Understanding Solvation in the Low Global Warming Hydrofluoroolefin HFO-1234ze Propellant

Hydrofluoroolefins (HFOs), with zero ozone-depleting effect and very low global warming potential, are considered to be the next-generation high-pressure working fluids. They have industrial relevance in areas including refrigeration and medical aerosols. One major challenge expected in the replacement of existing working fluids with HFOs is the solubility and solvation of additives in such hydrophobic and oleophobic low dielectric semifluorinated solvents. The study of the solvation of chemistries that represent those additives by HFOs is, therefore, of great relevance. In this work, we systematically investigate how the polarity and structure of fragments (the tail, t) that represent those additives affect their binding energy (E-b) with HFO-1234ze (1,1,1,3-tetrafluoropropene) (the solvent, s; E-b(st)). We also compare and contrast those results with those for the working fluids that are most widely used in the industry, the hydrofluoroalkanes (HFAs) HFA-134a and HFA-227. Three main chemistries were investigated: alkanes, ethers, and esters. It was found that HFO-1234ze interacts quite favorably with ethers and esters, as indicated by their E-b(st), while E-b(st) with alkanes was much lower. While ether and ester groups showed little difference in E-b(st), the much lower self-interaction energy between ether tailtail fragments (E-b(tt)) is expected to result in improved solubility/solvation of those groups in HFO-1234ze when compared with the more polar ester groups. The ratio E-b(st)/E-b(tt) is defined as the enhancement factor (E-enh) and is expected to be a better predictor of solubility/solvation of the tail fragments. The branching of the tail groups upon the addition of pendant CH3 groups did not significantly affect the solvation by the propellant. At low branching density (one CH3 pendant group), it did not affect tailtail self-interaction either. However, at high enough branching (two CH3 groups), steric hindrance caused a significant decrease in Ebtt and thus an increase in Eenh, suggesting that branching may be used as a strategy to enhance solvation in HFO propellants. Finally, the solvation behavior of HFO-1234ze was found to be similar to that of HFA-134a, thus suggesting similar considerations may apply for both propellants, when solvation properties are of a concern to the application.