zeta-potentials of planar silicon plates covered with alkyl- and fluoroalkylsilane self-assembled monolayers

zeta -potentials of planar Si plates covered with an alkyl- or a fluoroalky l 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 prepa red on SiO2/Si substrates by a chemical vapor deposition (CVD) method in wh ich an alkylsilane, that is, octadecyltrimethoxysilane [CH3(CH2)(17)Si(OCH3 )(3), i.e. ODS] or a fluoroalkylsilane, that is, (heptadecafluoro-1,1,2,2-t etrahydrodecyl) trimethoxysilane [CF3(CF2)(7)CH2CH2Si(OCH3)(3), i.e. FAS] w as used as a precursor. The SAM-covered Si plates were found to be 35-65% l ess charged than the SiO2/Si plate. Furthermore, SAM formation reduced surf ace 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 (similar to pH 2.0). These results are attributable to a reduction in the density of surface si lanol (Si-OH) groups on the SiO2/Si substrate. Si-OH groups were consumed d ue to the formation of siloxane bondings with the organosilane molecules wh ich compose the SAMs. Finally, the IEPs of the ODS- and FAS-SAM covered sur faces were found to be almost identical to those of the PE and PTFE bulk su rfaces. (C) 2001 Elsevier Science B.V. All rights reserved.