The opposing effect of butanol and butyric acid on the abundance of bromide and iodide at the aqueous solution-air interface.

The efficient oxidation of iodide and bromide at the aqueous solution-air interface of the ocean or of sea spray aerosol particles had been suggested to be related to their surface propensity. The ubiquitous presence of organic material at the ocean surface calls for an assessment of the impact of often surface-active organic compounds on the interfacial density of halide ions. We used in situ X-ray photoelectron spectroscopy with a liquid micro-jet to obtain chemical composition information at aqueous solution-vapor interfaces from mixed aqueous solutions containing bromide or iodide and 1-butanol or butyric acid as organic surfactants. Core level spectra of Br 3d, Na 2s, C 1s and O 1s at ca. 160 eV kinetic energy and core level spectra of I 4d and O 1s at ca. 400 eV kinetic energy are compared for solutions with 1-butanol and butyric acid as a function of organic concentration. A simple model was developed to account for the attenuation of photoelectrons by the aliphatic carbon layer of the surfactants and for changing local density of bromide and iodide in response to the presence of the surfactants. We observed that 1-butanol increases the interfacial density of bromide by 25%, while butyric acid reduces it by 40%, both in comparison to the pure aqueous halide solution. Qualitatively similar behavior was observed for the case of iodide. Classical molecular dynamics simulations failed to reproduce the details of the response of the halide ions to the presence of the two organics. This is attributed to the lack of correct monovalent ion parameters at low concentration possibly leading to an overestimation of the halide ion concentration at the interface in absence of organics. The results clearly demonstrate that organic surfactants change the electrostatic interactions near the interface with headgroup specific effects. This has implications for halogen activation processes specifically when oxidants interact with halide ions at the aqueous solution-air interfaces of the ocean surface or sea spray aerosol particles.

Rapid Water Transport through Organic Layers on Ice.

Processes involving atmospheric aerosol and cloud particles are affected by condensation of organic compounds that are omnipresent in the atmosphere. On ice particles, organic compounds with hydrophilic functional groups form hydrogen bonds with the ice and orient their hydrophobic groups away from the surface. The organic layer has been expected to constitute a barrier to gas uptake, but recent experimental studies suggest that the accommodation of water molecules on ice is only weakly affected by condensed short-chain alcohol layers. Here, we employ molecular dynamics simulations to study the water interactions with n-butanol covered ice at 200 K and show that the small effect of the condensed layer is due to efficient diffusion of water molecules along the surface plane while seeking appropriate sites to penetrate, followed by penetration driven by the combined attractive forces from butanol OH groups and water molecules within the ice. The water molecules that penetrate through the n-butanol layer become strongly bonded by approximately three hydrogen bonds at the butanol-ice interface. The obtained accommodation coefficient (0.81 ± 0.03) is in excellent agreement with results from previous environmental molecular beam experiments, leading to a picture where an adsorbed n-butanol layer does not alter the apparent accommodation coefficient but dramatically changes the detailed molecular dynamics and kinetics.

Mixed DPPC/POPC Monolayers: All-atom Molecular Dynamics Simulations and Langmuir Monolayer Experiments.

To elucidate the consequences of the saturated-unsaturated nature of lipid surface films, monolayers formed by an equimolar mixture of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipids are investigated in a wide range of surface pressures. As such mixtures share some features with naturally-occurring surfactants, for example the lung surfactant, the systems are studied at the temperature relevant for human body. All-atom molecular dynamics simulations and Langmuir trough experiments are employed. The binary lipid mixture is compared with the corresponding one-component systems. Atomistic-level alterations of monolayer molecular properties upon lateral compression are scrutinized. These involve elevation of lateral ordering of lipid chains, modulation of chain and headgroup orientation, and reduction of lipid hydration. The presence of the unsaturated POPC in the DPPC/POPC mixture reduces the liquid expanded-liquid condensed coexistence region and moderates the phase transition. Simulations predict that nanoscale lipid de-mixing occurs with small transient DPPC clusters emerging due to local fluctuations of the lateral lipid arrangement. A vertical sorting of lipids induced by lateral compression is also observed, with DPPC transferred toward the water phase. Both the conformational lipid alterations due to monolayer compression as well as the existence of lateral dynamic inhomogeneities of the lipid film are potentially pertain to dynamic and non-homogeneous lipid interfacial systems.

Investigation of Mixed Surfactant Films at Water Surface Using Molecular Dynamics Simulations.

Multicomponent Langmuir monolayers are important models of organic coatings of naturally occurring water-vapor interfaces such as the surfaces of oceans or aerosol particles. We investigated mixed monolayers comprised of palmitic acid, C15H31COOH (PA) and 1-bromoalkanes of different chain length (C5, C10, and C16) at the air-water interface employing classical molecular dynamics simulations. Different composition ratios and lateral compression of the monolayers were considered. The structural parameters, such as density profiles, and deuterium order parameter, evaluated as functions of composition and the lateral film packing, provide microscopic information about organization and dynamics of the mixed monolayers. Simulations demonstrate that stable and well mixed monolayers are formed by the mixtures of PA and BrC16H33 (BrCl6), whereas the two considered shorter bromoalkanes, BrC5H11 (BrC5) and BrC10H21 (BrC10), do not form stable films. This is in accord with earlier experimental studies. Under high lateral pressures, in PA/BrC10 mixed systems molecules of the bromoalkane readily flip in the monolayer and subsequently leave the film, while the molecules of the longer BrC16 are expelled from the PA film but no flipping occurs. These results suggest that the film collapse under pressure is preceded by squeezing-out of bromoalkanes from the PA monolayer.

Mechanism of anisotropic surface self-diffusivity at the prismatic ice-vapor interface.

Predictive theoretical models for mesoscopic roughening of ice require improved understanding of attachment kinetics occurring at the ice-vapor interface. Here, we use classical molecular dynamics to explore the generality and mechanics of a transition from anisotropic to isotropic self-diffusivity on exposed prismatic surfaces. We find that self-diffusion parallel to the crystallographic a-axis is favored over the c-axis at sub-melt temperatures below about -35 °C, for three different representations of the water-water intermolecular potential. In the low-temperature anisotropic regime, diffusion results from interstitial admolecules encountering entropically distinct barriers to diffusion in the two in-plane directions. At higher temperatures, isotropic self-diffusion occurring deeper within the quasi-liquid layer becomes the dominant mechanism, owing to its larger energy of activation.

Lack of Aggregation of Molecules on Ice Nanoparticles.

Multiple molecules adsorbed on the surface of nanosized ice particles can either remain isolated or form aggregates, depending on their mobility. Such (non)aggregation may subsequently drive the outcome of chemical reactions that play an important role in atmospheric chemistry or astrochemistry. We present a molecular beam experiment in which the controlled number of guest molecules is deposited on the water and argon nanoparticles in a pickup chamber and their aggregation is studied mass spectrometrically. The studied molecules (HCl, CH3Cl, CH3CH2CH2Cl, C6H5Cl, CH4, and C6H6) form large aggregates on argon nanoparticles. On the other hand, no aggregation is observed on ice nanoparticles. Molecular simulations confirm the experimental results; they reveal a high degree of aggregation on the argon nanoparticles and show that the molecules remain mostly isolated on the water ice surface. This finding will influence the efficiency of ice grain-mediated synthesis (e.g., in outer space) and is also important for the cluster science community because it shows some limitations of pickup experiments on water clusters.

Interactions of gaseous HNO3 and water with individual and mixed alkyl self-assembled monolayers at room temperature.

The major removal processes for gaseous nitric acid (HNO3) in the atmosphere are dry and wet deposition onto various surfaces. The surface in the boundary layer is often covered with organic films, but the interaction of gaseous HNO3 with them is not well understood. To better understand the factors controlling the uptake of gaseous nitric acid and its dissociation in organic films, studies were carried out using single component and mixtures of C8 and C18 alkyl self-assembled monolayers (SAMs) attached to a germanium (Ge) attenuated total reflectance (ATR) crystal upon which a thin layer of SiOx had been deposited. For comparison, diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) studies were also carried out using a C18 SAM attached to the native oxide layer on the surface of silicon powder. These studies show that the alkyl chain length and order/disorder of the SAMs does not significantly affect the uptake or dissociation/recombination of molecular HNO3. Thus, independent of the nature of the SAM, molecular HNO3 is observed up to 70-90% relative humidity. After dissociation, molecular HNO3 is regenerated on all SAM surfaces when water is removed. Results of molecular dynamics simulations are consistent with experiments and show that defects and pores on the surfaces control the uptake, dissociation and recombination of molecular HNO3. Organic films on surfaces in the boundary layer will certainly be more irregular and less ordered than SAMs studied here, therefore undissociated HNO3 may be present on surfaces in the boundary layer to a greater extent than previously thought. The combination of this observation with the results of recent studies showing enhanced photolysis of nitric acid on surfaces suggests that renoxification of deposited nitric acid may need to be taken into account in atmospheric models.

Adsorption, mobility, and self-association of naphthalene and 1-methylnaphthalene at the water-vapor interface.

The adsorption, mobility, and self-association of naphthalene (NPH) and 1-methylnaphthalene (1MN), two of the simplest polycyclic aromatic hydrocarbons (PAHs), at the surface of liquid water at 289 K were investigated using classical molecular dynamics (MD) simulations and free energy profile calculations across the water-vapor interface. Both NPH and 1MN, which exhibit a strong preference to be adsorbed at the water-vapor interface, are found to readily self-associate at the water surface, adopting mostly configurations with distinctly nonparallel arrangement of the two monomers. The additional methyl group of 1MN represents only a minor perturbation in terms of the hydration properties, interfacial orientation, and self-association with respect to NPH. Implications of the observed self-association behavior for fluorescence spectroscopy of NPH and 1MN in aqueous interfacial environment are discussed.

Spectroscopic properties of benzene at the air-ice interface: a combined experimental-computational approach.

A combined experimental and computational approach was used to study the spectroscopic properties of benzene at the ice-air interface at 253 and 77 K in comparison with its spectroscopic behavior in aqueous solutions. Benzene-contaminated ice samples were prepared either by shock-freezing of benzene aqueous solutions or by benzene vapor-deposition on pure ice grains and examined using UV diffuse reflectance and emission spectroscopies. Neither the absorption nor excitation nor emission spectra provided unambiguous evidence of benzene associates on the ice surface even at a higher surface coverage. Only a small increase in the fluorescence intensity in the region above 290 nm found experimentally might be associated with formation of benzene excimers perturbed by the interaction with the ice surface as shown by ADC(2) excited-state calculations. The benzene associates were found by MD simulations and ground-state DFT calculations, although not in the arrangement that corresponds to the excimer structures. Our experimental results clearly demonstrated that the energy of the S0 → S1 electronic transition of benzene is not markedly affected by the phase change or the microenvironment at the ice-air interface and its absorption is limited to the wavelengths below 268 nm. Neither benzene interactions with the water molecules of ice nor the formation of dimers and microcrystals at the air-ice interface thus causes any substantial bathochromic shift in its absorption spectrum. Such a critical evaluation of the photophysical properties of organic contaminants of snow and ice is essential for predictions and modeling of chemical processes occurring in polar regions.

Molecular dynamics simulations of small halogenated organics at the air-water interface: implications in water treatment and atmospheric chemistry.

Free energy profiles associated with transfer of chlorinated and brominated halomethane molecules from the gas phase across the water-vapor interface to the aqueous phase were calculated using classical molecular dynamics simulations. The investigated species include chloromethane (CH3Cl), bromomethane (CH3Br), dichloromethane (CH2Cl2), dibromomethane (CH2Br2), chloroform (CHCl3), and bromoform (CHBr3). The employed halomethane force field was tuned by scaling up the atomic charges to reproduce the experimental hydration free energies. The computed free energy profiles have a minimum at the water-vapor interface of about 12-15 kJ·mol(-1) relative to full hydration in the bulk liquid. This implies that the halomethanes exhibit enhanced interfacial concentrations in systems with large surface area per unit volume, such as air bubbles dispersed in water or water droplets dispersed in air. Implications for water treatment as well as for atmospheric chemistry are discussed.

Surface organization of aqueous MgCl2 and application to atmospheric marine aerosol chemistry.

Inorganic salts in marine aerosols play an active role in atmospheric chemistry, particularly in coastal urban regions. The study of the interactions of these ions with water molecules at the aqueous surface helps to elucidate the role of inorganic cations and anions in atmospheric processes. We present surface vibrational sum frequency generation (SFG) spectroscopic and molecular dynamics (MD) studies of aqueous MgCl(2) surfaces as models of marine aerosol. Spectroscopy results reveal that the disturbance of the hydrogen bonding environment of the air/aqueous interface is dependent on the MgCl(2) concentration. At low concentrations (< 1 M) minor changes are observed. At concentrations above 1 M the hydrogen bonding environment is highly perturbed. The 2.1 M intermediate concentration solution shows the largest SFG response relative to the other solutions including concentrations as high as 4.7 M. The enhancement of SFG signal observed for the 2.1 M solution is attributed to a larger SFG-active interfacial region and more strongly oriented water molecules relative to other concentrations. MD simulations reveal concentration dependent compression of stratified layers of ions and water orientation differences at higher concentrations. SFG and MD studies of the dangling OH of the surface water reveal that the topmost water layer is affected structurally at high concentrations (> 3.1 M). Finally, the MgCl(2) concentration effect on a fatty acid coated aqueous surface was investigated and SFG spectra reveal that deprotonation of the carboxylic acid of atmospherically relevant palmitic acid (PA) is accompanied by binding of the Mg(2+) to the PA headgroup.

Comparison of selected polarizable and nonpolarizable water models in molecular dynamics simulations of ice I(h).

We present a molecular dynamics simulation study in which we determined the melting point of ice I(h) for the polarizable SWM4-NDP water model (Lamoureux et al., Chem. Phys. Lett., 2006, 418, 245-249) and compared the performance of several popular water force fields, both polarizable and nonpolarizable, in terms of melting temperature, stability and orientational structuring of ice. The simulations yield the melting temperature of SWM4-NDP ice as low as T(m) = 185 ± 10 K, despite the quadrupole moment of a SWM4-NDP water molecule being close to the experimental gas phase value. The results thus show that the dependence of T(m) on the molecular quadrupole, observed for the three- and four-site water models, is generally lost if polarization is explicitly included. The study also shows that adding polarizability to a planar three-charge water model increases orientational disorder in hexagonal ice. In addition, analysis of the tetrahedral order in bulk ice reveals a correlation between the pre-existing degree of orientational disorder in ice simulated using different polarizable and nonpolarizable models and the melting temperature of the models. Our findings thus suggest some new considerations regarding the role of polarization forces in a crystalline solid that may guide future development of reliable polarizable water models for ice.

Arrhenius analysis of anisotropic surface self-diffusion on the prismatic facet of ice.

We present an Arrhenius analysis of self-diffusion on the prismatic surface of ice calculated from molecular dynamics simulations. The six-site water model of Nada and van der Eerden was used in combination with a structure-based criterion for determining the number of liquid-like molecules in the quasi-liquid layer. Simulated temperatures range from 230 K-287 K, the latter being just below the melting temperature of the model, 289 K. Calculated surface diffusion coefficients agree with available experimental data to within quoted precision. Our results indicate a positive Arrhenius curvature, implying a change in the mechanism of self-diffusion from low to high temperature, with a concomitant increase in energy of activation from 29.1 kJ mol(-1) at low temperature to 53.8 kJ mol(-1) close to the melting point. In addition, we find that the surface self-diffusion is anisotropic at lower temperatures, transitioning to isotropic in the temperature range of 240-250 K. We also present a framework for self-diffusion in the quasi-liquid layer on ice that aims to explain these observations.

Comparative molecular dynamics study of vapor-exposed basal, prismatic, and pyramidal surfaces of ice.

We present the results of molecular dynamics simulations in which ice I(h) slabs with free basal, prismatic, 28° pyramidal, and 14° pyramidal facets are exposed to vapor. All simulations were carried out at 250 K using a six-site intermolecular potential. Characteristics common to all facets include spontaneous development of a quasi-liquid layer (QLL) within ∼10 ns and QLL stratification into outer (ε(1)) and inner (ε(2)) sublayers having on average two and three hydrogen bonds, respectively. Vapor pressure, based on the rate of escape of molecules from the QLL to the vapor phase, is found to be greatest for the 14° pyramidal and basal facets (∼230 Pa), while significantly lower values are obtained for the prismatic and 28° pyramidal facets (∼200 Pa). The geometric thickness of the QLL also varies between facets, with the 14° pyramidal having the greatest thickness. The free prismatic and pyramidal facets exhibit significant anisotropic diffusivity, in-plane motion being faster in the trans-prismatic direction than in the basal-to-basal direction. The in-plane diffusion length is greatest for the 28° pyramidal facet and smallest for the prismatic facet. This diversity of facet-specific properties provides a rich set of possibilities for mechanisms of ice crystal growth and ablation.

The ice-vapor interface and the melting point of ice I(h) for the polarizable POL3 water model.

We use molecular dynamics simulations to determine the melting point of ice I(h) for the polarizable POL3 water force field (Dang, L. X. J. Chem. Phys.1992, 97, 2659). Simulations are performed on a slab of ice I(h) with two free surfaces at several different temperatures. The analysis of the time evolution of the total energy in the course of the simulations at the set of temperatures yields the melting point of the POL3 model to be T(m) = 180 ± 10 K. Moreover, the results of the simulations show that the degree of hydrogen-bond disorder occurring in the bulk of POL3 ice is larger (at the corresponding degree of undercooling) than in ice modeled by nonpolarizable water models. These results demonstrate that the POL3 water force field is rather a poor model for studying ice and ice-liquid or ice-vapor interfaces. While a number of polarizable water models have been developed over the past years, little is known about their performance in simulations of supercooled water and ice. This study thus highlights the need for testing of the existing polarizable water models over a broad range of temperatures, pressures, and phases, and developing a new polarizable water force field, reliable over larger areas of the phase diagram.

Water adsorption on hydrophilic and hydrophobic self-assembled monolayers as proxies for atmospheric surfaces. A grand canonical Monte Carlo simulation study.

Grand canonical Monte Carlo simulations are used to determine water adsorption on prototypical organic surfaces as a function of relative humidity at 300 K. Three model surfaces formed by well-ordered self-assembled monolayers (SAMs) of alkanethiolate chains on gold are investigated: (i) a smooth hydrophobic surface of methyl-terminated C(7)-CH(3) SAM; (ii) a rough hydrophobic surface of randomly mixed two-component SAM, composed of equal fractions of C(5)-CH(3) and C(7)-CH(3) chains (C(5)/C(7)-CH(3) SAM); and (iii) a smooth hydrophilic surface of carboxyl-terminated C(7)-COOH SAM. The all atom CHARMM22 force field is used for the SAM chains together with the SPC/E model for water. No noticeable water adsorption is observed on the smooth hydrophobic surface up to saturation. The mild surface roughness introduced by the uneven chain length of the two components constituting the C(5)/C(7)-CH(3) SAM has no significant effect on the surface hydrophobicity, and the rough hydrophobic surface also remains dry up to the point when water condensation occurs. In contrast, water readily adsorbs onto the hydrophilic surface by forming hydrogen bonds with the COOH groups of the substrate. In addition, hydrogen bonding with pre-adsorbed water molecules contributes to the mechanism of water uptake. Under low humidity conditions, water is present on the hydrophilic surface as individual molecules or small water clusters and, with increasing relative humidity, the surface coverage grows continuously beyond a monolayer formation. The adsorbed water film is observed to be rather inhomogeneous with patches of bare surface exposed. The amount of water constituting a stable adsorption layer prior to condensation is estimated to consist of about 2-5 molecular layers. Detailed analysis of the simulation results is used to obtain important insights into the structure and energetics of water adsorbed on highly oxidized organic surfaces exposed to ambient air of increasing relative humidity.

Effect of magnesium cation on the interfacial properties of aqueous salt solutions.

Sodium chloride solutions have been used extensively as a model of seawater in both theoretical and experimental studies of the chemistry of sea salt aerosol. Many groups have found that chloride anions are present at the air-solution interface. This observation has been important for the development of a mechanism for the heterogeneous production of molecular chlorine from chloride in sea salt aerosol. However, while sodium chloride is a major constituent of seawater, it is by no means the only salt present. Seawater contains one Mg(2+) for every eight Na(+). Mg(2+) is naturally occurring in ocean waters from mineral deposits in the Earth's crust and biological sources. Mg(2+) forms a hexahydrate structure, rather than contact ion pairs with chloride anion, and this impacts the ordering of water in solution. In this study, we use molecular dynamics simulations, ab initio calculations, and vibrational sum frequency generation (SFG) spectroscopy to explore the effect of the Mg(2+) cation and its tightly bound solvation shell on the surface propensity of chloride, ion-ion interactions, and water structure of the air-solution interface of concentrated chloride salt solutions. In addition, we provide molecular level details that may be relevant to the heterogeneous reactions of chloride in deliquesced sea salt aerosols. In particular, we show that the presence of the divalent Mg(2+) cation does not modify the surface propensity of chloride compared to Na(+) and hence, its availability to interfacial reaction, although some differences in the behavior of chloride may occur due to specific ion interactions. In this work, we also discuss the SFG free OH band at the surface of salt solutions and conclude that it is often not straightforward to interpret.

Solvation of magnesium dication: molecular dynamics simulation and vibrational spectroscopic study of magnesium chloride in aqueous solutions.

Magnesium dication plays many significant roles in biochemistry. While it is available to the environment from both ocean waters and mineral salts on land, its roles in environmental and atmospheric chemistry are still relatively unknown. Several pieces of experimental evidence suggest that contact ion pairing may not exist at ambient conditions in solutions of magnesium chloride up to saturation concentrations. This is not typical of most ions. There has been disagreement in the molecular dynamics literature concerning the existence of ion pairing in magnesium chloride solutions. Using a force field developed during this study, we show that contact ion pairing is not energetically favorable. Additionally, we present a concentration-dependent Raman spectroscopic study of the Mg-O(water) hexaaquo stretch that clearly supports the absence of ion pairing in MgCl(2) solutions, although a transition occurring in the spectrum between 0.06x and 0.09x suggests a change in solution structure. Finally, we compare experimental and calculated observables to validate our force field as well as two other commonly used magnesium force fields, and in the process show that ion pairing of magnesium clearly is not observed at higher concentrations in aqueous solutions of magnesium chloride, independent of the choice of magnesium force field, although some force fields give better agreement to experimental results than others.

Microscopic wetting of mixed self-assembled monolayers: a molecular dynamics study.

Molecular dynamics simulations are used to study the evolution of the organization of water molecules on the flat surface of well-ordered self-assembled monolayers (SAMs) of eight-carbon alkanethiolate chains bound to a gold substrate, as the character of the surface is finely tuned from completely hydrophobic to completely hydrophilic, and as the level of hydration is increased from submonolayer to the equivalent of about two monolayers of water. The hydrophilicity of the SAM surfaces is increased by randomly replacing methyl-terminated alkanethiolate chains with carboxylic acid-terminated chains. We report on the evolution of the structure of the surfaces of the SAMs, both in the absence and presence of water, and the organization of water molecules and the extent of wetting of the surfaces, as the fraction of hydrophilic groups is increased. The results suggest that on the flat organic surfaces with a small fraction of the hydrophilic components the hydrophilic spots serve as nucleation sites, resulting in the growth of a larger number of (smaller) water droplets compared to the completely hydrophobic surface, whereas on the surfaces with a large fraction of the hydrophilic component the uptake of water proceeds via a water film growing, at first, over the hydrophilic domains and, eventually, bridging over the hydrophobic patches, and spreading out over the entire surface. We discuss the implications of these processes on the properties of the organic aerosols in the atmosphere.

Surface residence and uptake of methyl chloride and methyl alcohol at the air/water interface studied by vibrational sum frequency spectroscopy and molecular dynamics.

Vibrational sum frequency generation (VSFG) spectroscopy and molecular dynamics (MD) simulations are used to study the surface residence and organization of gas-phase methyl halide and methyl alcohol molecules adsorbed to the air/water interface, while Raman spectroscopy is used to detect the uptake of the gas-phase species into the bulk aqueous phase. Spectroscopy results reveal the presence of methyl alcohol in the bulk and at the surface. Methyl chloride is detected in the bulk, but not at the surface. This indicates that methyl alcohol adsorbs to the aqueous surface in a layer that is ordered, in agreement with previous studies, and is also readily taken up into the bulk aqueous phase, whereas methyl chloride adsorbs, but, while being taken up into the bulk liquid, has lower surface number density and/or forms a more disordered surface layer than methyl alcohol. MD simulations show that methyl halide molecules transition readily between the gas phase and interface, resulting in significantly shorter residence times at the surface for the methyl halides relative to methyl alcohol. Both the geometries that the methyl species adopt at the interface and the interactions between the methyl species and the interfacial water molecules differ for the halides and the alcohol. Complementary studies of butyl species show similar results: butyl alcohol adsorbs to the aqueous surface in a layer that exhibits a certain degree of order corresponding to the chains aligned along the surface normal, while a markedly more disordered surface layer and shorter residence times are observed in MD simulations for the butyl halides as compared to the alcohol. Desorption from the interface was found to be less frequent for the butyl halides than for the methyl halides by MD simulations. Although Raman studies show uptake of the butyl alcohol into the bulk phase, neither Raman studies nor MD simulations provide any evidence for uptake of the butyl halides into the bulk phase. The profound difference in preferred orientations between alkyl halides and alcohols at the aqueous surface, with the halogen atom of the alkyl halides being to a large degree exposed to the vapor phase, is likely to have consequences for chemistry of alkyl halides adsorbed on the surface of atmospheric aerosol particles.