Molecular Dynamics Electrospray Simulations of Coarse-Grained Ethylammonium Nitrate (EAN) and 1-Ethyl-3-Methylimidazolium Tetrafluoroborate (EMIM-BF4)

Abstract

In this work, the use of molecular dynamics as a predictive tool for modeling the atomistic behavior of electrospray propulsion is discussed. 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF 4 ) and ethylammonium nitrate (EAN) were considered as two limits of ionic liquid (IL) propellants that tend to operate in an ion versus a droplet mode. The emission modes were found to depend on the electro-chemical properties of the IL propellant. The aprotic EMIM-BF 4 -based electrospray emitted primarily monomers and trimers as the dominant species and only small quantities of droplets. In contrast, trimers were the dominant emitted species in the protic EAN emissions with a significantly large contribution from droplets towards the total emission current, suggesting that EMIM-BF 4 -based colloid thrusters operate in ion mode and EAN-based devices operate in the droplet mode. Furthermore, the formation of the Taylor cone was found to depend on the mass flow rate and the external electric field strength. This paper provides a framework that can be extended for use to simulate any other ILs or their combinations.Aerospace 2018, 5, 1 2 of 18 mode [10]. On the other hand, ethylammonium nitrate (EAN) is classified as a protic IL, which are known to be environmentally more benign than other organic solvents [11] and easier to synthesize compared to aprotic ILs [12]. EAN has been reported to emit in the droplet mode, producing a jet of nanodroplets [13]. The electro-chemical properties of the ILs used as propellants and their mass flow rates determine the mode of operation of the colloid thruster. Extensive research has been performed to determine the emission modes of the colloid thrusters. Since future propellants may use mixtures of protic and aprotic types of ILs, molecular dynamics (MD) electrospray simulations can be used to analyze the differences in the emission behavior specific to these two types of ILs.A large number of ILs can be used as propellants for electrosprays [14,15], and the availability of a large number of inter-atomic potentials also makes it possible to model virtually any of the ILs. However, MD simulations scale on the order of O(N 2 ), where, N is the number of atoms, making MD very expensive to model large simulations (>10,000 atoms) typically used to study electrosprays. In this work, effective field coarse graining method [16,17] was used to derive coarse-grained potential of EAN, and its efficacy for the electrospray processes was verified by successfully performing MD simulations of Taylor cone formation and subsequent emission of ion-species. Previous works [16][17][18] have derived coarse-grained (CG) potentials to verify bulk IL physical properties or have used approximations to modify the CG potential to perform electrospray simulations [19,20].Prior to designing colloid thrusters, it is necessary to have a simulation-based ability to predict thrust, required voltages and the mode of operation. MD is a particle-based method that allows one to understand the nature of electrosprays at ...

Gas-Phase Binding Energies and Dissociation Dynamics of 1-Alkyl-3-Methylimidazolium Tetrafluoroborate Ionic Liquid Clusters

“…Molecular mechanics approaches have been used to model ILs in ESI plumes to elucidate information regarding the influence of the specific IL on efficiency and thrust. − The reliability of such modeling is highly dependent on the accuracy of the parameterization of the molecular mechanics force fields employed, which rely on thermochemistry derived from high-level quantum mechanics calculations and experimental measurements when available. An important motivating factor for the present work is to provide accurate BDEs of IL clusters for use in such modeling and simulation of space propulsion using ILs.…”

Polyatomic Ion-Induced Electron Emission (IIEE) in Electrospray Thrusters

“…In thruster applications, EMI + is typically accelerated under a 2-10 kV potential [5] to reach velocities in the order of 10 6 cm s −1 , at or below the classical secondary electron emission threshold velocity [30]. A similar analysis to the present study could be performed for dimers or trimers of EMI -Im, as these species often have a non-negligible contribution to the beam composition (though their susceptibility to fragmentation can be high) [43,44]. Cationic monomers were chosen as a starting point to both obtain key results and more thoroughly validate chosen methods, as data for cationic surface impacts are more available in the literature.…”

Intermolecular interactions in clusters of ethylammonium nitrate and 1-amino-1,2,3-triazole

“…EAN is chosen as a model DeEP component as it has seen previous wide application as an IL propellant. [69][70][71][72][73][74]…”

“…EAN is chosen as a model DeEP component as it has seen previous wide application as an IL propellant. 69–74 1-AT acts as the HBD for this DeEP. In this study, the most relevant intermolecular interactions of (EAN) n IL clusters and (EAN) x :(1-AT) n ( n = 1–5) mixtures are studied using the EFP method.…”