The aggregation behavior of the cationic fluorocarbon surfactants 1,1,2,2-tetrahydroperfluoroalkylpyridinium chloride and 2-hydroxy-1,1,2,3,3-pentahydroperfluoroalkyldiethylammonium chloride in aqueous solution have been studied using cryo-transmission electron microscopy as the main technique. The effects on aggregate structure of factors such as surfactant and salt concentration, counterion type, and alkyl chain length were investigated. Similar to hydrocarbon surfactants the fluorocarbon surfactants self-assemble into various aggregates such as micelles, threadlike micelles, vesicles, and other lamellar aggregates. A distinctive property of the fluorocarbon surfactants is their tendency to form structures with little curvature, such as cylindrical micelles and bilayer structures. Even a very small reduction of the repulsion between the headgroups is sufficient to accomplish a sphere to rod transition, so that a solution of globular micelles in water is turned into a highly viscous, sometimes viscoelastic solution on the addition of low concentrations of a simple salt. The differences between the fluorocarbon and hydrocarbon surfactants are discussed.

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The concentration dependent properties exhibit a break point due to the micellization.The CMC of surfactant is usually determined by plotting those properties as a function of concentration. The other important property of ionic surfactant solutions is the degree of micellar counterion dissociation, which has been used to be quantified by the two-site model (7,8). From this simple model one can calculate the α by using the concentrations of "free" ions in the aqueous bulk phase and "bound" ions in micellar phase. The model has successfully The MQAE probe was developed to study the transport of chloride ion in liposomes (11).The MQAE are highly polar and membrane impermeant, which has suitable physical properties for measurement of the concentration of free chloride ion. When the cationic MQAE is introduced to cationic surfactant solutions, the MQAE will tend to partition in aqueous bulk phase due to the low octanol-water partition coefficient in addition to the electrostatic effect.The fluorescence behavior of water-soluble MQAE can be expected to related to the chloride ion concentration in aqueous bulk phase. Fig. 2 shows the Stern-Volmer plots for quenching of MQAE fluorescence by various cationic surfactants. The fluorescence intensities of MQAE 5 were found to decrease in the presence of CTAC, TTAC, TDBAC and DAC. The observed quenching can be ascribed to the free chloride ion dissociated from the surfactant. All experimental data gave linear plots below their CMCs. The slopes of plots, K SV , below CMC are summarized in Table 1. The K SV for CTAC agreed very closely with that of NaCl. The K SV for surfactant monomer tend to decrease with decreasing length of the alkyl chain. The decrease in

Time-resolved fluorescence quenching studies of nonionic, anionic, and cationic micelles have been performed to compare two surfactant quenchers, a fluorocarbon surfactant quencher, N-(1,1,2,2-tetrahydroperfluorodecanyl)pyridinium chloride (HFDePC) and a hydrocarbon quencher of similar hydrophobicity, C16PC, N-hexadecylpyridinium chloride. The concentration dependence of the apparent aggregation numbers informs on the interaction between the surfactants, which always was repulsive for the fluorocarbon quencher in hydrocarbon micelles, except for the case with a nonionic micelle, where the effectively attractive electrostatic interaction dominated at low ionic strength. The simple theory (Almgren, M.; Hansson, P.; Wang, K. Langmuir 1996, 12, 3855) suggests that the interaction parameter from the slope of the apparent aggregation number versus mole fraction quencher should be the same as the interaction parameter describing the change of the critical micelle concentration with composition according to regular solution theory (Rubingh, D. H. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press:  New York, 1979; Vol. 1, p 337). The results show that it is not so; not even the sign of the interaction parameter is always the same. The reasons for the difference are discussed. For the weight average aggregation number, obtained by extrapolation to zero quencher concentration, the two quenchers gave results within about 10% for a given surfactant; the values with the fluorocarbon quencher seemed to be systematically lower, as if the aromatic probe strongly avoided micelles containing fluorocarbon quenchers. From solubility studies, pyrene was found to prefer a C16TAC micelle over a HFDePC micelle by a factor of 60. The preference of pyrene for micelles without fluorocarbon surfactants was utilized to show the demixing into fluorocarbon-rich and hydrocarbon-rich micelles in a mixture of lithium perfluorononanoate and lithium dodecyl sulfate and in cetyltrimethylammonium chloride and HFDePC.

A group contribution method is proposed to predict the cmc of aqueous binary mixtures of surfactants. The interaction parameters of the molecule's functional groups, including hydrophilic groups, were determined from the cmc values of mixed systems. The calculated cmc's were in good agreement with the experimental ones. The cmc curves in a series of mixed systems, e.g., lithium perfluorooctanesulfonate (LiFOS)-lithium dodecyl sulfate (LiDS), -lithium tetradecyl sulfate (LiTS), and -lithium hexadecyl sulfate (LiHS), were fitted by the same interaction parameters. The mutual solubilities of fluorocarbon and hydrocarbon surfactants in the micellar phase were also studied. The hydrocarbon-rich micelles solubilized the fluorocarbon surfactants to a certain extent, whereas the fluorocarbon-rich ones solubilized the hydrocarbon species sparingly. We could predict the cmc of nonideal binary mixtures by the use of cmc, .Kg, molecular structure data of pure components, and the group interaction parameters.