We present a comprehensive study of interfacial crystallization (IFC) arising from the ionic assembly of halide salts of both amino amide and peptoid monomer bases. The structures form readily at both air-liquid and liquid-liquid interfaces, and is best displayed by halide salts of Nab, Nle and Leu-NH2. By comparing across multiple aromatic and aliphatic sidechains, it has been possible to identify that overlapping noncovalent interactions in the forms of ionic coordination with the halide counter ion, dispersion, and hydrogen bonding together support this phenomenon. The specific structural requirements, and by implication how these noncovalent interactions are accentuated, seem to be quite general, with all aliphatic peptoid and amino acid amide bases with sufficiently large sidechains forming IFC structures. In the case of Nl, the absence of IFC was explained using QM calculations and MD simulations, which revealed that steric restrictions in sampling around the 𝜒 torsion exist for Nl but not Nab/Nle. We could conclude that a degree of steric freedom about this torsion is critical to IFC formation in aliphatic amphiphiles. Importantly, IFC was shown to be controlled by interfacial ion migration and coordination, resulting in alternating hydrophobic and ionic stacked layers. This was confirmed by the structural model gathered from single X-ray diffraction crystallography and TOF-SIMs characterization. Furthermore, the centrality of ionic coordination was confirmed based on complementary experimental and MD simulation studies with different anions. The emergence of macroscale IFC from amidated peptide and amino acid monomers, which represent a diverse set of minimalistic self-assembly building blocks, makes this phenomenon a readily accessible mode by which supramolecular structures can be fabricated. IFCs are also shown to be remarkable as plane-filling and reversible systems with favorable mechanical properties. Future work could focus on developing various applications, including its use as catalyst supports (e.g., through co-crystallization), and the reversible 'sealing off' of interfaces in non-equilibrium assembly systems.