Hydrated polyamide membrane and its interaction with alginate: a molecular dynamics study.

The properties of the hydrated amorphous polyamide (PA) membrane and its binding with alginate are investigated through molecular dynamics simulations. The density of the hydrated membrane, surface morphology, and water diffusion near and inside the membrane are compared to other studies. Particular focus is given to the steered molecular dynamics (SMD) simulation of the binding between the PA membrane and an alginate model. The PA surface composition is determined on the basis of experimental measurements of the oxygen/nitrogen (O/N) ratio. The surface model is built using a configurational-bias Monte Carlo technique. The consistent valence force field (CVFF) is used to describe the atomic interactions in the membrane-foulant system. Simulation results show that the carboxylate groups in both the PA surface and alginate exhibit strong binding with metal ions. This binding mechanism plays a major role in the PA-alginate fouling through the formation of an ionic binding bridge. Specifically, Ca(2+) ions have stronger binding with the carboxylate group than Na(+) ions, while the binding breakdown time is shorter for Ca(2+) than Na(+) because of the comparably higher hydration free energy of Ca(2+) ions with water molecules.