Simulations of Polyelectrolyte Interactions in Salt

Charged polymers, polyelectrolytes (PEs), are versatile synthetic materials with applications ranging from water treatment to fuel cells. PEs are abundant also in nature as many biological macromolecules, most notably DNA, are charged polymers. A characteristic property of PEs is that oppositely charged PEs attract and readily form complexes with each other, or other charged molecules and surfaces. This ability to complex is the basis of many applications of PEs. It can be utilized to build up thin, multilayered films consisting of up to hundreds of layers of oppositely charged PEs. Another application of PE complexation is in gene therapy, where DNA-polycation complexes can be used as means of delivering the genetic material into a cell. PE complexation is sensitive to the presence of additional salt. Salt can e.g. speed up the equilibration of PE complexes and dissolve polyelectrolyte multilayers. This thesis aims to elucidate the mechanisms through which salt affects PE interactions in complexation. This is done by using both all-atom molecular dynamics (MD) simulations of PE complexes and Monte Carlo simulations of a simplified model where the opposite charge PEs are modelled as rigid, charged rods. The MD simulations of this thesis demonstrate how salt dissolves a DNA-polycation complex by breaking the PE-PE contacts, and explain why multivalent ions are more effective in dissolving the complex compared to monovalent ions. The connection between the polyanion-polycation charge ratio in the complex and the complex behavior in salt is also investigated. Decreasing the polycation charge density seems to destabilize complexes in salt solutions. The MC simulations are used to extensively map out the interactions of oppositely charged PEs as function of salt concentration, salt valency, and PE charge ratio. Two mechanism leading to repulsion between oppositely charged rods are discovered: asymmetric overcharging and repulsion due to osmotic contributions. The latter is accompanied by a formation of a double minimum in the free energy landscape of the two approaching, oppositely charged rods. The location and depth of the secondary minimum are affected by salt concentration, salt type and the ratio of rod charges. The results thus suggest ways of tuning the interactions in PE systems by controlling the PE charges and the salt content of the solution. The findings of this thesis can be used to better understand and design materials based on PE complexation, such as the polycationic DNA carriers. In addition, this thesis presents a methodological advance related to the modelling of electrostatics in simulations. This novel modification of the Ewald summation enables more efficient simulations of charged, cylindrical macromolecules and facilitates the comparison of the simulation results to approximate theories utilizing similar geometries.