Nuclear Magnetic Resonance Studies in Lyotropic Liquid Crystals

88 Pages·1975·3.29 MB·English

by  C. L. Khetrapal

NMR
Basic Principles and Progress
Grundlagen und F ortschritte
Volume 9
Editors: P. Diehl E. Fluck R. Kosfeld
With 18 Figures
Springer-Verlag
Berlin . Heidelberg . New York 1975
Professor Dr. P. DIEHL
Physikalisches Institut der Universitat Basel
Professor Dr. E. FLUCK
Institut fUr Anorganische Chemie
der Universitat Stuttgart
Professor Dr. R. KOSFELD
Institut fUr Physikalische Chemie
der Rhein.-Westf. Technischen Hochschule Aachen
ISBN-13: 978-3-642-45475-2 e-ISBN-13: 978-3-642-45473-8
001: 10.1007/978-3-642-45473-8
Library of Congress Cataloging in Publication Data. Nuclear magnetic
resonance studies in lyotropic liquid crystals. (NMR, basic principles
and progress; v.9) Cover title. Includes bibliographical references and
index. 1. Liquid crystals-Spectra. 2. Nuclear magnetic resonance
spectroscopy. I. Khetrapal, Chunni Lal, 1937-. II. Series. QC490.N2
vol. 9 [QD923] 538'.3s [548'.9] 75-16370
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© by Springer-Verlag Berlin' Heidelberg 1975
Softcover reprint of the hardcover 1s t edition 1975
The use of general descriptive names, trade marks, etc. in this publication,
even if the former are not especially identified, is not be taken as a sign
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Nuclear Magnetic Resonance Studies
in Lyotropic Liquid Crystals
C. L. KHE'QlAPAL
Raman Research Institute
Bangalore, India
A.C.KuNWAR
Raman Research Institute
Bangalore, India
A. S. TRACEY
Department of Physics
University of Basel
KlingelbergstraBe 82
4056 Basel, Switzerland
P. DIEHL
Department of Physics
University of Basel
Klinge1bergstraBe 82
4056 Basel, Switzerland
Contents
Part I. Introduction . . . . 3
1. Lyotropic Liquid Crystals 3
2. Basic Principles. . . . . 9
2.1. Spectra of Lyotropic Phases 9
=t
2.1.1. Nuclei with Spin I 9
t
2.1.2. Nuclei with Spin I> 10
2.2. Spectra of Molecules Dissolved in Nematic Lyotropic Phases 11
2.2.1. The Hamiltonian . . . . . . . . . . . . . . . . 11
2.2.2. Direct Coupling and Degree of Order . . . . . . . 12
2.2.3. Interpretation of the Dipolar Couplings and Obtainable
Structural Information. . . . . . . . . . . . . . . .. 13
2.2.4. Chemical-shift Anisotropy . . . . . . . . . . . . . .. 14
2.2.5. Anisotropic Contribution of the Indirect Spin-spin Couplings 14
2.2.6. The Quadrupole Interaction . . . . . . . . . . . . .. 14
2 Contents
3. Experimental ...... . 15
3.1. Preparation of Samples 15
3.2. NMR Measurements . 16
3.2.1. Phases without Additional Solutes. 16
3.2.2. Solutes Dissolved in Nematic Phases. 17
Part II. Studies of Lyotropic Liquid Crystals. 19
4. Introduction . . . . ... . . . . 19
5. Applications . . . . . . . . . . . . . 21
5.1. Critical Micelle Concentration . . . 21
5.1.1. Chemical-shift Measurements. 21
5.1.2. Spin-lattice Relaxation Time Measurements. 23
5.2. Spectral Changes and Phase Transitions . . . . . 24
5.3. Proton and Deuteron Magnetic Resonance in Hydrated Fibrous
Materials . . . . . . . . . . '. . . . 28
5.4. Self-diffusion in Lyotropic Liquid Crystals . . . . 29
5.5. Interactions ofIons in Anisotropic Media . . . . 31
5.5.1. Ordering of Spherical and Tetrahedral Ions. 31
5.5.2. Ion Binding and Ion Competition 32
5.5.3. Halide Ions. . . . . . . . . . . . . . . 33
5.5.4. Alkali Ions. . . . . . . . . . . . . . . 33
5.5.5. Proton, Deuteron and Alkali Resonance Studies 34
5.5.6. PMR of Nonionic Surfactants in the Presence of Anionic
Surfactants. . . . . . . . . . . . . . . .. 35
5.5.7. 14N Quadrupole Interactions . . . . . . . . . . . . . . 35
5.6. Alkyl Chain Motion in Lyotropic Liquid Crystals. . . . . . . . 36
5.6.1. The Concept ofthe Order Parameter as Applied to the Hydro-
carbon Chain. . . . . . . . . . . . . . . . . . 36
5.6.2. Structure and Dynamics of the Hydrocarbon Region. 36
5.6.2.1. Micellar Solutions . 36
5.6.2.2. Anisotropic Phases. 37
5.7. Sonicated Lamellar Systems . . 41
Appendix to Part II: Systems Reported. . 42
m.
Part Studies of Molecular and Ionic Species Dissolved in the Nematic
Phase of Lyotropic Liquid Crystals . 47
6. Introduction. . . . . . . . . . 47
7. Applications. . . . . . . . . . . . . 48
8. Order in Nematic Lyotropic Phases . . . 54
8.1. Some General Comments Concerning the Order Parameter 54
8.2. Molecular and Ionic Species in Aqueous Phases 55
8.2.1. Benzenes and Related Compounds . . . . . . . . 55
8.2.2. Ionic Species as Solutes. . . . . . . . . . . . . 58
Appendix to Part III. Compounds Studied and Information Derived 59
Acknowledgements 74
References . . . . 74
Part I. Introduction
1. Lyotropic Liquid Crystals
The class of compounds known as thermotropic liquid crystals has been widely
utilized in basic research and industry during recent years. The properties of
these materials are such that on heating from the solid to the isotropic liquid
state, phase transitions occur with the formation of one or more intermediate
anisotropic liquids. The unique and sometimes startling properties of these
liquid crystals are the properties of pure compounds. However, there exists a
second class of substances known as lyotropic liquid crystals which obtain their
anisotropic properties from the mixing of two or more components. One of the
components is amphiphilic, containing a polar head group (generally ionic or
zwitterionic) attached to one or more long-chain hydrocarbons; the second
component is usually water. Lyotropic liquid crystals occur abundantly in
nature, particularly in all living systems. As a consequence, a bright future seems
assured for studies on such systems. Even now, many of the properties of these
systems are poorly understood. It is the purpose of this review to consolidate the
results obtained from nuclear magnetic resonance studies of such systems and
to provide a coherent picture of the field.
Probably the most familiar example of a lyotropic liquid crystal is soap in
water. A common soap is sodium dodecylsulphate where an ionic group
(sulphate) is attached to a hydrocarbon chain containing twelve carbons. The
sulphate head group is sufficiently soluble in water to allow complete dispersion
of the soap in dilute solutions. As the concentration of soap increases, the hydro
phobic paraffin chains tend to associate preferentially with one another since they
are quite insoluble in water. At a concentration known as the critical micelle con
centration, aggregates of the alkylsulphate ions form stable entities known as
'micelles'. The nonpolar hydrocarbon chains occupy the interior of the micelles
with the ionic head groups on the surface (Fig. 1) where they can interact
efficiently with the solvent. This type of behaviour is typical not only for soaps
but also for such naturally occurring materials as phospholipids. The critical
micelle concentration and the aggregation number depend on many factors
the nature of the polar head group and of the hydrocarbon chain, and the
presence or absence of electrolytes, to mention a few.
As the concentration of the soap increases, an anisotropic liquid crystalline
material may be formed. There are many articles describing the formation,
4 Introduction
Fig. 1. A representation of the isotropic micellar phase. The polar groups are on the surface
of the micelle, the hydrocarbon chains occupy the interior. [Reprinted from the J. Soc.
Cosmetic Chern. 19, 581 (1968) with permission from the copyright owners and the author]
properties and uses of such systems [1-7]. Several types of anisotropic liquids
exist and a systematic classification has been made from X-ray studies [8-10].
The most common lyotropic meso phase (neat soap) has a lamellar structure.
The hydrocarbon chains form the superstructure with the polar groups lying
along the interface with water. The arrangement is such that the hydrocarbon
chains are perpendicular to the interface and each layer is approximately two
hydrocarbon chain lengths thick (Fig. 2).
If a lamellar phase is irradiated with high-frequency sound, an isotropic
solution may be formed. Sonication does not destroy the lamellar structure, but
causes the formation of closed bilayer structures called vesicles (Fig. 3). They
are spheroidal in shape and enclose a volume of water dependent on the diameter
of the vesicle and separated from the interstitial water.
With changing composition, the lamellar structure often becomes unstable
and cylindrically shaped aggregates may form in a hexagonal packed structure
(Fig. 4). The polar head groups in this case lie on the surface of the cylinder with
the hydrocarbon chains in the interior (middle soap). The hexagonal and the
lamellar anisotropic liquids are by far the most common lyotropic phases.
Lyotropic Liquid Crystals 5
Water
NEAT
Fig. 2. A representation of the anisotropic lamellar phase. The hydrocarbon superstructure
layers are of indefinite extent and are separated by the interstitial water. [Reprinted from the
J. Soc. Cosmetic Chern. 19, 581 (1968) with permission from the copyright owners and the
author]
Fig. 3. A representation of the isotropic vesicular phase. Each vesicle contains a volume of
water which is separated from the interstitial water
6 Introduction
MIDDLE
Water
Fig. 4. Ar epresentation of the anisotropic hexagonally packed cylindrical phase. Cylinders are
of indefinite length and separated from one another by the interstitial water. [Reprinted from
the 1. Soc. Cosmetic Chern. 19, 581 (1968) with permission from the copyright owners and
the author]
However, for some soaps, nematic liquid crystals capable of being ordered by a
magnetic field also exist. Although it is generally assumed that such phases have
a cylindrical superstructure resembling the hexagonal phase, this certainly is not
clearly established. An optically isotropic phase with cubic symmetry may also
form at times.
In the presence of relatively small amounts of water an inverted hexagonal phase
may occur, particularly in the presence of an organic solvent. In this case, the
cylinders are formed in a hexagonal packing such that the polar groups still
occupy the cylindrical surface but the inner part consists of water and the
hydrophobic groups occupy the space between the cylinders. Similarly, inverted
micelles may be formed in hydrocarbon solvents [11,12].
For a fuller description of these and other intermediate phases, the reader is
referred to the literature [13-18].
It is interesting to note that on a micro scale, the structure of the various
phases in lyotropic liquid crystals is similar and that three regions of the system
Lyotropic Liquid Crystals 7
Fig. 5. A representation of the interface region. This region is similar for the isotropic and
anisotropic phases
are clearly defined, as illustrated in Fig. 5. Region 1 in Fig. 5 consists essentially
of hydrocarbon chains, throughout which only small amounts of water and ions
may be. dispersed. Region 2 is an interface region where the relatively immobile
polar head groups are located. They interact strongly with the solvent and the
counterions and keep the system in solution. Region 3 is formed by interstitial
water and some counterions. For zwitterionic species such as phospholipids, of
course, no unattached counterions are present.
All three regions of the lyotropic liquid crystals are of interest and subject to
investigation by NMR techniques. Effects of concentration, of addition of ionic
and molecular species and of protein dispersions have been and are being in
vestigated by NMR. Ion binding and ion competition for binding sites in region 2
are of interest, as is transport of ions and molecules across region 1. Structures of
the various regions have also been investigated by NMR.
This monograph is divided into the introduction and two sections: the first
describing studies of the type mentioned above, the other concerning itself strictly
with studies in the nematic mesophase. Nematic phases are of particular interest
since they may be ordered by a magnetic field and thus provide a homogeneous,
highly ordered anisotropic matrix. The structure of the dissolved molecules or ions
may then be determined from intramolecular dipolar interactions [19-35] which do
not average to zero in anisotropic liquids. This method was proposed by SAUPE
and ENGLERT [36] in 1963 when they observed that the IH-NMR spectrum of
benzene in a thermotropic liquid crystalline nematic phase had a relatively
complex appearance compared to a single-line spectrum in an isotropic phase.
The interpretation of this spectrum led to the discovery of a new method for the
determination of molecular geometries. It provides the only available technique
for the precise determination of the relative arrangement of nuclei in the liquid
8 Introduction
phase and is the most recent addition to the earlier existing list of methods for
the determination of molecular structure, e.g. X-ray, neutron and electron
diffraction and microwave spectroscopy.
During the span of about a decade since its discovery, the method has been
extensively used. Initially, investigators were mainly concerned with under
standing the scope and limitations of the method and hence only simple mole
cu1es, whose structures were already known, were studied and the results
compared with those obtained by other methods. During the last few years,
considerable progress has been made towards understanding the experimental
and theoretical aspects of the method. As a consequence, a broader applicability
has been developed. Attempts have been made to discover different types of
liquid crystalline materials, to simplify the spectra with the help of selective
isotopic substitution followed by heteronuc1ear spin decoupling, to apply
vibrational corrections to the observed dipolar couplings and to understand the
anisotropic contributions of the indirect couplings [25,37-46]. The greater part of
the work has been done on thermotropic liquid crystals. The results are sum
marized in the literature [19-45].
The first nematic lyotropic solvent used in NMR experiments was the one
suggested by LAWSON and FLAUTT [47] in 1967. It is formed by a mixture of
sodium C or C alkylsulphate, the corresponding alcohol, sodium sulphate
g lO
and water (or deuterated water) in approximate ratios of 8: 1: 1: 10. The addition
of sodium sulphate to the nematic pure ternary phase causes separation of the
phase into a smectic and a nematic state [48,49]. Addition of more sodium
sulphate or of some solutes recombines the two into a single nematic phase. Such
solutes can be conveniently studied by NMR [47]. Other nematic lyotropic
phases have also been used [50-52].
Synthetic polypeptides (NH-CHR-CO)n in the oc-helical conformation may
form a lyotropic liquid crystal when the polypeptide concentration exceeds a
certain critical value. A sufficiently strong magnetic field then arranges them with
the polypeptide helix axes aligned parallel to the field [53-55]. The use of such
materials in NMR experiments [56-63] is discussed in more detail in part III
of this review.
The use oflyotropic nematic mesophases for the determination ofthe geometry
of molecular or ionic species has certain advantages over thermotropic phases.
For instance, polar molecules, which are quite insoluble in a thermotrepic solvent,
often readily dissolve in a lyotropic one. Ions may be readily investigated in
lyotropic but not in thermotropic systems. Generally, nematic lyotropic meso
phases may be spun about an axis perpendicular to the direction of the magnetic
field (the arrangement in conventional spectrometers) without destroying the
molecular order. This results in highly resolved NMR spectra for the dissolved
molecules. However, there is a type of nematic lyotropic phase which cannot be
spun in conventional spectrometers without destroying the orientation [46]. The
spinning of both types of these phases without destruction of the molecular order
is permissible in spectrometers with cryogenic magnets. Lyotropic nematic
phases are more sensitive than thermotropic phases to solvent-solute interactions.
This is a disadvantage for studying dissolved molecules. On the other hand, much
sharper transitions allow the use of considerably lower concentrations than are