publications
publications by categories in reversed chronological order. generated by jekyll-scholar.
2024
- JPC B
Structural Properties of [N1888][TFSI] Ionic Liquid: A Small Angle Neutron Scattering and Polarizable Molecular Dynamics StudyShehan M. Parmar, William Dean, Changwoo Do, and 4 more authors2024In this study, we investigate the quaternary ammonium-based ionic liquid (QAIL), methyltrioctylammonium bis(trifluoromethylsulfonyl)imide, [N1888][TFSI], utilizing small angle neutron scattering (SANS) measurements and polarizable molecular dynamics (MD) simulations to characterize the short- and long-range liquid structure. Scattering structure factors show signatures of three length scales in reciprocal space indicative of alternating polarity (k ∼ 0.44 Å–1), charge (k ∼ 0.75 Å–1), and neighboring or adjacent (k ∼ 1.46 Å–1) domains. Excellent agreement between simulation and experimental scattering structure factors validates various simulation analyses that provide detailed atomistic characterization of the different length scale correlations. The first solvation shell structure is illustrated by obtaining radial, angular, dihedral, and combined distribution functions, where two dominant spatial motifs, N+···N– and N+···O–, compete for optimal packing around the polar head of the [N1888]+ cation. Intermediate and long-range structures are governed by the balance between local electroneutrality and octyl chain networking, respectively. By computing the charge-correlation structure factor, SZZ, and the spatial extent of the octyl chain network using graph theory, the bulk-phase structure of [N1888][TFSI] is characterized in terms of electrostatic screening and apolar domain formation length scales.
@article{parmar2024_N1888_TFSI, author = {Parmar, Shehan M. and Dean, William and Do, Changwoo and Browning, James F. and Klein, Jeffrey M. and Gurkan, Burcu E. and McDaniel, Jesse G.}, title = {Structural Properties of [N1888][TFSI] Ionic Liquid: A Small Angle Neutron Scattering and Polarizable Molecular Dynamics Study}, journal = {The Journal of Physical Chemistry B}, volume = {128}, number = {45}, pages = {11313-11327}, doi = {10.1021/acs.jpcb.4c06255}, url = {https://doi.org/10.1021/acs.jpcb.4c06255}, year = {2024}, type = {journal} } - JPC B
Liquid Structure and Hydrogen Bonding in Aqueous Hydroxylammonium NitrateDaniel D. Depew, Ghanshyam L. Vaghjiani, Shehan M. Parmar, and 1 more author2024Hydroxylammonium nitrate (HAN) has emerged as a promising component in ionic liquid-based spacecraft propellants. However, the physicochemical and structural properties of aqueous HAN have been largely overlooked. The purpose of this study is to investigate the hydrogen bonding in aqueous HAN and understand its implications on these properties and the proton transfer mechanism as a function of concentration. Classical polarizable molecular dynamics simulations have been employed with the APPLE&P force field to analyze the geometry of individual hydrogen bonds and the overall hydrogen-bonding network in various concentrations of aqueous HAN. Radial distribution functions (RDFs) and spatial distribution functions (SDFs) indicate the structural arrangement of the species and their hydrogen bonds. Projections of water density and the orientation of its electric dipole moment near the ions provide insight into the hydrogen-bonding network. The incorporation of water into the hydrogen-bonding network at high ion concentrations occurs via interstitial accommodation around the ions immediately outside the first solvation shell. While ion pairs are observed at all concentrations considered, the frequency of Ha···On hydrogen bonds increases substantially with the ion concentration. The findings contribute to a better fundamental understanding of HAN and the precursors of reactivity, crucial to the development of “green” spacecraft propellants.
@article{depew_parmar2024_HAN_H2O, author = {Depew, Daniel D. and Vaghjiani, Ghanshyam L. and Parmar, Shehan M. and Wang, Joseph J.}, title = {Liquid Structure and Hydrogen Bonding in Aqueous Hydroxylammonium Nitrate}, journal = {The Journal of Physical Chemistry B}, volume = {128}, number = {3}, pages = {824-840}, doi = {10.1021/acs.jpcb.3c05623}, url = {https://doi.org/10.1021/acs.jpcb.3c05623}, year = {2024}, type = {journal} }
2023
- JPC B
Structural Properties of HEHN- and HAN-Based Ionic Liquid Mixtures: A Polarizable Molecular Dynamics StudyShehan M. Parmar, Daniel D. Depew, Richard E. Wirz, and 1 more author2023ACS Editors’ Choice® is a collection designed to feature scientific articles of broad public interest.
Molecular dynamics simulations of binary mixtures comprising 2-hydroxyethylhydrazinium nitrate (HEHN) and hydroxylammonium nitrate (HAN) were conducted using the polarizable APPLE&P force field to investigate fundamental properties of multimode propulsion (MMP) propellants. Calculated densities as a function of temperature were in good agreement with experiments and similar simulations. The structural properties of neat HEHN and HAN–HEHN provided insights into their inherent, protic nature. Radial distribution functions (RDFs) identified key hydrogen bonding sites located at N–H···O and O–H···O within a first solvation shell of approximately 2 Å. Angular distribution functions further affirmed the relatively strong nature of the hydrogen bonds with nearly linear directionality. The increased hydroxylammonium cation (HA+) mole fraction shows the influence of competitively strong hydrogen bonds on the overall hydrogen bond network. Dominant spatial motifs via three-dimensional distribution functions along with nearly nanosecond-long hydrogen bond lifetimes highlight the local bonding environment that may precede proton transfer reactions.
@article{parmar2023_HAN_HEHN, author = {Parmar, Shehan M. and Depew, Daniel D. and Wirz, Richard E. and Vaghjiani, Ghanshyam L.}, title = {Structural Properties of HEHN- and HAN-Based Ionic Liquid Mixtures: A Polarizable Molecular Dynamics Study}, journal = {The Journal of Physical Chemistry B}, volume = {127}, number = {40}, pages = {8616-8633}, keywords = {ionic liquids, polarizable force fields, molecular dynamics}, doi = {10.1021/acs.jpcb.3c02649}, url = {https://doi.org/10.1021/acs.jpcb.3c02649}, year = {2023}, type = {journal} }
2022
- JEP
Molecular Dynamics Simulations of Ion Extraction from Nanodroplets for Ionic Liquid Electrospray ThrustersTakaaki Enomoto, Shehan M. Parmar, Ryohei Yamada, and 2 more authors2022Molecular dynamics (MD) simulations were performed for ion extraction from electrospray thrusters to investigate relevant extraction processes numerically. To approximate the electrospray jet tip, a simulation domain consisting of 4-5 nm-sized ionic liquid droplets was used. The extracted ion angles and kinetic energies from EMI–BF4 (1-ethyl-3-methylimidazolium tetrafluoroborate) and EMI–Im (1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide) droplets were quantified by applying uniform electric fields of 1.3–1.7 V nm−1. The MD simulations are in great agreement with simulations presented in the literature and consistently show a greater preference for monomer emission than reported experimentally. At field strengths above 1.5 V nm−1, apparent droplet fracturing and breakup lead to an increase in ion angular velocity distributions. Greater mobility of EMI–BF4 ions than EMI–Im was also observed, indicative of the crucial role of cation-anion hydrogen bond strengths in ion extraction and beam composition between different propellants.
@article{parmar2022_EMIM_EMI-BF4, author = {Enomoto, Takaaki and Parmar, Shehan M. and Yamada, Ryohei and Wirz, Richard E. and Takao, Yoshinori}, title = {Molecular Dynamics Simulations of Ion Extraction from Nanodroplets for Ionic Liquid Electrospray Thrusters}, journal = {Journal of Electric Propulsion}, volume = {1}, number = {13}, doi = {10.1007/s44205-022-00010-1}, url = {https://doi.org/10.1007/s44205-022-00010-1}, year = {2022}, type = {journal} }
