Lyotropic mesomorphism in micellar liquid crystals formed from aqueous solutions of the salts of pentadecafluorooctanoic acid : a thesis presented in accordance with the requirements for the degree of Doctor of Philosophy, Department of Chemistry and Biochemistry, Massey University, New Zealand
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Date
1990
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Massey University
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Abstract
High resolution phase diagrams for the caesium pentadecafluorooctanoate
(CsPFO)/water and the ammonium pentadecafluorooctanoate(APFO)/heavy water
systems have been determined, together with a partial phase diagram, i n the liquid
crystal phase transition regime, for the APFO/water system. In all three systems a
discoidal nematic (N) phase occurs over an extensive concentration and temperature
range. This phase is intermediate to an isotropic micellar solution phase (I), to higher
temperatures/lower concentrations and a micellar lamellar phase (LD) to lower
temperatures/higher concentrations. The distinctions between the phase diagrams lies in
the temperature and composition of the phase transitionS . A variety of
techniques have been used to delineate these temperatures including 2H, 14N and 133Cs
NMR spectroscopy, DSC and electrical conductivity. The NMR method i s the main
technique and 133Cs NMR in particular has proved to be an excellent nucleus for the
precise detection of phase transition temperatures. It is the first time 133Cs N MR has
been used for this purpose. The NMR measurements show a preference for counterion
binding to sites of lowest surface curvature.
Isotope effects are shown by both the CsPFO and the APFO systems. The overall
effect on substituting heavy water for water is to raise the phase transition temperatures,
by about 3 K at a volume fraction amphiphile <1> = 0.1 and by about 1 K at <1> = 0.4. The
isotope effect is caused by an increase in micelle size in heavy water as a result of an
enhanced hydrophobic effect. Isotope effects are also shown in a study undertaken on
the thermodynamics of micellization in the CsPFO/water system. The erne's in heavy
water at a given temperature occur at higher concentrations than those in water, a result
which is opposite to the isotope effect in hydrocarbon surfactant systems. It is shown
that small changes in both the fraction of bound ions and the aggregatio n numbers
between the two systems have a profound effect on the calculated thermodynamic
parameters. A full understanding of the thermodynamics of micellization would require
precise determinations of these quantities.
The effect of substituting Cs+ ions for NH; ions is to raise the temperature of the liquid
crystal phase transitions by about 23 K at a given volume fraction. The reason for this
is that the micelles in the APFO/water system are smaller than those in the CsPFO/water
system for any given volume fraction and temperature. This is probably due to the
greater ability of the Cs+ ions to reduce the electrostatic repulsion between the anionic
head groups in the micelle which leads to a reduction in micelle surface curvature and
an increase in micelle size.
2H NMR has been used to monitor magnetic-field induced order in an isotropic solution
of discoidal micelles of CsPFO on approaching a transition to a nematic phase. The
field induced order is revealed as a quadrupole splitting of the isotropic signal which is
first observed, at the field strength of the experiment (6.34 T), at a temperature
approximately 80 mK higher than the upper boundary to the transition (TIN) . The
splitting increases rapidly with decreasing temperature and diverges as a hypothetical
second-order transition to the nematic phase is approached at T*. The divergence
follows a (TrN-T*)-1 dependence as predicted by molecular field theory. At TIN the
divergence is quenched and in the biphasic region the quadrupole splitting is constant.
This facilitates the precise determination of TrN and (TrN-T*). This latter quantity,
which was measured over the surfactant weight fraction w range of w = 0. 1 5 to w=
0.35, approaches zero (i.e becomes second order) only at infinite dilution. TrN-TNI,
which is more easily measured than TIN-T* is shown to be an alternative measure of the
strength of the isotropic to nematic transition.
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Keywords
Liquid crystals, Pentadecafluorooctanoic acid, Lyotropic mesomorphism