Hydroxylammonium nitrate (HAN) is used as a task-specific, protic ionic liquid that offers lower vapor pressures and reduced toxicity with increased performance when in formulation as compared to the commonly used hydrazine monopropellant for in-space propulsion applications. Depending on a thruster’s mode of operation, propellants can experience complex thermolytic, catalytic, or electric field-induced decomposition that heavily depends on the IL’s mixture properties. To assess a propellant’s thermophysical characteristics, a nonreactive, polarizable force field, APPLE&P, has been developed to enable molecular simulations of new candidate HAN-based mixtures. In this work, an equilibrated, condensed-phase, HAN-H2O mixture was surrounded by vacuum slabs to elucidate fundamental interfacial phenomena. The simulation was carried out for 100 nanoseconds in the NVT ensemble at room temperature to allow sufficient time for ion relaxation within the system. Structural properties, such as species number density along the interface-normal direction, were quantified to show distinct separation in bulk and interfacial regions. Local rearrangement of the cations and anions at the surface layer was visualized and species aggregation and void formation were observed. Surface tension was calculated based on the pressure tensor output and was in agreement with similar IL mixtures with reported experimental measurements. Cumulative distribution functions of evaporated species at the surface layer indicated a near-constant flux of escaping water molecules. The results reveal the effects of a vacuum interface on the local structure and dynamics of a HAN-H2O mixture. Thus, a fundamental understanding of liquid-vacuum interfaces supports analysis of more complex but practical systems of interest, including ILs at solid-liquid interfaces, in thermal environments, and in vacuo under externally applied electric fields.

Structural, thermophysical, and dynamic properties of a hydroxylammonium nitrate (HAN) and water mixture at the liquid-vacuum interface

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.