Fluctuational equation of state and hypothetical phase diagram of superheated water and two imidazolium-based ionic liquids

The particular low-temperature variant of fluctuational EOS with constant excluded volume b and temperature-dependent-coefficient a(T) is applied to water and two imidazolium-based ionic liquids: [bmim][PF6] and [pmmim][Tf2N]. The former demonstrates the regular linear correlations of liquid density ρ(T) (decreasing) and isobaric heat capacity (increasing) at atmospheric pressure P = 101,325 kPa. The latter has some trend to density maxima near the melting point similar to low-temperature density anomaly in water. Such a behavior, if it exists in ionic liquids, implies the reentrant spinodal form with the presence of minimum at negative pressures in the range from triple up to normal boiling point. Only the above input data have been used to predict qualitatively both types (regular and anomal) of a hypothetical gas-liquid phase diagram for ionic liquids and water. To control the quantitative results one needs the thermodynamically consistent choice of the excluded volume coefficient b0. The method developed for this aim can be useful to predict the phase diagram and solubility of low-molecular substances in imidazolium-based ionic liquids.