Table Of ContentA C S S Y M P O S I U M SERIES 447
Fourier Transform Infrared
Spectroscopy in
Colloid and Interface Science
David R. Scheuing, EDITOR
Clorox Technical Center
Developed from a symposium sponsored
by the Division of Colloid and Surface Chemistry
of the American Chemical Society
at the 199th National Meeting
Boston, Massachusetts, April 22-27, 1990
American Chemical Society, Washington, DC 1990
In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Library of Congress Cataloging-in-Publication Data
Fourier transform infrared spectroscopy in colloid and interface science
David R. Scheuing, editor
p. cm—(ACS Symposium Series, 0097-6156; 447)
"Developed from a symposium sponsored by the Division of Colloid
and Surface Chemistry at the 199th national meeting of the American
Chemical Society, Boston, Massachusetts, April 22-27, 1990."
Includes bibliographical references and index.
ISBN 0-8412-1895-1
1. Surface chemistry—Congresses
3. Infrared spectroscopy—Congresses
spectroscopy—Congresses. I. Scheuing, David R., 1952- .
II. American Chemical Society. Division of Colloid and Surface
Chemistry. III. Series
QD506.A1F68 1991
541.3'.3—dc20 90-22913
CIP
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In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
ACS Symposium Series
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In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Foreword
THE ACS SYMPOSIUM SERIES was founded in 1974 to provide
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However, verbatim reproductions of previously published
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In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Preface
IN THE PAST DECADE, IMPROVEMENTS IN infrared spectroscopic instru
mentation have contributed to significant advances in the traditional
analytical applications of the technique. Progress in the application of
Fourier transform infrared spectroscopy to physiochemical studies of col
loidal assemblies and interfaces has been more uneven, however. While
much Fourier transform infrared spectroscopic work has been generated
about the structure of lipi
able on the subjects o
adopted by synthetic surfactants in water. In the area of interfacial chem
istry, much of the infrared spectroscopic work, both on the adsorption of
polymers or proteins and on the adsorption of surfactants forming so
called "self-assembled" mono- and multilayers, has transpired only in the
last five years or so.
Vibrational spectroscopy, through its sensitivity to both intra- and
intermolecular interactions, is useful in studies of the "geometric" aspects
of molecular packing in both colloidal assemblies and adsorbed layers.
Interest remains high—judging from the current literature—in relating
the size and shape of surfactant and lipid molecules to those of micelles,
bilayers, and vesicles, and in the thermodynamics of colloidal assembly.
Increased understanding of the details of intermolecular interactions in
such systems will benefit in development of commercial products contain
ing such structures and in comprehension of naturally occurring struc
tures such as cell membranes. The interactions of polymers or proteins
with solid surfaces, which may be studied in situ with Fourier transform
infrared spectroscopy, affect areas as diverse as lubrication, corrosion, and
the development of medical implant devices.
The preparation of a book requires the cooperation of many people.
The enthusiastic response I received from both authors and reviewers was
especially gratifying, and eased my tasks considerably. I also wish to
express my gratitude to the Clorox Company, which provided financial
assistance in sponsoring the original symposium. Thanks are also due to
all my colleagues at Clorox, especially Jim Rathman and Jeff Weers, for
their moral support and encouragement in this project.
vii
In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Finally, I thank my wife and children for providing the luxury of time
and for their support for my work on this volume.
DAVID R. SCHEUING
Clorox Technical Center
Analytical Research and Services Department
Pleasanton, CA 94588
September 14, 1990
viii
In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Chapter 1
Fourier Transform Infrared Spectroscopy
in Colloid and Interface Science
An Overview
David R. Scheuing
Clorox Technical Center, 7200 Johnson Drive, Pleasanton, CA 94588
This chapter reviews the wide range of colloidal systems amenable to
investigation by FT - IR spectroscopy. Molecular level information
about the interactions of amphiphilic substances in aggregates such as
micelles, bilayers, and gels can be obtained and related to the
appearance and stability of the various phases exhibited. The
interactions of polymers, surfactants and proteins with interfaces,
which substantially modify the solid - liquid or liquid - air interface
in many important industrial and natural processes, can also be
monitored using FT - IR.
Several themes of the application of FT-IR to studies of colloidal particles and
interfaces have appeared over the last decade or so. The purpose of this chapter is to
attempt to draw together examples of such research topics for both the practicing
spectroscopist and workers in the field of colloid and surfactant science. Major
advances have been made in recent years in the areas of spectroscopic data handling
(which have affected the "look" and "feel" of the analytical laboratory in general)
and sample handling (various new optical accessories). These advances have made
FT-IR a truly accessible technique, with tremendous potential for application to
research in areas of considerable economic, as well as fundamental, importance.
Synthetic surfactants are ubiquitous in the modem world, appearing in a wide
variety of consumer cleaning products, in foods and cosmetics, and in industrial
products and processes such as metal cleaning and enhanced oil recovery. The
design of novel surfactant - based products or processes featuring "improved"
performance or decreased environmental impact requires an understanding of the
relationship of molecular structure to surfactant phase behavior. In the case of
"natural amphiphiles" such as the phospholipids, a single phase, the bilayer, is of
paramount importance. Significant advances in the understanding of the molecular
forces which stabilize these aggregates have been made recently using vibrational
spectroscopy. In nature, membrane bilayers incorporate significant amounts of
additional substances such as proteins and cholesterol. Although more complex,
such systems are also beginning to yield to spectroscopic analysis. Besides being of
fundamental interest, membrane bilayer chemistry affects areas of considerable
economic opportunity, considering pharmaceutical chemistry alone.
Many of the spectroscopic concepts developed in studies of bilayers are
applicable to other molecular aggregates such as surfactant miceUes and gels. The
0097-6156/91/0447-0001$06.25/0
© 1991 American Chemical Society
In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
2 FTIR SPECTROSCOPY IN COLLOID AND INTERFACE SCIENCE
evidence for this statement lies, hopefully, in this chapter, and in the publications of
several workers participating in both fields.
Synthetic surfactants and polymers are probably most often used to modify the
characteristics of a solid surface, i.e., they function at the solid - liquid interface,
such as in the processes of detergency, lubrication, or the formation of adhesive
bonds. The performance of modem FT-IR spectrometers is such that many new
applications to the characterization of the solid - liquid interface, particularly in
kinetics studies, are possible. Reflection - absorption spectroscopy and attenuated
total reflectance (ATR) techniques have been applied to "wet" interfaces, even the
air - water interface, and have figured prominendy in recent studies of "self -
assembled" mono - and multilayers.
Interestingly, protein adsorption is also a field of biological interfacial
chemistry which parallels that of synthetic materials at the solid - liquid interface. A
number of spectroscopic advances have been made which allow FT-IR to be used in
kinetic monitoring of protein adsorption on metals and "biocompatible" polymers. In
addition to providing in - sit
also yield information about
layers.
In the various sections of this chapter, I will briefly describe the major
characteristics of FT-IR, and then relate the importance of these characteristics to
physiochemical studies of colloids and interfaces. This book is divided into two
major areas: studies of "bulk" colloidal aggregates such as micelles, surfactant gels
and bilayers; and studies of interfacial phenomena such as surfactant and polymer
adsorption at the solid-liquid interface. This review will follow the same
organization. A separate overview chapter addresses the details of the study of
interfaces via the attenuated total reflection (ATR) and grazing angle reflection
techniques.
FT-IR Brief Description
Interferometry. A detailed description of spectrometer designs, of which there are
several now commercially available, is beyond the scope of this book. There are
several fine texts to which workers new to die field, or interested non - practitioners,
may refer.d - 3).
Several major points should be mentioned. In FT-IR interferograms are recorded,
and the infrared spectra computed from the interferograms, via a fast Fourier
transform algorithm introduced relatively recently (4). It is the replacement of the
monochromator of earlier spectrometers by an interferometer which is primarily
responsible for the improved performance of FT instruments.
The use of an interferometer, the concept of which was first described by
Michelson in 1891 (5), allows all of the radiation interacting with the sample to fall
on the detector during all of the time of measurement, i.e., data from au spectral
frequencies of interest is collected simultaneously, which yields the so-called
multiplexing, or Felgett's advantage (6). The measurement of a single interferogram
can be made rapidly, resulting in a large savings in time of measurement. As is
typically done in chemical spectroscopy, interferograms are repeatedly obtained and
averaged, to yield spectra of very high signal to noise ratio (increasing with the
square root of measurement time, or number of scans, assuming the typical case of
constant velocity of the moving mirror of the interferometer).
Jacquinot's advantage refers to the improved signal to noise ratio in spectra
measured with interferometry due to the greater signal size present at the detector.
Griffiths (2) points out that the magnitude of Jacquinot's advantage in modem
spectrometers had been exaggerated in earlier times. Theoretical calculations which
indicate 200 to 2000 - fold sensitivity advantages for FT- IR are somewhat
misleading, since overall spectrometer performance, in the case of FT instruments,
In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
1. SCHEUING Overview 3
must take into account limitations imposed by detector design, detector foreoptics,
and beamsplitter efficiencies. In many cases, instrument performance is limited not
by optical throughput, but by the dynamic range of the analog to digital converter
used, since there can be an enormous difference in signal intensity between the
centeiburst and in the wings of the digitized interferograms being recorded (1).
Given that the multiplexing advantage can be used to obtain spectra of
samples exhibiting large energy losses, such as aqueous solutions, thin films, or
powders, another aspect of FT-IR also emerges as important. The precision of the
digitization intervals of the interferograms, and hence in the computed spectra, is
extremely high. Laser light fringing methods are commonly employed in
monitoring the position of the moving mirror in the Michelson interferometers used
in modern FT-IR spectrometers. The precision with which resulting infrared bands
arc digitized is such that it can be ignored as a source of error in the determination of
the frequency of many bands in the spectra of condensed phase samples. The
determination of the frequency of a Lorenztian band is limited, for practical
puiposes, by the signal to nois
noise ratio of 1000, which
frequency determination is < 0.01 cm", for a band recorded at 4 cm" nominal
resolution. Such results certainly suggest, taking such band frequency (and width)
information into account, together with interpretations of band shifts, mat FT-IR can
indeed be an "information - rich" technique.
Spectral Manipulation Techniques. Many sophisticated software packages are
now available for the manipulation of digitized spectra with both dedicated
spectrometer minicomputers, as well as larger main - frame machines. Application
of various mathematical techniques to FT-IR spectra is usually driven by the large
widths of many bands of interest. Fourier self - deconvolution of bands, sometimes
referred to as "resolution enhancement", has been found to be a valuable aid in the
determination of peak location, at the expense of exact peak shape, in FT-IR spectra.
This technique involves the application of a suitable apodization weighting function
to the cosine Fourier transform of an absorption spectrum, and then recomputing the
"deconvolved" spectrum, in which the widths of the individual bands are now
narrowed to an extent which depends on the nature of the apodization function
applied. Such manipulation does not truly change the "resolution" of the spectrum,
which is a consequence of instrumental parameters, but can provide improved visual
presentations of the spectra for study.
The "price" paid for deconvolution of spectra is an increase in the noise level,
as well as the potential for producing "ringing" baselines or subsidiary lobes on the
wings of real spectral features. In the development of the Fourier deconvolution
technique over the past several years, considerable efforts have been made in
investigation of the effects of signal to noise ratio, selection of the weighting
function used for deconvolution, and comparisons of this approach with that of even
- order derivative manipulations (8-13). A recent demonstration that peak areas are
practically unaffected by proper application of deconvolution techniques suggests
that quantitative measurements employing deconvolved spectra are possible (14).
Other manipulations of spectra are also possible, and are used with varying
degrees of success. Discussions of curve fitting (15), factor analysis (16.17),
derivative formation and smoothing (18) have all appeared. Curve fitting has been
applied to several weak bands in the spectrum of a micellar surfactant solution (19).
and work on improvements in fitting second derivative spectra has continued (20).
As stated in an article on the origin of artifacts in deconvoluted spectra, however,
one must avoid the pitfalls in mathematical manipulation of spectra "... in order to
remain in the field of spectroscopy without entering that of spectrology." (21).
In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
4 FOR SPECTROSCOPY IN COLLOID AND INTERFACE SCIENCE
Difference Spectroscopy. Spectral subtraction, or difference spectroscopy, is the
final important topic of this general description. In studies of "bulk" samples such as
micelles, bilayers, or vesicles, water (or D0), which is an intense absoiber
2
throughout most of the mid-infrared spectrum (4000-400 cm"1), is the solvent. The
subtraction of the spectrum of liquid water from that of an aqueous solution of an
analyte of interest can be accomplished routinely with many of the dedicated
minicomputers used with modem FT-IR spectrometers. The spectrum of liquid
water is recorded under the same conditions (cell pathlength, temperature, cell
window type) as the samples of interest, and digitally stored, This reference
spectrum of water can then be scaled by an appropriate multiplicative factor (usually
near 1.0) and subtracted from the sample spectrum, to produce a spectrum of the
analyte of interest (22,23). Residual noise in the difference spectra of aqueous
samples of high water content often persists above 3100 cm_1,in the region of the
intense H-O-H stretching bands, which is usually not too severe a limitation.
Successful subtraction of the H-O-H bending band of water near 1640 cm"1 is
usually of more concern, becaus
important bands due to ester
pathlength of the cell employed to less than 25 micrometers aids in controlling the
intensity of this band, and improves subtraction results in this frequency range. The
necessity for working with such relatively short pathlengths, which are
accomplished in ordinary transmission cells through the use of thin gaskets,
prompted the development of several optical accessories base on attenuated total
reflection (ATR) optics. Infrared radiation propagating inside a multiple internal
reflection element (IRE) of an ATR accessory interacts with the external medium
(sample solution) to a distance of only 1 micrometer or so. By adjusting the
dimensions of the IRE, and hence the number of internal reflections, an ATR
accessory can provide an effective pathlength from 10 to 20 micrometers, a range
which is ideal for use with aqueous solutions. Several designs, in which the IRE is
conveniently located within an easily filled chamber, have appeared (24,25). Studies
of the adsorption of proteins from aqueous solutions, which necessitated the
subtraction of the water bending band, have been conducted using ATR sampling
optics (26). The adsorption of proteins onto an IRE surface also points to a potential
pitfall in the use of such accessories in studies concerned with "bulk" samples.
Adsorption of a surfactant onto the IRE will provide spectra containing information
about both bulk and adsorbed species, and this potential distortion of the spectra
should be avoided in physiochemical studies of bulk phases.
Subtraction of the spectrum of liquid water, even of moderate band intensity,
can also be complicated by solute-water interactions which cause a shift in the
H-O-H bending bands, making a complete nulling of the band in the difference
spectrum impossible (23). As discussed further below, in bulk phase samples such as
microemulsions or inverse micelles of moderate water content, significant
information about aggregate structure is obtained from shifts in the water bands.
Spectral subtraction can also be used to enhance the more subtle differences
between two samples by the nulling of spectral features common to two spectra,
which leaves the differences between samples as excursions from an otherwise
featureless baseline. Interpretation of such difference spectra is usually done in
conjunction with measurement of other spectroscopic changes, such as band shifts or
intensity changes, that are produced in a series of spectra of a bulk phase sample
perturbed in some manner. Examples will be discussed further below in the various
subsections.
Characterization of Colloidal Aggregates
Bilayers. A considerable body of literature exists on the application of FT-IR (and
Raman) spectroscopy to characterization of the phase behavior of both naturally
occurring and synthetic phospholipids. Such compounds have the structure typified
In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
