ζ-Potentials of planar silicon plates covered with alkyl- and fluoroalkylsilane self-assembled monolayers

ζ-potentials of planar Si plates covered with an alkyl- or a fluoroalkyl organosilane self-assembled monolayer (SAM) were measured over a pH range of 3–11 by means of an electrophoretic light scattering spectrophotometer, and were compared with those of Si plates covered with native oxide (SiO2), polyethylene (PE) and polytetrafluoroethylene (PTFE). The SAMs were prepared on SiO2/Si substrates by a chemical vapor deposition (CVD) method in which an alkylsilane, that is, octadecyltrimethoxysilane [CH3(CH2)17Si(OCH3)3, i.e. ODS] or a fluoroalkylsilane, that is, (heptadecafluoro-1,1,2,2-tetrahydrodecyl) trimethoxysilane [CF3(CF2)7CH2CH2Si(OCH3)3, i.e. FAS] was used as a precursor. The SAM-covered Si plates were found to be 35–65% less charged than the SiO2/Si plate. Furthermore, SAM formation reduced surface acidity. The isoelectric points (IEPs) of the ODS- and FAS-SAM surfaces observed at pH 3.5–4 were higher than that of SiO2/Si (∼pH 2.0). These results are attributable to a reduction in the density of surface silanol (Si–OH) groups on the SiO2/Si substrate. Si–OH groups were consumed due to the formation of siloxane bondings with the organosilane molecules which compose the SAMs. Finally, the IEPs of the ODS- and FAS-SAM covered surfaces were found to be almost identical to those of the PE and PTFE bulk surfaces.