Recognition events at the molecular level, between small molecules and crystal surfaces, can be translated into observable micro- and macroscopic features and materials properties. To this end, a family of bis-urea dicarboxylic acids hydrogelators were designed and synthesized to study the effect of a gel microenvironment, which differs in regards to diffusion, ion activities, and water “structure” as compared to a solution, on the growth of calcium carbonate crystals. The gel was characterized using SEM, cryo-TEM and X-ray diffraction which revealed that at the microscopic level the gel was composed of fibers. This data also was used to model the molecular packing in the gel.

Calcium carbonate crystals formed in these organic hydrogels appeared to initially grow and then become etched over time. If crystals were removed from the gel after 3.5 hours, at which point they showed no sign of etch pits, and then placed in aqueous solutions, etch pits, similar to those observed on crystals that are left in the gel, were observed. Control calcite crystals exposed to similar conditions (water or buffer) show no significant dissolution after equivalent times. A probable cause of the increased solubility of gel-grown calcite is the occlusion of gelator material inside the growing crystal lattice. This system allows the opportunity to study the unique properties a gel has for influencing the nucleation and growth of inorganic crystals and to gain insight into key factors governing biomineralization.

In related work, an oligopyridine foldamer, which projects carboxylates from one face to provide a geometrical match to the carbonates in the calcite structure, was synthesized. At low concentrations, the trimeric structure has a striking effect on the morphology of calcite crystals grown in its presence. In the presence of a related monomer, only calcite rhombohedra are formed, indicating that it is the ordered array of carboxylates that causes the morphological changes, via a specific interaction between the foldamer and the newly expressed faces of the growing calcite crystals. The new faces were identified as the {01l} where 0.5 < l < 1, which are roughly parallel to the c axis of calcite making them good candidates for the interaction of acidic macromolecules such as the trimeric foldamer.