Surfactant solubility and aggregate orientation in hydrofluoroalkanes

Abstract

Purpose: To find surfactants soluble in the two hydrofluoroalkane (HFA) propellants, HFA-134a and HFA-227ea; to compare surfactant solubility in the two propellants with those in 2 H,3 H-decafluoropentane (DFP) in order to assess latter's suitability as a liquid model propellant and to investigate surfactant aggregation and aggregate orientation in HFAs. Methods: To assess surfactant solubility, HFA was added to a known amount of surfactant until dissolution was visibly apparent. An iodine solubilization method was used to determine surfactant aggregation behaviour in DFP. Fluorescence spectroscopic investigations on the surfactant orientation in aggregates were carried out in HFAs using a microviscosity sensitive fluorescent probe (1,3-dipyrenylpropane). The aim was to assess viscosity changes in the microenvironment of this lipophilic probe upon incorporation into surfactant aggregates. Results: Soluble surfactants could be found among the polyoxyethylene-ethers and POE–PPO-block copolymer surfactants. Solubility in DFP appears to correlate with solubility in HFA-134a, but not HFA-227ea. Iodine solubilization indicates micellization of Brij 30 in DFP at a cmc (type II association behaviour). L-44 in DFP, on the other hand, does not exhibit a well defined cmc, but shows continuous surfactant aggregation (type I association behaviour). The fluorescence spectroscopic studies showed evidence for probe incorporation into surfactant aggregates in HFAs. Conclusions: DFP proved to be a good model for HFA-134a only. An L 1-aggregate orientation was shown for surfactants in HFAs and is in marked contrast to the chlorofluorocarbon propellant where a L 2-aggregate orientation exists.