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

The generalized identification of truly interfacial molecules (ITIM) algorithm for nonplanar interfaces

We present a generalized version of the ITIM algorithm for the identification of interfacial molecules, which is able to treat arbitrarily shaped interfaces. The algorithm exploits the similarities between the concept of probe sphere used in ITIM and the circumsphere criterion used in the α-shapes approach, and can be regarded either as a reference-frame independent version of the former, or as an extended version of the latter that includes the atomic excluded volume. The new algorithm is applied to compute the intrinsic orientational order parameters of water around a dodecylphosphocholine and a cholic acid micelle in aqueous environment, and to the identification of solvent-reachable sites in four model structures for soot. The additional algorithm introduced for the calculation of intrinsic density profiles in arbitrary geometries proved to be extremely useful also for planar interfaces, as it allows to solve the paradox of smeared intrinsic profiles far from the interface. © 2013 American Institute of Physics

Poissonian and non Poissonian Voronoi Diagrams with application to the aggregation of molecules

The distributions that regulate the spatial domains of the Poissonian Voronoi Diagrams are discussed adopting the sum of gamma variate of argument two. The distributions that arise from the product and quotient of two gamma variates of argument two are also derived. Three examples of non Poissonian seeds for the Voronoi Diagrams are discussed. The developed algorithm allows the simulation of an aggregation of methanol and water.Comment: 18 pages 10 Figure

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