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Russian Journal of Applied Chemistry, Vol. 76, No. 2, 2003, pp. 203!206. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 2, 2003,
Original Russian Text Copyright + 2003 by Varvarin, Belyakova.
AND ION-EXCHANGE PROCESSES
Method for Determining the Concentration of Isolated
Silanol Groups on Silica Surface with Dimethylchlorosilane
A. M. Varvarin and L. A. Belyakova
Institute of Surface Chemistry, National Academy of Sciences of Ukraine, Kiev, Ukraine
Received July 15, 2002
Abstract-The chemical interaction between dimethylchlorosilane and silica of varied degree of hydroxyla-
tion was subjected to a systematic study. The optimal conditions for complete substitution of isolated silanol
groups with dimethyl hydride silyl groups were found. It is suggested that the reaction of dimethylchloro-
silane with the surface of silicon dioxide can be used to determine the concentration of free silanol groups
in the surface layer of silica.
It is known that main active centers on the surface
of silica are isolated =Si3OH groups [1, 2]. Most of
chemical reactions occurring on the silica surface
involve the silanol groups of silicon dioxide .
The silanol groups on the silica surface play an im-
portant part in adsorption and ion exchange [1, 2, 5].
Residual silanol groups on the surface of chemically
modified silicas strongly affect the adsorption, chro-
matographic, and catalytic properties of silica-contain-
There exist various methods for determining the
concentration of silanol groups on the silica surface
. These methods are mostly based on a chemical
reaction between the silanol groups and an appropriate
reagent, with subsequent quantitative analysis of
the reaction products formed.
In this study, it is suggested that the concentration
of isolated silanol groups on the silica surface should
be determined by means of chemical reaction between
dispersed silicon dioxide and dimethylchlorosilane
Silica was studied in the form of aerosil subjected
to vacuum treatment at 200, 400, or 600oC. These pre-
treatment temperatures are frequently used in studying
the adsorption and chemisorption properties of silicon
As established previously , the chemical reaction
of silica with DMCS proceeds quantitatively with
only free silanol groups involved:
=Si3OH + (CH
H + HCl8.
To determine the optimal conditions for complete
substitution of isolated silanol groups with dimethyl
hydride silyl groups, the interaction between DMCS
vapor and surface of silica vacuum-treated at 200,
400, or 600oC was studied in relation to temperature
(Fig. 1) and time (Fig. 2) of reaction. The conditions
Fig. 1. Degree of substitution of free OH groups, a, vs.
temperature T of reaction between DMCS and surface of
aerosil vacuum-treated for 2 h. Reaction duration 1 h. Vac-
uum-treatment temperature (oC): (1) 200, (2) 400, and
Fig. 2. Degree of substitution of free OH groups, a, vs.
duration t of reaction between DMCS and surface of aero-
sil. Vacuum pretreatment of silica for 2 h at (1) 600,
(2) 400, and (3) 200oC. Reaction temperature (oC): (1) 400,
(2) 350, and (3) 200.