Molecular Dynamics Study of the Vaporization of a Finite Ionic System
By: Nuno Galamba
From: Univ. Lisbon
At: Complexo Interdisciplinar, Anfiteatro
[2010-01-06]
A great interest in the characterization of the vapor of ionic liquids has emerged in recent years driven by the possibility of distilling some room temperature ionic liquids previously thought to be nonvolatile. While the thermodynamics as well as a microscopic description of the vapor of room temperature ionic liquids, is only recently the subject of experimental and theoretical examination, the vapor composition of molten salts, in particular alkali halides, has long been accessed experimentally. Nonetheless, the importance of polymeric species in the measured vapor pressure of solid and molten alkali halides was always subject of controversy among experimentalists. Further, mass spectrometric and infrared spectral studies lend significantly different vapor populations for alkali halides. In the present work the melting of a microcrystal and subsequent vaporization of a NaCl drop are considered through molecular dynamics simulations. The vaporization of the ionic drop is carried out for a single isochor at increasing temperatures until vaporization is complete, and gaseous NaCl forms. The vapor composition is examined in terms of the molecular species present in the vapor and the results are compared with those deduced from experiments. It is found that the vapor of the model ionic drop is composed of neutral molecular species, the most abundant of which, ranging from simple NaCl monomers (ion pairs) to non-linear polymers, (NanCln) n=2-4. Moreover, the results show that non-linear polymers are significantly more abundant than their linear isomers at all temperatures and the observed fraction of non-linear trimers is significantly larger than that inferred from experiments. Finally, the formation of a low density ionic gas is observed to take place at very high temperatures (~3500 K).