Self-assembly and self-organisation of discotic micelles formed in aqueous solutions of the salts of fluorinated carboxylic acids : Department of Chemistry and Biochemistry, Massey University, New Zealand, submitted in accordance with the requirements for the degree Doctor of Philosophy
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Date
1994
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Massey University
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Abstract
The phase behaviour of a series of discotic micellar liquid crystal systems found in aqueous solutions of perfluorinated carboxylic acids has been determined mainly by the use of 2H and 133Cs NMR. All the systems exhibited an isotropic micellar solution phase which underwent transitions to first a micellar nematic N+D and then a micellar lamellar LD phase on decreasing temperature or increasing the surfactant concentration. It was found that the self-organisation could be understood within the framework of a hard particle interaction and the changes in the phase transition temperatures on the addition of salt, change of counter-ion, and substitution of different chain length fluorocarbons were the result of changes in the micellar self-assembly. The effect of amphiphile chain length was investigated by studying the caesium salts of perfluorinated carboxylic acids with chain lengths of 7, 9 and 10 carbons, in 2H2O. A comparison with the caesium pentadecafluorooctanoate (CsPFO)/2H2O system shows the four systems exhibit universal phase behaviour with the phase transition temperatures at any given volume fraction of surfactant being simply displaced in temperature. In addition, the variation in the axial ratio of the discotic micelles along the transition lines has been established from the magnitudes of the 2H quadrupole splittings of 2H2O. It has been shown that for any given thermodynamic state of the system axial ratios of the discotic micelles decrease with increasing chain length, i.e. the micelle aggregation number increases. At the phase transition temperatures, however, the micelles have the same axial ratio irrespective of the length of the fluorocarbon chain. The influence of inorganic electrolyte on the mesophase behaviour has been studied in the CsPFO/CsCl/2H2O and ammonium pentadecafluorooctanoate (APFO)/NH4Cl/2H2O systems. The addition of electrolyte has the effect of increasing the phase transition temperatures in both systems. 2H NMR has been used in conjunction with low angle x-ray scattering to probe changes in the micelle structure as a function of electrolyte. It has been shown that electrolyte promotes growth of the micelle, i.e. a decrease in the axial ratio. But, as in the case of changing the chain length, the axial ratio of the discotic micelle is the same at the phase transition temperatures for any given volume fraction of amphiphile. The role of the counter-ion and co-ion have also been established by 133Cs and 35Cl NMR. The effect of counter-ion has been investigated by progressively substituting APFO for CsPFO on a mole for mole basis. The Cs+ ions are shown to be preferentially "bound" over NH+4 ions, but the micelle size is the same at the phase transition temperatures. Co-ion effects on the phase behaviour of the added salt systems has been probed by substitution of Cl- ions with F-, Br-, and I- ions. It has been shown that the co-ion has no significant effect on the phase behaviour.
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Micelles, Solution (Chemistry), Carboxylic acids