Photochemistry in Microheterogeneous Systems

394 Pages·1987·23.365 MB·English

by  K. Kalyanasundaram (Auth.)

Photochemistry
in Microheterogeneous
Systems
K. KALYANASUNDARAM
Institute of Physical Chemistry
Swiss Federal Institute of Technology (EPFL)
Lausanne, Switzerland
1987
ACADEMIC PRESS, INC.
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United Kingdom Edition published by
ACADEMIC PRESS INC. (LONDON) LTD.
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Library of Congress Cataloging in Publication Data
Kalyanasundaram, K.
Photochemistry in microheterogeneous systems.
Includes index.
1. Photochemistry. 2. Microchemistry. I. Title.
QD715.K35 1986 541.3*5 86-10841
ISBN 0-12-394995-5 (alk. paper)
PRINTED IN THE UNITED STATES OF AMERICA
98765432 1
Foreword
The subjects of this monograph have a long history, longer than that of
chemistry itself. They are the basis of many familiar things such as foods
and soaps and are at the heart of living matter. They used to form part of
the discipline called colloid chemistry, which was concerned mainly with
their gross physicochemical and thermodynamic properties rather than with
their structure at the molecular level, although there were notable excep­
tions among which is the work of Irving Langmuir on monomolecular
films, which must be regarded as having pioneered the whole field.
Over the last one or two decades the study of these substances has
flourished to such an extent that it is only a small exaggeration to say that a
new kind of chemistry has been born. It is the chemistry of aggregates of
molecules, often very large numbers of them, in which the structure of the
molecules, along with the solvent or other surrounding medium, determines
the structure of the aggregate. But in spite of the rather large number of
molecules that it contains, the aggregate is still small, usually of colloidal
size, hence the name "microheterogeneous."
Photochemistry and its associated spectroscopic and optical observations
are not readily carried out in normal macroscopic heterogeneous systems
where light scattering makes quantitative measurements very difficult.
Microheterogeneous systems on the other hand, whose particles are usually
much smaller than the wavelength of light, form clear solutions having no
scattering problems. Furthermore, the study of the photophysical proper­
ties of colored molecules incorporated into a microheterogeneous system
often provides valuable structural information about the aggregate, indeed
this may be the only method available because the aggregates often lie in an
awkward size range and cannot be removed from their environment.
ix
Foreword x
There were several reasons for the recent growth of interest in the
microheterogeneous systems and their photochemistry in particular. First
and most important, it has turned out to be extremely interesting scien­
tifically because of the way that aggregates can be designed, on the basis of
the known structure of their individual components, to have a varied range
of structures, properties, and shapes. Second, new phenomena of great
potential practical importance have been discovered, such as the optical
properties of liquid crystals and the catalytic properties of zeolites and
polyelectrolytes. Third, the great interest in photochemical methods for the
storage of solar energy opened up paniculate absorbers as photoelec-
trochemical contenders. Last, and now the main stimulus, is the realization
of the overwhelming importance of lipid membrane and vesicular structures
in all biological systems. The supreme example of photochemistry in
microheterogeneous systems is, of course, the photochemistry of the
photosynthetic unit.
Dr. Kalyanasundaram has a wide experience of most aspects of this new
field, having worked in several of the principal laboratories and made some
notable contributions himself. This monograph provides an extensive
review of the large amount of recent work in these diverse topics, but it is
very readable and will be found interesting and useful to all who wish to
learn of the rapid progress in a fascinating area of science.
SIR GEORGE PORTER P.R.S.
Preface
As new instruments become available and as newer, more sophisticated
techniques are created, we witness their novel application to increasingly
complex chemical and biological systems. The combined efforts of scientists
with very different backgrounds involved in these novel applications drive
the evolution of new disciplines. The topic of this monograph concerns one
such area, namely, application of photophysical and photochemical pro­
cesses and techniques to the study of various microheterogeneous systems
of chemical and biological interest.
This short monograph was written to provide an introduction to the sub­
ject of photochemistry in microheterogeneous systems for the student at the
graduate level and to review the recent, significant developments in the field
for the practicing chemist. It should be equally useful to those who intend to
broaden their research in this new and exciting field. The systems con­
sidered are of interest and utility to those in a wide spectrum of research in
specialized fields from chemistry to biology: colloids, interfaces, catalysis,
kinetics, polymers, biomembranes, photochemistry, and photobiology, to
name a few.
There are two potential approaches that we can take in discussing the
photochemistry in microheterogeneous systems (MHS): We can consider
each photophysical and chemical process separately and discuss its occur­
rence and applications to different forms of the MHS, or we can choose a
certain type of MHS and outline how the existing knowledge of the systems
and photoprocesses can profitably be employed to gain a better understand­
ing of the systems and processes. We choose this latter approach for three
principal reasons: (1) each system is unique in having different static and
xi
Preface xii
dynamic properties; (2) systems of increasing complexity are readily handled
as extensions in a logical manner (chronologically the evolution of the subject
has been on these lines!); and (3) most researchers' interests lie on one or
more types of the MHS.
We consider a variety of simple, organized systems that are structurally
well characterized. They are "microheterogeneous" in that they are hetero­
geneous at the microscopic level with the presence of charged interfaces in
hydrophilic or hydrophobic domains. In all of them there is some kind of
self-organization and order that we want to exploit. The motivations of
these studies are numerous, but we can single out two main, complementary
ones: (1) to use the existing knowledge of the photophysical and
photochemical processes to probe the dynamic and static properties of these
organized systems and (2) to use the information available about these
systems to study excited-state, molecular processes under novel microen-
vironments. Success in both of these areas has been phenomenal and is
growing all the time.
Partially reflecting the early origin and maturity of aggregated systems
composed of surfactants and lipids, a major portion of the book deals with
normal and inverted micelles, vesicles and liposomes, monolayers, black
lipid membranes, and liquid crystalline solvents. This is followed by over­
views of newer topics of current research with organic and inorganic
polymers, e.g., neutral and ionic polymers, polyelectrolytes, ion-exchange
membranes, polyaluminosilicates such as zeolites and clays, poly sugar s
such as cyclodextrins, polyethers such as crown ethers and crpytands, and
oxides such as alumina and silica.
It is a pleasure to express my gratitude to the many people who helped me
directly and indirectly. Sincere thanks go to the "trio" who induced
research interests in the topic of the book and encouraged my participation:
Professors J. K. Thomas, Sir G. Porter, and M. Grätzel. Special thanks are
also due to Professors D. G. Whitten, N. J. Turro, J. H. Fendler, and Dr.
R. Humphry-Baker for undertaking the onerous task of reading the manu­
script and offering invaluable comments and suggestions. Professor Harry
B. Gray richly deserves my gratitude and appreciation for providing a very
pleasant and stimulating environment for a major portion of this book to
evolve in sunny California. Finally, I would like to thank my wife Uthira
for willingly foregoing several evenings and weekends, which rightfully
belonged to her, and for her assistance in typing several versions of the
manuscript. This book is dedicated to our parents who cared and suffered
so much to give us an excellent education.
Chapter 1
Introduction
This monograph is concerned with studies of unimolecular and bimol-
ecular reactions of electronically excited molecules in nonhomogeneous
media. There are two major goals for these studies: to use the existing
knowledge on the photophysics and photochemical processes to probe the
static and dynamic properties of a wide variety of organized microhetero-
geneous systems (systems of chemical, industrial, and biological interest) and
conversely to use the known information on these systems to examine the
excited-state processes under novel environments. The goal of this work is to
illustrate the enormous progress that has been made in recent years in both of
these areas.
The complementary nature of these two goals requires some knowledge
in each of these areas. In order to provide the necessary background so that
the present monograph is self-supporting and to set the stage for detailed
discussions on the photochemistry in microheterogeneous systems per se,
we briefly overview each of these areas. Readers who desire more elabo­
rate discussions on the fundamentals or finer details on these background
areas are well advised to consult comprehensive texts in these areas
(photochemistry,1-9 microheterogeneous systems10-56). As systems become
increasingly complex, no single technique can provide all the answers and even
unambiguously resolve different processes. There are numerous physical or
chemical methods that we can utilize to supplement. Techniques such as
magnetic resonance (NMR, ESR), diffraction methods (X-ray, neutron),
electron microscopy, electrical (EMF, conductance, capacitance) and pulsed
methods such as stopped flow, temperature-jump, pressure-jump, and ultra­
sonic relaxation are to mention a few. Often it is imperative that we use the
1
2 1 Introduction
information derived from one technique to gain insight into the results from
another.
1.1 Micro heterogeneous Systems, an Overview
The microheterogeneous systems that we will consider are numerous, and
they can be broadly classified into two major types: molecular aggregates
composed of surfactants or lipids and organic or inorganic polymeric systems
and supports. Figure 1.1 presents schematically a broad subclassification of
these systems as they are discussed in subsequent chapters. Organized
molecular assemblies such as micelles, vesicles, microemulsions, and others
have been quite intensely studied in the past decade. The field has largely
matured, and the available photochemistry literature is fairly extensive.
Micellar systems are often used as simpler model systems to study and
understand larger, more complex aggregates. Consequently, a good pro­
portion of our discussion concerns these systems, especially the simpler
micellar systems. Herein we broadly survey the general features of various
forms of microheterogeneous systems that we shall be dealing with. Later, as a
prelude to the discussion of photochemistry, we will elaborate further on
various structural and dynamical properties of interest in each chapter.
A few remarks on the usage of the term microheterogeneous are pertinent
here. The systems under consideration are "heterogeneous" at the "micro­
scopic" level. The implications are numerous. The solute distribution can be
inhomogeneous throughout the entire volume of the solution/aggregate.
There may be hydrophobic or hydrophilic cavities/cages/pockets/
pools/pores that can sequester (or eliminate) the solutes. There may be
charged interfaces where electrostatic effects can play a dominant role in
influencing the solute distribution and their reactions. The systems or the
process of dissolution of solutes in their interiors can be dynamic such as
the continuous dissolution and reformation of the aggregates, entry, and
exit of the solutes so that the solutes can experience some time-averaged
effects due to the aggregates. It is important to note, however, that a good
majority of these systems provide optically transparent (nonturbid) solu­
tions readily amenable to photochemical investigations by steady-state and
pulsed photolysis methods.10
Surfactant and lipid molecules with one or more long alkyl chains (with at
least six méthylène units) and a polar headgroup are called amphiphathic
molecules:
-^^-^-^^^^^^^^^^ X = headgroup (NMe OSO3, etc.)
3>
/^\^\^s^\^v^—^y X ^^-\^ = hydrocarbon tail —(CH )
2 n
ω
CO ο
(/)
*" ω
ωΩ ο.
CO
ο(Λ .υc
< Linear and Cyclic Polyethers
(Crown Ethers and Cryptands) [Chapter 9]
Linear and Cyclic Polysugars
(Cyclodextrins, Starch, etc.) [Chapters 9-10]
<Λ
Ε ο
ω Linear and Cyclic Polysilicates
<>/ )0 0I (Clays, Zeolites, etc.) [Chapters 9-10]
υ 0)
Φh π03
>.- C
<■>■ 1 ) Ion Exchange Membranes
CL·
[Chapter 8]
Neutral Polymers and Polyelectrolytes
[Chapter 8]
Monolayers, Black Lipid Membranes, and
Liquid Crystalline Solvents [Chapter 7]
V)
E
CD
<>/) Surfactant, Lipid Vesicles, and Liposomes
(ΌCΛD Ϊ**· [Chapter 6]
CO ΓΝ
σ>
ω
ω
< Q.
j_ -CCO Inverse Micelles and Microemulsions
CO (> [Chapter 5]
3
υ
ω
°
\Έ
Normal Micelles
[Chapters 2-4]
4 1 Introduction
Their unique structure confers in them both hydrophobic and hydrophilic
properties. This gives them the fundamental property to form association
structures of différent types, some of which are shown in Fig. 1.2. Depending
on the nature and length of the hydrocarbon chain, the headgroup, concentra­
tion, temperature, and other additives, the size (aggregation number) and
shape of these aggregates may vary. The existence and boundaries sepa­
rating different phases (forms) of the aggregates are best derived from
studies of phase diagrams such as those shown in Figs. 1.3 and 1.4. Detailed
discussions on the features of these phase diagrams and their roles are
beyond the scope of this monograph. However, for those who envisage
working with any of these organized assemblies, it is advisable that they
carefully examine (or at least be aware of!) the phase diagrams for the systems
of interest. During the studies of one particular system, variations in the
surfactant/lipid concentration, temperature, etc. can lead to crossing of phase
boundaries and drastic changes in the size and shape of the molecular
aggregates under investigation.
The simplest systems to consider are those consisting of just two com­
ponents, a surfactant or lipid dispersed in water. Single-chain surfactants
monolayer spherical rod-like
micelle micelle
micelle w/o microemulsion o/w microemulsion
multicompartment vesicle
FIG. 1.2. Oversimplified representation of various forms of aggregated systems composed
of surfactants and lipids. [From Fendler.10 Copyright ©1982 John Wiley and Sons, Inc.]