Effect of molecular weight and end-group nature on the solubility of ethylene oxide oligomers in 2H, 3H-decafluoropentane and its fully fluorinated analogue, perfluoropentane

Abstract

Fluorinated liquids possess high chemical and physical stability, are tolerated by the human body and, therefore, show great promise in biomedical fields; however, they require extensive formulation. Phase diagrams are reported here for a series of ethylene oxide oligomeric additives in 2H,3H-perfluoropentane (HPFP), a non-chlorofluorocarbon fluorinated liquid regarded as a model propellant for pressurized metered-dose inhalers. Over a wide range of temperatures and concentrations, dihydroxyl end-capped poly(ethylene glycols) (PEGs) exhibited a lower critical solution temperature (LCST) that was strongly molecular weight dependent. In contrast, monomethyl (and thus monohydroxy) and dimethyl end-capped poly(ethylene oxides) were fully miscible with HPFP over the same temperature and concentration ranges, suggesting that the phase behaviour was dominated by end-group/solvent interactions. By systematically substituting HPFP for the fully fluorinated analogue perfluoropentane, the ability of these end-groups to interact with the solvent was perturbed and LCST-type behaviour was induced in the previously fully miscible monomethyl and dimethyl end-capped PEGs. Concomitantly, with increasing perfluoropentane content, the LCST of the dihydroxyl end-capped PEGs was driven to lower temperatures. Therefore, the phase behaviour of these systems may be controlled by 'tuning' the end-group structure of the ethylene oxide oligomers, and varying the hydrogen bonding capabilities of the fluorinated solvents.