8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P1.48Fri 911:10-14:00How strongly ionic are room temperature ionic liquids?A corresponding-states analysis of the surface tensionVolker C. Weiss, 1 Berit Heggen, 2 and Frédéric Leroy 21 Universität Bremen, Bremen Center for Computational Materials Science, Am Fallturm 128359, Bremen, Germany2 Technische Universität Darmstadt, Darmstadt, GermanyThe Coulomb interactions among charged particles in ionic fluids are very different in terms ofstrength and range from the omnipresent dispersion forces acting also among neutral particles. Differenttypes of interactions give rise to characteristic corresponding-states behavior. For example,simple inorganic molten salts, such as NaCl, are known to show much lower values of Guggenheim’scorresponding-states surface tension at a given reduced temperature than neutral fluids [1].Recently, the critical parameters of a few typical room temperature ionic liquids (RTILs) have beenestimated [2, 3] and applied to analyze the surface tension of RTILs within a corresponding-statesapproach [3, 4]. Quite surprisingly, these reduced surface-tension data were found to be nearly thesame as those for neutral fluids, that is, markedly larger than the values for alkali halides! SinceRTILs are complex molten salts (the ions usually consist of several atoms), we suggest that dispersionforces, rather than Coulomb interactions, are responsible for the observed behavior. To testthis hypothesis, we have simulated an ionic model fluid in which the attractive dispersion interactioncan be varied in strength relative to the Coulomb interaction [5]. Our results indicate that theregular behavior of simple fluids is found once the dispersive part of the interaction between unlikeions exceeds 20% of the Coulomb attraction. Estimates of the relative contributions in NaCl and intypical RTILs explain why the latter behave like simple neutral fluids, whereas the former showsthe characteristic deviations of strongly ionic fluids. [1] V. C. Weiss and W. Schröer, J. Chem.Phys. 122, 084705 (2005). [2] L. P. N. Rebelo et al. , J. Phys. Chem. B 109, 6040 (2005). [3] V.C. Weiss, B. Heggen, and F. Müller-Plathe, J. Phys. Chem. C 114, 3599 (2010). [4] V. C. Weiss,J. Phys. Chem. B 114, 9183 (2010). [5] F. Leroy and V. C. Weiss, J. Chem. Phys. 134, 094703(2011).48