Nanojets, Electrospray, and Ion Field Evaporation: Molecular Dynamics Simulations and Laboratory Experiments

The energetics, interfacial properties, instabilities, and fragmentation patterns of electrosprays made from formamide salt solutions are investigated in a mass spectrometric vacuum electrospray experiment and using molecular dynamics (MD) simulations. The electrospray source is operated in a Taylor cone-jet mode, with the nanojet that forms being characterized by high surface-normal electric field strengths in the vicinity of I V/nm. Mass-to-charge ratios were determined for both positive and negative currents sprayed from Nal-formamide solutions with solute-solvent mole ratios of 1:8.4 and 1:36.9, and from Ki-formamide solutions with mole ratios of 1:41 and 1:83. The molecular dynamics simulations were conducted on isolated 10 nm Nal-formamide droplets at mole ratios of 1:8 and 1:16. The droplet was subjected to a uniform electric field with strengths ranging between 0.5 and 1.5 V/nm. Both the experiments and simulations demonstrate a mixed charge emission regime where field-induced desorption of solvated ions and charged droplets occurs. The macroscopic parameters, such as average mass-to-charge ratio and maximum surface- normal field strengths deduced from the simulations are found to be in good agreement with the experimental work and consistent with electrohydrodynamic theory of cone-jets. The observed mass spectrometric Na+ and I solvated ion distributions are consistent with a thermal evaporation process, and are correctly reproduced by the simulation after incorporation of the different flight times and unimolecular ion dissociation rates in the analysis. Alignment of formamide dipoles and field-induced reorganization of the positive and negative ionic charges in the interfacial region are both found to contribute to the surface-normal field near the points of charge emission. In the simulations the majority of cluster ions are found to be emitted from the tip of the jet rather than from the neck region next to the Taylor cone.
Pub. in Journal Physical Chemical A., v112, p9628-9649, 2008. Support by the Air Force Office of Scientific Research as part of the Space Miniaturization Theme and under task 2303ES02. Sponsored in part by Department of Energy grant.