Analysis of pH-dependent Structure and Mass Transfer Characteristics of Polydopamine Membranes by Molecular Dynamics Simulation

Detailed atomistic structures are constructed for polydopamine membranes containing different amounts of catechol and quinone groups to investigate the effect of pH value in the membrane casting solution on sorption and diffusion of small gas molecules (water and propylene) in the membranes. Interactions between dopamine oligomers are calculated, and it is found that the interactions decrease from − 2356.52 kJ·mol− 1 in DOP-1 to − 1586.69 kJ·mol− 1 in DOP-3 when all of the catechol groups are converted to quinone groups. The mobility of polymer segments and free volume properties of polydopamine membranes are analyzed. The sorption quantities of water and propylene in the membrane are calculated using Grand Canonical Monte Carlo method. The sorption results show that water adsorbed in DOP-1, DOP-2 and DOP-3 are 17.3, 18.6 and 20.0 mg water per gram polymer, respectively, and no propylene molecule can be adsorbed. The diffusion behavior of water molecules in the membrane is investigated by molecular dynamics simulation. The diffusion coefficients of water molecules in DOP-1, DOP-2 and DOP-3 membranes are (1.80 ± 0.52) × 10− 11, (3.40 ± 0.64) × 10− 11 and (4.50 ± 0.92) × 10− 11 m2·s− 1, respectively. The predicted sorption quantities and diffusion coefficients of water and propylene in the membrane present the same trends as those from experimental results.

Graphical AbstractThe molecular simulation results reveal that water-accessible free volume voids in the gaps among multilayer structures stacked by dopamine oligomers tend to connect each other and form water channels, providing pathway for the rapid diffusion of water molecules.Figure optionsDownload as PowerPoint slide