Reversible binding of multivalent ions to thermo-responsive micellar systems

Abstract

Binding of multivalent ions is of major interest for many fields of water purification technology, ranging from household applications to drinking water processes. Current technology is based on the use of ion exchange resins, which have the major drawback that a chemical regeneration step is necessary during the process, leading to environmentally undesired waste streams. In this thesis, two novel polymer micellar systems are described that have the special ability to reversibly bind multivalent ions, thereby avoiding the need of a chemical regeneration step. Both systems contain thermo-responsive polymer blocks and anionic polymer end-groups, which give these systems the ability to bind and release multivalent ions as a function of temperature. The first system consists of carboxylic acid end-standing triblock Pluronic At temperatures below the critical micellization temperature (CMT) the Pluronic molecule and multivalent cations are present in solution as unimers or free molecules. The affinity of isolated anionic groups for multivalent cations is very low. Increasing the temperature above the CMT brings the anionic groups present on different surfactant chains together, thus allowing for the sequestering of the multivalent cations. This binding has been described and analyzed using isothermal titration calorimetry and a conductometric analysis method. The second surfactant system is constituted of a hydrophobic polystyrene (PS) block and a hydrophilic block. Two different hydrophilic monomers were used to generate the hydrophilic block: poly(N-isopropylacrylamide) (PNIPAM) and poly(di-ethyleneglycolacrylate) (PDEGA). The synthesis of this surfactant system involves the Atom Transfer Radical Polymerization of the different monomers. In order to bind multivalent ions, the synthesized surfactants are provided with a negatively charged functional end group. Results show that the synthesis of the surfactant of PS and PDEGA was most successful. The aggregation behavior of the anionic PS/PDEGA block copolymers in water is evaluated by construction of a Zimm plot, by light scattering and cryoTEM measurements. With respect to size and aggregation behavior, the results of these methods endorse each other. The thermo-sensitive properties of the block copolymers in water are investigated using UV-Vis spectroscopy. The block copolymer has a Lower Critical Solution Temperature (LCST), which value depends on the block length and the concentration in water. Micelles consisting of the PS/PDEGA surfactants form negatively charged surfaces as a result of the carboxylate end-groups of the surfactant Therefore, they have the ability to bind positively charged ions. Because of the LCST of PDEGA, there is a sudden change in the solubility of the micelles when the temperature is increased. As a result, the dielectric permittivity of the water around the micelle changes, which leads to an apparent pKa-shift of the carboxylate groups. Results of dialysis experiments show that the surfactant binds and releases calcium ions reversibly by altering the temperature of the water to values below and above the LCST. The hydrophobic-induced pKa-shift of the PS/PDEGA micelles is confirmed by a Self Consistent Field analysis. In this analysis, the properties of the above described polymer micelles is investigated as a function of the solvent quality for the corona chains. The use of PS/PDEGA micelles is probably most feasible in water softening applications, since the micelles can bind hardness ions at room temperature. The available water streams at an elevated temperature can be used to regenerate the water softener. Possible modifications to make this novel technique commercially more attractive involves an increase in charge density of the anionic active groups and immobilization of the thermo-reversible system on an inert, insoluble carrier. Considering the worldwide trend towards environmental friendly applications and the growing interest of thermo-reversible polymers, the long-term perspective of anionic thermo-reversible polymers with a hydrophobic pKa-shift seems promising.