A comprehensive computational study and simulation of innovative zwitterionic materials for enhanced poly (vinylidene fluoride) membrane hydrophilicity

Abstract

The goal of this study is to design a novel zwitterionic (ZW)-poly (vinylidene fluoride) (PVDF) membrane with high hydrophilicity potential using the pair interaction energy decomposition analysis (PIEDA) integrated with fragment molecular orbital (FMO) method. In addition, the differential hydration and efficiency of salt rejection of the novel zwitterion and original PVDF were investigated using molecular dynamics simulation (MDS). Within this study computational methods were applied to investigate the performance of zwitterionic moieties derived from three different anionic groups in the ZW head, specifically, carboxylate, sulfonate, and phosphate. This approach was used in addition to the inclusion of a linker between the ZW head and the PVDF backbone, such as trimethyl ammonium groups and hydroxyl group for an increase in PVDF membrane hydrophilicity. The quantum chemical calculations were employed to examine the hydration structure of moieties, the number of hydrogen bonding instances, and hydration free energy. The interactions between the ZW moieties on PVDF membranes with water molecules confirmed that they depended on the charged groups and the chemical groups between charged groups. The results pointed to differences in hydrophilicity, membrane water uptake due to their structural properties depending on the types of anionic groups involved, polar groups between charged groups, and the hydrophilic groups as a linker between charged groups of the zwitterions to the PVDF polymer backbone. The double zwitterionic PMAL®-C8-CB–OH–SB-PVDF was formed through protonated carboxyl group on backbone of copolymer PMAL®-C8, and protonated nitrogen atom of amide group. This double zwitterion showed strong electrostatic interactions between individual water and secondary ammonium and Oxygen of carboxybetaine, compared to PMAL®-C8-OH-SB-PVDF model. The simulated results using MDS confirmed the hydrophilicity of PMAL®-C8-CB–OH–SB-PVDF and showed that the positive and negative centers of zwitterionic polymer chains on PVDF membrane surface can interact with the ions, contributing into the increase of charge density. Our designed hydrophilic zwitterion PVDF membrane, and especially the double zwitterion membrane, is an exciting development that can be used in a broad range of water applications.