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

Abstract

Abstract.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 has 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.

Molecular Dynamics Simulations of the Interaction between Water Molecules and Aggregates of Acetic or Propionic Acid Molecules

“…Thus, the distributions of the binding energy of an acid molecule with all the other acid molecules ( ), of a water molecule with all the other water molecules ( ) and of an acid molecule with all the water molecules ( ) have been calculated. The results are discussed in comparison with our previous studies on dicarboxylic acid-water [24,25] and formic acid-water [26] aggregates, in order to better characterize the possible influence of the O:C ratio on the ability of organic aerosols to act as cloud condensation nuclei, as inferred from the analysis of recent experimental results [8]. Note that in the temperature range of the simulations, acetic and propionic acids are in the solid phase and should exhibit a crystalline structure that could have been evidenced by the study of some radial distribution functions and by careful examination of the hydrogen bond network in the results of our molecular dynamics simulations.…”

“…A total number of 55 simulations have thus been conducted to understand the phase behavior of the binary acid-water mixtures. Note that, as in our previous works, [24][25][26] the temperatures considered here go to much lower values than what is encountered in the troposphere, to shed light on all the possible structural changes in the studied molecular aggregates.…”

“…Such a behavior has also been observed for oxalic acid molecules, which have been shown to form demixed droplets at high temperature and high water contents [24], contrarily to malonic acid molecules. [25] Of course, it should be reminded that the simulation results depend on the model used to describe the interactions between the different species in the simulation box, i.e., not only the water-water and acid-acid, but also the acid-water interactions. Although the accuracy of the former one have been widely investigated in the literature (see for instance Vega et al [40]), the validity of the acid-acid and, even worse, of the acid-water interactions has not been widely studied.…”

“…Lennard-Jones parameters and partial charges for the acid molecules were taken from the OPLS-AA library, [34,35] and their geometries were determined using the Automated Topology Builder (ATP) tool. [36][37][38] In order to be consistent with our previous studies, [24][25][26] water molecules have been represented by the five-site TIP5P model. [39] These molecules have been kept rigid in the simulations, whereas no constraint has been applied to acid molecules.…”

“…Further simulations with larger numbers of acid molecules in the box showed the same clustering behavior, at least until 240 molecules (the maximum of molecules that was considered here), as already observed for formic and malonic acids. [25,26] We have thus chosen to work with aggregates made of 120 acid molecules, which appeared as a good compromise between the relevance and the time cost of the simulations performed with large number of additional water molecules.…”