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Physical and Theoretical Chemistry
Solid State Molecular Science
Professor John White
http://rsc.anu.edu.au/research/white.php
Neutron and X-ray scattering methods
developed by this research group are used to study the structure and
dynamics on the nanometre and picosecond space/time scales.
Adsorption, self-assembly at interfaces, polymers, the imitation of
biomineralisation phenomena using "template" molecules and,
most recently, the structure and denaturation of proteins at
interfaces are current areas of interest. The insights gained are
used to guide chemical synthesis in making new materials with
interesting physicochemical properties. One recent highlight has
been the first determination of the thermodynamic parameters for
protein denaturation in the 50 Angstroms surface layer of a protein
solution. By comparison with denaturation in the bulk, the
contribution of the surface forces can be measured quantitatively.
Another is the first measurement of the interfacial structure of an
emulsion surface by neutron reflectivity. "Micro breaking"
of the emulsion at the interface with a solid surface has been
detected.
Our
collaboration with Orica Ltd on the structure and stability of high
internal phase oil/water emulsions has produced scientifically
interesting and practically useful information. We continue to show
that structural relationships at the nanoscale have importance for
rheological and other properties. Scattering methods continue to give
new quantitative information about how the surfactant stabilises
these systems. Studies by the same methods on milk fat membrane in
collaboration with the Dairy Research Corporation and Food Science
Australia have continued using selective deuteration of membrane
components.
Surface Structure of Protein Solutions
We
have made the first studies of thermal denaturation at the air-water
interface of protein solutions. Measurements up to 70°C were
possible using the energy dispersive reflectometer at the ISIS pulsed
neutron source. The energy dispersive nature of this instrument has
provided the unique advantage for these measurements to overcome the
effects of evaporation and condensation. Denaturation of both
lysozyme and b-lactoglobulin in the 50 Angstroms surface layer occurs
at temperatures up to 20°C lower than in the bulk solution.
Furthermore by using the thickness of the denaturing layer as an
ordered parameter we have been able to calculate the surface
contribution to the enthalpy of denaturation of these proteins. The
work has important consequences for protein as templates in biology
and in connection with protein fouling of surfaces. Work has begun
on the chemical denaturation of interfacial layers. This looks
promising. (with M.J. Henderson, A. Perriman, and S.A. Holt [ISIS,
Rutherford Appleton Lab., UK])
Synthesis and Characterisation of Emulsions using Pure and Mixed
Surfactants
Our
previous work has shown that mixtures of surfactants may be important
for controlling both the rheology and the stability of high internal
phase emulsions. Work this year on several series of polyisobutylene
surfactant and SMO surfactant mixtures have shown that such mixtures
can produce properties unobtainable with pure surfactants. Our
methods of neutron contrast variation have been used to identify the
interfacial structures from many of these mixtures. Correlation of
these nanostructure measurements with rheological, surface tension
and other measurements is revealing interesting phenomena. In
related work the properties of these surfactants at the air-water
interface has been studied by classical methods and reflectometry.
This has shown that for insoluble surfactants in general, it is
necessary to invoke imperfections in the spread monolayers such as
aggregation, lateral segregation and polynya formation. (with M.J.
Henderson, P.A. Reynolds, and E.P. Gilbert [ANSTO, Lucas Heights])
Growth of Highly Ordered Mesoporous Films
Our
studies on the growth of highly ordered silicate films have now
disentangled two mechanisms leading to film growth under different
physical conditions. The millisecond refelctometer in our lab (see
below) has greatly aided the study of the mechanism. We are now able
to record data on the time scale of a few minutes (for the whole
reflectivity pattern) and different phases in the growth process for
hermetically sealed air-water interfaces have been recorded.
Currently the mechanism of the "switch" for the surfactant
sphere to rod transition is under study since short induction times
are now accessible with our instruments. An exciting recent
development is the growth of titanium dioxide films at the
air-water interface using new chemistry. (with M.J.
Henderson, D. King)
Millisecond Reflectometer Development
The
tests on the "Bragg rotor" version of the millisecond
reflectometer have now been successfully concluded and a major report
written. At the same time the instrument has been moved to the GX-13
rotating anode source to obtain the highest possible laboratory X-ray
intensities. At that source we have also tested dispersive energy
diode detectors as an alternative to the Bragg rotor system for "low
count rate" situations. By working away from the
critical edge, counting with either the "Iglet" detector or
the avalanche photo-diode system (developed for the Bragg rotor) is
now possible. We have found that for the avalanche
photo-diode system count rates up to 0.5 MHz are acceptable without
serious double quantum counting and with the Iglet count rates (at 2%
energy resolution) up to 50 kHz are acceptable. With these systems
we have been able to record X-ray reflectivity data, with correct
scale factor, and able to be processed by our suite of
programs. (with T.L. Dowling, M.J. Henderson, D.J. King)
Ion and Solvent Transfers at Nickel Hydroxide Films Exposed to LiOH
The
intercalaction/deintercalation of ion and solvent within the Ni(OH)2
film matrix during oxidation and reduction is crucial to the response
time of any device based on this material, e.g. the alkaline battery.
Our previous study of this solid/liquid interface enabled a unique
solution to the ion and solvent transfers during the charge/discharge
cycle -under potentiodynamic control- using the combined
electrochemical quartz crystal microbalance and probe beam deflection
instrument (constructed at the University of Leicester). In order to
reveal potential vs. time-dependent processes, we have now
applied a potentiostatic control function and extracted the time
dependencies of ion and solvent fluxes in a chronoamperometric
experiment. Combining the EQCM/PBD-derived fluxes of individual
mobile species with their molar volumes, the implied changes in film
volume, driven by redox switching can be estimated. (with M.J.
Henderson, and A.R. Hillman, H. French [U. Leicester, UK])
X-ray
Reflectivity Study of Milk Proteins (dominated by ß-casein)
at the Air-milk Serum Interface and their Response to Fat Content and
Temperature
The
surface structure of dispersed emulsions play a key role in stability
of the system. Proteins being one of the most important surface
active components in foods stabilise interfaces by self-interaction,
resulting in a stiff visco-elastic adsorbed layer. These
interactions are sensitive to disruptive effects of lipids. Previous
kinetics studies by the group (Holt, S.A. and White, J.W., 1999.
Phys. Chem. Chem. Phys., 1, 5139-5145) using the X-ray
reflectivity method to investigate the surface adsorption of milk
proteins indicate that ß-casein had a
stronger affinity for the air-liquid interface compared to whey
proteins. It has been shown that initially a dense protein layer,
with the thickness of 20Å is formed, then a second more diffuse
layer with lower volume density of protein follows. Here we report
the conformational changes occurred at the air-milk serum interface
due to the effects of milk fat content, temperature and the milk
preparation technique (i.e. homogenisation vs
microfluidisation).
In the effect of fat content on the adsorption of protein into the
interface the key conclusion is that at lower temperatures the
surface composition remains unchanged. The compositional changes,
however, become significant at room temperature indicating adsorption
of less reflective-water-soluble components into the surface layer.
Repulsive interactions between casein aggregates are also involved.
Microfluidised samples, having the advantage of smaller particle
size, prove to be more stable to fat or temperature effects compared
to the corresponding homogenised milks. (with R. Heidari)
Small Angle X-ray Scattering Studies on the Milk Fat Globule Membrane
(MFGM)
The
functional properties of fats are very important in many food
systems. The type of dispersion, the solid-liquid fat ratio and the
occurring crystal form have a strong influence on the behaviour of
the entire edible fat system. The milk fat globule membrane (about
10nm in cross-section) consists of a complex mixture of proteins,
glycoprotein, enzymes, phospholipids, triglycerides, cholestrol and
other minor lipids. It is generally presumed that saturated fats and
triglycerides contract when they solidify or transform from an
unstable to a more stable crystalline form. However, considerable
expansion (up to 20%) in many saturated triglycerides and
hydrogenated fats have also been reported (Hvolby 1974).
Present
study on MFGM using small angle scattering method has consistently
indicated the presence of a crystal with a Bragg spacing of 41Å
and a melting point of 37ºC corresponding to a minor
triglyceride in milk. The small angle scattering from this crystal,
revealed in all milk samples tested here (i.e. both
homogenized and microfluidised milks), has not yet been reported in
the literature. More detailed SAXS experiments with simulated MFGM
will provide quantitative information on possible involvement of
other MFGM components in the formation of the observed crystal and
its biochemical significance. (with R. Heidari, T.L. Dowling)
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