C. Toubin

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.

Dynamics of ice layers deposited on MgO(001): Quasielastic neutron scattering experiments and molecular dynamics simulations

The dynamical behavior of a thin film of ice Ih deposited on MgO(001) surface has been investigated both experimentally and theoretically. Incoherent neutron quasielastic scattering experiments, using uniform MgO powders, show that a quasiliquid water layer of monolayer thickness exists at T=265 K. The translational mobility of this layer, with a diffusion coefficient Dt=1.5×10−5 cm2 s−1, is close to that of liquid water. At T=270 K, the thickness of the quasiliquid layer increases to about two layers, showing no appreciable change in the Dt value but an increase of the rotational mobility from 6×109 s−1 to 1.2×1010 s−1. Classical molecular dynamics simulations are performed to determine the translational and orientational order parameters and diffusion coefficients of the supported ice film as a function of temperature within 190 and 270 K, and to compare the results with those obtained for bulk ice. It is shown that the whole supported ice film is much more disordered than bulk ice, with melting tempera...

Adsorption of HF and HCl molecules on ice at 190 and 235 K from molecular dynamics simulations: Free energy profiles and residence times

Constrained molecular dynamics simulations are carried out to compare the adsorption/incorporation mechanism of two (nonionizable) hydrogen halide acid molecules HF and HCl on/in ice at typical stratospheric temperatures (190 and 235 K). From the determination of the free energy profiles, it is shown that the free energy barrier to molecular HF incorporation is higher than that for molecular HCl. This difference is interpreted as resulting from the formation of two strong H bonds between HF and its water neighbors, while a single, more flexible, H bond with water favors the penetration for HCl.

Structure and dynamics of ice Ih films upon HCl adsorption between 190 and 270 K. I. Neutron diffraction and quasielastic neutron scattering experiments

Neutron diffraction and quasielastic neutron experiments are performed to investigate the effect of HCl adsorption on the structure and dynamics of an ultra-thin ice Ih film (5 H2O bilayers thick) deposited on a crystalline MgO(001) substrate. Three HCl coverages have been studied 0.3, 0.6, and 1 monolayer (ML) in the temperature range 190–270 K. At 0.3 and 0.6 HCl monolayer, no mobility is measured at T⩽220 K. A translational mobility, which is a signature of a liquid phase, is observed at T=250 K. This phase occurs 15 K below the surface melting temperature of the bare ice film. The fraction of mobile molecules represents 30% (0.3 ML HCl) and 45% (0.6 ML HCl) of the film. At 1 HCl monolayer and T=220 K, HCl–dihydrate coexists with ice Ih, whereas at T=250 K the ice film becomes amorphous and only 9% of the film is mobile. The results are discussed within the context of atmospheric chemistry.

Structure and dynamics of ice Ih films upon HCl adsorption between 190 and 270 K. II. Molecular dynamics simulations

Classical molecular dynamics simulations are carried out between 190 and 250 K on an ultrathin ice film doped by HCl deposition with a coverage varying from 0.3 to 1.0 monolayer. These conditions are similar to those defined in the experiments described in the companion paper. Within the assumption that the hydracid molecule remains in its molecular form, the order parameters and the diffusion coefficients for the H2O molecules are determined in the HCl doped ice film, and compared to the experimental data. The residence times of HCl at the ice surface are also calculated. Below 200 K, the HCl molecules are found to remain localized at the ice surface, while above 200 K, the HCl diffusion inside the film is easy and leads to a strong disorder of the ice structure. Although the formation of hydrates cannot be interpreted by the present calculations, the lowering of the ice melting temperature by 15 K measured in neutron experiments for an HCl doped ice film is qualitatively explained by simulation results.

Adsorption of small polar molecules as a probe of the surface electric field created by water layers supported by MgO(100): a theoretical study

Abstract The adsorption process of molecular pollutants on water layers supported by well-defined ionic surfaces can be used, in a first approximation, to modelize precursor mechanisms implied in the pollution of ice, as studied in glaciology or in polar stratospheric clouds. We use theoretical methods to calculate the electric field and field gradients at the surface of ordered mono and bilayer phases of ice observed at low temperature (≤200 K) on MgO(100). We analyze the adsorption properties of several pollutants (N 2 , CO 2 , HCl, HOCl) as a function of the site geometry and field intensity on the basis of semi-empirical potentials and optimization procedures. The layer structure strongly influences the intensity and the direction of the electric field experienced by the pollutant, and the stable adsorption site appears to be a compromise which tends to maximize the field effects and the coordination of the pollutant with the water molecules of the outermost layer. Moreover, the ionic support polarizes strongly the closest water layer and enhances consistently the water field, but its influence becomes rapidly negligible when the substrate–water layer distance increases.

Time study of pollutants at the surface of ice at 200 K

Abstract Constrained molecular dynamics simulations are performed to determine residence and transfer times of HCl and HOCl pollutants in a thin ice film at 200 K. We calculate the lifetime of small hydrogen bonded HCl–water aggregates which can induce proton transfer and HCl ionization. It is shown that rapid nanoscale pollutant adsorption/penetration in ice could be a relevant process at the origin of the reactions leading to the chlorine production.

The passage of small molecules through a water film supported by MgO(100): Transfer times from molecular dynamics simulations

The passage of small pollutant molecules (HCl, CO2) through a thin water film supported on a MgO substrate at 300 K has been studied by constrained classical molecular dynamics simulations. The calculated free energy profile of the pollutants exhibits two minima, one at the gas/liquid film interface, and the other inside the film near the ionic substrate. Lifetimes of the pollutants in these two sites have been characterized by unconstrained simulations. The residence times in these sites are in the range of a few tens of picoseconds. The transfer times from one site to the other, and the times spent by the pollutants in the liquid and at the liquid/gas interface (∼ one hundred ps) are always twice longer for CO2 than for HCl. This difference is interpreted in terms of correlated dynamics of HCl and H2O due to hydrogen bond interactions with water. The duration of the hydrogen bond Cl–H⋅⋅⋅O increases significantly (∼2 ps) at the film surface with respect to its value inside the film (<1 ps).