Molecular Dynamics Study On Ion Specific Effect Of Polyzwitterionic Brushes

It is well-known that polyzwitterionic brush-modified solid surfaces can effectively resist protein adsorption. Generally, the hydration layer of polyzwitterionic brushes formed via electrostatic interactions or hydrogen bonds is thought to be responsible for their excellent protein resistance properties. On the other hand, polyzwitterions usually have the so-called antipolyelectrolyte effect. For example, polyzwitterionic brushes would adopt a collapsed conformation at a low ionic strength with a low extent of hydration due to the electrostatic inter/intrachain association. In contrast, polyzwitterionic brushes would exhibit an extended conformation at a high ionic strength with a relatively high extent of hydration because the electrostatic inter/intrachain dipole-dipole interaction is weakened by the increase of ionic strength. Considering that the hydration of polyzwitterionic brushes is related to their conformation, their protein resistance properties should also be correlated with the conformation of the brushes.It has been widely recognized that the conformation of polyzwitterionic brushes is affected by ionic strength, pH, and temperature. Indeed, the conformation of polyzwitterions is also influenced by ion type. In other words, there is the specific ion effect in the system. In this thesis, we will studied the salt effects on the conformational behavior of PSBMA brushes in the different saline solution by using the molecular dynamics simulation method.Because the extent of hydration of the anions increase following the series F- > Cl- > Br- > I-, and the quaternary ammonium on the zwitterionic unit is a weakly hydrated group, a stronger interaction between the quaternary ammonium group and the weaker extent of hydration of anion would cause a weaker inter/intrachain dipole-dipole interaction, thereby leading to a stronger interaction between the negatively charged sulfonate group and the water molecules, resulting in a higher extent of hydration of the grafted chains. For the control of protein adsorption/desorption, our studies show that the brushes can more effectively resist the protein adsorption in the presence of a more chaotropic anion and a more chaotropic anion can also more effectively induce the protein desorption from the surface of the brushes.