The emissions of Taylor cones from a wide range of ionic liquids (ILs) have been tested in vacuo in an attempt to identify what physical properties favor the purely ionic regime (PIR). This regime is well known in the case of Taylor cones of liquid metals. For nonmetallic liquids, it has been previously observed in conventional (capillary tube) electrospray sources at room temperature only for the room temperature molten salt (ionic liquid) EMI-B F4 (EMI=1 -ethyl-3-methylimidazolium). A large number of other ILs and their mixtures have been studied here, most of which (but not all) are unable to reach the PIR at room temperature. Based on these results and additional theoretical considerations, strong support is assembled for the notion that the PIR is favored by ILs not only of high electrical conductivity but also of high surface tension. This hypothesis is confirmed by tests with three recently synthesized ILs, EMI-Ga Cl4, EMI-C (CN)3, and EMI-N (CN)2, all of which combine exceptional surface tension and electrical conductivity, and all of which reach the PIR at room temperature far more readily than EMI-B F4.
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We are thankful to Professor Martinez Sanchez (MIT) for many contributions to the electrospraying aspects of the problem and to Professor J. Wilkes (U.S. Air Force Academy) and Dr. A. Robertson (Cytec) for their kind gift of several ionic liquid samples. Our work has been supported by the U.S. AFOSR through Grant Nos. F-49620-01-1-0416 and FA9550-06-1-0104 at Yale, and Yale subcontracts to STTR grants to the companies Busek and Connecticut Analytical Corporation.
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