Polyelectrolytes and their counterions studied by EPR spectroscopy [Elektronische Ressource] / von Dariush Hinderberger

Informations

• Publié par: johannes_gutenberg-universitat_mainz
• Publié le: 01 janvier 2004
• Nombre de lectures: 10
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Polyelectrolytes and Their Counterions Studied by
EPR Spectroscopy
Dissertation
zur Erlangung des Grades
„Doktor der Naturwissenschaften“
am Fachbereich Chemie und Pharmazie
der Johannes-Gutenberg-Universität
in Mainz
von
Dariush Hinderberger
geboren in Heidelberg
Mainz 2004
Dekan: Prof. Dr. R. Zentel

Tag der mündlichen Prüfung: 02. März 2004 Der Gründliche.
Ein Forscher ich? O spart dies Wort! -
Ich bin nur schwer - so manche Pfund!
Ich falle, falle immerfort
Und endlich auf den Grund!
Friedrich Nietzsche
In memoriam Friedrich Hinderberger
Contents
. Introduction.……………………………………………………………………….…………. 1
1 Fundamentals of Electrostatic Interactions and Polyelectrolytes in Solution…………….4
1.1 Theoretical description of electrolyte solutions…………………………………………4
1.1.1 Primitive model and Poisson-Boltzmann theory…………………………….... 4
1.1.2 Debye-Hückel approximation……………….………………………………… 6
1.2 Polyelectrolytes in solution……………………………………………………………...7
1.2.1 Static chain properties: concepts………………………………………………..7
1.2.2 Counterion condensation.………………………………..…………………….. 11
1.2.3 The cell model…………………………………………………………………..13
1.2.4 Odijk-Skolnick-Fixman (OSF) theory………………...….……………………. 14
2 Fundamentals of EPR Spectroscopy on Polyelectrolytes……………………………..…… 16
2.1 Introduction: resonance condition.……….……………………………………………...16
2.2 Spin Hamiltonian and types of interactions...………………………………………..….17
2.3 Spectral analysis of continuous wave (CW) EPR…..…………………………………...22
2.4 Time evolution of spin ensembles…..………………………………………………….. 30
2.4.1 Density operator formalism……………………………………………………..32
2.4.2 Product operator formalism
2.4.3 Basics of pulse EPR measurements…...……………………………………….. 35
2.5 Fourier Transform (FT) EPR...…………………………………………………………. 36
2.6 Pulse EPR methods based on the primary echo…………...…………………………….37
2.6.1 Field-swept, ESE-detected spectra……………………………………………...38
2.6.2 2-pulse electron spin echo decay (ESED)
and envelope modulation (2-p ESEEM)……………………………………….. 39
2.6.3 Double electron-electron resonance (DEER)…………………………………...42
2.7 Pulse EPR methods based on the stimulated echo………………………………………45
2.7.1 3-pulse electron spin echo envelope modulation (3-p ESEEM)……………….. 46
2.7.2 Ratio analysis of 3-p ESEEM………………………………………………..… 48
3 Spin Probing and EPR Spectroscopy of Polyelectrolyte-Counterion Interactions……….51
3.1 Investigated systems………………………….………………………………………….51
3.1.1 Studied polyelectrolytes and spin probes………………………………………. 51
3.1.2 Variation of solvent…………………………………………………………….. 57
3.2 Electrostatic attachment of spin probes to a model rigid-rod polyelectrolyte………….. 58
3.2.1 Localized electrostatic attachment of FS to cationic Ru-centers………………. 58
3.2.2 Local attachment geometry and dynamic electrostatic attachment (DEA)…….. 62
3.3 Electrostatic attachment of spin probes to flexible polyelectrolytes in solution….……..65
3.3.1 Dynamic electrostatic attachment – from site binding to territorial binding of
counterions……………………………………………………………………...67
3.3.2 Quantification of changes in CW and FT EPR spectra…………………………76
3.3.3 Agglomeration of spin probes close to single polyelectrolyte chains and build-
up of concentration gradient…………………………………………………… 82
3.3.4 Effect of DEA on polyelectrolyte chain conformation…………………………90
3.3.5 Indirect observation of polyelectrolyte chain dynamics……………………….. 96
4 Interpretation Based on Models from Polyelectrolyte Theory.……………………......…..101
4.1 Radial distribution of spin probes around polyelectrolyte chains……………………….101
4.1.1 The charged cylindrical cell model……………………………………………..102
4.1.2 Spin probe radial distributions from analysis of CW EPR spectra……………..104
4.1.3 Spin probe radial distributions from a generalized scaling approach…………..114
4.2 Counterion distribution along a 1-dimensional chain and reduced number of effective
charges………………………………………………………………………………….. 116
4.2.1 a 1-dimensional chain………………………….. 117
4.2.2 Modification of the linear extended chain model……………………………… 120
4.3 Distribution of network-forming counterions…………………………………………...123
4.3.1 TAM spin probe distribution characterized from broadening of high-field ESE-
detected spectra…………………………………………………………………123
4.3.2 “Zip-like” clustering of TAM spin probes……………………………………...127
5 Conclusions and Outlook…………...…………………………………………….....…….….131
. Appendix……………………………………………………………………………………… 136
. References and Notes………………………………………………………………………… 141
. List of Abbreviations and Symbols…………………………………………………………..147
. Acknowledgments……………………………..………………………………………………149
. Summary……………..………………………..………………………………………………150
. Curriculum Vitae……………………………..……………………………………………… 151

. Introduction
Polyelectrolytes are macromolecular substances that are soluble in water or other ionizing solvents and
dissociate into macromolecular ions that carry multiple charges (polyions) together with an equivalent
amount of ions of small charge and opposite sign (Figure i). They play an important role in fields of
scientific research as diverse as molecular biology
1-6 - - and nanotechnology. Many biological
-
+ + + macromolecules such as DNA or proteins are +++ +
- - - polyelectrolytes and use electrostatic interactions to
-
trigger and control structural changes or binding of 15Fig. i. Scheme of the “physicist’s view” of a
5 polyelectrolyte that is dissociated into a polyion small molecules. Synthetic polyelectrolytes are
and small counterions in polar solvents;
also applied commercially in cosmetics, fuel cells,
7,8and food and oil industry.
Highly charged biological as well as synthetic polymeric materials have been under extensive
experimental and theoretical investigation for several decades and it is commonly acknowledged that
their interesting structural properties stem from the delicate balance of two opposing interactions.
First, gain in entropy upon release of counterions leads to highly charged polymers, and
electrostatic repulsion between the like charges on each repeat unit leads to a preference for extended
conformations of the polyelectrolyte chain. Second, polyelectrolytes are usually dissolved in water or
other solvents with high dielectric permittivity, which are poor solvents with respect to the polymer
backbone (hydrocarbons). Poor solubility is equivalent to an attractive hydrophobic interaction
1between repeat units on the polyelectrolyte, which favors more collapsed structures.
Polyelectrolytes are probably the least understood class of macromolecules, which is
remarkable when considering their importance in molecular biology and materials science. Despite
numerous studies on polyelectrolytes, the combined effects of polyelectrolyte-counterion and
polyelectrolyte-solvent interactions on polyelectrolyte structure are not fully comprehended. This is
mainly due to the importance of a multitude of intertwined length scales in polyelectrolytes as
compared with uncharged polymers, which makes theoretical and experimental investigations of the
9latter ones much simpler. It is known that the screening of intramolecular electrostatic repulsion by
1 Introduction 2
oppositely charged, in particular multivalent, counterions can lead to a dominance of hydrophobic
10,11,12attraction and thus to more collapsed, globular chain conformations. Within the last decade
theoretical investigations of the aforementioned interactions, driven by increasing computing power
and sophisticated algorithms, have been carried out and led to predictions of pearl-necklace-like
13conformations in solution that could not yet be convincingly verified by experiments.
As in many fields of polymer science, abstractions are made from the actual chemical
structure of the polyelectrolyte (see Fig. i) in order to be able to physically describe these complex
15materials. Such a simplified “physicist’s view” of the materials may, however, not be sufficient to
account for all of the observed polyelectrolyte features. To develop chemically more realistic models,
it is necessary to gain insight into specific interactions in the polyion-counterion-solvent system. The
interactions between the polyion and the small counterions are of particular interest, as they
predominantly determine chain conformation.
Characterization methods that probe macroscopic properties (such as conductivity) or can
characterize long-range order (light-, x-ray-, and neutron-scattering) have been applied extensively in
1,3,14 the past. Scattering experiments are usually carried out under addition of large amounts of inert
salt (e.g. 1M NaCl), which effectively screens Coulomb interactions not only along one
polyelectrolyte chain but also between different chains. This is necessary to damp intermolecular
components of the scattering function. Information about chain conformations, such as the radius of
2 0.5 2 0.5gyration, <R > , or the mean square end-to-end distance <R > of chains can be gained by G
scattering methods, whereas a detailed, local picture of electrostatic interactions between
polyelectrolyte and counterions cannot be obtained.
16Magnetic resonance methods, such as electron paramagnetic resonance (EPR) spectroscopy
on spin-carrying