Self-assembled monolayers of positional isomers of (12-mercaptododecyloxy)phenol: The role of molecular structure on interfacial properties

Self-assembled monolayers of 3-(12-mercaptododecyloxy)phenol and 4-(12-mereaptododecyloxy)phenol are prepared in an effort to probe the role of the orientation of the terminal hydroxyl group on interfacial properties. Differences in the interfacial properties of these isomeric monolayers are characterized by reflection-absorption infrared spectroscopy, reductive desorption, and contact angle measurements. The coverage of 4-(12-mercaptododecyloxy)phenol is 40%-60% lower than that of 3-(12-mereaptododecyloxy)phenol, suggesting that structural differences between these monolayers contribute to the monolayer organization. This is supported by differences in the infrared spectra which show that the monolayers have different organization at the interface. Contact angle measurements using solutions buffered at pH 7 show that monolayers of 4-(12-mereaptododecyloxy)phenol have a lower surface energy than monolayers of 3-(12-mercaptododecyloxy)phenol. Contact angle titrations of these monolayer-modified surfaces also have distinctly different behavior. The titration curve of 3-(12-mercaptododecyloxy)phenol has a clear transition from low pH to high pH with an inflection near pH 9, while 4-(12-mercaptododecyloxy)phenol does not reach a limiting value at high pH. These results are interpreted in terms of interfacial structural differences between the isomeric monolayers.