Structure of Hydrophilic Self-Assembled Monolayers: A Combined Scanning Tunneling Microscopy and Computer Simulation Study

Monolayers of thiols self-assembled on Au (111) can be made hydrophilic by functionalizing the surface with polar endgroups. The authors present scanning tunneling microscopy (STM) images of 2 such hydrophilic monolayers, one terminated by hydroxyl (mercaptoundecanol) and the other by amino groups (mercaptododecylamine). Both surfaces have a striped appearance but the period of the pattern is different. The authors examine the role of the formation of H bonds between the mols. in the layer and with polar coadsorbates (H2O and solvent) by comparing to STM images of nonpolar self-assembled monolayers and the results of mol. dynamics simulation. The structure of the amino-terminated layer is compatible with a H bond-induced reconstruction of the clean layer. The more dense stripe pattern obsd. for the hydroxyl-terminated layer is explained better by coadsorption with solvent (EtOH). Simulation shows that the ordered H bond configurations that might be stable for dry layers are largely dissolved by wetting. Similar wetting effects may also be responsible for the difficulties of obtaining mol. resoln. in STM imaging of hydrophilic layers. [on SciFinder (R)]