2D Interfacial Crystallization Stabilized by Short-Chain Aliphatic Interfaces

We report the discovery and in-depth investigation of interfacial crystallization (IFC), the assembly and formation of membrane-like crystalline sheets from both chiral amino acid and achiral <i>N</i>-substituted glycine "peptoid" amide monomers selectively at vapor-liquid and liquid-liquid interfaces. This is the first assembly process known to be shared by two peptidomimic families of molecules with crucial backbone differences. A series of AFM, SEM, TOF-SIMS, FTIR, X-ray crystallography, counterion screening experiments, QM calculations, and MD simulation studies identified that IFC is based on the assembly of single monomer layers with alternating molecular orientations, which results in bilayers of unit thickness 1.2-1.6 nm consisting of internal hydrophobic planes and ionic interfaces cocrystallized with halide salt ions. The assembly is underpinned by, paradoxically, the dynamic freedom of attached side chains, especially those of aliphatic designs. Growth of these bilayers then fills entire interfaces, limited only by the size of the container. The fundamental observation of the interface-filling nanostructures and the simplicity of the monomer chemistry involved suggest that IFC may have applications in the convenient formation of interface-sealing supramolecular barriers and, more broadly, tunable 2D layered materials.