Molecular dynamics simulation study of ethylene glycol, ethylenediamine, and 2-aminoethanol. 2. Structure in aqueous solutions

In this article, we report a molecular dynamics (MD) simulation study of the local structure in aqueous solutions of ethylene glycol (EG), ethylenediamine (ED), and 2-aminoethanol (AE), where the results are presented and discussed in a comparative manner. Four compositions (organic solute mole fractions of X = 0.03, 0.1, 0.3, and 0.8) were investigated for each compound. The OPLS-based potential models, as described in our companion paper [A. V. Gubskaya, P. G. Kusalik, J. Phys. Chem., 2004, 108, XXXX] are used to represent the ethane derivatives and the SPC/E model is used for water. Thermodynamic, dynamic, and molecular structural characteristics are calculated and compared whenever possible with previous theoretical and experimental findings. The local structure in the solutions of interest is examined by means of radial and spatial distribution functions (RDFs and SDFs, respectively). The revealed three-dimensional picture of hydration confirms the presence of hydrogen-bonding arrangements that are comprised of both strong and weak hydrogen bonds, reminiscent of those found in pure liquids of EG, ED, and AE. Several interesting trends in the behavior specific to water are noted. These include a tendency for the association of water molecules in water-poor solutions, the maintenance of tetrahedral coordination in water-rich solutions, and a clear preference for hydrophilic hydration of these organic solutes at high concentrations. Despite the presence of a hydrophobic hydrocarbon backbone within the compounds studied, there is no specific tendency for hydrophobic self-association observed; however, there is some evidence of such self-association in those systems where the organic molecules adopt a trans (open chain) conformation such as in ED solution. This work again clearly shows the ability of SDFs to provide detailed insights into the local structure of strongly associated molecular liquids, while, at the same time, revealing the structural complexity that results for solutions of conformationally flexible molecules.