Molecular Simulations On Polyelectrolyte-Colloid Solutions

Due to a range of related important applications, such as colloidal stability, electrokinetics, viscosity of suspension and the complexations of biological system, the investigation on solution containing colloids and polyelectrolytes gains a lot of scientific attention in the last decade. Our work can be divided into three parts: Monte Carlo(MC) simulations on complex of polyelectrolyte with oppositely charge spherical colloids, phase behavior of spherical polyelectrolyte brush in solution containing oppositely charge linear polyelectrolytes and MC simulations on the complexation of lysozymes and branched polyelectrolyte.A canonical Monte Carlo simulation is performed to investigate the microstructure and the electrical double layer (EDL) of polyelectrolytes around macroions in the bulk systems based on the primitive model. We explore the influences of particles size, chain length and charge density of polyelectrolytes on microscopic behavior of the macroions-polyelectrolytes systems. The simulation results show that the surface charge density and the chain length of the polyelectrolytes are two key factors to affect the microstructure of polyelectrolytes around the macroions and potential of mean force between the macroions as well as the zeta potential of the spherical EDL constructed by polyelectrolytes. The high surface charge density of a polyelectrolyte leads to the polyelectrolyte acting as a bridge for the aggregation of macroions, causing the presence of the attraction between macroions. The polyelectrolytes with long chain length present a cooperativity effect for the adsorption of the polyelectrolytes on the surface of the macroions. Furthermore, the two key factors both induce the overcharge of the macroions. The longer the chain length and the higher surface charge density of the polyelectrolytes, the stronger is the overcharge.In addition, due to the development of synthesis technology of polymer, a great range of polyelectrolyte with "vagarious" structures appear, including the polyelectrolyte brush system. On basis of a coarse grained model, we investigate the conformational behavior of a spherical polyelectrolyte brush (SPB) in solution containing oppositely charged linear polyelectrolytes. Our results obtained from Brownian Dynamics (BD) simulations show that with increasing amount of linear polyelectrolytes the SPB undergoes the process of swelling→collapse→re-swelling. The collapse of the SPB is due to the replacement of confined counterions by linear polyelectrolytes and is well described within a theoretical mean field approach. This replacement and a strong correlation between linear chains and SPB chains lead to a drop in the osmotic pressure inside the SPB. The re-swelling is caused by further adsorption of linear chains and counterions. This in turn results in an enhanced excluded volume effect within SPB. A weak charge inversion of the SPB complex is observed. With increasing length of linear polyelectrolytes the collapse of the SPB and its re-swelling is shifted towards lower concentrations of linear chains at which both effects occur. An increasing grafting density induces a multilayer structure of adsorbed linear chains and SPB chain segments. The packing process in turn increases the thickness of the SPB. We find that adsorbed linear polyelectrolytes are significantly denatured compared to the free ones in solution.Finally, based on the discretely charged sphere model of lysozyme, release behavior of lysozyme from the branched polyelectrolyte-lysozyme complexation is investigated by adding salt and changing the pH values of the solution. It is found that with the increase of the salt ionic strength of the solution, the lysozymes are gradually released from the oppositely charged polyelectrolyte due to the screening of electrostatic attraction between the two ionic species by the adding salt. Interestingly, there exists a critical salt ionic strength at which all proteins are released from the branched polyelectrolyte, and the polyelectrolyte-protein complexation is broken completely. Beyond the critical value, the increase of the salt ionic strength causes self-association of the proteins released from the branched polyelectrolyte-protein complexation. The self-association of the protein is detrimental in biological system. By calculating the second virial coefficient, we found that the optimal salt content for the dispersion of proteins coincides with the critical ionic strength, because the second virial coefficient reaches its maximum at the critical ionic strength. Similarly, increasing pH value of the solution can also release the lysozymes from the polyelectrolyte, because the increase of pH value of the solution changes the charge distribution and net charge of the lysozyme, and weakens the attraction between lysozymes mediated by polyelectrolyte, and finally leads to the dissolution of the complexation of branched polyelectrolyte with lysozymes in strong alkaline solution. In addition, by exploring the effect of architecture of the polyelectrolyte on the release behavior of proteins, we found that it is more difficult to release proteins from the branched polyelectrolyte than from the linear polyelectrolyte.