(179l) Rational Surfactant Design for Hydrofluoroolefins (HFO-1234yf and HFO-1234ze(E)) , the Next Generation Refrigerant/Propellants: A Molecular Dynamics Approach

Hydrofluoroalkane (HFA) refrigerants/propellants have a relatively high global warming potential (GWP), at 1430 for HFA-134a and 3200 for HFA-227.  With the advent of a series of global warming-related regulations, new alternatives to the traditionally used HFAs are thus being sought.  Hydrofluoroolefins (HFOs), with their zero ozone depleting potential, and low GWP - 6 for HFO-1234ze and 4 for HFO-1234yf, are promising next generation working fluids in a wide range of applications, from foams, to medical aerosols, to refrigerants.  The design of HFO-philic species and surface active agents for HFO-based systems is thus of great relevance.

In this work we use atomistic molecular dynamics simulations to probe the microstructure of HFO interfaces, including HFO-air (vacuum) and HFO-water, and to study the activity of surfactants at those interfaces.  We investigate a series of non-ionic surfactants, with alkyl- [representative of the Brij surfactant class (CH₃(CH₂)_n(OCH₂CH₂)_nOH)],and propylene oxide-based HFO-philic moieties.  The surface tensions for both HFOs are seen to match the experimental results within a wide range of temperature (270-300 K).  The interfacial tensions for HFO-1234ze(E)-water and HFO-1234yf-water are 40.6±1.0 and 44.4±1.0 respectively.  With a surfactant coverage of 112Ų/surfactant the interfacial tension undergo a drop of 12.0 and 11.3 respectively for HFO-1234ze(E)-water and HFO-1234yf-water.  No experimental results are available yet for comparison.  The surfactant results will be discussed at varying surface coverage, and a detailed analysis of the structure of the surfactant-modified interfaces will be provided.