Critical Parameters and Surface Tension of the Room Temperature Ionic Liquid [bmim][PF6]: A Corresponding-States Analysis of Experimental and New Simulation Data

The surface tension gamma of the room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6] in short, has been evaluated in the framework of Guggenheim's corresponding-states analysis using experimental data taken from the literature as well as new simulation data. The critical temperature is estimated to be T-c = 1100 K +/- 100 K from the surface-tension data, which are analyzed in terms of the empirical expressions of Guggenheim and of Eotvos. Adopting this Value of T-c a critical density of rho(c) = 0.35 g cm(-1) +/- 0.04 g cm(-3) is obtained frorn the experimental coexistence curve using the linear-diameter rule. The estimates of rho(c) obtained from our simulation data are lower by 13-25%. According to Guggenheim, the reduced surface tension gamma(red) = c gamma/T-c rho M-2/3(c)r(-2/3)), where M-r denotes tile molar mass and c a constant, is supposed to be a universal function of the reduced temperature T/T-c for all pure fluids. While many nonpolar, weakly polar, and moderately polar liquids adhere to this principle of corresponding states, deviations frorn Guggenheim's master Curve are seen for strongly polar, hydrogen-bonding Substances and for simple inorganic molten salts, Such as NaCl and KCl. We find that, despite its ionicity, the data ror [bmim][PF6], both from experiment and front simulation, follow the master Curve for at most moderately polar liquids, thereby indicating significant differences frorn (lie physicochemical properties of simple inorganic molten salts.