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

Abstract

Molecular dynamics simulations of the liquid-vapour interface of water-methylamine mixtures of eight different compositions, including neat water, are performed on the canonical (N,V,T) ensemble at 280 K. The molecules constituting the first three individual molecular layers beneath the liquid surface are identified by the Identification of the Truly Interfacial Molecules (ITIM) method. The results indicate that methylamine molecules are strongly adsorbed in the first, and somewhat depleted in the second molecular layer, while the composition of the third layer agrees well with that of the bulk liquid phase. On the other hand, methylamine molecules do not show considerable self-association within the surface layer. The orientational preferences of the methylamine molecules at the liquid surface are clearly governed by the requirement of maximizing their hydrogen bonding interaction. As a consequence, methylamine molecules point by their apolar CH3 group straight to the vapour, while by the potential hydrogen bonding directions of the NH2 group flatly to the liquid phase. Further, within the surface layer, methylamine molecules stay, on average, noticeably farther from the bulk liquid phase than waters. Increasing methylamine mole fraction leads to the gradual breaking up of the lateral percolating H-bonding network of the surface molecules. Finally, methylamine molecules accelerate, while water molecules slow down the exchange of both species between the liquid surface and the bulk liquid phase. Further, methylamine molecules slow down the lateral diffusion of each other, and even prevent water molecules from showing noticeable lateral diffusion within the surface layer. The reason for this latter effect is that the mean residence time of the water molecules at the liquid surface becomes considerably shorter than the characteristic time of their lateral diffusion in the presence of methylamine.