Effect of Final Monomer Deposition Steps on Molecular Layer-by-Layer Polyamide Surface Properties

A current challenge to desalination membrane technology is the inability to precisely control the properties of the polyamide selective layer due to the complexity of interfacial polymerization. In this study, we investigate the ability of molecular layer-by-layer (mLbL) assembly, an alternative polyamide fabrication technique, to create polyamide surfaces with tunable chemistry. We explore the influence of terminating monomer, monomer deposition time, monomer size, and the presence of underlying ionizable functional groups on mLbL-derived polyamide surface properties. AFM colloidal probe measurements, contact angle titrations, QCM cesium adsorption experiments, and XPS data show that polyamide films terminated with m-phenylenediamine or trimesoyl chloride for 20-30 s are chemically similar. Increasing terminating monomer deposition time or using a smaller, more reactive monomer results in more distinct colloidal-probe adhesive interactions, contact angle titration curves, negative charge densities, and near surface atomic compositions. By optimizing the final monomer deposition steps, both amine-rich and carboxyl-rich polyamide surfaces can be fabricated, which has implications for the application of mLbL assembly to membrane-based desalination.