pH-dependent complexation of polyelectrolytes and spherical nanoparticles
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- Institutt for fysikk 
The adsorption and desorption of polyelectrolytes (PEs) on other macromolecules are utilized in a number of different industries and biological applications. Electrostatic interactions are important in these systems, and if any of the constituents are pH-dependent, the interactions can be tuned by changing the pH of the solution. In this thesis, the complexation of annealed, i.e. pH-dependent, PEs with oppositely charged nanoparticles (NPs) have been investigated using coarse-grained Monte Carlo simulations. In particular, the conformational properties and charge of complexes formed under different conditions have been looked into. This has been done mainly in the context of the stability of NP suspensions, and in some part inspired by PE multilayer systems. By considering multiple PE chains with varying length, the effect of PE concentration in presence of a NP is studied. It is found that the degree of adsorption depends both on chain length and concentration. Longer chains adsorb more monomers and are able to overcharge the complex. Titration curves are shown to depend both on PE length and concentration, with more complex titration curves forming. These occur due to a high initial rate of ionization on adsorption on the NP, followed by a slower rate as the PE neutralizes the NP, while increasing again due to exclusion of PE monomers from the complex when it becomes sufficiently overcharged. The annealed PEs are also compared with quenched PEs of similar fractional charge, in the form of homopolymers. A difference in adsorption and the resulting conformational properties is found, especially at low fractional charge. This is seen both for PEs in presence of a single NP, and between two oppositely charged NPs. This behaviour can be attributed to the charge regulation of the annealed PE. It is also found that in systems where both PE and NP are titratable, the difference in the acid dissociation constant between the oppositely charged PE and NP have a strong influence on the interaction strength, where larger differences enhance the charge of both macromolecules, and also increase the pH-range where adsorption occurs. In addition, different PE architectures are studied. Branching of the PEs are shown to affect the charge behaviour due to a more compact architecture, while also displaying a larger window of adsorption compared to a linear PE. This is likely due to the difference in conformational entropy, as the linear PE has a higher conformational entropy which penalizes adsorption. The work done in this thesis extends upon work which has been done previously, much of which has been on systems with quenched PEs and NPs, and improves our understanding for the role of electrostatic interactions in these systems. The results found here can potentially aid in the evaluation of experimental data and improve the design of function-specific systems.