IFC at Air-Liquid and Liquid-Liquid Interfaces We initially discovered interfacial crystallization (IFC) at the air-liquid interface for the peptoid base Nle (Scheme 1; see associated explanation of monomer naming convention) as a bromide salt (i.e., Nle.HBr, Figure ) during recrystallization purification in acetonitrile (ACN): methanol (MeOH) (~ 2:1 v/v and T ≥ 50 o C), as part of a wider effort to synthesize a library of small peptoids. It was found that when this solution was left to slowly cool (e.g., in a beaker covered with watch-glass for slow evaporation), floating, transparent crystalline sheets formed at the air/liquid interface. Over time these grew to ultimately fill the surface (Figure ) and can fuse to form very large structures, e.g., across ~15.5 cm 2 for a 50 mL beaker (Figure and Video S1). More rapid cooling and evaporation in an uncovered set-up, produced interfacial structures that appeared more disordered, though the interface was similarly filled (Figure ). The process was found to be fully reversible, by the addition of methanol and warming as it is in essence a conventional recrystallization system. Seeking to establish the interfacial nature of IFC, we cooled the ACN: MeOH in a sealed vial, after which hexane, an immiscible solvent with lower density, was gradually added down the side of the container, and similar layers were also found at the biphasic liquid-liquid interface (Figure ). Thus, IFC appears to be a general phenomenon across chemical phase boundaries. Optical, scanning electron, and atomic force microscopy inspections of the transparent Nle.HBr IFC structures revealed these to be composed of terraced layers, that extend from the microscale down to the nanoscale (Figures 1A, S5-29). The crystal edges were found to be straight within the micron to the milli-and centimeter length scales. AFM and SEM measurements reveal a combination of larger straightedged terraces, which are clearly observed in line-scans of atomic force microscopy (AFM) images, and discreet layers with a uniform height of 1.5~1.6 nm steps (Figures and, and S50-53). Notably these discreet layers have curved edges which we propose as being indicative of the crystal growth pathway, in which successive layers stack on top of each other within the solution phase. As this occurs lateral growth of individual layers is slowed, giving rise to curved terrace steps on the surface. While floating at the liquid interface crystalline sheets can fuse or co-exist, and this gives rise to boundaries of individual crystallites which are observed in optical microscopy. We further prepared a set of related peptoid base bromide salts (Scheme 1) to understand the generality of the IFC phenomenon. Together with Nle, Nab, Nf, and Nfe all exhibited IFC. Visually, all species formed transparent crystal layers with well-defined edges (Figures , S5-29; see also Table for ACN: MeOH mixing ratios). Like Nle, Nab, also readily formed IFC layers, regardless of their linear or branched sidechains. However, Nl crystals were not formed at the interface but in solution and vial sides. They were also visually opaque due to light scattering from a crenulated surface morphology on the microscale (Figure ), suggesting a different crystal structure (see AFM and FTIR characterization below). IFC layers of the aromatic Nf and Nfe would form, but eventually settle into the solution. This observation suggests that the aromatic species give rise to crystals which have denser packing verses the aliphatic species and hence they do not float at the interface. In fact, Nfe and its analogues feature prominently in a number of peptoid assemblies, and the additional flexibility of its longer ethylene sidechain linkage to the aromatic group may simply aid in optimizing structural packing. Nanoscale Surface Morphology, Mechanical Stiffness, and Hydrogen Bonding AFM measurements of Nab and Nl confirmed the overall macro-and microscale observations. Specifically, like Nle, Nab exhibited a layered surface morphology composed of microscale terraces (Figure ) and step heights of 1.2 -1.4 nm (ESI, Figures ). In contrast, Nl surfaces showed no clear structure (Figure ). AFM was also used to probe the crystals' mechanical properties. Nab and Nle exhibited similar stiffnesses of 7 GPa and 6 GPa, respectively. These stiffness values are high compared to several tripeptide fibers/crystals that we and other have reported previously (e.g., previously, Adler-Abramovich reported a stiffness of 3.1 GPa for Boc-FF). However, they remain significantly softer than the Young's Modulus found for FF nanotubes (19-30 GPa) or indeed that we found for the tripeptide DYF-NH2 (~33 GPa). In contrast, the maximum stiffness measured for Nl was around 3 GPa (Figure ). Taken together, clearly Nl is structurally distinct from its linear aliphatic isomer (Nab) and ethyl sidechain analogue (Nle). Its low mechanical stiffness, irregular crystal surface and opacity implied that the internal packing of Nl clearly differed from that of Nab and Nle which are alike in these properties. To probe the difference, we analyzed hydrogen bonding, dispersion contributions and molecular conformation to assess the influence of aliphatic sidechain architecture over IFC propensity. In this effort we characterized the crystals by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Focusing on carbonyl stretch region (~1700 cm -1 ) and amide N-H bending region (~1620 cm -1 ), we found that all Nle, Nab, Nf, and Nfe IFC structures exhibited the hydrogen bonding bands at ~1690 and ~1620 cm -1 (Figures and). For Nl, these bands were instead at ~1680 and ~ 1605 cm -1 . These lower wavenumbers indicate that the hydrogen bonding network within Nl is different from the other compounds in this study. To further characterize this, we compared the ratio of N-H bend: carbonyl stretch absorbance's. For Nl this was ~0.45 but consistently around ~0.8 for all other aliphatic and aromatic peptoid bases (Figure 2F-G and Figure ), with the exception of Nfe which is around 0.6. We interpret this as indicating that the hydrogen bonding density in Nl is not only distinct but reduced in strength when compared to other peptoid bases which is consistent with the significantly lower stiffness measured by AFM. Packing within the crystal structure was thought to be a possible cause of this measured difference, and so we next sought to assess whether the dispersive contributions from a given aliphatic sidechain architecture might be descriptive of IFC propensity.