Porous Polymer Adsorbent Media Constructed by Molecular Dynamics Modeling and Simulations: The Immobilization of Charged Ligands and Their Effect on Pore Structure and Local Nonelectroneutrality

A molecular dynamics modeling and simulation approach is presented and employed to construct porous dextran polymer ion-exchange adsorbent media. Both the activation step of the surface of the pores of the dextran polymer layer grafted on an agarose surface and the immobilization of charged ligands on the activated surface of the porous dextran polymer layer are considered. For the systems studied in this work, the activation step modifies slightly the pore structure of the base, nonactivated porous dextran polymer, while the immobilization of the ligands on the activated pore surface of the dextran layer changes significantly the pore structure of the activated dextran layer. The density distributions of the counterions and immobilized charged ligands along the direction of net transport in the adsorbent media constructed in this study are found to be nonuniform. The variables that affect the shape and magnitude of the density distributions of the counterions and immobilized charged ligands as well as the total number of charged ligands that can be immobilized on the activated porous dextran layer are identified and presented in this work. Furthermore, the data clearly show that there is local nonelectroneutrality in the porous dextran polymer ion-exchange adsorbent media, and this result has very important practical implications for the operation and performance of separation systems involving ion-exchange adsorbent media (e.g., ion-exchange chromatography systems). Also, the results of this work suggest approaches for (1) controlling the immobilization process of charged ligands and (2) constructing and studying the behavior of chromatographic polymeric monoliths and packed bed columns having a gradient of density of functionalities along the axis of the chromatographic polymeric monolith or packed bed column.