Microscopic and Thermodynamic Properties of the HFA134a−Water Interface:  Atomistic Computer Simulations and Tensiometry under Pressure

A combined computational and experimental approach is used to determine the interfacial thermodynamic and structural properties of the liquid 1,1,1,2-tetrafluoroethane (HFA134a)−vapor and liquid HFA134a−water (HFA134a|W) interfaces at 298 K and saturation pressure. Molecular dynamics (MD) computer simulations reveal a stable interface between HFA134a and water. The “10−90” interfacial thickness is comparable with those typically reported for organic−water systems. The interfacial tension of the HFA134a|W interface obtained from the pressure tensor analysis of the MD trajectory is in good agreement with the experimental value determined using in situ high-pressure tensiometry. These results indicate that the potential models utilized are capable of describing the intermolecular interactions between these two fluids. The tension of the HFA134a|W interface is significantly lower than those typically observed for conventional oil−water interfaces and similar to that of the compressed CO₂−water interface, observed at moderate CO₂ pressures. The MD and tensiometric results are also compared and contrasted with the HFA134a|W and chlorofluorocarbon−water tension values estimated from a parametric relationship. This represents the first report of the interfacial and microscopic properties of the (propellant) hydrofluoroalkanes (HFA)|W interface. The results presented here are of relevance in the design of surfactants capable of forming and stabilizing water-in-HFA microemulsions. Reverse aqueous microemulsions in HFA-based pressurized metered-dose inhalers are candidate formulations for the systemic delivery of biomolecules to and through the lungs.