Adsorption and aggregation of semifluorinated alkanes in binary and ternary mixtures with hydrocarbon and fluorocarbon solvents

The phase behavior, adsorption, and aggregation of semifluorinated alkanes (SFAs) of general structure F(CF2)(n)(CH2)(m)H (abbreviated to FnHm) in hydrocarbon and fluorocarbon solvents has been investigated. The SFAs form a novel class of structurally primitive surfactants, and their behavior in mixtures with hydrocarbon and fluorocarbon solvents is compared with that of conventional surfactants in mixtures with oil and water. Miscibility studies of hydrocarbon and fluorocarbon solvents of different chain lengths show that the free energies of transfer of -CH2- from hydrocarbon to fluorocarbon phases and -CF2- from fluorocarbon to hydrocarbon are 1.1 and 1.4 kJ mol(-1) respectively. These values are approximately one-third of the free energy of transfer of a -CH2- group from alkane to water. Hence, SFAs are expected to be only weakly amphiphilic when compared with conventional surfactants of similar chain lengths in oil + water systems. This expectation is confirmed by SFA solubility and aggregation measurements which show that SFAs aggregate weakly in both hydrocarbon and fluorocarbon solvents to give aggregates with aggregation numbers in the range 2-10. SFAs adsorb at the hydrocarbon-air and hydrocarbon-fluorocarbon interfaces but not at the fluorocarbon-air surface. A limited number of two-phase ternary mixtures of SFA with hydrocarbon and fluorocarbon solvents were studied. No evidence for the formation of ''middle phase'' microemulsions was observed, again consistent with the relatively weak amphiphilicity of SFAs.