Molecular dynamic simulation of dicarboxylic acid coated aqueous aerosol: structure and processing of water vapor

Abstract

Organic monolayers at the surfaces of aqueous aerosols play an important role in determining the mass, heat transfer rate and surface reactivity of atmospheric aerosols. They can potentially contribute to the formation of cloud condensation nuclei (CCN) and are involved in a series of chemical reactions occurring in atmosphere. Recent studies even suggest that organic-coated interfaces could have played some role in prebiotic biochemistry and the origin of life. However, creating reproducible, well-characterized aqueous aerosol particles coated with organic films is an experimental challenge. This opens the opportunity for computer simulations and modeling of these complex structures. In this work, molecular dynamics simulation was used to probe the structure and the interfacial properties of the dicarboxylic acid coated aqueous aerosol. Low molecular weight dicarboxylic acids of various chain lengths and water solubility were chosen to coat a water droplet consisting of 2440 water molecules. For malonic acid coated aerosol, the surface acid molecules dissolved into the water core and formed an ordered structure due to the hydrophobic interactions. The acid and the water are separated inside the aerosol. For other nanoaerosols coated with low solubility acids, phase separation between water and acid molecules was observed on the surface of the particle. To study the water processing of the coated aerosols, the water vapor accommodation factors were calculated.

Water and formic acid aggregates: A molecular dynamics study

“…In the present simulations, only large mixed droplets of formic acid and water molecules have been found at temperatures relevant for Tropospheric conditions, a behavior that is thus rather similar to that obtained for the malonic acid (O:C = 4/3)-water systems. 21,26 The comparison between these three acid molecules thus indicates that, unfortunately, using the O:C ratio for predicting the ability of carboxylic acids to act as water nuclei might be not as straightforward as previously inferred. 27 At least, it seems that this conclusion cannot be generalized to mono and dicarboxylic acids because the affinity of these molecules for water depends not only on the O:C ratio, but also on the number of carboxylic groups and of the internal geometry of the acid molecules.…”

“…[8][9][10][11][12][13][14][15][16][17][18] A few molecular dynamics simulation studies have thus been recently devoted to a detailed investigation of the behavior of large droplets made of water and organic molecules used as surrogates for organic aerosols. Either big water droplets coated by various organic molecules [19][20][21][22][23][24] or the reverse situation, i.e., large organic aggregates interacting with surrounding water molecules 25,26 have been considered in the corresponding theoretical works, as a function of temperature. These two approaches showed however some similar results especially regarding the state of mixture of the aerosol particles.…”

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

“…This approach can be related to the very recent experimental work of Schill and Tolbert, [47] who tried to provide simple parametrizations of organic ice nucleation efficacy by using the O:C ratio as a proxy for characterizing the organic aerosol hydrophilicity. It can be also related to the recent work of Ma et al, [24] in which molecular dynamics simulations have been performed to characterize dicarboxylic coated aqueous aerosols.…”

The effect of low solubility organic acids on the hygroscopicity of sodium halide aerosols

“…Special effort has been carried out to study the morphology and phase partitioning of aerosol particles consisting of hydrophobic and hydrophilic phases (Ciobanu et al, 2009;Kwamena et al, 2010;Reid et al, 2011;Veghte et al, 2013). On the other hand, molecular dynamics calculations are becoming a commonplace theoretical approach in atmospheric aerosol modelling that includes sea salt particles mixed with organic molecules (Ma et al, 2011;Sun et al, 2012Sun et al, , 2013Takahama and Russell, 2011).…”