Nucleation and Growth of Molecular Crystals on Self-Assembled Monolayers

Self-assembled organosulfur monolayers with different interfacial chemical functionalities influence the heterogeneous nucleation and growth of malonic acid crystals. Nucleation rates of malonic acid (HOOCCH₂COOH), as deduced from the number of crystals per unit area and mean crystal size, are significantly more rapid on organosulfur monolayers terminated with carboxylic acid groups ([Au]-S(CH₂)₁₁COOH and [Au]-S(CH₂)₁₅COOH) than on corresponding monolayers terminated with methyl, methyl ester, or ethyl ester functionalities. X-ray diffraction reveals that the orientation of malonic acid crystals is affected by the monolayer composition. The malonic acid (001) plane, described by truncated hydrogen-bonded chains, is oriented parallel to the [Au]-S(CH₂)₁₁COOH monolayer interface. However, this orientation is absent on [Au]-S(CH₂)₁₁CH₃and [Au]-S(CH₂)₁₁COOCH₃. The observed behavior is consistent with interfacial stabilization of malonic acid prenucleation aggregates by interfacial hydrogen bonding and surface polar forces when carboxylic acid monolayers are employed.