1070-4272/03/7606-0888 $25.00 C 2003 MAIK [Nauka/Interperiodica]
Russian Journal of Applied Chemistry, Vol. 76, No. 6, 2003, pp. 888!891. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 6, 2003,
Original Russian Text Copyright + 2003 by Airapetyan, Khachatryan.
OF SYSTEMS AND PROCESSES
Effect of Method Used for Synthesis of Silica Sols on Pore
Characteristics of Silica Gels on Their Base
S. S. Airapetyan and A. G. Khachatryan
Yerevan State University, Yerevan, Armenia
Received December 3, 2002; in final form, May 2003
Abstract-Problems associated with synthesis of silica gels with prescribed pore characteristics from sta-
bilized silica sols with controlled colloid particle size, obtained in the metastable solubility region of
the system SiO
O (8.53 9.5), are considered.
Conventionally, silica gels are synthesized from
alkali metal silicates by precipitation with an acid
. The silica gels prepared using this technique
have a number of disadvantages: poor reproducibility
of pore characteristics, impossibility to control growth
of primary structure-forming colloid particles in
the course of gelation, necessity for removal of a con-
siderable amount of alkali metal cations, etc.
Strict requirements (narrow pore size distribution,
reproducible pore characteristics, high mechanical
strength, etc.) are imposed on sorbents and supports
for high-efficiency liquid chromatography. However,
silica gels prepared from soluble silicates cannot
satisfy these requirements completely.
Directed synthesis of sorbents with prescribed pore
characteristics has always been and still is a topical
task. Therefore, a search for methods that could solve
the problem of control and reproducibility of pore
characteristics is an important issue.
A possible way to control the porous structure of
silica gels is to use sols of silicic acid as starting
material [1, 638]. The size distribution of colloid
particles of a silica sol determines, in the end, the pore
size distribution (in gelation without precipitation),
and the size of particles themselves determine the spe-
cific surface area of the silica gels obtained.
There exist various methods for obtaining silica
sols . These methods use polysilicic acid (PSA)
prepared by ion-exchange conversion of a liquid glass
solution (3.53 4.0 wt % SiO
O = 3.25) as
starting material .
To gain better understanding of how SiO
particles grow in the SiO
O system, it is necessary
to consider some questions associated with nuclea-
tion of a new phase. A silica solution is subjected to
homogeneous nucleation. At high pH values, associ-
ates of silicate ions, which contain up to 5310 SiO
units, are mostly present in silicate solutions. For
silica, there exists a certain possibility of formation
of a dense octameric structure, which can form col-
loid particles about 1 nm in size. As the SiO
ratio becomes greater than 2 : 1, three-dimensional
corpuscular [colloid species] start to form in solution,
which eventually become nuclei .
It has been shown that nucleation of a new phase in
early stages of polycondensation, with the number and
size of nuclei depending on the initial supersaturation
in the system, results in that, further, this process starts
to occur by the heterogeneous mechanism [18, 19].
Nucleation occurs in the SiO
O system at low
supersaturation values. The equilibrium solubility of
silica in water at room temperature is 0.007 wt % (in
terms of SiO
) . Consequently, to ensure growth of
colloid particles of silicon dioxide, it is necessary to
clearly delineate the metastable solubility region in
O system. Particles grow in the metasta-
ble state, and both the particle growth and nucleation
occur at high degree of supersaturation .
The polysilicate fraction is characterized by narrow
polyion size distribution, which is due to the fact that
small particles are more soluble than large particles.
This leads to fast change in particle size distribution,
to the point of total homogeneity .
Nucleation and further growth of colloid SiO
ticles at high pH values make it possible to work with
more concentrated solutions, easily control the meta-
stable region by varying the pH of the medium (which