A Study of the Packing Density and Molecular Orientation of Bimolecular Self-Assembled Monolayers of Aromatic and Aliphatic Organosilanes on Silica
M B. Smith, K Efimenko, Daniel A. Fischer, S E. Lappi, P E. Kilpatrick, Jan Genzer
Bimolecular self-assembled monolayers (SAMs) of aromatic and aliphatic chlorosilanes were self-assembled onto silica and their characteristics were established by contact angle measurement, near-edge X-ray adsorption fine structure spectroscopy, and Fourier transform infrared spectroscopy. Three aromatic constituents (phenyltrichlorosilane, benzyltrichlorosilane, and phenethyltrichlorosilane) were studied in combination with four aliphatic co-adsorbates (butyltrichlorosilane, butyldimethylchloro-silane, octadecyltrichlorosilane, and octadecyldimethylchlorosilane). The aromatic moieties varied in the length of the polymethylene (PM) linker between the phenyl ring and the silicon atom; the aliphatics differed in the lengths of their PM chains and in their head-group chemistries (mono- and trichloro- silanes). Our results demonstrate that, while SAMs made of trichlorinated organosilanes are densely packed, SAMs prepared from monochlorinated species are less dense and poorly ordered. The best orientation and packing was detected in SAMs made of octadecyltrichlorosilane. In mixed systems, trichlorinated aromatics and trichlorinated aliphatics formed SAMs with highly tunable compositions; their surfaces were compositionally homogeneous with no large-scale domain separation. The homogeneous nature of the resulting SAM was a consequence of the formation of in-plane siloxane linkages among neighboring molecules. In contrast, when mixing monochlorinated aliphatics with trichlorinated aromatics, molecular segregation occurred primarily because of structural incompatibilities of the molecules at the head group; the molecules were incapable of locking into place through polysiloxane polymerization at the silica surface. While the two shortest aromatic species did not display significant changes in orientation upon mixing with aliphatics, the aromatic species with the longest polymethylene spacer, phenethyl trichloro silane, displayed markedly different orientation behavior in mixtures of short- and long-chain aliphatics.
, Efimenko, K.
, Fischer, D.
, Lappi, S.
, Kilpatrick, P.
and Genzer, J.
A Study of the Packing Density and Molecular Orientation of Bimolecular Self-Assembled Monolayers of Aromatic and Aliphatic Organosilanes on Silica, Langmuir
(Accessed November 30, 2023)