The local structure in liquid methylamine and methylamine–water mixtures

Abstract

Molecular dynamics (MD) computer simulations are carried out on liquid methylamine and on 10 and 30 wt % aqueous solutions of methylamine. The local three-dimensional structure in these liquid systems is investigated using standard one-dimensional radial distribution functions and the fully three-dimensional measure of the local structure around the molecule, spatial distribution functions. Time correlation functions for the linear and angular motion of methylamine in the molecular coordinate system are also explored. From this analysis, a detailed structural picture emerges, revealing local molecular environments in these liquids that are both complex and varied. Hydrogen bond balance is found to play a key role in determining preferred arrangements. Strong hydrogen bonds are formed around the amino group in the pure liquid and in aqueous solution. At the same time, there is a strong hydrophobic association of methyl groups. The hydration structure in aqueous solution is found to be particularly rich, where in addition to the usual H-bonding nearest neighbors in the first hydration shell of the amino group, there are bridging water molecules and a novel type of distinctly non-H-bonding neighbors. The hydration structure in aqueous methylamine solution differs substantially from that found in methanol–water liquid mixtures.