Vesela Malinova
Despite progress during the last decade, the behavior of charged macromolecules, polyelectrolytes, including their interactions is not yet completely understood. In particular, interactions with oppositely charged surfaces, which play a significant role in many practical applications and are of central importance in life processes, need more investigation. This situation has motivated the selection of the topic of this doctoral thesis. The study is dedicated to interactions of positively charged polyelectrolytes with negatively charged surfaces considering two types of surfaces, flat amphiphile monolayers and porous surfaces of microspheres. The research aimed at quantitative evaluation of the influence of macromolecular dimensions, chemical structure, electrostatic parameters, and surface characteristics on the adsorption process. As a prerequisite, a series of poly(vinylbenzyltrialkylammonium chloride) model polyelectrolytes has been synthesized. The characteristics were five defined contour lengths with three different types of substituents at the quaternary ammonium group each in addition to uniform charge density and narrow chain length distribution. The final polyelectrolytes were the quaternization products of precursors synthesized by controlled radical polymerization. Two experimental techniques were employed to quantify the influence of chemical, macromolecular, and electrostatic parameters as well as geometrical characteristics on the adsorption of polyelectrolytes on porous microsphere surfaces. These were adsorption on microspheres suspended in aqueous phase and microspheres packed in a chromatography column. During the first technique the total surface comes immediately in contact with the polyelectrolyte molecules, whereas the second technique followed the rules of chromatography. For the first time, the contour length of the polyelectrolyte, the Debye length, the pore dimensions, as well as the chemical structure were likewise considered in a comprehensive study to describe the interaction of polyelectrolytes with well-characterized porous surfaces. A polyelectrolyte charge to surface charge ratio was defined and served to classify different adsorption behavior. Three models have been proposed to illustrate the adsorption behavior based on experimental findings and model calculations. The first one identifies the Debye length in addition to the contour length as dominating parameter, which decides whether a polyelectrolyte molecule can enter a pore or not. The second quantifies the influence of the contour length. Finally, the third describes the concentration influence on the type of molecule organization, layer formation, on the outer surface or inside of pores with diameters exceeding considerably the contour length. For amphiphile monolayer, for the first time, the influence of chain length and substituent at the quaternary ammonium group on the adsorption process was quantified. Based on the surface pressure, pressure-area and pressure-time isotherms measured, a model describing the monolayer arrangement on polyelectrolyte solutions was proposed. A second model correlates the increase of the area per amphiphile molecule and the decrease of the surface pressure with the size and hydrophobicity of the substituents. In addition chain length influence was evident. The results of the thesis contribute to progress in basic research in the field of polyelectrolytes, while also an impact on technical applications such as surface modification, chromatographic processes or improvement of materials is expected. Precise data for comparison with theoretical model calculations are provided, which were not available before.
EPFL2005
