Amines are organic derivatives of ammonia and form structural moieties for a variety of drugs commercially available in the market. The presence of lone pair of electrons on the nitrogen atom makes it a suitable donor of electrons for hydrogen bonding, which influences the thermodynamic properties of amines. Amines have complex phase diagrams with other fluids like alcohols, nitriles and ketones. Diethylamine exhibits binary polyazeotropy with methanol. This phenomena represents a condition for which more than one stable azeotrope exists for a given temperature and pressure. The complexity is compounded when the system shows a temperature dependence on type of azeotropic behavior. This system displays multiple (minimum pressure and maximum pressure) azeotropy at 398 K but only shows a minimum pressure azeotrope at 348 K and 297 K. A variety of hydrogen bonding occurs for this system involving N, O, and H atoms. Diethylamine forms a maximum pressure azeotrope with acetone and also with acetonitrile.

In this work, histogram-reweighting Monte Carlo simulations in the grand canonical ensemble are used to determine the vapor-liquid coexistence curves, vapor pressures and critical points for pure component diethylamine and acetonitrile as well as mixture pressure composition diagrams. The TraPPE force field is used to describe the interactions in diethylamine, acetonitrile, acetone and methanol. Non-bonded interactions are represented by Lennard-Jones (LJ) potentials and partial charges, while harmonic potentials are used to control bond angle bending. The united-atom approach for diethylamine and acetonitrile has small deviations on the prediction of pure component phase diagram compared to the explicit atom representation originally used in the parameterization of the force fields. For acetone and methanol the force field parameters developed under the united-atom scheme was used. The TraPPE force field predicts a maximum pressure azeotrope for diethylamine+methanol for all temperatures studied. This contradicts experimental findings, where a double azeotrope is seen for 398 K, and minimum pressure azeotropes are seen at other temperatures. The TraPPE-UA force field is able to predict the maximum pressure azeotrope for the diethylamine + acetonitrile mixture, while it fails to predict the maximum pressure azeotrope in the diethylamine+acetone system. Simulations in the isobaric-isothermal ensemble were used to determine the microscopic structures predicted by TraPPE at different temperatures and corresponding pressures for the three binary mixtures. Radial distribution functions extracted from the NPT simulations showed evidence of self-association as well as cross association of the molecules by hydrogen bonding. Methods of improving the phase equilibrium predictions are discussed.