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THE AUSTRALIAN NATIONAL UNIVERSITY
RESEARCH SCHOOL OF PHYSICAL SCIENCES
DIFFUSION RESEARCH UNIT
ANNUAL REPOP.T 1974
40/1975
Head of Unit (Senior Fellow) Dr. R. Mills, M.Sc.(N.Z.),Ph.D.
(Wash. St. Louis) , D.Sc.
Fellow
Research Fellow
Visiting Fellow
Research Assistant (II)
Staff Changes
Dr - L.A. Woolf, M.Sc.(W.A.), Ph.D.(N.E.)
Dr. K.R. Harris, Ph.~.(Adclaide) · (from 4/3/74)
Dr. H.J.lf. Hanleys Ph.D.(Lond.)ARIC (until Nov. 1974)
r1r. P. Back, B.Sc. (Adelaide)
GENERAL
Dr. K.R. Harris joined the Unit as a Research Fellow in Harch
.
Conferences
Members of the Unit were involved in the organisation of · a
three day Sympo~ium on "The Physics and Chemistry of Fluids" which
was held in Canberra in May under the auspices of the P~yal
Australian Chemical Institute. The Symposium was attended by 70
Australian and overseas participants and was generally regarded as
a very successful one. At the conclusion of the Meeting Dr. R.
Mills was elected first Chairman of the newly-formed Physical
Chemistry DbTision of the RAC!.
RES&\PCtI PROGRAMME
Introduction
Research in the Diffusion Research Unit is centred around the
meas-urement and interpretation of self-diffusion in liquids. Over
the past ten years we have been concentrating on bringing the
isotopic tracer method to a state of accuracy where we are able to
produce new and significant data on a continuing basis. Concurrent
with this tbc Unit has been working on two difficult and long-term
projects, diffusion in cryo-genic liquids and the nuclear magnetic
resonance technique of measuring liquid diffusion. This year has
seen both these major projects finally brought to the operative
state after many delays and frustrations. A further component of a
theoret'ical nature in out' programme is the cal-culation of
intermolecular potentials for polyatomic molecules, which is a
joint project with the Computer Centre.
Tnis year the continuing isotopic tracer studies have produced
some very interesting information. Pressure diffusion studies in
light and heavy water have given insight into certain structural
features of water. Ionic tracer diffusion work which is being done
jointly with a group in Karlsruhe, Germany, allows a different
approach to the description of water structure in concentrated
electrolytes. Work has also continued
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on isotopic mass effects in li'quid diffusion. A large number of
experi-ments carried out during the year indicate that there is
virtually no iso-topic mass effect and is in accord with recent
theoretical predictions. This conclusion contradicts recent results
for uranium isotope separation in liquids by South African workers
who ascribe the separative process to molecular diffusion.
High pressure diffusion
Self-diffusion measurements in water up to 2.2 kbar for
temperatures from 277 to 31 8 K have been completed. There i s an
ano alous pressure dependence of the self-diffusion coefficient
which does not directly correspond with the effect of pressure on
the shear viscosity. This effect is believed to be more marked in
heavy water and studies in that liquid are in progress.
Previous high pressure self-diffusion measurements in benzene
have been extended to a higher temperature. An improved agreement
is apparent between these new measurements and a theoretical
relationship between the self-diffusion coefficient and the shear
viscosity.
Uork is continuing on the extension of the pressure range of the
diffusion equipment to 4 kbar and to the development of apparatus
for high pressure NMR studies.
Friction coefficients
Friction coefficients are basic transport coefficients derived
from the thermodynamics of irreversible processes. Previous work in
this area has not produced the expected correlations between
transport processes and proved generally inconclusive.
Octamethylcyclotetrasiloxane and carbon tetrachloride are near-
spherical molecules of disparate size for which good mutual
diffusion and activity coefficient data exist over a range of
temperatures. Therefore measurement of tracer diffusion
coefficients of both species will enable friction coefficients to
be cal-culated over a range of temperatures for the fi~st time.
Both isotopic and UNR techniques are being used in this study which
it is hoped will be more successful in relating the various
macroscopic transport coefficients.
Diffusion in electrolytes containing structure-breaking ions
Certain electrolytes which contain large structure-breaking ions
such as I- or cs+ have viscosity minima at fairly high
concentrations. During the year a large number of ionic diffusion
coefficients have been measured with diaphragm cells. We have found
that the separate ionic species and water all react differently in
their change in diffusion rate with concentration in these systems.
For example , in KI , the cation is accelerated much more than the
anion and in CsCl the anion is the more affected. An explanation in
terms of the breaking down of water structure around the ions has
been offered for these effects. This work is being carried out
jointly with Professor H.G. Hertz, 0f the University of
Karlsruhe.
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Isotopic effects in liquid diffusion
There has been some controversy in the literature as to the
existence and nature of isotopic diffusion effects in liquids. ·
Friedman has predicted from theoretical considerations · that if
existent they would be very small. Work here in normal and
deuterated hydrocarbons which have been tritiated so allowing their
tracer diffusion coefficients to be measured 9 has con-firmed this
ex~ectation. Further work in progress on the tracer-diffusion of
the 1125 ~ I 31 and csl3 4 ~ csl37 pairs indicate that isotopic
effects are absent here also. This work is of some practical
interest since there have recently been reports that urani~cl
isotopes have been separated by electrolyte diffusion. Our results
would contradict these reports.
Rare gas diffusion
The transport properties of the rare gases are of considerable
interest to theorists because these systems and argon in particular
most nearly approach ideal fluids. Present data are sparse and of
doubtful accuracy. A large cryostat has been built in the Unit over
a number · of years and after initial measurements in a diaphragm
cell mode has -been converted to a capillary type. The machine was
operational in the latter mode by late December although there have
been many problems mainly related to sealing at low temperatures
which have impeded progress.
NMR measur.a~ents of self-diffusion
The ~n1R technique for measuring self-diffusion by spin-echo
atten-uation is a very powerful one but has suffered in the past
through both inaccuracy and imprecisio~. An NMR machine. has ~een
built in the Unit which it is hoped will ov.ercome these two
problems. At the end of the year a start was ~ade to calibrate 'it
with isotopic data generated within the Unit. Self-diffusion data
for dense gases and liquids will then .be measured over wide
density ranges. As a preliminary step to these measu~ements a
literature survey was made of ail· prev1ous self-diffusion data in
simple fluids and an analysis presented in tabular form as a Uni~
report.
Theoretical studies
Present ~ork on the theory of liquids is in the statistical
mechan-ics of rigid p~lyatomic molecules. During the past year two
papers des-cribing the calc~~ation of second virial coefficients of
rigi4 poly-atomics have been written. Som~ µew formulae have been
derived for the 2 and 3 body 9 long range tensor interactions of
dipole polarizable quadrupolar molecules. A study has been made of
the influence of these and other forces upon the structure and
sublimation energy of crystalline benzene. A paper has been
completed describing the general nature of transport processes in
isotropic polyatomic fluids. In this paper Kubo expressions are
derived for the generalized transport coefficients defined in the
paper.
Work is just beginning on the numerical evaluation of transport
coefficients of dilute polyatomics and on the determination of the
macroscopic equilibriu:u:: properties of liquid benzene.
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PUBLICATIONS
Evans, D.J. and Watts ~ R.0. 1 "Water-water pair interactions
and the s9cond virial coefficient of steam11
Molecular Physics, 2e. 1233-1240 (1974)
Evans, D.J. and Watts, R.o. 1 "Benzene-benzene interactions. I
11
Molecular Physics. 28 . 0000 (1974)
Harris, K.R •• i'fills , R. , Hanley, H.J. M. 2 and Woolf , L.A.
nThe self-diffusion of simple fluids Tabulated values for a rgon
and methane"
DRU Report RR2 (ANU Press), 1-32 (1974)
Hertz, H.G. 3• Holz, M.3 and Mills, R. 11The effect of structure
on ion self-diffusion in concentrated electrolyte solutions 11
Journal de Chimie Physique, Z!_, 0000 (1974)
Mills, R. "Self-diffusion measurements in simple and complex
liquids c'
Molecular Motions in Liquids. J. Lascombe (ed.) (D. Reidel
Publishing Co., Dordrccht). 391- 401 (1974)
J.."dlls, R. " Isotopic effects in liquid diffusion:'
Proceedings of the International Meeting on Isotope Effects in
Physical and Chemical Processes. R.V. Bucur (ed.) (Editura Dacia,
_Cluj) 0000 , (1974)
Woolf, L.A. "Self-diffusion in water to 2100 bar at 2s0 c11
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Journal of Chemical Physics, .§.!_, 1600-1601 (1974)
1. Member of the Computer Centre. 2. Based on work done while a
Visiting Research Worker 3. Not a member of this University
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