Nucleation and growth of glycine crystals on self-assembled monolayers on gold

Control of crystal morphology is critical in many pharmaceutical and food a pplications. Here we show that SAMs and mixed SAMs of rigid thiols on gold can serve as nucleation planes and modify the morphology of glycine crystal s. Self-assembled monolayers (SAMs) and mixed SAMs of 4'-hydroxy-4-mercapto biphenyl, 4-(4-mercaptophenyl)pyridine, and their mixed SAMs with 4'-methyl -4-mercaptobiphenyl were prepared on gold (111) surfaces and used as templa tes for the nucleation and growth of glycine crystals. Glycine nucleates in the alpha-glycine structure independent of hydroxy or pyridine surface con centration. The crystallographic planes corresponding to the nucleation sur faces, for the different SAM surfaces under study, were determined by inter facial angle measurements. For nucleation on 100% OH surfaces, the glycine crystallographic plane corresponding to the nucleation is {011}, whereas fo r the 0 and 50% OH surfaces, the crystallographic plane corresponding to th e nucleation surface is a {hOl} face, probably {101}. For 25%, 75%, and 100 % surface pyridine concentrations, the crystallographic planes correspondin g to the nucleation are {010}, {121}, and {1105}, respectively. These diffe rences are attributed to differences in H-bonding between glycine molecules in the nucleating layer and the SAM surface. As interfacial H-bonding incr eases, the dipoles of glycine molecules within the crystal become more perp endicular to the SAM surface. The direction of dipoles of glycine molecules that nucleated on a pyridine surface are not as close to the surface norma l as those of molecules that nucleated on hydroxyl surface. This implies th at the overall H-bonding interactions between the CO2- and NH3+ groups of t he glycine and the hydroxyl groups of the SAMs surface are stronger than th ose between the NH3+ and the pyridine group.