Zeta Potential in Colloid Science. Principles and Applications

391 Pages·1981·10.258 MB·English

by  Robert J. Hunter, R. H. Ottewill and R. L. Rowell (Auth.)

COLLOID SCIENCE
Editors
R. H. Ottewill and R. L. Rowell
In recent years colloid science has developed rapidly and now frequently in­
volves both sophisticated mathematical theories and advanced experimental
techniques. However, many of the applications in this field require simple
ideas and simple measurements. The breadth and the interdisciplinary nature
of the subject have made it virtually impossible for a single individual to
distill the subject for all to understand. The need for understanding suggests
that the approach to an interdisciplinary subject should be through the per­
spectives gained by individuals.
The series consists of separate monographs, each written by a single author
or by collaborating authors. It is the aim that each book will be written at a
research level but will be readable by the average graduate student in chemi­
stry, chemical engineering or physics. Theory, experiment and methodology,
where necessary, are arranged to stress clarity so that the reader may gain
in understanding, insight and predictive capability. It is hoped that this
approach will also make the volumes useful for non-specialists and advanced
undergraduates.
The author's role is regarded as paramount, and emphasis is placed on
individual interpretation rather than on collecting together specialist articles.
The editors simply regard themselves as initiators and catalysts.
1. J. Mahanty and B. W. Ninham: Dispersion Forces.
2. Robert J. Hunter: Zeta Potential in Colloid Science.
3. D. M. Napper: Polymeric Stabilisation of Colloidal Dispersions.
ZETA POTENTIAL IN
COLLOID SCIENCE
Principles and Applications
ROBERT J. HUNTER
School of Chemistry, University of Sydney
Sydney, New South Wales
Australia
ACADEMIC PRESS
Harcourt Brace Jovanovich, Publishers
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Copyright © 1981 by
ACADEMIC PRESS LIMITED
Second printing 1986
Third printing (new paperback edition) 1988
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No part of this book may be reproduced in any form by photostat, microfilm, or any other means,
without written permission from the publishers.
British Library Cataloguing in Publication Data
Hunter, R. J.
Zeta potential in colloid science. - (Colloid
science, ISSN 0305-9723)
1. Colloids
I. Title. II. Series
541.3451 QD549 80-42268
ISBN 0-12-361960-2
ISBN 0-12-361961-0 (pbk)
Typeset by Kelmscott Press Ltd., 30 New Bridge Street, London EC4
Printed in Great Britain by the Alden Press, Osney Mead, Oxford
Preface
The concept of the zeta potential once occupied a pre-eminent place in col­
loid chemical theory but was for a long time under a cloud. More recently,
the availability of reproducible colloid systems and an integrated approach
to the study of surface properties have given us more confidence in its value
as a characterizing parameter. I hope this work will go some way towards re­
habilitating the concept and improving its usefulness. At least it should make
clear where the gaps in our understanding are widest.
The book was conceived in the (northern) winter of 1973-74 when I was on
study leave at Bristol University. Professor Ron Ottewil lhad assembled there
a group of visiting professors including Robert Rowell from Massachusetts
and Robert Fisk from Lehigh. We had all recognized for some time the need
for a modern treatment of colloid science, as no definitive work had appeared
since the two-volume work of Kruyt, written almost entirely by Professor
Theo Overbeek. Although several attempts had been made to provide a
fitting successor, it was by this time conceded that it was unlikely that anyone
who was sufficiently active in colloids research to write an authoritative text
would be able to find enough time to do so. It was therefore decided that the
alternative of a series of separate contributions in well-defined areas would
go some way towards meeting the need for a coherent treatment o fthe sub­
ject, whilst limiting the burden placed on each of the authors. Professors
Ottewill and Rowell agreed to act as editors of the series and Academic Press
agreed to publish it.
I had hoped to secure the assistance of my good friend and colleague Dr
(now Professor) Tom Healy to produce a work which would cover the
theory, measurement, interpretation and applications of electrokinetic tech­
niques in colloid science. Tom was very enthusiastic about the project, and
over the ensuing few years, as my introductory chapters on theory and
v
VI PREFACE
measurement grew and became more or less consolidated, he prepared ex­
tensive notes and preliminary drafts of some of the applications. Unfor­
tunately, the increasing demands placed on him by his accession to the Chair
of Physical Chemistry at the University of Melbourne made it impossible for
him to complete his contribution. In the (northern) winter of 1978-79,1 had
another study leave period, this time at the University of California at
Berkeley in the laboratories of Professor Douglas Fuerstenau, and there had
the opportunity of finishing off the manuscript, drawing on the notes which
Tom Healy had prepared and made available to me. I am very pleased to be
able to thank him for his generosity in that regard. At the same time, I had
the opportunity of discussing various ideas with Doug Fuerstenau and of
drawing on his extensive knowledge of the application of zeta potentials in
the study of mineral flotation and in his collection of reprints in the area. I
am indebted to him for his personal assistance and for the facilities he placed
at my disposal. In particular, I must express my deep appreciation to Mrs
Gloria Pelatowski of the Berkeley laboratory for preparing most of the
illustrations.
I am also indebted to Professors Ottewill and Rowell, who read the manu­
script and have offered many valuable comments. I have tried to incorporate
all of their suggestions but must, of course, take responsibility for any re­
maining inconsistencies and misconceptions. Various parts of the manuscript
were also read and corrected by Professor Bill Rüssel of Princeton, Dr Dirk
Stigter of the U.S.D.A. California, and Dr Neil Furlong of the University of
Melbourne, and I thank them for their help.
Soon after I began this work, there appeared a book called Electrokinetic
Phenomena by S. S. Dukhin and B. V. Deryaguin, in the series on Surface
and Colloid Chemistry, edited by Professor Egon Matijevic. The appearance
of a book in the same area by two such distinguished contributors caused us
to think very carefully about whether we should proceed, but after examining
the thrust of that work and noting the areas to which it had not addressed
itself we decided to press ahead. Since then I have had the opportunity of
showing some of my material to Professor Deryaguin and have been able to
take advantage of his valuable comments.
Finally I thank the several typists who prepared various parts of the
manuscript and the people at Academic Press for their continued interest in
the work and their expedition in its publication.
April, 1981 Robert J. Hunter
Chapter 1
Introduction
1.1 Origin and classification of electrokinetic effects
When two phases are placed in contact there develops, in general, a difference
in potential between them. If one of the phases is a polar liquid, like water, its
(dipolar) molecules will tend to be oriented in a particular direction at the
interface and this will generate a potential difference. If there are ions or
excess electrons in one or both phases, or ionogenic groups present, there will
be a tendency for the electric charges to distribute themselves in a non-
uniform way at the interface. The reasons for this behaviour and the nature
of the resulting distribution will be discussed below (Chapter 2). For the
present we need only note that except under very special conditions, the
region between two adjoining phases is always marked by a separation of
electric charges so that near to or on the surface of phase I there is an excess
of charge of one sign and the balancing charge is distributed in some way
through the adjoining surface regions of phase II (Fig. 1.1.).
It would be difficult to overestimate the importance of this process, be­
cause it is basic to an understanding of an enormous variety of natural
phenomena, particularly in the fields of colloid chemistry and electro­
chemistry. Phenomena such as electrode kinetics, electrocatalysis, corrosion,
adsorption, crystal growth, colloid stability and flow behaviour (both of
colloidal suspensions and of electrolytes through porous media) cannot be
properly treated without a knowledge of the distribution of charges and
dipoles in the interfacial region.
If the surface of phase I is positively charged, its electrostatic potential will
be positive with respect to the bulk of phase II ; if phase II is a liquid contain­
ing dissolved ions, then as one moves into phase II, the potential will decrease,
more or less regularly, until it becomes constant in the bulk liquid far from
1
2 ZETA POTENTIAL IN COLLOID SCIENCE
Phase I
Phase H
Fig. LI. Possible distribution of charges at an interface between two phases. Only the excess
charges in each phase are shown.
the surface of phase I. It is customary to take this constant potential in the
bulk of one of the phases (usually a liquid) as the reference or zero potential.
For aqueous systems, with which we will be principally concerned, the words
"far from" mean "at distances greater than about 5-200 nm (depending on
the electrolyte concentration)" (Fig. 1.2). The region where the liquid has a
positive electrostatic potential will accumulate an excess of negative ions
and repel positive ions of the electrolyte. It is this excess of negative ions
which gradually lowers the electrostatic potential (and the electric field) to
zero in the bulk electrolyte. The arrangement of (positive) charges on the
surface of phase I and the charges in the liquid phase II is referred to as the
electrical double layer at the interface.
When one of these phases is caused to move tangentially past the second
phase there are observed a number of phenomena which are grouped under
the title of "electrokinetic effects'". There are four distinct effects depending on
the way in which motion is induced. They are: electrophoresis, electro-
osmosis, streaming potential and sedimentation potential.
(a) Electrophoresis
If one phase consists of a liquid or gas in which the second phase is suspended
as particles of solid or liquid, then the particles can be induced to move by
Fig. 1.2. A possible electrostatic potential distribution in the liquid (phase 11) near a solid
surface. The point P would be at a distance of about l-50nm in most colloid systems of interest.
1. INTRODUCTION 3
Fig. 1.3. Electrophoresis. The motion of the particles can be followed by observing the move­
ment of the boundary between the cloudy suspension and the clear supernatant.
applying an electric field across the system (Fig. 1.3). This is called electro­
phoresis. Measurement of the velocity of the particles under a known external
field gives information about their net electric charge, or their surface
potential with respect to the bulk of the suspending phase.
(b) Electro-osmosis
When the solid remains stationary and the liquid moves in response to an
applied electric field this is called electro-osmosis. It occurs when the solid
is in the form of a capillary or a porous plug which is filled with the liquid.
The applied field acts upon the charges (usually ions) in the liquid, and as
they move in response to the field they drag the liquid along with them.
Measurement of the velocity of the liquid, or the volume of liquid transported
per unit current flow, again gives information about the net surface charge
or the electrical potential in the neighbourhood of the wall.
(c) Streaming potential
Instead of applying an electric field to cause liquid to move through a
capillary or porous plug, one can force the liquid through under a pressure
gradient. The excess charges near the wall are carried along by the liquid and
their accumulation down-stream causes the build-up of an electric field which
drives an electric current back (by ionic conduction through the liquid)
4 ZETA POTENTIAL IN COLLOID SCIENCE
against the direction of the liquid flow. A steady state is quickly established,
and the measured potential difference across the capillary or plug is called
the streaming potential. It is related to the driving pressure and to the potential
in the neighbourhood of the wall.
(d) Sedimentation potential
When charged colloidal particles are allowed to settle (or rise) through a
fluid under gravity or in a centrifugal field, a potential difference is generated ;
this is the sedimentation potential. Since each particle is usually surrounded
by a balancing atmosphere of opposite charge it might be expected that this
movement would not lead to a potential difference. As it moves, however,
the particle leaves behind its atmosphere and a new one is continuously
established by a flow of charge into one side and out of the other (Fig. 1.4).
Negative particles set up a field which is negative in the direction of their
motion and the steady state is established by a backflow of positive ions.
These phenomena have been studied for a very long time. The first obser­
vations of electro-osmosis occurred in the beginning of the nineteenth
century, not long after systematic studies of electricity first became possible.
(For a brief review of this early history see Dukhin and Deryaguin (1974),
Chapter 1.)
Fig. 14. Schematic picture of the current flow which generates the sedimentation potential.
1.2 The zeta potential and the surface of shear
In almost all electrokinetic phenomena a fluid moves with respect to a solid
surface. (An exception is the electrophoresis of emulsions.) For the most
part we shall be concerned with determining the relation between the velocity
of the fluid (which will generally vary with distance from the solid) and the
electric field in the interphase region. The electric field will be partly deter-
1. INTRODUCTION 5
mined by the surface charges on the solid and in the liquid but may also
include an externally imposed field, either generated deliberately by the
experimenter (electro-osmosis and electrophoresis) or arising out of the
motion of particles (sedimentation potential) or ions (streaming potential).
The relation between the potential (or the electric field) at any point and
the number of charges is given by Poisson's equation (Appendix 1.4). The
charges themselves will respond to three sorts of forces :
(i) the electrical potential ;
(ii) the diffusion force, tending to smooth out concentration variations;
(iii) the bulk movement of charge carried along by the flow of the liquid
(convective transport).
At the same time the liquid itself is subjected at each point to forces caused
by pressure gradients in the system and the electrical charges it contains, as
well as shear forces induced by neighbouring parcels of liquid moving with
different velocities.
Even with the powerful tools of vector calculus and high-speed computer
solution of the resulting differential equations it is still necessary to make a
number of significant simplifications and to treat, at least in the first instance,
highly idealized models of the real experimental systems. Nevertheless, a
great deal of interesting and valuable information can now be obtained from
electrokinetic measurements.
Theoretical treatments generally assume that the solid is either a sphere, a
cylinder, or a large flat plate; more rarely it may be a disc or ellipsoid. The
liquid is assumed to be Newtonian (i.e. its viscosity does not depend on shear
rate (see Appendix 3) and moving sufficiently slowly so that turbulence and
other non-linear effects are absent.
The most important concept which is introduced is that of the surface of
shear. This is an imaginary surface which is considered to lie close to the
solid surface and within which the fluid is stationary. In the case of a particle
undergoing electrophoresis, the surface of shear forms a sheath which en­
velopes the particle. All of the material inside that sheath forms the kinetic
unit so that the particle moves along with a certain quantity of the surrounding
liquid and its contained charge. Measurement of the electrophoretic mobility
(i.e. the velocity per unit electric field) therefore gives a measure of the net
charge on the solid particle.
The analysis of the forces on the solid or the liquid can be carried out in
terms of either charge or electrostatic potential. In the latter case one calcu­
lates the average potential in the surface of shear; this is called the electro-
kinetic or zeta potential, and is universally given the Greek symbol, zeta (ζ).
At the microscopic level it may be inferred from the finite dimensions of
ions and solvent molecules that the real slipping surface is likely to be a