Critical point behaviour in binary and ternary liquid mixtures with particular reference to rheological and interfacial properties in model mixtures for microemulsions.

The phase behaviour, rheological effects and interfacial properties of binary and ternary
liquid mixtures have been studied near critical points. In particular, measurements have
been made of the viscosity-at the bulk macroscopic level by capillary viscometry and at
the microscopic level by fluorescence depolarisatiorr-and of critical-point wetting and
adsorptiorr-at the solid-liquid interface using evanescent-wave-generated fluorescence
spectroscopy and at the liquid-vapour interface using specular neutron reflection. The
systems investigated have been mostly alkane + perfluoroalkane mixtures or
2-butoxyethanol + H20 or D20 mixtures, although in some cases hexamethyldisiloxane,
propanenitrile and perfluorooctyloctane have also been the components of mixtures. The
main outcomes of this study are:
• Macroscopic viscosity: The divergence to infinity in the shear viscosity of hexane +
perfluorohexane at the critical endpoint for approach along the path of constant
critical composition both from the single phase and along both limbs of the
coexistence curve is described well using the Renormalisation Group Theory critical
exponent y = 0.04. The correlation length amplitude obtained by fitting the sheargradient
dependence of the viscosity is ~o = (S.S±l.S) A.
• Microscopic viscosity: The product of the rotational correlation time and the
temperature 'tR"T, often taken as a measure of the microscopic viscosity, exhibits an
anomaly as the critical point is approached as a function of temperature. This
anomaly mirrors that in the macroscopic viscosity for some fluorescent dye probes,
but for others the anomaly is in the opposite sense indicating that other effects such
as solvent structure must playa part in the near-critical behaviour of'tR·T.
• Critical-point wetting at the solid-liquid interface: The wetting transition
temperature has been identified for heptane + perfluorohexane at the quartz-liquid
interface from fluorescence lifetime measurements of a probe. The wetting layer is of
the same composition as the bulk heptane-rich phase and the transition is tentatively
identified as first-order.
• Adsorption and wetting at the liquid-vapour interface: The surface structure of
several mixtures has been determined by neutron reflection. The results are in general
agreement with the expectations of critical-point wetting and adsorption. The surface
is complex and in some mixtures an oscillatory scattering length density profile
through the interface is required to model the reflectivity data.
• Ternary mixtures: The phase behaviour of three mixtures exhibiting tunnel phase
behaviour has been studied experimentally and various characteristics of the shape of
the twmel identified. A theoretical study on one of the mixtures predicts the drop in
temperature for the locus of maximum phase separation temperatures which is
observed experimentally.