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

Water adsorption around small acetic and propionic acid aggregates has been studied by means of molecular dynamics simulation in the temperature range of 100-265 K as a function of the water content. Calculations have shown that acetic and propionic acid molecules behave similarly and that both the temperature and the water content have a strong influence on the behavior of the corresponding systems. Two situations have been evidenced for the acid-water aggregates, corresponding either to water adsorption on large acid grains at very low temperatures or to the formation of droplets consisting of acid molecules adsorbed at the surface of water aggregates at higher temperatures and high water content. At low water content and high temperature, only a partial mixing between water and acid molecules has been observed. The results of the present simulations emphasize the need for further experimental and simulation works to achieve a better characterization of the effects of both temperature and humidity on the behavior of organic aerosols in the troposphere.