Russian Journal of Applied Chemistry, 2010, Vol. 83, No. 2, pp. 214−218.
Pleiades Publishing, Ltd., 2010.
Original Russian Text
V.I. Rakhimov, O.V. Rakhimova, M.P. Semov, 2010, published in Zhurnal Prikladnoi Khimii, 2010, Vol. 83, No. 2, pp. 217−221.
INORGANIC SYNTHESIS AND INDUSTRIAL
Inﬂ uence of Boric Acid on Aging of Freshly Prepared
V. I. Rakhimov
, O. V. Rakhimova
, and M. P. Semov
St. Petersburg State University, St. Petersburg, Russia
Ul’yanov (Lenin) St. Petersburg State Electrotechnical University (LETI), St. Petersburg, Russia
Received July 29, 2009
Abstract—The technique for kinetic examination of polycondensation of silicon alkoxides, which allows
determining the rate of reaction between ammonium molybdate and various associates of silica for systems in
which hydrolysis proceeds, was modiﬁ ed to examine similar systems after the gel point. Aging of freshly prepared
tetraethoxysilane-based gels was for the ﬁ rst time studied spectrophotometrically as inﬂ uenced by boric acid
additions. The proportions of various molecular species of silica in the examined systems at different times after
the gel point were determined in terms of the kinetic model of the molybdate reaction.
A signiﬁ cant role in modern chemistry of silica belongs
to development of techniques to determine not only the
amounts but also the forms of occurrence of silica in
speciﬁ c objects. The task may consist in determining the
degree of hydration and polymerization, distribution of
silica over molecular species, and functional groups that
are oxygen-bridged with silicon-oxygen tetrahedrons.
Silica structurization is conventionally discussed
in terms of the size and nature of the functional
groups linked to the silicon atom . The smallest
size is characteristic for alkoxides (–OR), hydroxy
group (–OH), or bridging oxygen (–O–Si≡). They
are followed by different types of silica oligomers
(dimers, trimers, tetramers, etc.), which can be linear,
branched, or cyclic. Next go associates that are longer
than oligomers identiﬁ ed from the molecular weight
and spatial conﬁ guration but shorter than polymers.
Modern science identiﬁ es silica species in solid objects
mainly by instrumental research methods, in particular,
NMR, small-angle X-ray scattering, gas chromatography,
and Raman spectroscopy (for the most comprehensive
bibliography, see ).
It is essential that not only the structural species of
silica be identiﬁ ed but also that their relative content and
evolution in time be determined. This allows predicting
the best pathways to obtain new materials with desired
properties from silica compounds using sol-gel techno-
logy based on hydrolytic polycondensation of silicon
alkoxides (alkoxysilanes). Studies of the time evolution
for molecular species of silica in the course of hydrolytic
polycondensation, both before and after the gel point, are
essential for preparing stable gels, characterizing their
structurization, and optimizing the subsequent procedure
of gel treatment to obtain end synthesis products.
Here, we modiﬁ ed the technique that we previously
proposed for kinetic examinations of early stages
of hydrolytic polycondensation of alkoxides with
the use of molybdate technique based on formation
of β-silicomolybdic heteropoly acid (HPA) . We
determined the kinetic parameters of the molybdate
reaction and evolution of the distribution of various
associates of silica in the course of polycondensation
after the gel point.
Sol-gel synthesis of materials from organosilicon
compounds involves adding compounds of various
elements such as aluminum, germanium, titanium,
zirconium, and boron to impart the desired properties to
the synthesis products. Among these additions, boron is
promising for preparation of chemically highly uniform
materials. This is of both applied (borosilicate systems
ﬁ nd widespread use in various materials technology
areas) and scientiﬁ c importance (the structure of such