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

Abstract

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 (SiO 2), polyethylene (PE) and polytetrafluoroethylene (PTFE). The SAMs were prepared on SiO 2/Si substrates by a chemical vapor deposition (CVD) method in which an alkylsilane, that is, octadecyltrimethoxysilane [CH 3(CH 2) 17Si(OCH 3) 3, i.e. ODS] or a fluoroalkylsilane, that is, (heptadecafluoro-1,1,2,2-tetrahydrodecyl) trimethoxysilane [CF 3(CF 2) 7CH 2CH 2Si(OCH 3) 3, i.e. FAS] was used as a precursor. The SAM-covered Si plates were found to be 35–65% less charged than the SiO 2/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 SiO 2/Si (∼pH 2.0). These results are attributable to a reduction in the density of surface silanol (Si–OH) groups on the SiO 2/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.