(367g) Water Desalination through Molecular Layer-By-Layer Membranes: Insights from Molecular Dynamics Simulations

Reverse osmosis (RO) membranes based on polyamide (PA) membranes synthesized via interfacial polymerization (IP) of trimesoyl chloride (TMC) and m-phenylenediamine (MPD) represent one of the most common membranes used for producing potable water from seawater. However, typical RO membranes synthesized via IP can have several drawbacks which can impact the efficiency of the membrane. In particular, IP membranes lack control over (1) film thickness, (2) surface roughness, (3) cross-linkage, and (4) local chemical composition, which influence the membrane performance and efficiency. As an alternative to these challenges, ultraâ??thin PA membranes synthesized via molecular layer-by-layer (mLbL) deposition have been introduced.

Although PA-mLbL have been studied in previous work, there are still challenges in both experimental characterization and understanding the relationship between membrane performance and membrane variables (grafting density and monomer ratio). In this work, molecular dynamics simulations were employed to characterize and to investigate membrane performance with respect to these membrane variables. We first applied a general simulated polymerization algorithm, Polymatic, developed in our group [Theor. Chem. Acc. 2013, 132, 1334] to virtually synthesize RO membranes via mLbL deposition on silicon substrates. The resulting simulated structures for twenty cycles are characterized by analyzing densities, membrane thickness, free volume, swelling, molecular orientations and local chemical compositions. Membrane performance was evaluated by analyzing water and salt dynamics in mLbL membranes with respect to grafting density and monomer ratio. This simulation study provided molecular level insight into water desalination using PA-mLbL membranes and illustrates that molecular simulations can play a crucial role in the understanding of next generation RO membranes synthesized via mLbL deposition.