An ab initio Based Structure Property Relationship for Prediction of Ignition Delay of Hypergolic Ionic Liquids

Abstract

AbstractCurrently, monomethyl hydrazine is the most widely used hypergolic rocket fuel. However, due to its high vapor toxicity, there is a need to develop low‐toxicity hypergolic fuels. Ionic liquids are one such potential category of fuels, since they are consistently characterized by ultra‐low vapor pressures, but designing ionic liquid propellants with ignition delay times that are comparable to that of monomethyl hydrazine is a challenge. This is because a fundamental understanding of the hypergolic nature of ionic liquids is far from clear. Quantitative structure property relationship (QSPR) represents a simplified design approach for quantitatively predicting the ignition delay times based on linear correlations using a set of descriptors, which, in this work, define electrostatics, hydrogen bonding, and other structural features of the ionic liquids. Experimental ignition delay times for a set of 41 ionic liquids were collected for QSPR development. Experimental measurements of the ignition delay times were then correlated to theoretical descriptors determined from quantum mechanical calculations. A number of multi‐descriptor linear equations were analyzed by regression of the ignition delay data, showing reasonable success. The ignition delay values were observed to spread over a wide range, in large part due to the presence of oxygen in the fuel molecule. The training data was thus split into two sets and refitted to a set of linear equations, showing a systematic improvement in the correlation coefficients. The success of the present QSPR results is encouraging, which should motivate further efforts to enlarge the training set to include experimentally measured properties of ionic liquids for developing highly predictive capabilities.