Water Interactions with Acetic Acid Layers on Ice and Graphite - The

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Water Interactions with Acetic Acid Layers on Ice and Graphite Panos Papagiannakopoulos, Xiangrui Kong, Erik S Thomson, and Jan B. C. Pettersson J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp503552w • Publication Date (Web): 30 May 2014 Downloaded from http://pubs.acs.org on June 4, 2014

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The Journal of Physical Chemistry

Water Interactions with Acetic Acid Layers on Ice and Graphite Panos Papagiannakopoulos#,*, Xiangrui Kong, Erik S. Thomson, and Jan B. C. Pettersson* Department of Chemistry and Molecular Biology, Atmospheric Science, University of Gothenburg, SE-412 96 Gothenburg, Sweden

#

also at Department of Chemistry, Laboratory of Photochemistry and Kinetics, University of Crete, GR-71 003 Heraklion, Greece

*

To whom correspondence should be addressed. E-mail: [email protected] (P.P.) or [email protected] (J.B.C.P); Tel. +46 31 7869072; Fax: +46 31 7721394.

1 ACS Paragon Plus Environment


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Abstract Adsorbed organic compounds modify the properties of environmental interfaces with potential implications for many Earth system processes. Here, we describe experimental studies of water interactions with acetic acid (AcOH) layers on ice and graphite surfaces at temperatures from 186 to 200 K. Hyperthermal D2O water molecules are efficiently trapped on all of the investigated surfaces, with only a minor fraction that scatters inelastically after an 80% loss of kinetic energy to surface modes. Trapped molecules desorb rapidly from both µm-thick solid AcOH and AcOH monolayers on graphite indicating that water has limited opportunities to form hydrogen bonds with these surfaces. In contrast, trapped water molecules bind efficiently to AcOH-covered ice and remain on the surface on the observational time scale of the experiments (60 ms). Thus adsorbed AcOH is observed to have a significant impact on water-ice surface properties and to enhance the water accommodation coefficient compared to bare ice surfaces. The mechanism for increased water uptake and the implications for atmospheric cloud processes are discussed.

Keywords: environmental molecular beam, H2O, carboxylic acid, aerosol, cirrus


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1. Introduction

Condensation and evaporation of water are key processes in the endless movement of water and energy within the Earth system. Although the unique physical and chemical properties of water in gas, liquid and solid form are generally acknowledged and well understood, interfacial properties are often distinctly different from those of the bulk materials.1-3 In the atmosphere, insoluble or incompletely soluble compounds tend to concentrate on the surface of aerosol and cloud particles and thereby affect heterogeneous processes including the exchange of water and other gases across the interface.4-7 The incomplete understanding of these effects introduces uncertainties into atmospheric models, and consequently limits our ability to forecast weather and climate.

Of particular relevance for these interfacial processes is the behavior of organic compounds. A multitude of organic compounds released to the atmosphere transform vis-à-vis

gas phase

reactions into products with low vapor pressures that condense on existing aerosol and cloud particles.2,8 Primary hydrophobic compounds undergo slow oxidation and become increasingly hydrophilic,9 and the hygroscopicity of particles changes with age and oxidation state of surfacebound compounds.8 Many reactions lead to the formation of carboxylic acids that are stable on the time scale of days or longer in the atmosphere.2,8 Acetic acid (CH3COOH; AcOH) and formic acid (HCOOH) are the most abundant organic acids in the atmosphere, and motivate the AcOH investigations of this study.10-12

Sources of atmospheric AcOH include primary emissions from soil, vegetation, and combustion processes including biomass burning. Additional, secondary production results from a range of atmospheric precursors.13 In the atmosphere AcOH is slowly oxidized and has an estimated life3 ACS Paragon Plus Environment


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time of a few days to weeks.14 Consequently it is distributed worldwide and plays an important role in tropospheric chemistry and atmospheric acidity. In the lower troposphere wet deposition is a major sink of AcOH, but more generally heterogeneous processes are likely to play a significant role in AcOH removal.14 Near surface, gas phase concentrations of 60-715 pptv have been observed in remote regions including Greenland and Antarctica, and production of AcOH in the snowpack has been suggested as a local source.13,15 In cold regions of the atmosphere like the upper troposphere, where AcOH has measured mixing ratios of 20–700 pptv,11 AcOH may condense on existing surfaces. For example, model simulations of gas uptake in cirrus clouds suggest that AcOH together with HNO3 and HCl are likely to adsorb on cloud particles.16 Sedimentation of particles to lower altitudes may partially remove the particle-bound components from the upper troposphere, but adsorbed acids may simultaneously affect cloud processes by modifying particle formation, growth and morphology. Studies carried out at high temperatures relative to those of the upper troposphere have shown that adsorbed AcOH may influence the structure of growing snow crystals.17

Acetic acid forms relatively strong hydrogen bonds and is highly miscible in liquid water,18 but like most other compounds it does not mix well with ice and tends to concentrate at surfaces.19 The interaction of AcOH and water ice surfaces has been investigated in several experimental14,19,20-27 and theoretical studies.21,28-30 In particular, the AcOH uptake coefficient, initial sticking coefficient, adsorption isotherms and enthalpies have been determined at low concentrations and temperatures (100-240 K) using flow tube techniques.14,20-24 Adopting the Langmuir adsorption model, it was found that the saturation surface coverage Nmax = 2.5⋅1014 molecules cm-2,31 and the adsorption enthalpy ∆Hads varies between -55( ± 9) and -73( ± 12) kJ


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mol-1.14,20,22,23 The initial sticking coefficient γ0 = 4.7⋅10-17exp(6496/T) cm between 198 and 208 K.24 In addition, the geometry and dynamics of AcOH molecules on ice have been examined by spectroscopic techniques,19,25-27,32 and by molecular dynamics and ab initio calculations.21,28-30 These studies show that AcOH molecules are deposited as monomers, cyclic dimers, or open dimers,20,23,32 and form one or two hydrogen bonds between the ice surface water molecules and the carboxylic group.21,25,27,28 They do not appear to diffuse significantly in the bulk ice, rather staying in the upper layers of the ice surface and causing only minor disorder of the surface structure.19 Adsorbed AcOH molecules may also form cyclic monohydrates and dihydrates in the presence of mobile surface water molecules.27,29,32-34 In related work, the properties of water molecules on pure AcOH surfaces have been investigated by density functional theory calculations, and sites with binding energies between 37.3 and 46.1 kJ mol-1 have been identified on different crystal facets.30

One interfacial process of both fundamental and environmental interest is the accommodation of water molecules on ice. The process remains incompletely understood and estimates of the accommodation coefficient vary by more than two orders of magnitude.35,36 We have recently characterized water accommodation and desorption kinetics on pure ice from 170 to 200 K, and confirmed that adsorption goes through a weakly bound surface state.35,37,38 We have also previously shown that adsorbed n-butanol reduces water accommodation on ice, although methanol has a limited effect.39 These studies have been made possible by a recently developed environmental molecular beam method (EMB), which provides a tool for detailed studies of interface dynamics and kinetics at vapor pressures into the 100 Pa range.40,41



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Here we present results from EMB studies of water interactions with AcOH-covered surfaces with the overall aim of characterizing the dynamics and kinetics of the interactions. The main focus is on water interactions with AcOH-covered ice, but the results are also contrasted with observations of water interactions with solid AcOH and AcOH monolayers on graphite. Of particular interest are the detailed character of water-AcOH interactions and the effect of adsorbed AcOH on water accommodation. The experiments cover the temperature range from 186 to 200 K and are carried out using D2O as a substitute for H2O to enhance the experimental sensitivity.

2. Experimental methods

A. Experimental setup The EMB apparatus used in the experiments has been described in detail elsewhere,40-42 and thus the methodology is only briefly presented here. A gas source produces short gas pulses into the six-chamber vacuum system of the EMB apparatus. Part of the gas passes through a skimmer and a subsequent mechanical chopper and forms a directed low density molecular beam. The beam source is run with a D2O:He gas mixture at a total pressure of 2⋅105 Pa and a partial D2O pressure of approximately 2.2⋅103 Pa, which produces a beam with mean kinetic energies of 31 ± 2 and 6.2 ± 0.3 kJ mol-1 for D2O and He, respectively.

The beam is directed towards a graphite surface (Advanced Ceramics Corp.; highly oriented pyrolytic graphite, grade ZYB, 12 × 12 mm2) located in the center of the main ultra-high vacuum


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(UHV) chamber. The UHV chamber has a background pressure of approximately 10-7 Pa that is primarily due to residual vapor from gases introduced during the experiments. The surface is enclosed by a separate, inner, environmental chamber surrounding the surface, which allows for elevated vapor pressures in the 100 Pa range. The finite pressures achieved with this system distinguish the method from traditional molecular beam experiments. The apparatus has been designed to minimize the molecular beam path length (28 mm) within the high-pressure zone, such that the attenuation of the beam due to gas collisions within the inner chamber becomes significant only above 10-1 Pa.40 The incident D2O/He beam collides with the surface at an angle of 45°, and the outgoing flux in the forward direction is monitored with a quadrupole mass spectrometer (QMS), also at an angle of 45° from the surface normal direction. The QMS is rotatable and is also used to measure in the incident beam.

Adsorbed layers are produced on the graphite substrate by introducing water and/or AcOH gas into the environmental chamber through two separate gas inlets. The thicknesses of the adsorbed layers are determined by monitoring the interference produced by the reflections of a diode laser (0.86 mW, 670 nm) from the surface.40,41 Thick AcOH layers are typically produced with an initial growth rate of approximately 20 monolayers per second (ML s-1), and the AcOH pressure is subsequently adjusted to maintain a layer thickness of approximately 1 µm. This corresponds to ca. 1300 ML assuming that 1 ML of AcOH consists of 2.5⋅1014 molecules cm-2 30 and the unit cell volume of crystalline AcOH is 3.02⋅10-22 cm3.43 Sub-monolayer and monolayer coverages of AcOH on graphite are produced by leaking gas into the inner chamber while the surface coverage is probed by elastic helium scattering. The method accounts for the fact that adsorbed gases attenuate the elastic scattering of He from graphite in a manner that is to a good approximation



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linearly proportional to the surface coverage, and has been applied in several recent studies where the methodology is described in more detail.41,44-46 In a similar manner to the AcOH layer formation, water ice layers are formed by water vapor deposition and are grown to a thickness of approximately 1 µm. In the present experiments, water ice is grown on a pre-formed monolayer of AcOH, while during the experiments the AcOH inlet is held constant. The graphite surface is cleaned between experiments by heating to 500 K, and clean surface conditions are routinely confirmed by elastic helium scattering after surface cooling to 200 K or lower.40,41

The absolute AcOH pressure is estimated by comparison with the absolute water vapor pressure at the same temperature and assuming an ionization efficiency of 2.32 higher for AcOH compared to water in the QMS.47 The estimated uncertainty in absolute AcOH pressure is ± 25% based on the typical agreement between literature values and observed QMS data for the present setup. Acetic acid may dimerize in the gas phase upon entering the inner chamber. However, it is estimated according to equilibrium coefficients for AcOH dimerization48,49 that in our experimental conditions (< 5⋅10-3 Pa and > 240 K in the gas phase near the surface) the concentration of dimers is less than 5%, which is ignored in the present studies.

B. Analysis The measured flux from the surface is recorded using a multi-channel scaler with a 10 µs dwell time, yielding time-of-flight (TOF) intensity distributions. The quantitative analysis of the TOF distributions relies on least-squares fitting assuming the data can be described by a combination of inelastic scattering (IS) and trapping followed by thermal desorption (TD).39 The TD is modeled as ordinary desorption, expressed in terms of measured intensity Fres as,


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= ,

(1)

where C1 is a scaling factor, k is the desorption rate constant, and t is time. The inelastic scattering distribution is assumed to have the form,48

I vt = C vt exp -

- !"

# $,

(2)

where C2 is a scaling factor, v(t) is the velocity calculated from t and the path length between the surface and the QMS, v% represents the peak of the inelastically scattered velocities, and vIS is,

v = &

'( )!" *

,

(3)

where the temperature TIS is a free fitting parameter that is indicative of the velocity spread of inelastically scattered particles, kB is the Boltzmann constant, and m is the molecular mass.

The absolute probability for D2O trapping followed by thermal desorption, PTD, is computed by normalizing each TD integral by the TD integral from a contiguously measured bare graphite case.39 Over the experimental temperature range the TD probability for hyperthermal D2O scattering from graphite is constant within the measurement uncertainty, and likewise the TD component has a cosine angular distribution independent of temperature.50 Desorption for the bare graphite surface is then linearly scaled by the sticking coefficient sgraphite = 0.73 ± 0.07 for



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D2O on bare graphite under the present conditions.39 Using this sticking coefficient as a scaling parameter PTD is computed as,

/012345

789: ⁄ 6,+,- = ./012345 6,-

/012345

=>?@ where I)< and 6,-

,

(4)

are the TD integrals in the AcOH-covered and clean graphite cases.

An error estimate is calculated from the integral values based on the 95% confidence intervals for the fitting parameters that contribute to the thermal decay function. The final error is based on propagating the error of each integral and the uncertainty in sgraphite.

3. Results

We have carried out EMB studies of D2O interactions with three types of surfaces, and the results consist of TOF distributions that are analyzed to determine probabilities for inelastic scattering and thermal desorption, and energy transfer in the gas-surface collisions. We begin by describing the results for water interactions with solid AcOH and AcOH monolayers on a graphite substrate, followed by the results for AcOH-covered water ice.

A. Water interactions with solid acetic acid Figure 1a shows a TOF distribution for D2O colliding with solid AcOH at 200 K. The temperature is far below the melting point of AcOH (289.9 K at 100 kPa)51 and the µm-thick layers are likely to be amorphous or polycrystalline considering the high deposition rate of 20


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ML s-1. The TOF distribution is overlaid by a two component non-linear least squares fitting of the data with each component (IS and TD) and the sum of the two components depicted. The IS peak is shifted in time compared to the incident beam distribution (Fig. 1c), which corresponds to hypothetical elastic scattering since the total beam path is the same as in the scattering experiments. A final average kinetic energy of 6.1 ± 0.6 kJ mol-1 is determined from the IS data indicating that scattered water molecules lose 80% of their kinetic energy in surface contact. The large energy loss is typical for water collisions with water ice and organic surfaces.39,42,52 The width of the TD distribution in Fig. 1a is mainly determined by the thermal velocity distribution of desorbing D2O and the surface residence time of adsorbed water molecules is less than 20 µs, which is the shortest residence time that can be resolved in the present experiments. Based on the comparison with TD from clean graphite, as described by Equation (4), the absolute TD probability is determined to be 0.71 ± 0.07. The comparable intensities of the IS and TD components in Fig. 1 results from the preferential IS in the forward direction as compared to a broad cosine distribution for TD. Note that the absolute IS probability cannot be directly determined with the present setup where the signal is measured at a single scattering angle of 45° from the surface normal direction. We conclude that water interactions with the solid AcOH surface are characterized by highly inelastic collisions and dominated by trapping of D2O molecules followed by rapid desorption. This suggests that the surface does not allow for strong bonds to develop between water and surface AcOH on the µs or shorter time scale of the interactions.

B. Water interactions with an acetic acid monolayer on graphite



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Experiments were also carried out with an AcOH monolayer on the graphite substrate, in contrast to the bulk AcOH studies described above. Organic compounds including fatty acids53 and alcohols54 form highly stable monolayers on graphite. Monolayers with high stability and unique properties are likely to form also for AcOH monolayers on graphite, with potential impact on water-surface interactions. The formation of an AcOH monolayer was investigated with elastic He scattering, which was used to probe the AcOH surface coverage and to determine adsorption isotherms between 160 and 190 K. Figure 2 shows a typical adsorption isotherm determined at a surface temperature of 190 K. The equilibrium pressure over bulk AcOH, P0, was determined experimentally as the pressure required to maintain a AcOH multilayer surface at constant thickness. Surface coverage is observed to increase rapidly with increasing pressure and a monolayer is formed at P/P0 ≈ 0.17. Note that a surface coverage above one cannot be determined since the elastic He scattering method only probes the fraction of the graphite surface that is uncovered. When the AcOH pressure is again decreased a large hysteresis effect is observed, with monolayer coverage maintained to P/P0 ≈ 0.05. The isotherm can be categorized according to the classification introduced by Brunauer et al.55 as a Type V. These isotherms indicate that intermolecular interactions are important and they have been observed for water and alcohols interacting with large surface area carbon systems like charcoal and activated carbon.56 Thus the results are consistent with the reversible formation of a highly stable AcOH monolayer on graphite, where molecules form strong intermolecular bonds within the adsorbed phase.

Figure 1b shows a typical TOF distribution for D2O collisions with an AcOH monolayer on graphite. The distribution is well described by the IS and TD components and comparable to the results for solid AcOH. The TD probability is 0.76 ± 0.07 and similar to the result for solid


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AcOH. The final average kinetic energy is 6.1 ± 0.6 kJ mol-1 (80% of the incident kinetic energy) and the collision dynamics are thus comparable for the two systems. The water interactions are similar for both water-AcOH bulk and water-AcOH monolayers, in that strong D2O-AcOH bonds do not easily form at either interface.

C. Water interactions with acetic acid-covered water ice The dynamics and kinetics of D2O interactions with AcOH layers on H2O ice were studied between 186 and 194 K and the results are compared to D2O interactions with pure ice that were recently investigated using the same method.35 Figure 3 shows TOF distributions for D2O colliding with bare and AcOH-covered water ice at 186 K. The experiments were carried out with long (7 ms) beam pulses and a total measurement window of 60 ms in order to capture the slow signal decay observed for the uncovered ice surface. Figure 3a illustrates that D2O collisions with bare ice result in efficient trapping followed by significant desorption on the 60 ms time-scale of the experiments, while IS is negligible. The observed TD probability is relatively low and a major fraction of the incident water molecules remains on the surface on the 60 ms time scale of the experiment, with an accommodation coefficient of 0.76 ± 0.07 determined from the data.35

Figure 3b illustrates that the adsorption of an AcOH layer on the ice surface has a strong effect on water uptake. The experiments were carried out at a steady-state AcOH pressure of 1⋅10-4 Pa, which is sufficient to maintain a monolayer of AcOH on the ice surface (see Figure 4 and the related discussion below). The TOF distribution is well described by a single inelastic peak and the TD observed for pure ice is no longer visible. The final average kinetic energy of scattered molecules is 6.2 ± 0.6 kJ mol-1 and the energy loss is comparable to the results for AcOH mono-



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and multilayers described above. The relative IS intensity is also comparable to the other two systems, indicating that the absolute IS probability is also low for this system. Therefore a major fraction of the incoming water molecules trap and remain on the ice surface for tens of milliseconds or more. Isotopic exchange between trapped D2O and surface H2O or AcOH molecules could potentially be an alternative explanation for the loss of D2O, but no desorbing HDO is observed in experiments eliminating isotopic exchange as a major sink of heavy water.

Experiments were also carried out at other partial pressures of AcOH to investigate the influence of AcOH coverage on the water-surface interactions. Figure 4 shows the relative IS intensity as a function of AcOH pressure at 186 K. The IS intensity increases by approximately a factor of three when the AcOH pressure is raised from 8⋅10-6 to 4⋅10-3 Pa. Figure 4 also includes a calculated adsorption isotherm at 186 K based on the results from von Hessberg et al. who determined the Langmuir isotherm for AcOH on ice in the temperature range from 187 to 227 K.22 The calculated values are slightly lower than values recommended by IUPAC,30 which emphasizes values determined at high T. The von Hessberg et al. isotherm suggests that all of the present experiments were carried out with an AcOH surface coverage θ ≥ 0.5. The highest IS values in Fig. 4 were obtained with a partial AcOH pressure above the equilibrium pressure over pure AcOH, and the observed values may have been affected by the formation of a new AcOHenriched surface phase.

No TD is observed for D2O on AcOH-coated ice in the temperature range from 186 to 194 K, and the bulk accommodation coefficient is concluded to be 1.00 ± 0.05 at these temperatures. The corresponding values for water accommodation on pure ice are 0.66-0.78 in this temperature


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range,35 and the results are thus distinctly different for the two systems (see Supporting Information for a graphical comparison between the two data sets). We must conclude that an adsorbed AcOH layer suppresses rapid water desorption from the ice surface.

4. Discussion

Water collisions with all three investigated AcOH surfaces are characterized by highly efficient energy transfer. Efficient energy transfer and very rapid dissipation of the collision energy have previously been observed in several similar systems including water interactions with pure water ice, methanol and butanol monolayers on ice,39 solid and liquid butanol,42 and thin alcohol films on graphite.52, and we do not anticipate bond breaking or mechanical penetration at impact to play a major role in the present studies. The energy loss is associated with a high trapping probability, and under thermal conditions (where incident kinetic energies are lower than used here) the trapping probability can be expected to be close to unity. In generalizing the results we conclude that water collisions with typical environmental gas-solid interfaces will result in efficient trapping and inelastic scattering can to a first approximation be ignored, i. e. surface accommodation is close to unity. Hyperthermal collision energies, as employed in the present study, thus primarily serve to explore the gas-surface interaction potential and the dynamics of the investigated systems.

The micrometer thick AcOH surfaces used here were prepared by rapid vapor deposition and the procedure is expected to result in heterogeneous and relatively rough surfaces. This should make available a range of surface sites with different binding properties. However, trapped water 15 ACS Paragon Plus Environment


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molecules rapidly desorb indicating that they rarely locate sites with high binding energies. The surface residence time cannot be resolved in the present experiments indicating that it is shorter than 20 µs. With the assumption of first order desorption kinetics described by the Arrhenius equation and a typical pre-exponential factor of 1013 s-1,57 the desorption energy is calculated to be ≤ 25 kJ mol-1. This is in agreement with ab initio calculations for an isolated H2O-AcOH cluster that yielded a binding energy of 19 kJ mol-1,33 and the present results suggest that adsorbed water molecules form a maximum of one hydrogen bond with the AcOH surface. The experiments do not rule out the existence of a minor surface population of strongly bound states that saturates during the initial phase of each experiment. Such a scenario would require strongly bound states to remain saturated between incoming gas pulses. Under the present conditions with a molecular beam pulse repetition rate of 120 Hz and assuming Arrhenius desorption behavior with a pre-exponential factor of 1013 s-1, a surface binding energy ≥ 35 kJ mol-1 would be required for desorption of strongly bound molecules to remain unresolved on experimental timescales.

The experimental observations of bulk AcOH are consistent with density functional theory calculations of water adsorption on different AcOH crystal planes.27 In these studies simulated orthorhombic AcOH crystals were cut along four characteristic surface planes and the surface structures were allowed to relax. Subsequent calculations yielded available surface bonding sites with binding energies between 37.3 and 46.1 kJ mol-1, while at the same time large surface areas are hydrophobic with low binding energies. On the [101] surface, a large valley between catemer chains is bordered by -C=O groups and stable structures represent the adsorption mode of water on each side of the valley. On the [111] surface, only one type of site is available and it is embedded between rows of dangling methyl groups. On the [110] surface, sites with a single


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hydrogen bond and a binding energy of 37.3 kJ mol-1 exist. The present experimental results are consistent with the picture that the polycrystalline surface is largely hydrophobic with limited availability of strongly bound surface states. However, the theoretical suggestion that strongly bound states also exist on the surface cannot be ruled out. States with binding energies of more than 35 kJ mol-1 may become occupied during the initial phase of these water adsorption experiments, in which case subsequent desorption would remain outside our observational window.

The experimental results for water interactions with an AcOH monolayer on graphite are similar to the results for solid AcOH. Crystalline monolayers of linear carboxylic acids with 6-14 carbon atoms have been studied with X-ray and neutron diffraction.53 In all cases, molecules are arranged in hydrogen-bonded dimers with their extended axes parallel to the surface plane. The monolayer crystal structures have unit cell dimensions similar to close-packed planes of bulk crystals, but with different molecular arrangements. Similar highly stable monolayer structures have also been observed for linear alcohol layers on graphite.54 In contrast to the large fatty acids that form dimer crystals, bulk AcOH forms infinite chains of molecules.43 However, earlier studies show that the lattice energies of the catemer and dimer structures for AcOH are very similar,58,59 and it is not known if AcOH forms dimers or infinite chains on graphite when the surface attraction influences the system. In any case, our results indicate that AcOH-AcOH bonds are strong within the surface layer and adsorbed water molecules are not capable of inducing the breaking of these bonds.

The pronounced effect of adsorbed AcOH on water uptake on ice is intriguing and the results suggest that the surface is hydrophilic, in sharp contrast to the hydrophobic surfaces discussed 17 ACS Paragon Plus Environment


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above. Related literature concerning the effects of adsorbates on water accommodation by ice is limited. We recently showed that a butanol monolayer on water ice results in a small reduction in water uptake while a methanol monolayer has no observable effect compared to pure ice.39 Those observations are in line with the expected effects of linear alcohols that are surface-active and well-known to reduce the evaporation rate of liquid water.60,61 Davy and Somorjai studied the effect of adsorbed gases on water desorption from ice at temperatures between 183 and 233 K.37 Adsorbed NH3 decreased the evaporation of ice, adsorbed HF increased the evaporation rate, while HCl had no observable effect. In related work, Nathanson et al. investigated the effect of adsorbed alcohols on concentrated sulfuric acid and observed that n-hexanol impedes water evaporation by up to 20% while n-butanol did not affect the evaporation rate.62,63 A recent molecular dynamics study suggests that the limited effect of adsorbed butanol is due to partial protonation of butanol within the surface layer, which roughens the surface and thereby affects water uptake.64

Such studies of related compounds suggest that the effect of AcOH on water uptake may depend importantly on the structure of AcOH on ice surfaces. The collective evidence from earlier experimental14,19,20-27 and theoretical studies21,28-30 indicates that AcOH binds relatively strongly and reversibly to the ice surface. Molecules do not diffuse into the ice bulk, but rather stay on or within the upper surface layers and cause only minor disordering of the surface structure.19 Allouche and Bahr showed that adsorbed water molecules may form hydrogen bonds to two AcOH surface molecules and several structures have similar stability.27 Picaud et al. used an alternative potential with a weaker AcOH-AcOH interaction that favored AcOH monomer adsorption on the ice surface.21 However, more recent experiments and theoretical treatments challenge these views. Recent experiments indicate that a substantial fraction of the AcOH 18 ACS Paragon Plus Environment

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molecules become deprotonated within the uppermost ice layers.19 This is in contrast to liquid water where AcOH tends to be neutral on the surface, while molecules deprotonate in the bulk liquid.65 In related work on frozen salt water surfaces, ions have been found to induce changes in the properties of the surface layer that have a major influence on gas uptake, and a similar process may be in operation on AcOH-covered ice.66 Furthermore, ab initio calculations indicate that the cis form of AcOH is stable on the ice surface, whereas the trans form is otherwise the preferred structure.67 These calculations also indicate that AcOH does not easily become deprotonated on the ice surface. Thus the present understanding of the AcOH structure on ice is unclear and views diverge with regards to the detailed structure and degree of protonation in the surface layer. The present study adds to the picture by showing that water readily binds strongly to the surface. Further theoretical studies will be required to elucidate the relation between surface structure and the uptake mechanism in greater detail.

Literature values for the accommodation coefficient of water on ice vary by more than two orders of magnitude,35 and the reasons for the large variation remain uncertain. It is clear that some surfactants like alcohols will act to reduce water uptake by making the ice surface more hydrophobic.39 The present study shows that adsorbed AcOH may work in the opposite manner and enhance water uptake. Thus adsorbates may induce both reductions and increases in water accommodation compared to pure ice. This observation makes plain that the importance of contaminants needs to be carefully considered in any water uptake experiments, as well as in atmospheric cloud models. Model simulations of gas uptake in cirrus clouds indicate that abundant gas components like HNO3, HCl, AcOH and formic acid are readily taken up by cloud particles.16 Uptake depends on their stability on the ice surface and high HNO3 and HCl concentrations can be expected under realistic conditions. The present study suggests that a finite 19 ACS Paragon Plus Environment


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AcOH coverage leads to increased ice growth rates compared to uncovered ice particles, and further studies are required to determine if the strong acids will have a comparable effect on water uptake.

5. Conclusions

We have studied D2O interactions with both bulk and monolayer AcOH on graphite and with AcOH-covered water ice using the EMB method. Hyperthermal collisions of D2O molecules with solid AcOH and with AcOH monolayers on graphite results in either inelastic scattering or trapping followed by rapid desorption (residence time < 20 µs). This indicates that the AcOH surface has a closed surface structure with limited opportunities for water to form hydrogen bonds with surface molecules, which is consistent with the high estimated intermolecular binding energy in AcOH of 66 kJ mol-1.29

On water ice, adsorbed AcOH is observed to substantially alter the D2O-surface interactions. A small fraction of incoming water molecules scatter inelastically from the AcOH layer, while trapped water molecules bind efficiently and remain on the ice surface on the time scale of the experiments (60 ms). The behavior is qualitatively different from what is observed on the other AcOH surfaces studied herein, and on pure ice surfaces where a weakly bound precursor state limits bulk accommodation.35 Adsorbed AcOH thus has a major impact on water-ice interactions and enhances water uptake on ice. This is in contrast to other surface-active organics that may produce a significant barrier for water uptake and lower the water accommodation coefficient.


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The results are of potential importance for ice particle growth in cirrus where AcOH is likely to adsorb to the surface of ice particles.

The study illustrates that adsorbed organic compounds may substantially modify the properties of environmental interfaces with potential implications for many Earth system processes. A theoretical description with predictive power remains to be developed for water-solid interactions in the atmosphere, and water uptake experiments should be carried out with additional adsorbates of atmospheric relevance. Theoretical studies including molecular dynamics simulations will help to further improve our understanding of water interactions with AcOH layers and to elucidate the detailed mechanism behind the observed enhancement of water adsorption. Knepp et al.17 concluded that the study of water uptake onto growing snow crystals in the presence of coadsorbers should be a prioritzed area of inquiry. We second that and conclude that further systematic studies are required to resolve the conditions in the atmosphere.

Acknowledgements This work was supported by the Swedish Research Council and the Nordic Top-Level Research Initiative CRAICC. PP thanks the Wenner-Gren Foundation for providing funding for an extended stay at the University of Gothenburg.

Supporting Information Description The SI contains a graphical comparison between the bulk accommodation coefficient for D2O on AcOH-covered ice (this study) and pure ice.35 This information is available free of charge via the Internet at http://pubs.acs.org.



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Figure captions

Fig. 1 Time-of-flight distributions for D2O inelastic scattering and thermal desorption from (a) solid AcOH and (b) an AcOH monolayer on graphite at 200 K. The time profile of the incident D2O beam is shown in (c). The black, violet and green curves show the total, inelastic and thermal desorption components, respectively, obtained from the non-linear least-squares data fitting as described in the text. The experimental data have been normalized to the incident beam intensity and smoothed with a seven point stepwise average. The estimated equilibrium pressure P0 over solid AcOH was 2⋅10-2 Pa.

Fig. 2 Sub-monolayer surface coverage of AcOH on graphite (Ts = 190 K) as a function of relative AcOH vapor pressure during layer growth (orange points) and shrinkage (green points). The equilibrium pressure P0 = 2⋅10-3 Pa was experimentally determined by measuring the steady-state pressure over a µm-thick layer of pure AcOH.

Fig. 3 Time-of-flight distributions for (a) pure thermal desorption of D2O from pure ice and (b) pure inelastic scattering of D2O from AcOH-covered ice surfaces, both at 186 K. The incident D2O beam profile is shown in (c). The AcOH pressure was 1⋅10-4 Pa.

Fig. 4 The relative intensity of D2O inelastic scattering from AcOH-covered ice as a function of AcOH pressure (left axis), and the calculated surface coverage θ (right axis), see text for further details. The surface temperature was 186 K. The dashed line indicates the equilibrium pressure P0 over pure AcOH.


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Figure 1

(a) AcOH multilayer/graphite

0.06 0.04 0.02 0.00

Intensity (arb. unit)

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(b) AcOH monolayer/graphite

0.06 0.04 0.02 0.00

(c) Beam 10000 5000 0 0

1

2 Time (ms)

3


4


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Figure 2


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Figure 3



Figure 4

1

Intensity (arb. unit)

0.1

Surface coverage

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0.01 10 - 6

0.1 10 - 5

10 - 4

10 - 3

Pressure (Pa)


10 - 2

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