Adsorption of H2O2 at the surface of Ih ice, as seen from grand canonical Monte Carlo simulations

Adsorption of H2O2 at the (0 0 0 1) surface of I h ice is investigated by GCMC simulations under tropospheric conditions. The results are in qualitative agreement with experimental data and reveal that the main driving force of the adsorption is the formation of new H2O2-H2O2 rather than H 2O2-water H-bonds. The isotherm belongs to class III and not even its low pressure part can be described by the Langmuir formalism. At low coverages H2O2 prefers perpendicular alignment to the surface, in which they can form three H-bonds with surface waters. At higher coverages parallel alignment, stabilized by H-bonds between neighbouring H 2O2 molecules, becomes increasingly preferred. © 2014 Elsevier B.V. All rights reserved

Adsorption of HCN at the Surface of Ice : A Grand Canonical Monte Carlo Simulation Study

Adsorption of HCN molecules at the surface of hexagonal (I-h) ice is studied under tropospheric conditions by a set of grand canonical Monte Carlo simulations. Although the adsorption isotherm is of Langmuir shape and the saturated adsorption layer is practically monomolecular, lateral interactions are found to have a non-negligible effect on the adsorption. The Langmuir shape of the isotherm can be rationalized by the fact that the interaction energy for HCN-water and HCN-HCN pairs is rather close to each other, and hence, monomolecular adsorption even in the presence of lateral interactions turns into a special case. At low surface coverages the HCN molecules prefer a tilted orientation, pointing by the N atom flatly toward the ice surface, in which they can form a strong O-H center dot center dot center dot N-type hydrogen bond with the surface water molecules. At high surface coverages, an opposite tilted orientation is preferred, in which the H atom points toward the ice phase and the HCN molecule can form only a weak C-H center dot center dot center dot O-type hydrogen bond with a surface water molecule. This orientational change is dictated by the smaller surface area occupied by a H than a N atom, and the corresponding energy loss is (over)compensated by formation of C-H center dot center dot center dot N-type hydrogen bonds between neighboring HCN molecules. The obtained results have several consequences both on the atmospheric effect of HCN and also on the possible prebiotic formation of precursor molecules of adenine. These consequences are also discussed in the paper

Molecular dynamics simulations of the water adsorption around malonic acid aerosol models

Water nucleation around a malonic acid aggregate has been studied by means of molecular dynamics simulations in the temperature and pressure range relevant for atmospheric conditions. Systems of different water contents have been considered and a large number of simulations have allowed us to determine the phase diagram of the corresponding binary malonic acid–water systems. Two phases have been evidenced in the phase diagrams corresponding either to water adsorption on a large malonic acid grain at low temperatures, or to the formation of a liquid-like mixed aggregate of the two types of molecules, at higher temperatures. Finally, the comparison between the phase diagrams simulated for malonic acid–water and oxalic acid–water mixtures emphasizes the influence of the O : C ratio on the hydrophilic behavior of the aerosol, and thus on its ability to act as a cloud condensation nucleus, in accordance with recent experimental conclusions

Amfipatikus molekulák agregációjának vizsgálata = Investigation of the aggregation properties of amphiphilic molecules

Munkánk során vizsgáltuk amfipatikus molekulák aggregátumait folyadék-gőz határfelületeken, lipid membránokban, illetve micellákban, fluid-fluid (folyadék-folyadék és folyadék-gőz) határfelületek tulajdonságait, illetve kis molekulák (pl. metanol, aceton, hangyasav) adszorpcióját fluid-fluid és szilárd-gőz határfelületeken. Vizsgálatainkat a perkolációszámításra vonatkozó metodológiai fejlesztések tették teljessé. Eredményeinkből 33, nemzetközi folyóiratban a projekt számának feltüntetésével közölt publikáció született. | In the project we have investigated aggregates of amphiphilic molecules at liquid/gas interfaces, in lipid membranes and in micelles, studied the properties of fluid/fluid (i.e., liquid/liquid and liquid/gas) interfaces, and the adsorption of various small molecules (e.g., methanol, acetone, formic acid) at fluid/fluid and solid/gas interfaces. Our investigations have been completed by methodological developments concerning percolation analysis. The results of these investigations have been published in 33 scientific papers in international journals by indicating the reference number of the project

Layer-by-layer and intrinsic analysis of molecular and thermodynamic properties across soft interfaces

Interfaces are ubiquitous objects, whose thermodynamic behavior we only recently started to understand at the microscopic detail. Here, we borrow concepts from the techniques of surface identification and intrinsic analysis, to provide a complementary point of view on the density, stress, energy, and free energy distribution across liquid ("soft") interfaces by analyzing the respective contributions coming from successive layers. © 2015 AIP Publishing LLC

Adsorption of Methylene Fluoride and Methylene Chloride at the Surface of Ice under Tropospheric Conditions: A Grand Canonical Monte Carlo Simulation Study

The adsorption of two halogenated methane derivatives, namely, methylene fluoride and methylene chloride, at the surface of Ih ice is studied by grand canonical Monte Carlo simulations under tropospheric conditions. The adsorption isotherms of the two molecules, differing only in the halogen atom type, are found to be markedly different from each other. Thus, while methylene fluoride exhibits multilayer adsorption and its adsorption isotherm belongs to class II according to the IUPAC convention, methylene chloride does not show considerable adsorption at the ice surface, as its condensation well precedes the saturation of even the first adsorbed molecular layer. Interestingly, both the surface orientation and the binding energy of the two types of adsorbed molecules are rather similar to each other; first layer molecules form one single hydrogen bond with the dangling OH groups of the ice surface. The strong differences in the adsorption behavior of methylene fluoride and methylene chloride are traced back to the different cohesions in the liquid phase and, hence, to the strongly different boiling points of the two molecules

Microscopic origin of the surface tension anomaly of water

We investigate the hydrogen bonding percolation threshold of water molecules at the surface of the liquid-vapor interface. We show that the percolation temperature agrees within statistical accuracy with the high-temperature inflection point of the water surface tension. We associate the origin of this surface tension anomaly of water with the sudden breakup of the hydrogen bonding network in the interfacial molecular layer

Structure of the adsorption layer of various ionic and non-ionic surfactants at the free water surface, as seen from computer simulation and ITIM analysis

Molecular dynamics simulations of the adsorption layer of five different surfactant molecules, namely pentanol, octanol, dodecanol, dodecyl trimethyl ammonium chloride, and sodium dodecyl sulphate have been performed at the free surface of water at two different surface densities, namely 1 μmol/m2 (corresponding to unsaturated adsorption layer), and 4 μmol/m2 (corresponding to saturated adsorption layer), on the canonical ensemble at room temperature. The surfactants have been chosen in such a way that the effect of their headgroup charge as well as alkyl tail length on the properties of the adsorption layer can be separately investigated. The results are analysed in terms of the molecular level structure of the adsorption layer; organisation of the different groups and molecules along the macroscopic surface normal axis as well as conformation and orientation of the apolar tail is investigated in detail. In addition, the roughness of the surface of the aqueous phase is also analysed, using the ITIM method for accurately locating the real, capillary wave corrugated surface of the aqueous phase. © 2014 Elsevier B.V. All rights reserved