High Solute Solubility and Emulsion Formation in Supercritical Hydrofluorocarbon Solvents

Supercritical (sc) CO2 is by far the most commonly used supercritical fluid. Reasons for this are that it is inexpensive, non-toxic, has low critical constants (Tc = 304.20 K, Pc = 72.8 bar) and is environmentally benign. The major problem encountered when using sc CO2 is the low solubility of polar solutes. Hydrofluorocarbon (HFC) solvents such as difluoromethane (HFC 32) and 1,1,1,2-terafluoroethane (HFC 134a) are much more polar than scCO2 and have easily accessible critical constants (HFC 32: Tc = 351.26 K, Pc = 57.82 bar; HFC 134a: Tc = 374.21 K, Pc = 40.59 bar). In addition HFCs are readily obtainable, non-toxic, non-ozone depleting, inert, nonflammable and highly compressible. These solvent properties indicate that they are idea solvents for supercritical applications. By altering temperature and pressure these solvents have been shown to be much more tuneable in terms of their ability to dissolve polar solutes than scCO2. We present the solubility of a variety of solutes in HFC 32 and show how these can be modelled using various equations of state. At high solute concentrations these models break down and it has been found that solute-solute clustering may result in the formation of supercritical emulsions. The hydrogen bonding ability of these solvents is quantified for the first time as a function of temperature and pressure and it is shown that emulsion formation is prevalent when hydrogen bonding is significant.