Comparison of model monomeric and polymeric alkyl stationary phases on silica using sum-frequency spectroscopy

Model monomeric and polymeric stationary phases were examined using sum-frequency spectroscopy and contact angle measurements in order to determine structural differences between the two stationary phases. Octadecyldimethylchlorosiloxane (ODMS) monolayers and ODMS/methyl siloxane (MS) mixed monolayers in contact with water and acetonitrile solvent were examined. The results showed that the alkyl chains in the ODMS and the ODMS/TMS monolayer are highly disordered and are weakly sensitive to the change in solvent environment. In order to determine whether the observed disorder in ODMS is due to steric effects of the silicon bonded methyl groups or the lack of crosslinking, “surface-bonded” ODS and “highly crosslinked” ODS were studied and compared with ODMS. Calculations using contact angle data on ODMS monolayers showed that water interacts 70% with methyl groups and 30% with methylene groups.