| 11. |
- Borgström, Johan
(author)
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Phase separation, aggregation and gelation of a charged helixforming polysaccharide in aqueous salt solution
- 1997
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Doctoral thesis (other academic/artistic)abstract
- The phase behaviour and gelation of the anionic polysaccharide kappa-carrageenan (KC) in different salt solutions was studied. The specific site binding of certain anions and certain cations to the KC helix was used as a means of controlling the interactions. In the presence of a binding anion (I-) the charge density of the helix increases and the aggregation of helices, and thereby the gelation, is prevented. By gradually introducing a site binding cation (Cs+) the effective charge density of the helix is reduced, as is the electrostatic repulsion between the helices. Most experiments were performed in 0.1 M salt solution of the mixed salts NaI/CsI. The molecular weight of the KC was varied. For a low molecular weight in 0.1 M NaI, a chiral nematic phase was observed above 5% KC. The volume of the chiral nematic phase was found to depend sensitively on the helical content which demonstrates a coupling between the coil-helix transition and the isotropic-nematic phase transition. From the liquid crystalline phase behaviour, the persistence length of the kappa-carrageenan helix was estimated to be in the range 90-120 nm. The development of the nematic phase was very slow for the high molecular weight KC, due to the high viscosity and slow diffusion for these large molecules. KC of a high molecular weight in 0.1 M NaI produced weak gels above 1% KC with - for KC - unusually low storage moduli. These “gels” were found to be caused by helix entanglements. The effect of mixing different proportions of specific and non-specific ions on the transition enthalpy was investigated experimentally by DSC for a wide range of salt compositions. Under conditions where no aggregation occurred (and thus no gelation), the variations in the observed enthalpies could be reproduced by Poisson-Boltzmann model calculations, taking into account the specific binding of ions to the helix. A change in the macroscopic properties, from isotropic/nematic phase separation to gelation, was found to occur above a certain cesium content in the mixed NaI/CsI salt solution. Studies on dilute solutions by cryo-TEM, viscometry, optical rotation and light scattering showed that at and above this molar ratio of cesium, the kappa carrageenan helices aggregate into rigid super-helical rods. These rods are not soluble, but self-aggregate in turn, giving rise to the gel. Thus, the properties of the gel seem to depend on the properties of this higher order kappa-carrageenan aggregate (the super-helical rod), rather than on the properties of the individual kappa-carrageenan helix.
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| 12. |
- Brinck, Johanna
(author)
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Adsorption of nonionic amphiphiles to solid surfaces
- 1999
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Doctoral thesis (other academic/artistic)abstract
- In this study, the adsorption of nonionic amphiphiles from aqueous solutions to solid, planar surfaces is investigated using null ellipsometry. This technique allows independent measurements of the amounts adsorbed at the surface and of the mean optical thickness of the adsorbed layer. The time resolution is sufficient for investigating the dynamics of adsorption and desorption in systems of this type. Detailed experimental studies of the processes involved in the formation and dissolution of pure and binary polyethylene glycol monoalkyl ether surface structures at the silica-water interface were carried out. A theoretical model was developed for interpreting the experimental data and for gaining an understanding of these processes at a molecular level. For the pure surfactant systems, the combination of theory and experiment led to an increased understanding of the influence that a number of surfactant and flow-cell related parameters have on the adsorption and desorption kinetics. An extension of the work to binary surfactant systems revealed the importance of considering the individual paths of adsorption and desorption for each component. Large changes in the surface composition upon rinsing were discovered in binary systems in which there was a significant difference in surfactant cmc. Studies of the addition of long-chain alcohols to aqueous solutions of polyethylene glycol monoalkyl ethers in contact with a silica surface demonstrated that small amounts of alcohol that are added can give rise to large increases in the total amount adsorbed. Just as with the binary surfactant systems, mixtures of surfactants with alcohol or with oil were shown to produce segregated desorption paths. Finally, adsorption from an aqueous solution of a fatty acid to a hydrophobic surface was found to be very sensitive to the pH of the solution. A close correlation was observed between the pH dependence of the fouling of the hydrophobic membranes and the experimentally determined adsorption at the planar hydrophobic surface.
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| 13. |
- Carlsson, Ingemar
(author)
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Electrostatic Interactions in Complex Liquids : A Thermodynamic Analysis Based on the Poisson-Boltzmann Equation and the Flexible Surface Model
- 1998
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Doctoral thesis (other academic/artistic)abstract
- The aim of this study was to analyse physico-chemical properties of some complex liquids using the Poisson-Boltzmann (PB) cell model; to extend the theory to cover bilayer systems with complex global geometry; finally to apply this new extended model to such bilayer systems. The PB cell model provides a practical tool to examine counterion association and phase behaviour for structures of simple geometries; cylindrical and spherical. The model applies to self-assembled aggregates of ionic surfactants as well as to a polyelectrolyte system. The framework of the PB cell model provides a consistent route to derive the electrostatic contribution to Helfrich's bending energy at finite salt- and surfactant concentrations. The bending energy concept, with the extension of a variable electrostatic contribution to the bending rigidity, the saddle splay modulus, and the product (bending rigidity)*Ho (where Ho is the spontaneous mean curvature) was adopted for the description of the fluid mono- and bilayer phases, bicontinous microemulsion and L3 (sponge), respectively. This formalism successfully describes many features of a ionic microemulsion system and the inclusion of both salt and surfactant concentration dependence was necessary for this purpose. The same formalism provided the basis for a model description of the ternary system AerosolOT/NaCl/Water. A theoretical phase diagram, containing the L3 phase in competition with the lamellar phase and dilute solution, was calculated. The calculations captured the features typical for L3: narrowness in the one phase region and the characteristic sequence of phase transitions.
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| 14. |
- Edén, Mattias, 1971-
(author)
-
Solid state NMR of multiple-spin systems
- 1999
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Doctoral thesis (other academic/artistic)abstract
- Many nuclear magnetic resonance (NMR) experiments subject the nuclear spins to time-periodic modulations, for example, mechanical sample rotation and irradiation by periodic radio-frequency pulse sequences. An efficient simulation algorithm (COMPUTE) has been proposed for calculating NMR spectra from such systems.Computation of NMR spectra of powders involves an average over all possible crystallite orientations. It is desirable that a reasonable numerical approximation to a full powder average is obtained, using a minimum number of orientational samples. This thesis discusses methods for selecting these. Furthermore, it has been demonstrated that averaging over one of the three orientational variables may be performed within the COMPUTE framework with minimum computational effort. In many cases, these advances combine to reduce the computational time by an order of magnitude compared to previous methods.This thesis also discusses excitation and exploitation of multiple-quantum coherences in the NMR of rotating solids. A method has been developed that determines the backbone torsion angle y in peptides and proteins, by using evolution of multiple-quantum coherence under heteronuclear 13C-15N dipolar interactions. It operates under magic-angle-spinning conditions, and has been implemented in double-quantum and triple-quantum versions. The technique has been demonstrated by determining y in fragments of the tripeptides gly-gly-gly and ala-gly-gly, with an accuracy of 5o-10o.
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| 15. |
- Eriksson, Leif
(author)
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Flocculation with Polyelectrolytes in Latex and Microbial Systems
- 1997
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Doctoral thesis (other academic/artistic)abstract
- Adsorption of cationic polyelectrolytes onto polystyrene latices was studied as function of polymer chemical composition and molecular weight, polymer and surface charge density and ionic strength. The ratio between polymer charges and surface charges was close to one when distances between polymer and surface charges were equal. Overcompensation of the surface charge was obtained at higher polymer charge densities due to surplus of positive charges at the adsorption site and at lower polymer charge densities due to formation of loops and tails. Different lengths of side chains carrying the cationic groups had effects on the way the polymers neutralised the surface. Flocculation experiments under the same conditions as for the adsorption gave minima in floc size and strength when the distance between polymer and surface charges was the same. With reference to surface force measurements it was concluded that the polymers, depending on charge density, played several and different roles in capturing, binding and adhesion trough traditional bridging, electrostatic double layer bridging and electrostatic attraction. With low charge density polymers dominance of traditional bridging led to flexible and shear resistant flocs with dense structure on small length scales and a less dense fractal structure on larger length scales. With higher charge density increasing adhesion between the primary particles resulted in fractal structure on all length scales and decreasing shear resistance. This was compensated however with a god reflocculation ability. Different behaviour of flocs under and after high shear was related to shear history and to the degrees of traditional bridging and adhesion respectively. Separation properties of activated sludge bioflocs were related to the degree of crosslinking and shrinkage of the biopolymer matrix binding the bacterial cells together. This depended on the availability of calcium ions and on the energy input to the system. Flocculation of monocultured bacteria with polycations proceeded in two steps. First the anionic extracellular polymers from the bacteria were precipitated to polyelectrolyte complexes which on recharging flocculated the cells. Depending on the distribution of extracellular polymers between surface and liquid phase differently broad effective dosage ranges were obtained. For conditioning of sludges with large contents of biopolymers it was necessary with high charge density polycations and dosage need was mainly spent on polymer complexation as with the pure bacteria. For sludges with more particular character lower charge density polymers were advantaeous.
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| 16. |
- Eskilsson, Krister
(author)
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Adsorption Properties of Triblock Copolymers at Solid Surfaces
- 1999
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Doctoral thesis (other academic/artistic)abstract
- The interfacial behaviour of nonionic triblock copolymers of the type poly(ethylene oxide-tetrahydrofurane-ethylene oxide) has been systematically examined at different types of surfaces. It is shown how the chemical nature of the substrate greatly influences the adsorption behaviour of block copolymers. Adsorption at hydrophilic surfaces resulted in the formation of micellar-like surface aggregates, whereas the same molecules were shown to form a monolayer at hydrophobic surfaces. It is further shown how the adsorbed layer characteristics changes the forces measured between copolymer covered surfaces. Rapidly repeated force curves were found to be perfectly reproducible for all systems studied. The relaxation time of the adsorbed layers is, therefore, relatively short. At hydrophobic surfaces the copolymers exhibit a mixed pancake-to-brush transition as a function of increased concentration. This transition is also reflected in both the adsorption kinetics and the interaction patterns. The main features of the force versus distance curve can be predicted from the adsorption isotherm. The copolymers form surface aggregates when adsorbed at a hydrophilic silica surface. The relatively strong interaction between the PEO blocks and the surface leads to a compositional discrepancy between bulk and surface aggregates. For copolymer samples with high average molecular weights, competitive adsorption between individual high molecular weight copolymers and surface aggregates was observed. Adsorbed molecules of the high molecular weight fraction restricted the formation of surface aggregates. Interaction forces between two copolymer-covered silica surfaces change from attraction to repulsion as a function of increased copolymer concentration. At low concentrations, PEO chains of copolymers tethered in the surface aggregates form bridges between the surfaces from relatively large distances. At higher concentrations, no free space exists at the surface and bridging can not occur, the interaction becomes purely repulsive for all surface-surface separations during both approach and separation.
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| 17. |
- Evenäs, Johan
(author)
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NMR studies on calcium-induced conformational transitions in calmodulin
- 1998
-
Doctoral thesis (other academic/artistic)abstract
- Nuclear magnetic resonance (NMR) spectroscopy was used in order to investigate the relationships between structure, dynamics and calcium binding in the intracellular regulatory protein, calmodulin. Calmodulin consists of two similar domains, each binding two calcium ions with positive cooperativity. The studies described in this thesis focus on the isolated C-terminal domain, TR2C, and two mutants thereof. TR2C consists of two calcium-binding helix- loop-helix motifs packed together in a parallel fashion. The three-dimensional solution structures of TR2C were determined with and without calcium ions bound.The two structures are similar in terms of secondary structure, but large reorientations of the helices occur upon calcium binding, yielding an open conformation with a large exposed hydrophobic surface. Furthermore, the structures of the calcium- free and calcium-loaded states of TR2C were shown to be highly similar to those of the corresponding domain of intact calmodulin, demonstrating the structural autonomy of the domains. In each of the two mutants, E104Q and E140Q, a conserved bidentate calcium-coordinating glutamic acid residue has been mutated to a glutamine in one of the two binding sites. Calcium titration data were obtained from NMR spectra and the binding constants were determined. Both mutants bind calcium sequentially, but in the opposite order, and the structural response was found to be different for the two calcium-binding sites. The calcium-free states of the mutants are very similar to the calcium-free state of wild-type TR2C. In contrast, the calcium-bound states are shown to undergo equilibrium exchange between at least two, approximately equally populated, conformations similar to those of the calcium-free and calcium- loaded states of wild-type TR2C. This conformational exchange occurs on the sub-millisecond time scale. From studies of the backbone fluctuations on pico- second to nanosecond time scales of the calcium- loaded state of the E140Q-mutant, the presence of any significantly populated unfolded intermediate substates could be excluded. Investigations of the temperature dependence of slower dynamics on micro- second to millisecond time scales indicated that the exchange does not involve one single structural transition. The use of off-resonance rotating frame spin relaxation experiments yielded an improved description of the exchange.
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| 18. |
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| 19. |
- Forsman, Jan
(author)
-
Theoretical Studies of Surface Forces and Phase Equilibria
- 1998
-
Doctoral thesis (other academic/artistic)abstract
- The interaction between large hydrophobic and hydrophilic surfaces in aqueous solution is investigated, using Metropolis Monte Carlo simulations and density functional theory. Special attention is devoted to the solvation force and its dependence on the nature of the solvent-solvent and solvent-surface interactions. In order to facilitate simulations of surface forces in aqueous systems, methods are developed to ensure that equilibrium with a bulk solution of the pure solvent is maintained at all surface-surface separations. The results suggest that the range and strength of the surface forces in simple solvents are mainly governed by the cohesivity of the solvent. In this respect, water is not unique and interactions similar to the attractive "hydrophobic force" and the repulsive "hydration force" are likely to exist in many other solvents. The high cohesivity in water makes these interactions stronger in aqueous solution than in most other systems. However, even though anisotropic properties of the solvent may have a large influence on the net repulsive force between solvophilic surfaces, hydrogen bonding is not a criterium for the existence of such a repulsion. Phase equilibria in heterogeneous systems are also studied, using simulations and density functional theory. Coexistence in confined geometries between a gas and a liquid phase, as well as between various "layered phases" is established. In the latter case, comparisons are made with the behaviour at an isolated surface. Phase diagrams are constructed and methods developed, to avoid phase transitions during the simulations.
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| 20. |
- Frost, Ray L., et al.
(author)
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Combination bands in the infrared spectroscopy of kaolins : a drift spectroscopic study
- 1998
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In: Clays and clay minerals. - 0009-8604 .- 1552-8367. ; 46:4, s. 466-477
-
Journal article (peer-reviewed)abstract
- Kaolinites with varying degrees of defect structures have been studied by both mid-infrared (IR) and near-IR diffuse reflectance spectroscopy (DRIFT). Difference bands were observed in the 2650to 2750-cm-1 region. This region coincides with the kaolinite-deuterated hydroxyl stretching region. Summation bands were observed in the near-IR spectra in the 4500- to 4650-cm-1 and in the 7050- to 7250cm-1 region. Each of the spectral regions of the summation and difference bands is both kaolin polytype and sample dependent. It is proposed that each of these sets of bands arises from the combination of the hydroxyl stretching frequencies and the hydroxyl deformation frequencies and, to a lesser extent, the silicon-oxygen symmetric stretching vibration of the siloxane layer. Additional difference bands of very low intensity were also observed at 2930 and 2856 cm-1. Combination bands were observed in all kaolinites at 2137 and 2227 cm-1. Each of the 3 major combination spectral regions was composed of 5 component bands corresponding to the 4 IR active and the 1 Raman active kaolinite hydroxyl stretching frequencies. Combination bands were also observed at ∼1932 and 1821 cm-1.
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