HYDROLYSIS AND CONDENSATION OF ALKYLMETHOXYSILANES - STUDIED BY MEANSOF THE LANGMUIR-BLODGETT TECHNIQUE AND ELECTRON-SPECTROSCOPY FOR CHEMICAL-ANALYSIS
J. Sjoblom et al., HYDROLYSIS AND CONDENSATION OF ALKYLMETHOXYSILANES - STUDIED BY MEANSOF THE LANGMUIR-BLODGETT TECHNIQUE AND ELECTRON-SPECTROSCOPY FOR CHEMICAL-ANALYSIS, Langmuir, 11(7), 1995, pp. 2652-2660
Hydrolysis and condensation reactions of octadecyltrimethoxysilane (C(
18)TMS) and octadecyldimethylmethoxysilane (C(18)DMMS) have been studi
ed by means of the Langmuir-Blodgett technique and electron spectrosco
py for chemical analysis (ESCA). C(18)TMS can form linear two-dimensio
nal polymers, while C(18)DMMS forms only dimers. The polymerization/di
merization rate seems to increase in the sequence pH 5.6 --> 11 --> 1.
At pH 11 the spread monolayer contracts when electrolyte is added and
becomes less compressible. This indicates decreased electrostatic rep
ulsion within the ionized monolayer and/or a higher level of polymeriz
ation/dimerization of the film molecules. C(18)DMMS could be compresse
d at pH 1 to a surface area of similar to 18 Angstrom(2), before the f
ilm enters a condensed phase. Constant surface pressure-area relaxatio
n experiments indicate that a nucleation process takes place in the C(
18)DMMS film at pH 1 for pi=20 mN m(-1). In contrast, the C(18)TMS fil
m at pH 1 was stable for more than 12 h at 40 mN m(-1). When two subse
quent compression steps are applied with a delay of 16 h, the C(18)TMS
films (pH 5.6 and 11) reproduced the condensed pH 1 film, with molecu
lar areas of similar to 20 Angstrom(2), while the C(18)DMMS film (pH 5
.6 and 11) was still in an expanded state. A condensed state in the tw
o-dimensional film indicates good packing and strong cohesion forces b
etween the hydrocarbon chains. The dimer-forming C(18)DMMS will not ac
hieve the same strong cohesion that we find in the fully polymerized C
(18)TMS films. For the C(18)TMS film at pH 11 a lowering of the barrie
r speed increased the polymerization rate, as observed by decreasing c
ompressibility and decreasing areas per molecule for a given surface p
ressure. Langmuir-Blodgett films analyzed by ESCA confirmed these resu
lts.