Dissertations, Theses, and Capstone Projects

Date of Degree

6-2025

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy

Program

Chemistry

Advisor

Jianbo Liu

Committee Members

Angelo Bongiorno

Steven Chambreau

Yu Chen

Subject Categories

Analytical Chemistry | Computational Chemistry | Physical Chemistry

Keywords

Ionic Liquids, Electrospray Ionization, Propellant, Electric Thrusters

Abstract

Dual-mode propulsion combines chemical and electric propulsion methods into a single compact system by sharing hardware and propellant for both modes, offering complementary advantages tailored to specific mission needs. A promising propellant candidate for dual-mode propulsion is a binary mixture of hydroxylammonium nitrate (HAN) and 2-hydroxyethylhydrazinium nitrate (HEHN) ionic liquids (ILs). Each of the two ILs was developed as a greener alternative to the traditional, yet toxic, hydrazine monopropellant used in chemical propulsion. Recently, their potentials in electrospray propulsion have garnered attention. This thesis focused on the investigation of reaction dynamics and kinetics of HAN, HEHN and their binary mixture in electrosprays, aimed at providing both fundamental chemistry knowledge of individual ILs and binary mixture and implications for applications of energetic ILs in rocket propulsion for Air Force aerospace missions. The thesis research included study of electrosprays of HAN, HEHN, and their mixture in positive and negative ion modes using a home-built electrospray ionization (ESI) guided-ion beam tandem mass spectrometry, augmented by classical and Born-Oppenheimer molecular dynamics simulations and reaction potential energy surface exploration. Each of these works is summarized below, following an Introduction of IL chemistry in Chapter 1 and Experimental and Computational Methods in Chapter 2.

Capitalizing on tandem mass spectrometry and collision-induced dissociation measurements, and augmented by dynamics simulations, Chapter 3 characterized the structures and reaction dynamics of the species present in the electrosprays of HAN under different conditions mimicking low earth orbit and outer space. A single HAN monomer adopts a stable covalent structure HONH2·HNO3. Spontaneous proton transfer (PT) within the HAN monomer can be induced by a charge and/or a dipole without the need of chemical interaction or physical contact, such as the presence of a NO3-, a water or a second HAN monomer within 3 – 5 Å or a H+ within 8 Å. Moreover, the addition of NO3- to HAN leads to the formation of a stable -O3N·HONH3+·NO3- anion in negative electrosprays. In contrast, when a H+ approaches the HONH2·HNO3 structure, dissociative reactions occur that lead to H2O, NO2 and HONH2 fragments (and their cations) but not intact HAN species in positive electrosprays.

HEHN has the potential to power both electric and chemical thrusters and provide a wider range of specific impulse needs. Chapter 4 investigated the formation and structures of HEHN cluster ions in positive electrospray. Measurements included compositions of primary ions in the electrospray plume and their individual collision-induced dissociation (CID) cross sections and threshold energies. The comparison between experiment and theoretical calculation was used to verify the structures for the emitted species [(HEHN)nHE + H]+, [(HEHN)n(HE)2 + H]+, [(HE)n+1 + H]+ and [(HE)nC2H4OH]+ (n = 0 - 2), of which [(HE)1-2 + H]+ dominates. Due to the protic nature of HEHN, cluster fragmentation can be rationalized by PT-mediated elimination of HNO3, HE and HE·HNO3, and the latter two become dominant in larger clusters. [(HE)2 + H]+ and [(HE)nC2H4OH]+ contain H-bonded water and consequently are featured by water elimination in fragmentation.

In Chapter 5, the work was extended to HEHN cluster formation and fragmentation in the negative mode for comparison. The negative electrospray of HEHN was dominated by the cluster series of [(HEHN)n(HNO3)0-1NO3]-. In both modes, the cluster ions were predominantly composed of m/z below 350; loss of HEH+×NO3- represents the most important cluster fragmentation pathway, followed by intra-ion pair PT-mediated HE and HNO3 elimination; and all clusters started to dissociate at threshold energies less than 1.5 eV. The overwhelming similarities in the formation and fragmentation chemistry of positively vs. negatively charged HEHN clusters may be attributed to inherent ionic nature and high electric conductivities.

The impact of mixing HAN and HEHN on the electrospray performance was examined in Chapter 6. The ESI mass spectra of this binary IL were measured in positive and negative ion modes, highlighting the changes in electrosprays upon the mixing of the two ILs and the formation of binary cluster ions containing both IL components. This was followed by characterizing fragmentation pathways and energetics of the binary cluster ions using CID mass spectrometry. To verify the structures of the clusters, direct dynamics trajectory simulations were employed to mimic IL reactions responsible for the formation and decomposition of specific clusters. The combined experiment and computations indicate that the positive electrospray of the HAN + HEHN mixture was dominated by single ions of HAH+ and HEH+. The negative electrospray of the mixture, on the other hand, was overwhelmingly dominated by NO3- originated from HAN, and the HEHN species comprised less than 1% of the ion plume.

As concluded in Chapter 7, mixing HAN and HEHN in electrosprays enhanced ion plume in the low-mass range while reduced clustering in the high-mass range. These changes were driven by intermolecular proton transfer from HAH⁺ to the HE moiety that would otherwise promote the formation of large HEHN clusters and shift the electrospray away from a purely ionic mode. This thesis will have an impact on the aerospace community as the IL propellant technology adoption begins to accelerate for the multi-mode propulsion applications and would be of interest to the general ion-molecule chemical-physics community.

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