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

Abstract

Monolayers of thiols self-assembled on Au(lll) can be made hydrophilic by functionalizing the surface with polar endgroups. We present scanning tunneling microscopy (STM) images of two 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. By comparing to STM images of nonpolar self-assembled monolayers and the results of molecular dynamics simulation, we examine the role of the formation of hydrogen bonds between the molecules in the layer and with polar coadsorbates (water and solvent). We find that the structure of the amino-terminated layer is compatible with a hydrogen bond induced reconstruction of the clean layer. The more dense stripe pattern observed for the hydroxyl terminated layer is better explained by coadsorption with solvent (ethanol). Simulation shows that the ordered hydrogen bond configurations that might be stable for dry layers are largely dissolved by wetting. It is argued that simular wetting effects may also be responsible for the difficulties of obtaining molecular resolution in STM imaging of hydrophilic layers. © 1994, American Chemical Society. All rights reserved.