Freestanding ultrathin films for separation of small molecules in an aqueous environment

Alternative separation methods operating in an aqueous environment are of increasing importance for further progress of molecular separation in life sciences and other industrial sectors working towards a biobased economy. By spincoating, membranes with thicknesses under 100 nm and 20 cm² surface area were prepared from an epoxy based resin. For the first time such ultrathin epoxy films were used for the selective separation of small molecules and metabolites within an aqueous environment. Initially, selectivity is demonstrated by the separation of two dyes of similar size (0.7 and 1.4 nm diameter). By variation of the precursor concentrations, both mechanical stability and selectivity for molecular transport are shown to be tunable. The observed transport properties of the different membranes correlated with their biaxial moduli and ultimate tensile strengths which were in the range of 0.3-3.5 GPa and 10-44 MPa, respectively. These observations agreed with the conclusion drawn from FTIR analysis that variations in the covalent crosslinking density determine the emergent properties. Finally, permeation rates for small molecules of industrial relevance were assessed to confirm a size based diffusion cutoff for compounds with hydrodynamic diameters below 2 nm.