showed that an acetylated FF peptoid analogue may form mismatched nanosheet layers on dilution of an acetonitrile stock solution with water (i.e., kinetic assembly), but may also form crystalline needles (thermodynamic assembly) via slow evaporation from a DMSO/water solution. Solvent modification can also be used to sensitively explore chemical libraries. In our recent work, only Fmoc-A but no other Fmoc-aliphatic amino acids formed crystals in mixed water/THF, which showed that larger aliphatic sidechains competed with Fmoc directed assembly. Elsewhere, Erdogan et al. demonstrated assemblies ranging from plates to wires from VA and AV dipeptides by adjusting the addition of pyridine and 2-propanol in their assembly solutions. Overall, these studies and others highlight the importance of taking a more holistic (solvent plus solute) view of assembly. Liquid-liquid, air-liquid, and liquid-solid phase boundaries arise in many chemical contexts. These interfaces themselves may direct the formation of structures not otherwise accessible and permit additional control over material functionality. Classic examples of this latter case include lipid bilayers , Langmuir-Blodgett films and self-assembled monolayers. However, the study of interfacial peptide assembly/crystallization has been relatively less explored despite some prominent demonstrations (e.g., peptoid nanosheets and the stabilization of emulsions by peptide self-assembly ). Herein we studied the recrystallization of bromide salts for a library of peptoid bases in acetonitrile/methanol solvent mixtures and in doing so serendipitously discovered mode of interfacial crystallization (IFC). We observed that IFC structures can be up to centimeters in length/diameter and grow to fill the entire interface of a container. To characterize the key intermolecular interactions, we studied molecules with varying aliphatic side chains, which undergo IFC to different degrees (Scheme 1), and provide generalizable molecular design insights for IFC. A mechanistic explanation for this growth process was then proposed by comprehensively studying IFC structures using AFM, FT-IR, time of flight secondary ionization mass spectrometry (TOF-SIMs), quantum mechanical (QM) calculations, molecular dynamics (MD), single crystal X-ray crystallography, and salt exchange experiments. Scheme 1. Library of amidated peptoid and amino amide salts studied in this work, as well as counter ions used in IFC salt anion selectivity study. The inset shows the structural distinction between an amino acid and its peptoid monomer counterpart. The peptoid bases are named such that the single letter code of the most closely related amino acid is used (e.g., Nl is a direct peptoid analogue of leucine) and further modifications, including sidechain extension is captured with subsequent letters (e.g., Nle, is the ethylsidechain analogue of leucine).