Effect of counterion and solvent on the self-assembly of perfluorocarbon surfactants : submitted in accordance with the requirements for the degree of Doctor of Philosophy
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Date
1998
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Massey University
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Abstract
This work investigates the effects of changing the counterion and the solvent on the self-assembly and self-organisation of anionic perfluorocarbon surfactants. Specifically, the following investigations were carried out: • The concentration dependence of the rheological behaviour of tetraethyl- and tetramethylammonium perfluorononanoate/H2O has been examined. The behaviour at intermediate concentrations is consistent with solutions of very long and entangled self-assembled rodlike micelles which is not observed in systems with unalkylated ammonium ions as counterions. At higher surfactant concentrations their rheological behaviour suggests that the rodlengths go through a maximum. At higher concentrations still the systems exhibit liquid crystalline phase behaviour which is consistent with the presence of discotic micelles i.e. a rod-to-disk transition must occur. • The effects of substituting D2O for H2O, and ethylene glycol (EG) for H2O, on the self-assembly of ammonium perfluorooctanoate (APFO) has been examined by variations in the electrical conductivity of their solutions as functions of temperature and surfactant concentration. The effect of substituting EG/H2O mixtures for H2O on APFO micellisation at 298 K was also examined. The results were interpreted within a phenomenological statistical thermodynamic model which showed that differences in self-assembly arise largely from modification of the solution/fluorocarbon interfacial tension. • The effects of changing the solvent from water to mixtures of water and the cosolvents formamide (FA), ethylene glycol (EG), N-methylformamide (NMF), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA) on the phase behaviour of the micellar liquid crystal caesium perfluorooctanoate (CsPFO)/D2O have been investigated using 133Cs and 2H NMR. With increasing concentration of cosolvent the isotropic I - to - discotic nematic ND and ND - to - smectic L sequences of transitions are displaced to lower temperatures, but the general phase behaviour is conserved. The efficacy of the cosolvents in depressing the phase transition temperatures parallels the order of their effect on the solution/fluorocarbon interfacial tension i.e. DMA>DMF>NMF>EG>FA. Measurement of the 2H quadrupole splittings of D2O reveal that increasing the cosolvent concentration at constant temperature results in a concomitant decrease in micelle size but the axial ratios of the discotic micelles have essentially singular values at both the I - to - ND and ND - to - L transitions. The effect of cosolvent on the phase transition temperatures can be qualitatively understood in terms of phase transitions driven by hard particle interactions and, separately, modifications in the micelle self assembly as a consequence of changes in the solution/fluorocarbon interfacial tension with the addition of cosolvent. • The effects of the cosolvents on the micellar size of APFO/H2O at high surfactant concentration have been investigated by small angle x-ray diffraction (SAXS) and have also been shown to arise from changes in the solution/fluorocarbon interfacial tension.
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Surface active agents, Organofluorine compounds