Micelles, Membranes, Microemulsions, and Monolayers

619 Pages·1994·22.713 MB·English

by  Avinoam Ben-Shaul (auth.), William M. Gelbart, Avinoam Ben-Shaul, Didier Roux (eds.)

Partially Ordered Systems
Editorial Board:
L. Lam . D. Langevin
Advisory Board:
J. Charvolin . W. Helfrich . P .A. Lee
J.D. Litster . D.R. Nelson· M. Schadt
Partially Ordered Systems
Editorial Board: L. Lam • D. Langevin
Solitons in Liquid Crystals
Lui Lam and Jacques Prost, Editors
Bond-Orientational Order in Condensed Matter Systems
Katherine J. Strandburg, Editor
Diffraction Optics of Complex-Structured Periodic Media
V.A. Be1yakov
Fluctuational Effects in the Dynamics of Liquid Crystals
E.!. Kats and V.V. Lebedev
Nuclear Magnetic Resonance of Liquid Crystals
Ronald Y. Dong
Electrooptic Effects in Liquid Crystal Materials
L.M. Blinov and V.G. Chigrinov
Liquid Crystalline and Mesomorphic Polymers
Valery P. Shibaev and Lui Lam, Editors
Micelles, Membranes, Microemulsions, and Monolayers
William M. Gelbart, Avinoam Ben-Shaul, and Didier Roux, Editors
William M. Gelbart Avinoam Ben-Shaul
Didier Roux Editors
Micelles, Membranes,
Microemulsions, and
Monolayers
With 220 Illustrations
Springer-Verlag
New York Berlin Heidelberg London Paris
Tokyo Hong Kong Barcelona Budapest
William M. Gelbart Avinoam Ben-Shaul Didier Roux
Department of Chemistry Department of Physical CRPP
and Biochemistry Chemistry Domaine Universitaire
University of California Hebrew University 33405 Talence Cedex
405 Hilgard Avenue 91904 Jerusalem France
Los Angeles, CA 90024-1569 Israel
USA
Editorial Board:
Lui Lam Dominique Langevin
Department of Physics Laboratoire de Physique ENS
San Jose State University 24 Rue Lhomond
One Washington Square F-75231 Paris, Cedex05
San Jose, CA 95192-0106 France
USA
Advisory Board:
Jean Charvolin Wolfgang Helfrich Patrick A. Lee
Institut Max von Laue-Paul Freie Universitat Berlin Massachusetts Institute of
Langevin Technology
John D. Litster David R. Nelson Martin Schadt
Massachusetts Institute of Harvard University F. Hoffman-La Roche
Technology &Co.
Library of Congress Cataloging-in-Publication Data
Micelles, membranes, microemulsions, and monolayers / [edited by] William M.
Gelbart, Avinoam Ben-Shaul, Didier Roux.
p. cm. - (Partially ordered systems)
Includes bibliographical references and index.
ISBN-13:978-1-4613-8391-8
1. Surface active agents. 2. Micelles. 3. Emulsions.
I. Gelbart, W. (William) II. Ben-Shaul, A. (Avinoam) III. Roux, D. (Didier)
IV. Series.
TP994.M53 1994
541.3'3-dc20 94-15496
Printed on acid-free paper.
©1994 Springer-Verlag New York, Inc.
Softcover reprint of the hardcover 1st edition 1994
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Production managed by Hal Henglein; manufacturing supervised by Vincent Scelta.
Camera-ready copy prepared from the editors' TeX files.
987654321
ISBN-13:978-1-4613-8391-8 e-ISBN-13:978-1-4613-8389-5
DOl: 10.1007/978-1-4613-8389-5
Preface
Over the course of the past one to two decades, the study of surfactant
solutions has been profoundly transformed by a dramatic infusion of new
ideas and techniques from Chemistry, Physics, and Materials Science. This
renaissance in fundamental research activity has been sparked largely by
the many connections and analogies that have been established between
micellar phases and micro emulsions on the one hand and polymer sys
tems and interfacial films on the other. Consequently, many otherwise in
tractable conceptual and technical problems arising in the self-assembly
area have now become feasible to study because of theoretical and exper
imental breakthroughs in the general field of complex fluids. For example,
the theory of critical phenomena and polymer structure/dynamics (includ
ing scaling and renormalization group ideas) has been especially useful, as
have new high-resolution scattering and magnetic resonance spectroscopies.
The purpose of this book is to develop a systematic account of the ex
citing developments referred to above. Part of our effort is devoted to pro
viding a general introduction to the broad range of phenomena involved
and part to offering a critical consensus of what is presently understood
and what is not. While the book consists of twelve chapters by different
authors, we have specifically edited them so that they reinforce one an
other in content, format, and notation. Almost every page of each chapter
contains an explicit reference to related sections of other chapters. A single
subject index at the back of the book refers to all chapters simultaneously.
In the remainder of this Preface we make some general remarks about the
physical systems and problems involved, and about how they are treated
in the present monograph.
First, though, a few words about the evolution of this particular volume.
For several years, the editors had been encouraged to write a book that
would go beyond the usual collections of disjointed articles and compendia
of conference proceedings, etc. These latter types of volumes, after all, do
not provide the "uninitiated" but interested reader with a sufficiently in
cisive or critical introduction to the field. Often, in explaining our work to
colleagues and visiting researchers in different areas of physical chemistry
and condensed matter science, we have been asked for instructive references
where they might follow up on these discussions. In response, we came up
with the idea of an edited volume, where the chapters would be written
by different experts in the various subfields but where each contribution
would be revised to complement and dovetail with all of the others, i.e.,
each author would have to agree beforehand to have their essay substan-
vi Preface
tively edited with this end in mind. While we are confident that none of
them has regrets in having assigned us these rights, we are also sure that
they will never forgive us for having taken so long before we buckled down
and completed this time-consuming job. Since many chapters were con
tributed for the first time as early as 1987 and 1988, and since several of
them were written by colleagues from France, we thought it appropriate
that the volume should appear in 1989, in time to celebrate the bicente
nary of the French Revolution. When 1993 came along and we still hadn't
found sufficient time for revising and transcribing all the texts, we decided
to commemorate instead the notorious Terror of 1794. The authors have
graciously managed to continue to speak to us over the past few years and
to humor us in our belief that the long delays involved might indeed make
the subject material more timeless, even if less timely. In most cases, refer
ences have been updated and paragaraphs added at the appropriate places
to follow up on the earlier work described and to apprise the reader of some
most recent developments.
A surfactant, or amphiphile-"loving" ("philo") "both" ("amphi")
molecule is made of two parts that have opposing natures: one is water
soluble (hydrophilic) and the other is oil soluble (hydrophobic). The hy
drophilic and hydrophobic parts of the surfactant are linked together by
a chemical bond and consequently cannot phase separate as they would if
the two parts were free. When such molecules are put in water they prefer
the (water/air) surface, where the hydrophilic "heads" and hydrophobic
"tails" lie, respectively, in and out of the water. Indeed, for a small amount
of surfactant, they practically all lie at the interface. As a consequence,
the (liquid-vapor) surface tension decreases as the concentration of surfac
tant increases. Then, at a certain concentration (the Critical Micelle Con
centration, or "CMC"), the surface tension levels off and remains nearly
constant. Careful study of this phenomenon confirms that the added sur
factant molecules no longer go preferentially to the surface but rather go
into solution in the bulk of the aqueous phase. There the molecules orga
nize as small aggregates-micelles-which are often globular in shape, the
tails comprising the interior and the heads coating the surface.
In both the very low concentration regime, where most of the added
molecules are at the surface, and in the higher concentration case, where
aggregates are formed, the physical phenomenon responsible for such be
havior is referred to as the hydrophobic effect and is due to a subtle balance
between intermolecular energies and entropies. (This is the same hydropho
bic effect that underlies the (classic) immiscibility of oil and water.) For
concentrations of surfactant that greatly exceed the CMC, there is a negli
gible number of molecules that sit at the surface or remain as monomers in
solution. The aggregated molecules, on the other hand, reveal themselves in
a large variety of structures. Indeed, upon increasing further the concentra
tion of surfactant, long-range ordered phases may appear, such as lamellar
or hexagonal states: these phases are commonly classified under the name
Preface vii
of lyotropic liquid crystals. Furthermore, adding oil to micellized surfactant
solutions leads to many different structures and phases of microemulsion.
The industrial applications of amphiphilic molecules were recognized very
early. The cleaning power of soaps is probably one of the oldest and best
known exploitations. But the practical uses of amphiphilic species have
increased dramatically in recent times, including a variety of important
applications in the pharmaceutical, cosmetics, and oil industries. Perhaps
most spectacular of all will be the use of surfactant layers (as Langmuir
Blodgett films) in the fabrication of new optical and electronic devices.
With compelling impetus from these developments, fundamental scientists
have also recognized the importance of studying surfactants in solution. A
large amount of work was devoted in the 1940s and 50s to understanding
the phase behavior and structural properties of many amphiphilic systems.
In addition, the discovery that biological membranes in living cells are inte
grally composed of lipids, which are basically "just" surfactant molecules,
has inspired many studies of bilayers (lamellae) as simple models for cell
membranes. In the 1960s, a remarkable series of x-ray studies (which even
now remain up to date) established the similarity of behavior between lipids
and "simple" amphiphiles and identified the different types of structures
that can be found in both biological and "ordinary" surfactant systems.
As already remarked, in the last decade or two, the field of surfactants
in solution has undergone profound changes. A good part of this rebirth is
due to the development of new understanding in the area of thermotropic
("neat") liquid crystals. Indeed, at the time in the early 1970s when the
modern era of liquid crystals was launched, lyotropic systems were actively
being ~nvestigated as a special class of these aligned fluids. The system
atic studies of micellar phase diagrams led to important discoveries such
as biaxial nematics (which to this day have not been discovered for ther
motropic systems), and in-plane defects in what had long been considered
as regular ("classical") lamellae. For obvious reasons, the main interest
of the liquid crystal community was focused on long-range ordered phases,
with scant attention paid to the isotropic solutions composed of small glob
ular ("spherical") micelles. The oil crisis in the 1970s, and the possibility
of using microemulsions for enhanced oil recovery, had (among many other
consequences) the effect of attracting the interest of an increasing number
of scientists to these fascinating disordered phases.
The many problematic definitions of microemulsions in the early liter
ature were a natural reflection of the scientific community's ignorance of
what they really were. Now that much more is known, having a precise
definition seems to be of less importance! We simply recall that for certain
types of surfactants, or combination of surfactants, it is possible to mix wa
ter and oil in all proportions with only a small amount of surfactant (just
a few percent in favorable cases). The thermodynamically stable phase ob
tained is liquid, isotropic, optically transparent, and is conveniently called
microemulsion. A great deal of work has been devoted to understanding
viii Preface
the structure and stability of such phases, and only now can a coherent
description of this state be given.
From all the work that has been done on the states of surfactants in
solution, there emerges a fundamental new concept: these phases are often
better described as phases of surfaces than as phases of particles. The ability
of surfactants in solution to aggregate can in fact be seen as the ability to
create surfaces in the bulk of the solution. This is true not only for the
lyotropic liquid crystal phases but even more so for the isotropic fluids
such as microemulsions. The statistical physics of fluctuating surfaces is a
field that is currently in initial but fast-moving stages of development: it is
too early to describe the full behavior of amphiphilic systems exclusively
in terms of phases of surfaces-but it is clearly the direction that will be
followed in the next few years. In pursuing this course, we will continue to
benefit from describing these phases by means of what is known and well
established in other fields, notably those of simple fluids, liquid crystals,
polymers, and quasi-two-dimensional systems.
In this spirit, we shall emphasize throughout this monograph both what
can be learned about amphiphilic systems via analogies with these other
areas and-still more importantly-what can not. We proceed, then, by
dividing the characteristic features of amphiphilic systems into two classes.
On the one hand, the properties of surfactant solutions correspond to well
understood behaviors of other systems but with a different range of phys
ical parameters. This is largely the case, say, for the existence of uniaxial
nematic phases (Chapter 3), the isotropic phase of interacting microemul
sion droplets that can be considered as an example of colloidal suspension
(Chapters 7 and 9), and the critical behavior of micellar and micro emulsion
systems (Chapter 11). But, on the other hand, entirely new concepts have
also to be developed. This is most notably the case for micellar growth in
dilute surfactant solutions (Chapters 1 and 2), size/alignment coupling in
micellar liquid crystals (Chapters 1 and 3), curvature frustration in systems
of parallel films (Chapter 4), fluctuations in dilute lamellar phases (Chap
ters 5 and 6), and bicontinuous states of microemulsions (Chapters 7-9).
Also, in systems of adsorbed surfactant monolayers, dramatically low in
terfacial tensions can occur (see Chapter 10), and the nature of the special
phase transitions that arise is due to the inter- (half 2-D/half 3-D) dimen
sionality of the interfacial film (Chapter 12). In these latter instances, we
are confronted by wholly new phenomena for which it is no longer sufficient
to make simple analogies with "ordinary" liquid crystal or polymer fluids.
We stress that the fundamental conceptual differences between micellar
solutions and "ordinary" colloidal suspensions do not arise only as special
cases involving extreme circumstances. Rather, they are often unavoidable,
dominating the observable properties under virtually all conditions. Even
in a dilute ("ideal solution") phase of micellar aggregates, for example,
new phenomena appear because the basic "particles" involved-micelles-
do not maintain their integrity as the concentration or temperature, say,
Preface ix
is varied. Instead, the equilibrium position of the exchange of molecules
between aggregates is shifted. That is, not only do we have more "parti
cles" as we add surfactant, but we also see a change in their average size.
The extent to which there are increases in the number of aggregates, versus
changes in their size, depends on the details of the surfactant and aqueous
solvent involved. At higher concentrations, where interactions between the
aggregates become important, the onset of long-range orientational and
positional order can be shown to enhance further the size of the micelles.
Clearly, we are dealing with a situation in which-unlike the ordinary col
loidal suspension-the experimentalist can only control the total number
of added molecules: the number of aggregates and hence the distribution of
sizes will be determined by the statistical thermodynamics of exchange and
the degree of long-range order.
After a long period during which the physics of amphiphilic systems
was considered as essentially descriptive, important breakthroughs in both
experimental and theoretical studies have made possible precise, quanti
tative accounts of many classes of these systems. On the experimental
side, it has become possible to probe directly the intramolecular struc
ture and dynamics of surfactants in aggregates. Selective deuteration and
relaxation spectroscopies, developed in the context of nuclear magnetic
resonance techniques, have been especially fruitful. Similarly, the overall
shapes and sizes of micelles-and the polymorphism and defect charac
teristics of their ordered phases-have been incisively investigated by a
concerted combination of static and dynamic light scattering, synchrotron
x-ray diffraction, and small angle neutron spectra. Novel fluorescence and
electron microscopies have also been developed and applied. On the the
oretical side, physicists and chemists have turned to amphiphilic systems
from the more "standard" areas of simple fluids, liquid crystals, polymers,
and thin films, bringing with them the powerful conceptual techniques of
many-body perturbation theories, continuum and scaling approaches, fluc
tuation and critical phenomena, symmetry analyses, and the statistical me
chanics of model hamiltonians. In this process, several classic problems and
phenomena-including ultra-low interfacial tensions and adsorbed mono
layer phase transitions-have finally been put on a firm conceptual footing.
There are basically two prevailing and complementary levels of phe
nomenological description of aggregates in solution. The first is couched in
terms of individual molecules and tries to deduce the structure and phase
behavior of their aggregates from the hamiltonian and free energy of such
molecules in an aqueous solvent. While in prinCiple such an approach could
proceed from detailed interaction potentials between each surfactant and
water molecule, followed by explicit molecular dynamics or Monte Carlo
simulations, in practice we must await several new generations of com
puter power before it will be feasible to carry out a priori calculations of
this kind with enough particles and for sufficiently long times. For example,
to describe the spontaneous formation of just a single micelle at low concen-
x Preface
trations (1O-5M, say), one needs to consider at least 106 water molecules
and calculate for as long as billions of picoseconds; furthermore, interac
tion potentials between all of the relevant surfactant and aqueous solution
species must be known to far better than current accuracy. Accordingly,
comprehensive approaches that start from the individual molecules have
necessarily been of the phenomenological kind in which spins on a lattice
are introduced, for example, with many-site interactions being used to keep
surfactant at the oil-water interface with its preferred curvature. By con
trast, the second level of description starts from the outset with already
formed aggregates of specified shapes. Here the focus is on the interface
between hydrophobic and hydrophilic regions, rather than on any single
molecular species. A free energy is associated directly with this interface,
featuring separately its bending (curvature) elasticity and its topological
entropy. The power of this continuum approach lies in its natural ease in
predicting the coupling between self-assembled (meso -scopic) structures
on the one hand, and phase transition (long -range) behavior on the other,
even as it gives up the possibility of describing structure and order on a
single molecule level.
The aim of the book is to present in a series of twelve chapters a care
fully chosen set of problems on which sufficient progress has been made to
provide agreed-upon starting points for further work in fundamental am
phiphilic science. We intend this monograph specifically to serve as an in
troduction for both young and established researchers who are interested in
moving into this "new" (renewed) and challenging field. As mentioned ear
lier, much effort has been devoted to cross-referencing the discussion, with
the intention of emphasizing the several common concepts that underlie
the broad range of physical effects covered. By emphasizing these central
themes we hope to focus further attention on the coupling of self-assembly
to thermodynamic and long-range ordering variables, the topology of de
fects, fluctuations, and structure of "inter"-dimensional (i.e., quasi 2D and
3D) systems, and phases of surfaces. All of these ideas playa crucial role
in understanding the basic physical properties of amphiphilic systems, and
point up the dramatic contrasts with "simple" fluids, "ordinary" liquid
crystals and colloidal suspensions, and "conventional" solid-state systems.