Molecular Dynamics Studies on Liquid/Vapor Interface Properties and Structures of 1-Ethyl-3-methylimidazolium Dimethylphosphate-Water

1-Ethyl-3-methylimidazolium dimethylphosphate ([Emim][Dmp])-water binary solution is one of the promising new working-pairs for absorption heat pump and absorption chillers, which are widely used to recover industrial waste heat. In the absorption process, the mass and heat transfer at the interface greatly depend on interface microscopic structure. Therefore, in order to understand the absorption process, it is very important to study the interface microscopic structure. The liquid-vapor interface properties, as well as the orientation of [Emim]+, [Dmp]-, water at the interface and its aqueous solution with different water mole fraction, were studied using classical all-atom force field by molecular dynamic simulations. The simulated bulk mass density fitted by hyperbolic tangent function for each system was in good agreement with the experiment data, with the relative deviation between simulated and experimental value within 2%. The simulated results indicate that anion is always distributed at the outmost layer of the interface, followed by cation and water molecule. In [Emim][Dmp], the tilt angle of imidazolium rings to the surface normal is in the range of 0° < θ < 12°; for most cation, their ethyl and methyl tilted toward gas phase and bulk, respectively, but for a few cation, their ethyl and the methyl take the opposite orientation. For anion, one methyl prefers to turn toward gas phase and another methyl (PC vector from P atom to C atom) lie nearly parallel to the surface, while one PO vector (from P atom to O atom) turns toward liquid bulk and another PO vector is nearly parallel to the surface. In aqueous solution of [Emim][Dmp], the tilt angle of the imidazolium ring to the surface normal becomes larger (0° < θ < 37°) at the interface, but almost all ethyl intend to tilt toward gas phase and the methyl tilt toward liquid bulk compared with pure [Emim][Dmp]. Two methyl in anion prefer to turn toward gas phase and its two PO vectors toward liquid bulk. This orientation indicates that pure [Emim][Dmp] absorb water in gas phase more easily than [Emim][Dmp]+H2O system does. Water molecules are distributed in the inner layer of the interface with two OH vectors (from O atom to H atom) tilting toward external surface.