The Effect of Hydrophilic Headgroup Chemistry on Peptoid‐Directed Mineralization of CaCO 3

Engineered mineralization of calcium carbonate (CaCO 3 ) provides one strategy for CO 2 capture removal. Developing additives to accelerate the process or control resulting polymorphs would enhance underground reservoir security and advance development of CaCO 3 ‐based building materials. Peptoids — sequence‐specific poly‐N‐substituted glycines — have been shown to accelerate CaCO 3 nucleation and growth, select for metastable polymorphs, and induce unusual morphologies, yet the underlying mechanisms remain poorly understood. Here, we investigate control of these phenomena by using amphiphilic di‐block peptoids with carboxyl (‐COOH) and amine (‐NH 2 ) rich hydrophilic blocks. We demonstrate that ‐COOH‐functionalized peptoids free in solution drive rapid crystallization of calcite directly from amorphous precursors, bypassing the metastable vaterite phase observed in control experiments. Systematically increasing the number of ‐COOH groups progressively tunes calcite morphology from perfect rhombohedra to truncated structures to spindle‐shaped mesocrystals. In contrast, ‐NH 2 ‐bearing peptoids preferentially stabilize vaterite. When assembled into crystalline nanosheets exposing the hydrophilic block, only ‐COOH‐rich peptoid sheets nucleate crystals, unequivocally identifying carboxyl moieties as the key templating group for calcite formation. These findings provide chemical insights into controls of amphiphilic peptoids over CaCO 3 formation, establishing a platform for design of peptoids to control crystallization and advancing CO 2 sequestration and sustainable materials synthesis through carbonate mineralization.