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SBIR-Phase I: Molecular Simulation Tools for Predicting Hypergolicity in Ionic Liquids Hypergolic liquid propellants play a key role in advanced space and missile applications. In these systems when oxidizer and fuel come in contact with each other the exothermic energy of mixing and the heat from spontaneous chemical reactions result in self-sustaining combustion leading to mixture ignition. There is a number of different oxidizer-fuel combinations used in hypergolic bipropellant formulations, while the fuel typically is hydrazine or one of its derivatives. While hydrazine-based propellants exhibit desirable chemical kinetics and energetics they have several disadvantages, motivating the search for alternative hypergolic propellants. One of the main disadvantages of hydrazine-based propellants is that they are highly toxic and carcinogenic which, combined with high volatility of reaction products, makes these propellants very hazardous and problematic for handling and application. Recently a novel class of ionic liquids (ILs) with high nitrogen content has received significant attention as novel energetic materials due to very high heats of formation. Energetic ILs possess a number of advantages over conventional hypergolic propellants such as high density, good oxygen balance, improved stability, low vapor pressure that results in reduced loss of material, and decreased hazards through formation of explosive fumes. Moreover, for application in hypergolic bipropellants it is not necessary to use an oxygen-balanced IL therefore allowing to tune combinations of cations and anions to achieve desired chemistry kinetics, thermodynamic and transport properties, as well as safety characteristics. In order to efficiently design novel IL hypergolic systems a fundamental understanding of preignition and ignition stages is needed. Particularly important is the interplay of physical phenomena (mixing and interfacial transport) and chemical kinetics (key initiation reactions, preignition intermediates, and ignition events) whose unraveling would greatly facilitate development of new ILs with desired chemical, physical and processing characteristics for application in hypergolic propellants. Capitalizing on our extensive experience in molecular dynamics (MD) simulation of ILs and energetic materials using fully atomistic reactive (ReaxFF) and polarizable non-reactive (APPLE&P) force fields we propose to adopt these methods for investigation and prediction of hypergolicity in IL-based fuels. In Phase I we propose to demonstrate that information obtained from MD simulations can be correlated with hypergolicity of a given combination of IL and oxidizer for a subset of ILs which has been already characterized experimentally. Utilization of both reactive and non-reactive force fields will allow efficient and accurate modeling of chemical reactions and thermophysical properties. |