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

Abstract

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.

A theoretical perspective on the structure and thermodynamics of secondary organic aerosols from toluene: molecular hierarchical synergistic effects

“…22,66 3.2 Key composition of stable SOA nanoparticles Pyruvic acid dominates synergistic effects. Although the significant thermodynamic advantage of organic acids in the formation and growth of SOA clusters has been reported frequently, 22,32,35 the synergistic effects are discussed here in detail. To shed light on the important role of a single component in SOA nanoparticle formation, different molar ratios of H 2 SO 4 /H 2 O are examined first.…”

“…Generally, DFT calculations just can predict the formation of a small cluster within a hundred atoms, while MD simulations are both mandatory and complementary to explore the nanoscale behavior of SOA nanoparticles. 30,31 For example, Roose et al 32 employed classical MD simulations to study the formation of submicrometer organic aerosols derived from carboxylic acids at various acid/water ratios and ambient temperatures. The SOA morphology and internal structure were then characterized by HBs, binding energy, and the surface ratio.…”

“…The SOA morphology and internal structure were then characterized by HBs, binding energy, and the surface ratio. 32,33 In addition, the combination of MD simulations and DFT calculations can systematically investigate the SOA formation behavior, including the interfacial adsorption of organics and the nucleation enhancement of sulfuric acid. 34,35 Specifically, sulfuric acid not only promotes the adsorption and accommodation of organics at the air/water interface but also accelerates the aqueous reaction of organics inside the interior region of nanoparticles.…”

Water Activity from Equilibrium Molecular Dynamics Simulations and Kirkwood-Buff Theory

“…The formation of H-bonded oligomers has also been observed from MD simulations of various mono-and dicarboxylic acids. 16,96,97 Self-association and oligomerization of organic compounds in aqueous systems itself is not an artifact of the simulations and can also be inferred from experiments; 98−100 however, cluster size and lifetimes have not been explicitly characterized to our knowledge.…”

The influence of saturation on the surface structure of mixed fatty acid-on-water aerosol: a molecular dynamics study

“…Molecular dynamics (MD) simulations are a popular method for modelling the structure of aerosol particles. The majority of MD studies of aqueous aerosol mimics have focussed on a single organic species, either pure or in conjunction with water, 19,[46][47][48][49][50][51][52][53][54] as opposed to looking at mixtures of species and how these interact with each other and the water core. In addition to this the focus is often on the overall structure of organic-on-water particles, as opposed to specifically what is present at the air-aerosol interface.…”