Hydrogen bonding and orientation effects on the accommodation of methylamine at the air-water interface

Abstract

Methylamine is an abundant amine compound detected in the atmosphere which can affect the nature of atmospheric aerosol surfaces, changing their chemical and optical properties. Molecular dynamics simulation results show that methylamine accommodation on water is close to unity with the hydrophilic head group solvated in the interfacial environment and the methyl group pointing into the air phase. A detailed analysis of the hydrogen bond network indicates stronger hydrogen bonds between water and the primary amine group at the interface, suggesting that atmospheric trace gases will likely react with the methyl group instead of the solvated amine site. These findings suggest new chemical pathways for methylamine acting on atmospheric aerosols in which the methyl group is the site of orientation specific chemistry involving its conversion into a carbonyl site providing hydrophilic groups for uptake of additional water. This conversion may explain the tendency of aged organic aerosols to form cloud condensation nuclei. At the same time, formation of NH 2 radical and formaldehyde is suggested to be a new source for NH 2 radicals at aerosol surfaces, other than by reaction of absorbed NH 3 . The results have general implications for the chemistry of other amphiphilic organics, amines in particular, at the surface of atmospherically relevant aerosols. Published by AIP Publishing. [http://dx

Elucidating the molecular mechanisms of Criegee-amine chemistry in the gas phase and aqueous surface environments

“…Considering that the aqueous surfaces, which are ubiquitous in fogs, cloud waters, microdroplets and aerosols, provide interesting reaction media for atmospheric chemistries, [38][39][40][41] we next examined the reaction between anti-CH 3 CHOO and CH 3 NH 2 on a water droplet of 191 water molecules within the framework of the BOMD simulations. The selection of these particular precursors for the interfacial reaction is inspired by the fact that recent studies predict them to remain undissociated on the water droplet, 44,54 thus indicating the possibility of an interfacial reaction between them.…”

Investigation of the liquid-vapour interface of aqueous methylamine solutions by computer simulation methods

“…In these studies we demonstrated the strong tendency of methylamine molecules for being adsorbed at icy surfaces, including their ability even for multilayer adsorption, and characterized the surface orientation of the adsorbed molecules as well as their hydrogen bonding with the surface waters [22,23]. Hoehn et al investigated the energetic and structural properties of a single methylamine molecule at air/water interface [24]. In this study, the solvation free energy profile of the methylamine molecule was found to exhibit a minimum at the interfacial region, whereas the magnitude of this effect (2.5-5.2 kJ/mol) was found to be rather sensitive to the force field combination used.…”

“…In this study, the solvation free energy profile of the methylamine molecule was found to exhibit a minimum at the interfacial region, whereas the magnitude of this effect (2.5-5.2 kJ/mol) was found to be rather sensitive to the force field combination used. Further, the interfacial orientation of methylamine as well as the hydrogen bonding characteristics of the water molecules surrounding the methylamine molecule at the interface of the infinitely dilute solution were also discussed [24]. However, we are not aware of any computer simulation study of the liquid-vapour interface of aqueous methylamine solutions of finite concentration, thus, to the best of our knowledge, the concentration effect on the interfacial properties of methylamine has been never discussed in the literature.…”

“…The X edge of the basic simulation box, being perpendicular to the macroscopic plane of the interface, has been 150 Å long, while the length of the Y and Z edges has been 50 Å in every case. Methylamine molecules have been described by the generalized AMBER force field (GAFF) [75], using the fractional charges developed by Hoehn et al [24], while water was modelled by the TIP4P/2005 potential [76]. Besides describing the properties of the corresponding neat liquids well, this model combination was shown to reproduce the experimental hydration free energy of methylamine [77] within error bars [24].…”

“…Methylamine molecules have been described by the generalized AMBER force field (GAFF) [75], using the fractional charges developed by Hoehn et al [24], while water was modelled by the TIP4P/2005 potential [76]. Besides describing the properties of the corresponding neat liquids well, this model combination was shown to reproduce the experimental hydration free energy of methylamine [77] within error bars [24]. Both potential models are rigid and pairwise additive, thus, the total energy of the system (apart from the long range correction) is given by the sum of the interaction energies of all molecule pairs.…”