Monolayers are prepared by self-assembly of 12-phenyldodecyl mercaptan or 11-phenoxyundecyl mercaptan on gold substrates. Reflection-absorption infrared spectra of these monolayers show that the 12-phenyldodecyl mercaptan monolayer forms a well-organized interface while 11-phenoxyundecyl mercaptan monolayers do not. Infrared spectra also suggest that the C1-C4 axis of the phenyl ring is largely parallel to the Au-substrate for 12-phenyldodecyl mercaptan layers, but has a more perpendicular orientation relative to the substrate for 11-phenoxyundecyl mercaptan layers. Despite these structural differences, Zisman analysis of contact angle data shows that these monolayers have similar surface energies, 27.1 (+/-6.7) mN/m for 12-phenyldodecyl mercaptan and 26.1 (+/-5.3) mN/m for 11-phenoxyundecyl mercaptan. Fowkes analysis of the contact angle data suggest that dispersive interactions account for all of the measured surface energy for both modified interfaces. The structural differences are, however, reflected in the lateral stabilization energy measurements where monolayers of 12-phenyldodecyl mercaptan are found to be 24 (+/-5) kJ/mol more stable than those of 11-phenoxyundecyl mercaptan. (C) 2004 Elsevier Inc. All rights reserved.