(500d) Rational Design of Hydrofluoroolefin-Philic Moieties: A Combined Computational and Experimental Approach
Hydrofluoroalkanes (HFAs) are currently the propellants of choice in many industrial applications, including refrigeration and in medical aerosols, as they have zero ozone depleting potential (ODP). However, HFAs have a relatively high global warming potential (GWP), at 1430 for HFA-134a and 3200 for HFA-227. As regulations become more stringent, new alternatives to HFAs are being sought. Hydrofluoroolefins (HFOs), with their zero ODP and very low GWP - 6 for HFO-1234ze and 4 for HFO-1234yf, are promising next generation working fluids. In many applications involving propellants, the dispersion of low solubility species is of essence. Because of the dual hydrophobic and lipophobic nature of HFOs, solute solubility is expected to be of major concern for this class of solvents. The design of HFO-philic moieties and surface active agents for HFOs is thus of great relevance in furthering the applicability of such environmentally friendly propellants.
In this work we use a combination of computational and experimental tools, including ab initio calculations, molecular dynamics (MD) simulations, and high pressure tensiometry to address HFO-philicity. Pair interaction binding energy from ab initio calculations was used to determine an HFO-philicity scale for the alkane, ether, and ester moieties, with the ether and ester moieties having the strongest interaction with HFO. The potential of such chemistries as HFO-philic tails in non-ionic surfactants was tested by determining their impact on the tension of the water-HFO interface, and the surfactant balance using both MD simulations and high-pressure tensiometry. The interfacial tension of HFO|H2O interface from MD and tensiometry was found to be in excellent agreement. The PO-based surfactant was capable of reducing the interfacial tension of the binary interface by 30.9 mN/m. The optimum balance for that surfactant class was determined to be at 60EO% by MD simulations and 50EO% by tensiometry. This study represents the first attempt to systematically design dispersing agents for HFOs. Reverse aggregates of water in HFO may find applications in the delivery of water-soluble or water-dispersible therapeutics using medical sprays and in other industrial applications.