Water and formic acid aggregates: A molecular dynamics study

Abstract

Water adsorption around a formic acid aggregate has been studied by means of molecular dynamics simulations in a large temperature range including tropospheric conditions. Systems of different water contents have been considered and a large number of simulations has allowed us to determine the behavior of the corresponding binary formic acid-water systems as a function of temperature and humidity. The results clearly evidence a threshold temperature below which the system consists of water molecules adsorbed on a large formic acid grain. Above this temperature, formation of liquid-like mixed aggregates is obtained. This threshold temperature depends on the water content and may influence the ability of formic acid grains to act as cloud condensation nuclei in the Troposphere.

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.…”

“…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.…”

“…[26] Indeed, in this case, a complete deliquescence of the formic acid aggregate was observed above 200 K. Although this different behavior could be a priori related to different water-acid interactions, we have also examined whether or not it depends on the simulation duration, which might have not been not long enough here to observe the whole deliquescence of the acetic and propionic aggregates.…”

“…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.…”