Classical Molecular Dynamics Study of Small-Chain Carboxylic Acid Aerosol Particles

The growth of small valeric (pentanoic) and glutaric (pentanedioic) acid aerosol particles from 20 to 500 molecules has been investigated at room temperature using classical molecular dynamics simulations. As a result of a higher propensity to form hydrogen bonds, glutaric acid aggregates are shown to be denser than their valeric counterpart. The addition of water molecules with water/acid ratios of 1:1 and 2:1 has then been studied in the case of the diacid. At a low water content, water primarily forms small islands on the surface. When the amount of water increases, it penetrates deeper into the aggregate but a significant fraction remains at the surface. A Connolly surface analysis reveals that the surface is mostly covered by hydrogen atoms from CH2 groups, with acidic hydrogens being saturated and not available at the surface, for both dry and wet particles. These atomic distributions could impact the reactivity of such particles with gas-phase oxidants and the uptake of trace gases