Hydrogen-bonding networks of dialkyl disulfides containing the urea moiety in self-assembled monolayers

Abstract

Self-assembled monolayers (SAM) of two newly synthesized dialkyl disulfides containing the urea moiety, 4-(3-octadecylureido)phenyl disulfide (ODPD) and 3-(octadecylureido)ethanedisulfide (ODED), were prepared and studied by infrared spectroscopy. Infrared reflection-absorption (IRRA) spectra of the SAMs showed almost no amide I band, whereas this band appeared strongly in diffuse reflectance infrared Fourier transform (DRIFT) spectra of the corresponding bulk systems. This suggests that the molecules in the SAMs form a hydrogen-bonding network in which the N-H and C=O bonds are parallel to the monolayer surface. Further evidence supporting the existence of hydrogen-bonding networks in the SAMs of ODPD and ODED was obtained from experiments in which octadecyl disulfide (ODDS) molecules, which act as barrier molecules, were added into the ODPD and ODED SAMs. Comparison of the IR spectra of SAMs with and without ODDS indicated that addition of ODDS degraded the hydrogen bonding between ODPD (or ODED) molecules. Furthermore, the orientations of the alkyl chains in the SAMs of ODPD and ODED were determined to probe the influence of the linker between the urea and sulfur moieties in the ODPD and ODED molecular structures (ODPD has a benzene ring as the linker and ODED has an ethylene linker). The analysis showed that the ODPD and ODED alkyl chains have tilt angles of 15.0degrees and 18.1degrees with respect to the surface normal, respectively. This result indicates that the molecular architecture of the SAMs containing the urea moiety is not affected by the type of linker; instead, hydrogen bonding and van der Waals interactions between the alkyl chains are the principal determinants of the SAM structure.