Techn. Hochsch., Diss., 2001--Aachen.

L'utilisation croissante d'acides organiques dans l'industrie alimentaire, chimique et pharmaceutique entraîne le développement de nouvelles technologies pour leur isolement, séparation et concentration à partir de solutions complexes. Les procédés électro-membranaires constituent une voie prometteuse. Afin d'intensifier ces procédés, il est nécessaire de mieux comprendre les mécanismes de transport de la solution d'ampholytes dans le système électromembranaire souvent couplé à des réactions chimiques. La composition des formes ioniques peut en effet varier en fonction du pH de la solution. Les principaux objectifs de ce travail sont l'étude du comportement des systèmes membranaires contenant des solutions d'ampholytes dans un état d'équilibre (sans force de transfert ou sous très faible courant alternatif), et hors d'équilibre en régime d'électrodialyse (application d'un courant). Dans les deux cas, l'approche comprend une partie expérimentale et une partie théorique de caractérisation de transport de solution complexe. Dans la cadre de la modélisation associée, on a développé un modèle de système membranaire qui permet d'accéder à la distribution des formes d'ampholyte à l'intérieur et à l'extérieur de la membrane en fonction des paramètres externes.Les résultats de la comparaison des données expérimentales et de simulation de systèmes membranaires montre et explique les spécificités des mécanismes de transfert des ions d'ampholyte associés aux changements du pH de la solution au cours de l'électrodialyse. / The wide application of organic acids in the food, chemical and pharmaceutical industry is responsible for the increased interest in the development of new technologies for their isolation, separation and concentration from the complex solutions. The electro-membrane processes are promising. The difficulty to understand the transport mechanisms of the amphoteric solution in the electromembrane system is the coupling of chemical reactions: the ionic forms composition can vary depending on the pH of the solution.The main objectives are the study of the behavior of membrane systems containing ampholyte solutions in a steady state (without transfer force or under very low AC) and in a non-equilibrium state such as in electrodialysis regime (applying a current). In both cases, a study includes the experimental and theoretical parts of characterization of the complex solution transport. In the context of modeling a model of the membrane system which can calculate the ampholyte form distribution inside and outside the membrane depending on the external parameters was developed.The comparison of experimental data and results obtained from the simulation of membrane systems containing ampholytes solutions, shows and explains the specific features in the transfer mechanism of ampholyte ions which associated with changes of the solution pH during electrodialysis and, as a consequence, with modification of ampholyte forms.

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Isoelectric trapping (IET) has become an accepted preparative-scale electrophoretic separation technique. However, there are still a number of shortcomings that limit its utility. The performance of the current preparative-scale IET systems is limited by the serial arrangement of the separation compartments, the difficulties in the selection of the appropriate buffering membranes, the effect of Joule heating that may alter separation selectivity and a lack of methods for the determination of the true, operational pH value inside the buffering membranes. In order to bolster the current membrane pH determination methods which rely on the separation of complex ampholytic mixtures, a fluorescent carrier ampholyte mixture was synthesized. The use of a fluorescent mixture allows for a reduced load of carrier ampholytes, thereby reducing a possible source of error in the pH determinations. A mixture of carrier ampholytes tagged with an alkoxypyrenetrisulfonate fluorophore was shown to have suitable fluorescence and ampholytic properties and used to accurately determine the pH of high pH buffering membranes under actual IET conditions. In a more elegant solution to the difficulties associated with pH determinations, a method utilizing commercial UV-transparent carrier ampholytes as the ampholyte mixture to be separated was developed. By using commercial carrier ampholytes and eliminating the need to synthesize, purify, and blend fluorescently tagged ampholytes, the new method greatly simplified the determination of the operational pH value of the buffering membranes. In order to address the remaining limitations, a new system has been developed that relies on (i) parallel arrangement of the electrodes and the collection compartments, (ii) a directionally-controlled convection system for the delivery of analytes, (iii) short anode-to-cathode distances, (iv) short intermembrane distances, and (v) an external cooling system. This system has been tested in four operational modes and used for the separation of small molecule ampholytic mixtures, for the separation of protein isoforms, and direct purification of a target pI marker from a crude reaction mixture.

Multidimensional characterization of polyelectrolytes and interpolyelectrolyte complexes in aqueous solutions Abstract: This PhD thesis is dedicated to the study of polyelectrolytes and their self-assembly in aqueous solutions. The morphology and ionization state of individual polymer chains were investigated as well as their co-assembly with oppositely charged species. First, the charge regulation of short and long weak polyelectrolytes in aqueous solution was investigated to deeper understand their pH-responsive properties. The changes of ionization degree and charge of oligopeptides composed of 5 amino acids with acid side-chains and 5 with basic side chains were followed upon varying the pH in order to reveal the effect of interactions between the like-charged and oppositely charged groups. It was shown that intra-molecular electrostatic interactions and conformational flexibility caused the suppression of the total charge and enhancement of ionization of the peptides. To get an insight into the distribution of local H+ concentration in the proximity of a polyelectrolyte chain, a modified polyelectrolyte structure was used with a fluorescent pH- indicator, covalently attached at the end of the chain. Ionization of the pH-responsive fluorophore revealed its effective pK which was compared with the local...