P. N. M. Hoang

Adsorption of acetone molecules on proton ordered ice. A molecular dynamics study

The adsorption of acetone molecules on a proton ordered ice Ih(0001) surface was studied using classical molecular dynamics simulations between 50 and 150 K. At low coverage, we show that acetone molecules form an ordered monolayer on this ice surface, which is stable for T⩽100 K. At higher temperature, it exhibits orientational disordering, though local translational order remains. Preliminary simulations at higher coverage indicates the formation of additional ordered layers above the first monolayer, which are also stable below 100 K. These results support previous conclusions on the acetone/ice interactions based on the interpretation of experimental data.

Geometry and dynamics of formic and acetic acids adsorbed on ice

Energy optimization at 0 K and constrained molecular dynamics simulations at 250 K have been carried out to study adsorption and incorporation of formic and acetic acids on/in ice. The results show that the adsorption and incorporation processes are highly influenced by the formation of two H-bonds between the carboxyl function and two water molecules. The free energy profiles indicate that the two acid molecules are strongly trapped at the ice surface and that the incorporation of formic acid is favored when compared to acetic acid. These data are discussed within the context of tropospheric conditions.

A molecular dynamics study of the structure of water layers adsorbed on MgO(100)

Molecular dynamics simulations are performed at various temperatures (150-300 K) and coverages (1-3 layers) on the adsorption of water on a clean MgO(100) surface using semiempirical potentials. At the monolayer coverage, a number of very stable (m×n) structures are obtained which differ only by the mutual orientations of the molecules. The p(3×2) phase observed above 180 K in low-energy electron diffraction (LEED) and helium atom scattering (HAS) experiments is shown to be the most stable at 200 K and above this temperature. It contains six inequivalently oriented molecules which lie flat above the cation sites with the hydrogens pointing approximately along the Mg rows. When the water coverage increases, a layer of icelike hexagonal structure within which the water molecules are hydrogen bonded is formed above the stable monolayer. This overlayer, which is stable at 150 K, is not hydrogen bonded to the stable monolayer. At 300 K it tends to break up and to aggregate into a 3D ice structure with strong h...

Adsorption of acetic acid on ice: experiments and molecular dynamics simulations.

Adsorption study of acetic acid on ice surfaces was performed by combining experimental and theoretical approaches. The experiments were conducted between 193 and 223 K using a coated wall flow tube coupled to a mass spectrometric detection. Under our experimental conditions, acetic acid was mainly dimerized in the gas phase. The surface coverage increases with decreasing temperature and with increasing concentrations of acetic acid dimers. The obtained experimental surface coverages were fitted according to the BET theory in order to determine the enthalpy of adsorption deltaH(ads) and the mololayer capacity N(M(dimers)) of the acetic acid dimers on ice: deltaH(ads) = (-33.5 +/- 4.2) kJ mol(-1), N(M(dimers)) = (l1.27 +/- 0.25) x 10(14) dimers cm(-2). The adsorption characteristics of acetic acid on an ideal ice I(n)(0001) surface were also studied by means of classical molecular dynamics simulations in the same temperature range. The monolayer capacity, the configurations of the molecules in their adsorption sites, and the corresponding adsorption energies have been determined for both acetic acid monomers and dimers, and compared to the corresponding data obtained from the experiments. In addition, the theoretical results show that the interaction with the ice surface could be strong enough to break the acetic acid dimers that exist in the gas phase and leads to the stabilization of acetic acid monomers on ice.

Grand canonical Monte Carlo simulation of the adsorption isotherms of water molecules on model soot particles

The grand canonical Monte Carlo method is used to simulate the adsorption isotherms of water molecules on different types of model soot particles. The soot particles are modeled by graphite-type layers arranged in an onionlike structure that contains randomly distributed hydrophilic sites, such as OH and COOH groups. The calculated water adsorption isotherm at 298 K exhibits different characteristic shapes depending both on the type and the location of the hydrophilic sites and also on the size of the pores inside the soot particle. The different shapes of the adsorption isotherms result from different ways of water aggregation in or/and around the soot particle. The present results show the very weak influence of the OH sites on the water adsorption process when compared to the COOH sites. The results of these simulations can help in interpreting the experimental isotherms of water adsorbed on aircraft soot.

Water adsorption isotherms on porous onionlike carbonaceous particles. Simulations with the grand canonical Monte Carlo method.

The grand canonical Monte Carlo method is used to simulate the adsorption isotherms of water molecules on different types of model soot particles. These soot models are constructed by first removing atoms from onion-fullerene structures in order to create randomly distributed pores inside the soot, and then performing molecular dynamics simulations, based on the reactive adaptive intermolecular reactive empirical bond order (AIREBO) description of the interaction between carbon atoms, to optimize the resulting structures. The obtained results clearly show that the main driving force of water adsorption on soot is the possibility of the formation of new water-water hydrogen bonds with the already adsorbed water molecules. The shape of the calculated water adsorption isotherms at 298 K strongly depends on the possible confinement of the water molecules in pores of the carbonaceous structure. We found that there are two important factors influencing the adsorption ability of soot. The first of these factors, dominating at low pressures, is the ability of the soot of accommodating the first adsorbed water molecules at strongly hydrophilic sites. The second factor concerns the size and shape of the pores, which should be such that the hydrogen bonding network of the water molecules filling them should be optimal. This second factor determines the adsorption properties at higher pressures.

Clustering of water molecules on model soot particles: an ab initio study

Clustering of water molecules on model soot particles is studied by means of quantum calculations based on the ONIOM approach. The soot particles are modelled by anchoring OH or COOH groups on the face side or on the edges of a graphite crystallite of nanometer size. The quantum calculations aim at characterizing the adsorption properties (structure and adsorption energy) of small water aggregates containing up to 5 water molecules, in order to better understand at a molecular level the role of these OH and COOH groups on the behavior with respect to water adsorption of graphite surface modelling soot emitted by aircraft.

Geometry of (NH3)n and (H2O)n aggregates adsorbed on well characterized MgO(100) and Si(111)−(1×1)H substrates

Abstract On the basis of semi-empirical potentials used to determine the structure of small gas phase clusters, we calculate the geometry of molecular aggregates (M) n ( n = 2−5) physisorbed on ideal and clean corrugated surfaces. The competition between intraadsorbate and adsorbate-substrate interactions is studied for the cases of ammonia and water aggregates adsorbed on H-passivated Si(111) and MgO(100) surfaces. It is shown that the adsorbate-substrate interactions are dominant for ammonia on Si(111) and for water on MgO(100) leading to planar aggregate geometries. However, the lateral interactions tend to prevent NH 3 aggregation on the ionic substrate as they are repulsive and to favor compact H 2 O aggregates on the semiconductor. These features are in qualitative agreement with recent experimental results.

A Comparitive Study of the Geometry of CO2 Monolayers Adsorbed on the Ionic Substrates NaCl and MgO(100)

Interaction potential calculations performed on the systems CO2/NaCl(100) and CO2/MgO(100) provide additional information on the adsorption characteristics deduced from data on low-energy electron diffraction, polarization infra-red spectroscopy, helium atom diffraction and thermodynamics. CO2 monolayer stability calculations agree fairly well with most of the experimental analyses which lead to stable (2 × 1) and (2 √2 × √2) R45° structures on NaCl and MgO substrates, respectively, but they rule out some other assignments.

Adsorption of Benzaldehyde at the Surface of Ice, Studied by Experimental Method and Computer Simulation

Adsorption study of benzaldehyde on ice surfaces is performed by combining experimental and theoretical approaches. The experiments are conducted over the temperature range 233-253 K using a coated wall flow tube coupled to a mass spectrometric detector. Besides the experimental way, the adsorption isotherm is also determined by performing a set of grand canonical Monte Carlo simulations at 233 K. The experimental and calculated adsorption isotherms show a very good agreement within the corresponding errors. Besides, both experimental and theoretical studies permit us to derive the enthalpy of adsorption of benzaldehyde on ice surfaces DeltaH(ads), which are in excellent agreement: DeltaH(ads) = -61.4 +/- 9.7 kJ/mol (experimental) and DeltaH(ads) = -59.4 +/- 5.1 kJ/mol (simulation). The obtained results indicate a much stronger ability of benzaldehyde of being adsorbed at the surface of ice than that of small aliphatic aldehydes, such as formaldehyde or acetaldehyde. At low surface coverages the adsorbed molecules exclusively lie parallel with the ice surface. With increasing surface coverage, however, the increasing competition of the adsorbed molecules for the surface area to be occupied leads to the appearance of two different perpendicular orientations relative to the surface. In the first orientation, the benzaldehyde molecule turns its aldehyde group toward the ice phase, and, similarly to the molecules in the lying orientation, forms a hydrogen bond with a surface water molecule. In the other perpendicular orientation the aldehyde group turns to the vapor phase, and its O atom interacts with the delocalized pi system of the benzene ring of a nearby lying benzaldehyde molecule of the second molecular layer. In accordance with this observed scenario, the saturated adsorption layer, being stable in a roughly 1 kJ/mol broad range of chemical potentials, contains, besides the first molecular layer, also traces of the second molecular layer of adsorbed benzaldehyde.