FLUOROCARBON HYDROCARBON INTERACTIONS IN MICELLES AND OTHER LIPID ASSEMBLIES, AT INTERFACES, AND IN SOLUTIONS

It is well known that interactions of fluorocarbons and hydrocarbons in liquids are highly nonideal. Some evidence is presented which indicates that the expression of hydrophobicity of fluorocarbon compounds, when compared to that of hydrocarbon compounds, is significantly different because of the greater participation of water-structure pertubation effects in the case of fluorocarbon compounds. The nonideality of the interactions of fluorocarbons and hydrocarbons is manifested in a great variety of systems involving interfaces, micelles and other lipid assemblies and in solution. Results from a number of different systems are briefly summarized. These discussions include (a) nonideality effects expressed in surface tensions and interfacial tensions against water of mixtures of fluorocarbon and hydrocarbon liquids, (b) a comparison of relative affinities of fluorocarbon and hydrocarbon surfactants at the air-water, hexane-water, and perfluorohexane-water interfaces, (c) differences in the nature of the adsorption of fluorocarbon and hydrocarbon surfactants to low energy solids and the effects of such differences on the wetting properties of surfactants, (d) nonideality in mixed micelles, microscopic phase separation in mixed micelles and the coexistence of mixed micelles of different compositions, (e) unusual properties of partially fluorinated amphiphiles, and (f) possible problems in the use of fluorine-labeled groups as spectroscopic probes. In addition some counterion effects in the micelle formation of perfluoroalkanoates are reviewed. Hydrogen ions as counterions confer greater stability on perfluoroalkanoates as opposed to dodecyl sulfonates. The hydrophobic stabilization of the micelles of perfluorooctanoic acid salts when the tetraethylammonium ion is used as the counterion has been shown recently to be superior to the stabilization of the dodecyl sulfate system. This observation suggests that nonideal fluorocarbon-hydrocarbon interactions may not be of great importance in hydrophobic processes in which contact is the primary event, as opposed to hydrophobic processes in which chain mingling is important, e.g. micelle formation. For contact hydrophobic interactions water-structure effects play a more significant role, and this factor may increase the effective hydrophobicity of fluorocarbons.