Thermodynamics and Solubility Modeling in Hydrofluoroalkane Systems

Abstract

The phase-out of chlorofluorocarbons (CFCs) has resulted in an expanding new area of research in alternative ozone friendly propellants, for example hydrofluoroalkanes (HFAs). The HFA solvent system is unique in that many CFC soluble compounds behave differently in the HFA alternatives, such as HFA-134a and HFA-227. The reason for the difference in solubility is not fully recognized. This work investigates the solubility of 22 compounds in HFA-227 with the addition of ethanol as a cosolvent. The physical properties of both solute and solvent were investigated in order to determine the effects on solubility. The solubilities of 5 compounds in HFA-134a were also investigated. A thermodynamic approach was utilized in order to look at the enthalpic and entropic effects on solubility in the propellant. Due to the high vapor pressure of propellants, a liquid model was utilized, owing to its ease of use in characterizing solubility. The correlation between the liquid model 2H,3H-decafluoropentane (DFP) and the propellants HFA-134a and HFA-227 was examined.The solubilities in HFA-227 with ethanol ranged from 0.001 to 3.282 %w/w, where the solubilities always increase when ethanol was added. The experimental solubilities were compared to calculated values obtained from ideal solubility and regular solution theory models. A clear correlation with the ideal solubility (melting point) combined with an intercept term and two physical properties was noted. A regression approach was also used to predict the activity coefficient in HFA-227 with 0 - 20% ethanol. These equations were combined with the extended ideal solubility equation, creating a useful predictive equation with AAE values ranging from 0.32 to 0.36, or factor errors of 2.09 to 2.29. The equations shown in this work are useful for the prediction of solute solubility in HFA-227/ethanol mixtures.Results in the liquid model DFP with 0 - 20% ethanol show that a regression equation results in a useful predictive equation for the solubilities in both HFA-134/ethanol and HFA-227/ethanol systems, where the AAE values ranged from 0.3 to 0.56, or factor errors of 2.0 to 3.6.The solubilities of a series of chlorobenzene compounds along with a group of hydrogen donating and/or accepting compounds was examined in HFA-134a. The entropic effects appear to be the limiting factor in the solubility of these compounds. The compounds capable of hydrogen accepting and donating exhibited negative enthalpy of mixing values when placed in HFA-134a, a stark contrast to the values obtained for the chlorobenzenes. This suggests HFA-134a is able to strongly interact with solutes capable of donating or accepting hydrogen.