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)11CH3 and [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.