Order-disorder transitions in solutions of discoid micelles

Abstract

X-ray scattering, electrical conductivity and NMR measurements are presented for a sample of caesium pentadecafluoroctanoate (CsPFO) (55 per cent by weight)/water (2H2O). A first order lamellar to nematic phase transition occurs at 325.13 (1) to 325.50 (1) K and a first order nematic to isotropic micellar solution transition occurs at 330.23 (1) to 330.83 (1) K. The fundamental mesogenic particle is shown to be a discrete discoid micelle in all three phases, that is, the lamellar phase consists of planes of discoid micelles. The thickness of the micelle is of the order 2.2 nm. In the isotropic micellar solution, the average diameter of the micelle grows slowly on cooling. At the isotropic to nematic transition, it increases discontinuously from 5.26 nm (NA = 88 ) at the upper boundary to 5.82 nm ( N A = 113) at the lower boundary, but there is no discontinuity in size at the nematic to lamellar transition where the diameter is 6.38 nm (N A =140). The growth of the micelle in the nematic phase appears to be driven mainly by the variation with temperature of the orientational order parameter (this is 0.35 at the nematic to isotropic transition and 0.72 at the nematic to lamellar transition). This behaviour is as predicted by the theories of Gelbart et al., yet the transition to the lamellar phase is not driven by the predicted explosive growth of the micelles into bilayers. The X-ray scattering patterns are characteristically different from those generally observed in the analogous thermotropic nematic and smectic A phases. It is argued that this is because the micelles (volume fraction 0.35) are relatively loosely packed: in fact, in all of the three phases they may well undergo large amplitude uncorrelated reorientational fluctuations. Short range orientational correlations are consequently either absent or very limited in their extent. This is manifest in the unexpectedly large values (0.92 to 0.71 in the nematic phase) of the orientational order parameters obtained from the X-ray measurements ; it is suggested that these order parameters are mainly determined by the long-range collective director fluctuations. Thus, the model which emerges for the nematic phase is one in which the individual orientational fluctuations of the micelles are essentially decoupled from the collective director fluctuations.