Surfactant Design for the 1,1,1,2-Tetrafluoroethane-Water Interface: ab initio Calculations and in situ High-Pressure Tensiometry

In situ high-pressure tensiometry and ab initio calculations were used to rationally design surfactants for the 1,1,1,2-tetrafluoroethane-water (HFA134a|W) interface. Nonbonded pair interaction (binding) energies (Eb) of the complexes between HFA134a and candidate surfactant tails were used to quantify the HFA-philicity of selected moieties. The interaction between HFA134a and an ether-based tail was shown to be predominantly electrostatic in nature and much more favorable than that between HFA134a and a methyl-based fragment. The interfacial activity of (i) amphiphiles typically found in FDA-approved pressurized metered-dose inhaler (pMDI) formulations, (ii) a series of nonionic surfactants with methylene-based tails, and (iii) a series of nonionic surfactants with ether-based tails was investigated at the HFA134a|W interface using in situ tensiometry. This is the first time that the tension of the surfactant-modified HFA134a|Winterface has been reported in the literature. The ether-based surfactants were shown to be very interfacially active, with tension decreasing by as much as 27 mNm^-1. However, the methyl-based surfactants, including those from FDA-approved formulations, did not exhibit high activity at the HFA134a|Winterface. These results are in direct agreement with the Eb calculations. Significant differences in interfacial activity are noted for surfactants at the 2H,3H-perfluoropentane (HPFP)|water and HFA134a|Winterfaces. Care should be taken, therefore, when results from the mimicking solvent (HPFP) are extrapolated to HFA134a-based systems. The results shown here are of relevance in the selection of surfactants capable of forming and stabilizing reverse aqueous aggregates in HFA-based pMDIs, which are promising formulations for the systemic delivery of biomolecules to and through the lungs.