Chitosan and its derivatives as building blocks in amphiphilic complexes and for precipitation of humic acids

A polyelectrolyte (PE) is a polymer with a large number of charges of uniform sign along its chain, while counterions are screening around. Oppositely charged small objects (macroion) will be attracted by the long-range electrostatic force to polyelectrolyte, which results from the release of counterions. Once the polyelectrolyte complexes such a macroion, that leads to substantial gain of entropy. This self-assembly produces colloidally stable complexes, which possess properties from both materials. When the small objects are surfactant aggregates, whose structures are stabilized by the hydrophobic interaction and contain therefore local hydrophobic domains as potential carrier system. Introduction of PE results in the concentrating of surfactant aggregates surrounding the PE macromolecule, furthermore modify the aggregate structure at the microscopic view, form secondary polyelectrolyte-surfactant complex (PESC) at the mesoscopic and alert the system properties e.g. transparency and viscosity at the macroscopic. Therefore, such systems are particularly interesting for applications in various fields, such as formulation in the fields of cosmetic, pharmacy, detergency etc., or in drug delivery etc. One part of this study focuses on the quaternized Chitosan-alkylethoxy carboxylate PESCs. Chitosan is a natural abundant biofriendly polycation, whose pH-sensitivity can be removed by quaternization of the amino group. The commercially available anionic surfactant family has different alkyl and ethoxy chain lengths, forming a spectrum of structures from small micelles to larger vesicles. The complexation reaction of the quaternized chitosan with alkylethoxy carboxylates produces a rich variety of potential complex structures. In the experiments, the type, molecular weight (MW) and charge density of PE and the hydrophobic and hydrophilic parts of surfactants were systematically varied, as well as the mixing ratio, quantified by the charge ratio of Z = [+]PE / ([+]PE + [-]Surfactant). Based on a thorough determination of the phase behavior and employing different methods such as light and neutron scattering, detailed structural information of the formed complexes in terms of size, MW and shape was obtained. This then allows systematic approaches to control the complexation process in order to achieve specific structures and properties. Another part of this work concentrates on the removal of anionic aromatic macromolecules, i.e. humic acid, from water with selected polyelectrolytes, aiming at the real application in water plant. The experiments were designed not only to find out the best material as coagulant and flocculant, but also to clarify the structure-performance relationship behind this process. The influence of the type, MW and shape of PE on the removal efficiency was studied. Small angle X-ray scattering (SAXS) was employed to study the hydrophobic domains at nanometer scale. With the experiments, a comprehensive picture of the formed complexes was obtained. By studying the complexation mechanism, efficient methods to control the pattern of structure formation can be revealed. As conclusion, quaternized chitosan may find interesting applications in formulation and water treatment.