rest of the system, bottom panel) and that of its contributions coming from the interaction with the other adsorbed molecules (middle panel) and with the ice phase (top panel). Solid lines, system I; dashed lines, system II; dotted lines, system III.

higher distances. The unimodality of the peaks even at the highest chemical potential value considered indicates, in accordance with the Langmuir-like behavior of the isotherm, that the saturated adsorption layer is still monomolecular. Further, the lack of the peak position shift indicates that no substantial change in the orientation of the adsorbed molecules occurs upon saturation. It is also seen that the density peak of the O atoms is located about 1 Å closer to the ice surface in every case than that of the CH3 groups. This finding indicates that the adsorbed acetaldehyde molecules prefer, on average, to point toward the ice surface by the O atom and away from it by the CH3 group. The fact that the peak-to-peak distance of the O and CH3 density profiles of 1 Å is considerably smaller than the intramolecular distance of the O atom and the CH3 group of 2.4 Å also suggests that the majority of the adsorbed molecules are probably tilted from the surface normal axis rather than pointing straight toward the ice phase by the O atom. This point is further discussed in a following subsection. 3.2.2. Energetic Background of the Adsorption. In order to get insight into the energetic background of the adsorption, we have calculated the distribution of the binding energy Ub of the adsorbed acetaldehyde molecules (i.e., the energy of their interaction with the rest of the system, in other words, the energy cost needed to bring the molecule to infinite distance from the rest of the system) in systems I−III. In addition, the Ubw and Ublat contributions to the total binding energy, coming from the interaction with the water molecules of the ice phase and from the lateral interaction with the other acetaldehyde molecules, respectively, have also been calculated. Thus, Ubw is the interaction energy of an adsorbed acetaldehyde molecule with all the water molecules (constituting the ice phase) of the system, while Ublat is the interaction energy of the given adsorbed molecule with all the other acetaldehydes. The P(Ub), P(Ubw), and P(Ublat) distributions obtained in systems I−III are shown in Figure 5. As is seen, at low surface coverage (i.e., in system I) the P(Ubw) distribution has a single peak around −35 kJ/mol. Considering the fact that the energy of a hydrogen bond is roughly −20 to −25 kJ/mol, this finding indicates that the adsorbed acetaldehyde molecules form one hydrogen bond

with the surface water molecules. The mean value of the distribution turns out to be −34.1 kJ/mol. This value can serve as an estimate for the heat of adsorption at infinitely low coverage. Upon saturation the peak of the P(Ubw) distribution gradually shifts to higher energies, being at about −29 and −23 kJ/mol in systems II and III, respectively, indicating the increasing competition of the adsorbed molecules. However, apart from a small fraction of the adsorbed molecules contributing to the small peak of P(Ubw) near zero in system III, the adsorbed acetaldehyde molecules still form one hydrogen bond with surface waters even at high coverages. The P(Ublat) distribution exhibits a large peak at zero energy in system I, reflecting the fact that at this low coverage the adsorbed molecules are typically well separated from each other. However, this distribution exhibits two clear shoulders at the negative energy side of the main peak, one at around −10 kJ/mol and another small one around −20 kJ/mol. These peaks correspond to acetaldehyde molecules having one and two near neighbors, respectively. Further, the relative orientation of these neighbors has to be such that it is favored by dipolar forces (e.g., head-to-tail or antiparallel dipole−dipole alignments). In systems II and III the P(Ublat) distribution exhibits a peak around −14 and −22 kJ/mol, respectively, indicating that upon saturation an increasing fraction of the adsorbed molecules has two such neighbors, and in the saturated adsorption layer this becomes the typical situation. The distribution of the full binding energy, P(Ub), is unimodal in every case, the peak position being shifted to lower energies upon saturation from about −38 kJ/mol in system I to −42 kJ/mol (system II) and −48 kJ/mol (system III). This result confirms, as it could be expected, that the increasing lateral interaction overcompensates the slight weakening of the acetaldehyde−ice interaction, occurring due to the increasing competition of the molecules upon saturation. 3.2.3. Orientation of the Adsorbed Molecules. The analysis of the density profiles and binding energy distributions already led to several conclusions on the orientation of the adsorbed 4202


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Figure 6. (a) Definition of the local Cartesian coordinate frame fixed to the individual acetaldehyde molecules in order to describe their surface orientation. X is the surface normal vector pointing away from the ice phase; ϑ and ϕ are its polar coordinates in this molecule-fixed local frame. (b) Orientational map of the adsorbed acetaldehyde molecules in systems I (left) II (middle) and III (right). Lighter colors indicate higher probabilities. (c) Illustration of the acetaldehyde orientations corresponding to the peak region of the maps. The plane of the molecule gets increasingly tilted upon moving along the peak region from lower to higher cos ϑ values.

following way. Its origin coincides with the C atom of the aldehyde group, axis x points along the CO double bond from the C to the O atom, axis z is perpendicular to the plane formed by the CH3 group and CO bond of the molecule, and axis y, being perpendicular to the above two axes, is oriented in such a way that the y coordinate of the CH3 group is negative. The definition of this local frame along with that of the polar angles ϑ and ϕ is illustrated in Figure 6. The P(cos ϑ,ϕ) orientational maps of the adsorbed acetaldehyde molecules are shown in Figure 6 as obtained in systems I−III. As is seen, the preferred orientation of the molecules shows only a slight dependence on the surface density of the adsorption layer. Thus, in system I the map exhibits a rather elongated peak around the ϕ value of 250°.

acetaldehyde molecules. However, the full description of their orientational statistics, as for any rigid molecules, requires the calculation of the bivariate distribution of two independent orientational variables.33,34 It has been shown that the angular polar coordinates ϑ and ϕ of the surface normal vector, X (pointing, by our convention, away from the ice phase), in an arbitrary, molecule-fixed Cartesian coordinate frame represent a suitable choice for such variables.33,34 It should be noted that, by definition, ϑ is a general spatial angle, while ϕ is an angle defined within a given plane (i.e., the xy plane of the local frame), and therefore uncorrelated orientation of the molecules with the ice surface only results in a uniform bivariate distribution if cos ϑ and ϕ are chosen to be the orientational variables.33,34 Here we define the local Cartesian frame in the 4203


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Figure 7. Possible hydrogen bonds between a water molecule located at the ice surface and an adsorbed acetaldehyde molecule aligned in one of its preferred orientations: (a) acetaldehyde molecule aligned perpendicular to the ice surface, (b) acetaldehyde tilted relative to the ice surface. Possible alignments of the neighboring acetaldehyde molecules are also illustrated: (c) acetaldehyde molecules in head-to-tail type dipole arrangement, (d) acetaldehyde molecules in antiparallel-like dipole arrangement. X is the surface normal vector pointing away from the ice phase.

This peak extends from cos ϑ = 0 to about cos ϑ = 0.6, corresponding to the ϑ range of about 50° ≤ ϑ ≤ 90°. Since, according to the above definition of the local Cartesian frame, ϑ is simply the angle formed by the plane of the adsorbed molecule with the plane of the ice surface, this finding means that the acetaldehyde molecules adopt orientations ranging from tilted by about 50° to perpendicular relative to the surface plane. The observed orientational preference of the acetaldehyde molecule is also illustrated in Figure 6. Contrary to ϑ, the value of ϕ is very strongly determined for the adsorbed molecules. Its value of about 250° indicates that the CH3 group is located as far from the ice surface as possible within the constraint of the tilt angle of the molecular plane, ϑ. In the particular case of ϑ = 90° (i.e., cos ϑ = 0) this means that the C−CH3 bond points straight away from the ice surface. It is also seen that upon saturation of the adsorption layer the peak of the P(cos ϑ,ϕ) orientational map becomes less elongated along the cos ϑ axis, extending to the cos ϑ values of about 0.4 and 0.2 in systems II and III, respectively. This finding indicates that as the adsorption layer gets increasingly crowded, the adsorbed molecules adopt, on average, less tilted orientations, and in the saturated adsorption layer the perpendicular orientation becomes clearly the preferred one. This is in accordance with the fact that, due to their increasing competition, the adsorbed molecules should occupy, on average, smaller surface area upon saturation.

To understand the physical background of the observed orientational preferences, it should be noted that surface water molecules of Ih ice have four preferred orientations.14 In one of these orientations an O−H bond stays perpendicular to the surface pointing by its H atom to the vapor phase, while in other preferred orientations at least one of the O−H bonds point flatly to the vapor phase, declining from the surface plane by about 20°. As is illustrated in Figure 7, the adsorbed acetaldehyde molecules that align perpendicular to the surface can accept a hydrogen bond from the former, while those being tilted by 50° relative to the ice surface can accept that from the latter type of water molecules. Finally, considering the observed orientational preferences, the favored dipole−dipole arrangement of two neighboring acetaldehyde molecules, seen from the lateral binding energy distributions, can be realized either by the two molecules being located behind each other, pointing by their CO bond to the same direction (head-to-tail-like arrangement), or by them being located next to each other, pointing by their CO bonds to the opposite directions (antiparallel-like arrangement). Both of these possible nearneighbor arrangements of the adsorbed acetaldehyde molecules are also illustrated in Figure 7. 4204


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4. EXPERIMENTAL RESULTS AND COMPARISON WITH SIMULATIONS Figure 8a shows the infrared spectra of ice exposed to increasing amount of acetaldehyde at 90 K, compared to the

For acetone, this hydrogen bond also induces a redshift of the νCO stretching frequency of −28 cm−1 with respect to the gas phase. A similar shift is observed for acetaldehyde. Figure 8b details the νCO band of acetaldehyde on ice, which can be decomposed into two Gaussian contributions. The strongly red-shifted band located at 1717 cm−1 (−26 cm−1 compared to the gas phase value of 1743 cm−1)43 is assigned to acetaldehyde molecules in contact with ice in the first adsorption layer, the CO group of which being hydrogen bonded to the dH sites. Such a binding configuration must keep the O atom close to the surface and pull the CH3 group away, in agreement with the distributions of the distance in the first adsorption layer depicted in Figure 4. The second band at 1727.5 cm−1 is assigned to the formation of an acetaldehyde multilayer phase, growing at the side of the monolayer (its contribution at 1727.5 cm−1 is labeled “solid phase” in Figure 8b). This phase, which is observed from the lowest coverage, is made of acetaldehyde molecules with their carbonyl group hydrogen bonded to the CH group of a neighboring molecule. Such binding configuration is significantly less energetic than when acetaldehyde is connected with ice, as evidenced by the smaller red-shift of the νCO stretching mode (−15.5 cm−1 with respect to the gas phase value). In the inset of Figure 8b, the intensity (peak area) of the monolayer component at 1717 cm−1 (blue circles) is plotted as a function of the exposure. It follows Langmuir behavior, as it can be well fitted with the Langmuir equation (see eq 3, blue dotted line), using adjustment constants that are specific to the experiments, and hence they cannot be compared with those used in the fit of Γ(Figure 2). This Langmuir-like behavior is expected for a monolayer surface adsorption, which confirms the assignment of the band at 1717 cm−1 to the monolayer contribution. It is also in excellent agreement with the adsorption isotherm presented in Figure 2, in which the experimental exposure range corresponds to the systems I and II of the simulation. The intensity of the multilayer component at 1727.5 cm−1 (red circles) is also plotted as a function of the exposure in the inset of Figure 8b. It steeply increases linearly as the molecules are provided, since no factor limits the condensation of this phase. The growth of this phase uses most of the impinging molecules, which explains why the monolayer is still incomplete at exposures as high as 5 langmuirs. The adsorption enthalpy (ΔHads), resulting from the interaction of the carbonyl function with ice, can be estimated from the infrared data, assuming a linear relationship between the frequency shift of the νCO stretching vibration and the strength of the hydrogen bond with water.44 In the case of acetone on ice, i.e., another molecule being also singly hydrogen bonded to ice through the carbonyl function, the νCO stretch is shifted by −28 cm−1 with respect to the gas phase.22,42 Temperature-programmed desorption allowed estimating the adsorption enthalpy of acetone on ice to be −39 ± 2 kJ mol−1,42 from which a proportionality coefficient of 1.39 ± 0.07 kJ mol−1 per cm−1 can be inferred. Assuming that this value, obtained for acetone, holds also for acetaldehyde, the observed shift of −26 cm−1 leads to the estimate of the binding energy of −36 ± 2 kJ mol−1 for acetaldehyde on ice at 90 K. This energy is typical of singly hydrogen bonded molecules and is in a good agreement with the experimental adsorption enthalpy of −34.7 kJ mol−1 reported by Hudson et al. on water ice between 120 and 180 K4 and of −36.4 ± 2.0 kJ/mol reported by Petitjean at 203 K.20 Further, this value is also in a good agreement with the simulation estimate of the heat of

Figure 8. (a) FT-RAIR spectra of acetaldehyde adsorbed at 90 K on water ice. (b) Evolution of the νCO stretching mode of acetaldehyde adsorbed on water ice. The intensities of the monolayer and multilayer components (see text) are plotted in the inset.

spectrum of solid acetaldehyde (with the corresponding assignments taken from the literature35−37). The water ice bands are the νOH stretching modes (3390 and 3300 cm−1), the bending mode δHOH at 1655 cm−1, and the stretching of the surface dangling H atoms (“dH sites”) at 3697 cm−1,38−41 this latter band being enlarged in the inset. The dH intensity decreases with acetaldehyde exposure, resulting from the formation of one hydrogen bond between an acetaldehyde molecule and the dH sites of the surface of ice. Similar extinction of the dH infrared intensity has been reported by Schaff et al.42 in the case of acetone on water ice, resulting from the hydrogen bonding between the CO group and a dH site. 4205


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adsorption at infinitely low coverage of −34.1 kJ mol−1, calculated as the mean value of the P(Ubw) distribution in system I (see section 3.2.2).

(BALATON). Financial support from the French Ministry of Research through the LEFE/CHAT program is gratefully acknowledged.

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5. SUMMARY AND CONCLUSIONS In this paper we presented, to our knowledge, the first combined computer simulation and infrared spectroscopy investigation of the adsorption of a small organic molecule, namely acetaldehyde, at the surface of ice. The experimental results turned out to be in very good agreement with the major features provided by the simulations. The results of the present computer simulation analysis revealed that, similarly to formaldehyde15 but in a clear contrast with benzaldehyde,21 the adsorption of acetaldehyde on ice follows the Langmuir behavior in the entire pressure range of the existence of vapor phase acetaldehyde. Correspondingly, the adsorption layer is found to be monomolecular up to the point of condensation. Experimental results confirmed that at least one component of the adsorption is Langmuir-like. Acetaldehyde molecules are attached to the ice surface by one single hydrogen bond, formed typically with the dangling H atoms of the surface water molecules. The plane of the acetaldehyde molecule is found to align preferentially perpendicular to the ice surface, however, it can also rather easily adopt tilted alignments with a tilt angle (relative to the ice surface plane) larger than 50°. Upon saturation of the adsorption layer the range of this tilt angle gets narrower, and the preference of the acetaldehyde molecule for the perpendicular alignment becomes stronger. Further, it is found that the CH3 group strongly prefers to point as straight away from the ice surface as possible within the constraint set by the alignment of the molecular plane. The analysis of the binding energy distribution as well as that of its ice and lateral contributions revealed that at high surface coverages lateral interactions contribute to the total binding energy comparably with the ice−adsorbate interaction. Lateral attraction originates from the dipolar interaction of the neighboring acetaldehyde molecules, the dipole vectors of which thus adopt head-to-tail or antiparallel-like relative arrangements. The heat of adsorption at infinitely low surface coverage (i.e., the binding energy of a single acetaldehyde molecule by the ice phase) turned out to be −36 ± 2 kJ/mol from the experiment, in a clear accordance with the simulation result of −34.1 kJ/mol. Considering that this value is about 20% lower than what was previously obtained for formaldehyde,15 and also the relative abundance of acetaldehyde in the upper troposphere, the present study clearly stresses the atmospheric importance of the adsorption of acetaldehyde molecules on ice grains.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Hungarian OTKA Foundation under Project No. 75328, by TAMOP under Project No. TAMOP-4.2.2/B-10/1-2010-0030, by the MTACNRS bilateral collaboration program, and by the Hungarian− French Intergovernmental Science and Technology Program 4206


Langmuir

Article

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Adsorption of Acetaldehyde on Ice As Seen from Computer Simulation

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