Temperature Dependence of the Concentration of Silanol Groups in Silica Precipitated from a Hydrothermal Solution

Abstract

The physicochemical characteristics of amorphous silica precipitated from a hydrothermal solution are investigated. The specific
surface of silica is determined by the Brunauer-Emmett-Teller method from the data on low-temperature nitrogen absorption.
The limits of the total water content are estimated according to the thermogravimetric data. The temperature dependences of
the concentration of surface and internal silanol groups in the range 200–1200°C are determined by comparing the thermogravimetric
data with the Zhuravlev physicochemical constants for the silica samples under consideration. A new type of amorphous silica
with a considerable concentration of internal water is revealed. It is established that the mechanisms of the removal of surface
and internal water differ from each other.

Full text

1087-6596/05/3105- © 2005 Pleiades Publishing, Inc.0661
Glass Physics and Chemistry, Vol. 31, No. 5, 2005, pp. 661–670.
Original Russian Text Copyright © 2005 by Fizika i Khimiya Stekla, Potapov, Zhuravlev.
INTRODUCTION
A colloidal silica dispersion is formed in a hydro-
thermal solution through a series of successive physic-
ochemical processes. The initial silica concentration
depends on the temperature at which the chemical equi-
librium of water and aluminosilicate minerals of rocks
is attained in a high-temperature hydrothermal field [1–
3]. At temperatures of 250–350
°
C, the total content
C
t
of silicon dioxide SiO
2
in water virtually corresponds to
the solubility of quartz (500–700 mg/kg). In solutions,
silicon predominantly occurs in the monomeric form as
H
4
SiO
4
orthosilicic acid molecules.
After ascending filtration in rocks or rise to the sur-
face in producing wells of geothermal electric power
stations, the solution becomes supersaturated with
respect to the solubility
C
e
of amorphous silica due to
the decrease in the pressure and temperature and partial
evaporation [3]. In this case, the total silica content
C
t
in the solution can be as high as 700–1500 mg/kg [3].
The solution supersaturation, which is equal to the dif-
ference (
C
s
–
C
e
) between the concentration
C
s
of
orthosilicic acid and the solubility
C
e
, is the driving
force of polycondensation of silicic acid molecules
with the formation of siloxane bonds and partial dehy-
dration [4]; that is,
(1)
or
(2)
Si
OH OH
OH OH + OH Si
OH
OH OH Si
OH OH
OH OSi
OH
OH OH + H2O
SimOm1–()
OH()
2m2+()
SinOn1–()
OH()
2n2+() Si mn+()
Omn1–+()
OH 2n2m2++()
H2O.++
Nucleation and polycondensation result in the for-
mation of colloidal particles consisting of
n
SiO
2
·
m
H
2
O hydrated silica in the solution. Surface silanol
groups SiOH dissociate with detachment of H
+
protons,
and the particle surface acquires a negative electric
charge. The electrostatic repulsive forces prevent the
particle coagulation and provide the stability of colloi-
dal silica in the hydrothermal solution.
Investigation into the physicochemical characteris-
tics of colloidal silica in the hydrothermal solution and
after its precipitation is required to improve a model of
formation of minerals in hydrothermal minerals [5, 6],
including ore minerals, and to develop a technology of
extracting and using silica in order to increase the effi-
ciency of geothermal electric power stations [7].
SAMPLE PREPARATION
AND EXPERIMENTAL TECHNIQUE
Silica samples were prepared by freezing a dis-
persed solution from wells of the Mutnovsk field. On
the snow surface, solution drops freeze and colloidal
silica particles were clustered in regions between ice
crystals. This led to a decrease in the distance between
Temperature Dependence of the Concentration of Silanol Groups
in Silica Precipitated from a Hydrothermal Solution
V. V. Potapov* and L. T. Zhuravlev**
* Geotechnological Research Center, Far East Division, Russian Academy of Sciences,
Severo-Vostochnoe sh. 30, POB 56, Petropavlovsk-Kamchatskii, 683002 Russia
** Institute of Physical Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, GSP -1, 119991 Russia
Received August 6, 2004
Abstract
—The physicochemical characteristics of amorphous silica precipitated from a hydrothermal solution
are investigated. The specific surface of silica is determined by the Brunauer–Emmett–Teller method from the
data on low-temperature nitrogen absorption. The limits of the total water content are estimated according to
the thermogravimetric data. The temperature dependences of the concentration of surface and internal silanol
groups in the range 200–1200
°
C are determined by comparing the thermogravimetric data with the Zhuravlev
physicochemical constants for the silica samples under consideration. A new type of amorphous silica with a
considerable concentration of internal water is revealed. It is established that the mechanisms of removal of sur-
face and internal water differ from each other.

662
GLASS PHYSICS AND CHEMISTRY
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POTAPOV, ZHURAVLEV
particles and acceleration of their coagulation. As a
result, a gel-like material that consisted of a mixture of
precipitated silica with snow was formed on the sur-
face. After drying at 105
°
C, the mixture transformed
into a finely dispersed powder. Precipitated silica was
removed from the surface and dried with the use of geo-
thermal heat. The density of gel-like silica taken from
the snow surface was equal to 2.0 g/cm
3
. After drying
for 12–16 h at 110
°
C, the material transformed into a
finely dispersed powder, whose density was equal to
0.22–0.24 g/cm
3
.
Amorphous silica hydroxylated to different degrees
can find wide use in science and engineering [8]. In the
general case,
≡
Si–OH silanol groups (silanols) are
formed on the surface due to the two main thermody-
namically favorable processes [8]. First, these groups
are formed in the course of synthesis, for example, con-
densation polymerization of Si(OH)
4
(Fig. 1a) when a
supersaturated solution of monosilicic acid transforms
into polysilicic acids with the subsequent formation of
SiO
2
sols and gels containing OH groups on the sur-
face. The final product after drying (xerogel) retains
totally or partially silanol groups on the surface. Sec-
ond, silanol groups can be formed through rehydroxy-
lation of thermally hydroxylated silica upon treatment
with water or aqueous solutions (Fig. 1b).
Different types of groups on the surface and in the
bulk of silica are shown in Fig. 2 [8]. These are surface
free single (isolated) silanol groups
≡
SiOH (
Q
3
type);
surface free geminal (isolated) silanol or silanediol
groups =Si(OH)
2
(
Q
2
type); vicinal bridging silanol
groups, i.e., hydrogen-bonded surface single silanol
groups, single geminal groups, and their combinations;
≡
Si–O–Si
≡
siloxane bridges with the O atom on the
surface (
Q
4
type); and internal silanol groups located
within the skeleton and (or) in very thin ultrami-
cropores of silica. Therefore, amorphous silicas on the
surface contain only two main types of OH groups: sin-
gle and geminal groups, which, in turn, are subdivided
into isolated, free, and hydrogen-bonded vicinal silanol
groups [8].
The properties of dispersed amorphous silica as an
adsorbent are primarily determined by the porous struc-
ture and the chemical reactivity of the surface. Note that
the chemical reactivity depends on the concentration of
OH groups, i.e., on the concentration of all silanol
groups and their species, the temperature and energy
distribution of silanol groups, and the presence of Si–
O–Si siloxane bridges, whereas the surface morphol-
HO OHSi
O
H
H
O
HO OSi
O
H
H
O
OHSi
O
H
H
O
H
O
H
O
OHO Si
O
Si
O
OH
OHO Si Si OH
OO
H H
O
OSi
Si Si
H
H
H
O
O
O
Si
H
H
OH
H
O
O
+H2O
Si
Si
Si
Si
Si
Si OSi
Si
O
Si Si
O
Si Si
OO
HH
O
H
H
(a)
(b)
Fig. 1.
Schemes of formation of the silica surface structure (
≡
Si–OH silanol groups) in the course of (a) condensation polymeriza-
tion and (b) rehydroxylation.
H
O
Si
Si
Si
Si
O
HO
O
H
H
Si
O
H
OH
Si – OH Si OH
Si O
H
Si
OOO
Si Si
Si
Isolated silanol
groups
Q
3
Geminalsilanol
groups
Q
2
Surface siloxane
groups
Q
4
Vicinal silanol
groups
Vicinal silanol
groups
Internal silanol
groups
Fig. 2.
Types of silanol groups and siloxane bridges on the
surface of amorphous silica and internal OH groups.

GLASS PHYSICS AND CHEMISTRY
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No. 5
2005
TEMPERATURE DEPENDENCE OF THE CONCENTRATION 663
ogy is predominantly governed by the synthesis proce-
dure and the conditions of subsequent treatment of sil-
ica. Silanol groups
≡
Si–OH can be located on the sur-
face (surface silanols) and in the bulk of amorphous
silica particles and (or) in very thin ultramicropores
(less than 1 nm in diameter) accessible only for the
smallest molecules, for example, water molecules
(internal silanols). In [8], it was clearly demonstrated
that OH groups on the surface of large transport pores
easily, rapidly, and completely enter into the deuterium
exchange reaction with D
2
O heavy water (introduced in
the form of vapor or liquid) at room temperature. How-
ever, this process virtually does not involve bound
water contained inside the SiO
2
skeleton and isotope
exchange with silanol groups in ultramicropores pro-
ceeds at a very low rate.
Internal silanol groups (Fig. 2) not responsible for
the surface processes are widely occurring species in
amorphous silica. The internal silanol groups
≡
Si–OH
are disregarded when it is necessary to calculate the
surface concentration of OH groups (the silanol number
α
OH
, OH/nm
2
) and to study the reactive behavior of sur-
face silanol groups, because only the latter groups play
a decisive role in different surface processes.
Silanol groups within the skeleton and ultrami-
cropores of silica can be formed in different ways (we
consider only a number of them).
(i) For silica prepared by condensation of low-
molecular polysilicic acids, individual silanol groups
can remain in the inorganic polymer network due to
incomplete polycondensation, if this reaction is per-
formed without required partners. Moreover, in silica
synthesized from sodium silicate, a number of OH
groups can be captured by the silica skeleton during
aggregation of primary small-sized particles and then in
the course of ageing of the SiO
2
gel.
(ii) According to Iler [4], in the case when colloidal
particles are formed through growth in an alkali solu-
tion (pH ~ 9), sodium atoms can be adsorbed on parti-
cles simultaneously with SiO
2
precipitation. This
favors capture of silanol groups by the silica structure.
(iii) In the case of pyrogenic silica, large spherical
particles (globules) 10–20 nm in diameter are formed
via aggregation of 1–2-nm elementary primary glob-
ules produced by hydrolysis in a flame at high temper-
atures. Since primary particles contain a number of sur-
face silanol groups, these OH groups can be captured
inside large globules of the final product.
(iv) Sindorf and Maciel [9] demonstrated that the
appearance of internal silanol groups can be explained
by diffusion of H
2
O molecules inside SiO
2
solid parti-
cles (to a depth of 15 nm) at elevated temperatures.
(v) One of the most widespread types of treatment
that lead to the formation of silanol groups in the skel-
eton and ultramicropores of silica is hydrothermal
treatment. Hydrothermal treatment of silica species is
accompanied by complex processes of dissolution and
reprecipitation of silica and water diffusion inside the
solid phase. This results in the formation of secondary
structurally modified silicas that have pores with a dif-
ferent geometry and can hold silanol groups or bound
water within particles and in ultramicropores.
For example, by using hydrothermal treatment in an
autoclave at the stage of SiO
2
hydrogels, Doremus [10]
prepared a series of silica gels that had a mesoporous
structure and a dense packing of particles but did con-
tain ultramicropores. These samples involve silanol
groups on the surface and inside silica particles.
With the use of hydrothermal treatment at the xero-
gel stage, Chertov
et al.
[11] produced silica gels in
which particles had different porous (globular, tran-
sient, sponge) structures. Similar samples contain sil-
anol groups on the surface and inside silica particles
and in ultramicropores.
Upon hydrothermal treatment of initial pyrogenic
silicas (aerosils), Gorelik
et al.
[12] synthesized a series
of aerosilica gels with different porous structures. Sil-
anol groups on the surface and within silica particles
were also contained in these materials.
In [13], modified glasses with different porous
structures were prepared by different types of treat-
ments, including long-term boiling of initial porous
glasses in water. Apart from surface OH groups, silanol
groups were revealed inside particles and in ultrami-
cropores.
It should be expected that internal silanol groups
should be contained in different amorphous silicas
formed as solid deposits on walls of wells, pipes, and
thermal equipment of thermal and electric power sta-
tions operating with the use of natural geothermal heat
carriers (fields in Russia, New Zealand, Japan, United
States, Philippines, Mexico, Iceland, Italy, etc.) [1–3].
The chemical composition of silica sample AK1b
prepared in our experiments was as follows (wt %):
SiO
2
, 81.13; TiO
2
, 0.02; Al
2
O
3
, 0.41; Fe
2
O
3
, 0.07; FeO,
0.09; MnO, MgO, and CaO, not found; Na
2
O, 0.60;
K
2
O, 0.29; H
2
O, 10.93 (drying loss at 105
°
C); calcina-
tion loss at 1000
°
C, 6.03; and P
2
O
5
, 0.06 (
Σ
, 99.63).
The silica dioxide content in the samples after sub-
traction of losses by drying at 105
°
C and calcination at
1000
°
C varied from 95.0–97.69 to 99.02 wt %. The
total content of calcium, aluminum, and iron did not
exceed 0.6%.
The silica samples precipitated by freezing of the
hydrothermal solution had an amorphous structure, as
can be judged from the amorphous halo at 0.387–
0.40 nm in the X-ray diffraction pattern (Fig. 3a). After
calcination at 1000
°
C, amorphous silica transformed
into the crystalline phase with a cristobalite structure
(Fig. 3b).
The IR spectra of the precipitated samples were
recorded on a Bruker Vector 22/N Fourier-transform IR
spectrometer in the wave number range 250–4250 cm
–1
.
The IR spectra in the range 250–1200 cm
–1
involve three

664
GLASS PHYSICS AND CHEMISTRY
Vol. 31
No. 5
2005
POTAPOV, ZHURAVLEV
maxima attributed to the vibrations of Si–O–Si bonds
in SiO
2
tetrahedra: two low-intensity maxima at 500
and 750–850 cm
–1
and one intense maximum at 1096–
1104 cm
–1
(Fig. 4). All the IR spectra in the range
1200–4000 cm
–1
contain two low-intensity peaks at
1600–1640 and 2344–2368 cm
–1
and one intense peak
at 3440–3480 cm
–1
. These peaks are associated with the
vibrations of hydroxyl groups. However, the band at
3750 cm
–1 that, according to the data available in the lit-
erature, is characteristic of ≡Si–OH free silanol groups
was not found in the spectra due to the overlapping with
the bands of vicinal silanol groups and, possibly,
adsorbed water molecules. The shape of the IR spectra
and the location of the two main peaks at 1096–1104
and 3440–3480 cm–1 are characteristic of different
forms of amorphous silicon dioxide.
The thermal analysis of silica sample AK1b was
performed on a Perkin-Elmer Pyris Diamond TG/TGA
thermal analyzer (heating rate, 20 K/min) in air (Fig. 5).
The optical reflectances of the dispersed silica sur-
face were measured with an MSFU-K microscope
spectrophotometer in the wavelength range 400.0–
760.0 nm. The optical reflectance (whiteness) for the
geothermal silica samples varied from 91–95 to 94–
98%. As the wavelength increases, the reflectance
increases in the following way: 480.0 nm, 0.9402;
500.0 nm, 0.9531; 520.0 nm, 0.95029; 540.0 nm,
0.95394; 560.0 nm, 0.96534; 580.0 nm, 0.96592;
600.0 nm, 0.97425; 620.0 nm, 0.97911; 640.0 nm,
0.97679; 660.0 nm, 0.97182; 680.0 nm, 0.97274; and
700.0 nm, 0.98265.
The surface area and volume of pores in the silica
samples were determined using a Micrometrics ASAP-
2010N porosity analyzer (USA) from the data on low-
temperature nitrogen adsorption. The method is based
on the measurement of the nitrogen adsorption iso-
therms at the liquid-nitrogen temperature [14]. In the
course of experiments, the change in the weight of the
dispersed sample was measured and used to determine
the amount of adsorbed nitrogen at a specific relative
pressure of nitrogen P/P0 in a cell with the sample (P is
the nitrogen pressure in the system, P0 is the nitrogen
10
841
20 30 40 50 60
θ, deg
0
137
0
Ix
(a)
(b)
Fig. 3. X-ray powder diffraction patterns of the silica sample (a) before and (b) after calcination at 1000°C.
0.2
1000 2000 3000 40000cm
–1
0.4
0.6
0.8
1.0
472
1104
800 1632 2368
3440
Fig. 4. IR spectrum of the silica sample.

GLASS PHYSICS AND CHEMISTRY Vol. 31 No. 5 2005
TEMPERATURE DEPENDENCE OF THE CONCENTRATION 665
saturation vapor pressure at a given temperature). The
volume V of adsorbed nitrogen was determined initially
with a sequential increase in the relative pressure P/P0
from 0.01 to 1.0 (adsorption curve) and then with a
decrease in the relative pressure P/P0 from 1.0 to 0.04
(desorption curve). The dependence V(P/P0) for silica
sample AK1b is plotted in Fig. 6.
The experimental curves obtained belong to adsorp-
tion isotherms of the IV type [14]. The curves are con-
vex in the vicinity of zero, become concave after pass-
ing through an inflection point at P/P0 = 0.3–0.5, and
are again convex in the vicinity of P/P0 = 1.0 (Fig. 6).
The specific surface was determined by the
Brunauer–Emmett–Teller (BET) and Brunauer–Dem-
ing–Halsey (BDH) methods. The specific surface was
calculated using the Brunauer–Emmett–Teller equation
for polymolecular adsorption of vapor from the experi-
mental data obtained in the range of the relative pres-
sures P/P0 = 0.0 –1.0 [14].
RESULTS AND DISCUSSION
Within the classical theory of adsorption and des-
orption [14], the determined dependences V(P/P0) were
used to calculate the differential distributions of vol-
umes Vp and surfaces Sp of pores with diameters dp in a
specific range and also the total volumes and total sur-
faces of pores with diameters from 1.7 nm to a given
diameter dp. Furthermore, we calculated the differential
dependences dVp/dlog(dp) and dSp/dlog(dp). The curves
dVp/dlog(dp) and dSp/dlog(dp) exhibit maxima at dp =
18.0 and 11.7 nm, respectively.
The pore parameters obtained by the adsorption
method for the samples of dispersed geothermal silica
are listed in Table 1. The designations in Table 1 are as
follows: volumes 1 and 2 (used for calculations of coef-
ficients) are the cell volumes automatically measured
by the instrument at room temperature and after dip-
ping in a Dewar vessel with liquid nitrogen, respec-
tively; SS is the specific determined at the fixed relative
pressure P/P0 = 0.200; SBET is the total specific surface
of pores according to the BET method; SMP is the spe-
cific surface of micropores with a diameter of an order
of 1.7 nm; SAC is the total specific surface determined
200
–100
400 600 800 1000
–50
0
50
100
150
200
–350
–300
–250
–200
–150
–100
–50
0
50
100
150
200
3-DTA2-DTG
88
90
94
96
98
100
92
1-TG
68°C
1
2
3
T, °C
Fig. 5. (1) Thermogravimetric, (2) differential thermogravimetric, and (3) DTA curves for the silica sample.
0.2 0.4 0.6 0.8 1.0
100
50
0
150
200
250
300
350
400
450
500
550
600
650
700
P/P0
VN, cm3/g
1
2
Fig. 6. Nitrogen (1) adsorption and (2) desorption curves for
the silica sample.

666
GLASS PHYSICS AND CHEMISTRY Vol. 31 No. 5 2005
POTAPOV, ZHURAVLEV
by the BDH method from the adsorption curve for
pores with diameters from 1.7 to 300.0 nm (the BDH
surface area); SDC is the specific surface determined by
the BDH method from the desorption curve for pores
with diameters from 1.7 to 300.0 nm; VS is the total vol-
ume of pores with diameters smaller than 40.0 nm at the
fixed relative nitrogen pressure P/P0 = 0.950; VMP is the
volume of micropores with a diameter of an order of
1.7 nm; VAC is the total volume determined by the BDH
method from the adsorption curve for pores with diam-
eters from 1.7 to 300.0 nm (the BDH volume); VDC is
the total volume determined by the BDH method from
the desorption curve for pores with diameters from 1.7
to 300.0 nm; dBET = 4VS/SBET is the mean pore diameter;
dA = 4VAC/SAC is the mean pore diameter calculated
from the volume and specific surface determined by the
BDH method from the adsorption curve; and dD =
4VDC/SDC is the mean pore diameter calculated from the
volume and specific surface determined by the BDH
method from the desorption curve.
It can be seen from the data presented in Table 1
that, for silica sample AK1b prepared from the solution
of wells of the Mutnovsk field, the specific surface is as
large as 300 m2/g, the porosity is equal to 1.1 g/cm3, and
the mean pore diameter lies in the range 12.7–16.6 nm.
The specific surface and volume of micropores in the
geothermal silica samples appear to be relatively small.
The ratio between the specific surface of micropores
and the total specific surface of pores falls in the range
0.09–0.107, and the ratio of the micropore volume to
the total pore volume is even smaller (0.005–0.0085).
The specific surfaces and volumes of pores as a
function of their diameter are given in Table 2. As can
be seen from this table, the diameters of pores in sample
AKb1 of geothermal silica lie in a narrow range. The
differential distribution of volumes over pore diameters
exhibits a maximum at dp = 33 nm, and the differential
distribution of surfaces over pore diameters has two
close approximately identical maxima at 13 and 9 nm.
Pores with diameters in the ranges dp = 5.18–20.61 nm,
dp = 5.18–26.47 nm, and dp = 5.18–40.00 nm occupy
71.1, 79.8, and 88.3% of the total volume, respectively.
In turn, pores with diameters in the ranges dp = 5.18–
26.47 nm and dp = 5.18–40.00 nm account for 60.9 and
76.4% of the total specific surface. These pore parame-
ters provide a sufficiently high reactivity of the precip-
itated material and rapid homogeneous complete disso-
lution of silica in technological processes.
Knowing the specific surface of silica SBET (m2/g)
and the weight percent ∆ (wt %), i.e., the total
mass loss due to removal of water and OH groups in the
course of thermogravimetric analysis, it is possible to
determine the total concentration δOH (OH/nm2) of all
silanol groups on the surface and in the bulk of silica
per unit specific surface of sample AK1b; that is,
(3)
By setting SBET = 300 m2/g, assuming that the tem-
perature of complete removal of all silanol groups is
equal to 1000°C, and taking into account the thermo-
gravimetric data, we calculated the total concentrations
δOH (on the surface and in the bulk) per specific surface
of the sample at different temperatures (Table 3).
Many researchers have investigated different
aspects of dehydroxylation and rehydroxylation of
amorphous silica surfaces. This is explained by the fact
that their chemical properties, which are predominantly
determined by the concentration, distribution, and reac-
tivity of surface silanol groups ≡Si–OH, are of crucial
theoretical and practical importance.
The dependences of the silanol number αOH, T for the
surface on the temperature TPHT of preliminary heat
treatment under vacuum according to the deuterium
exchange data (the subscript T indicates the total silanol
number without separating into isolated free, geminal
free, and vicinal OH groups) are presented in Fig. 7 and
Table 3. This dependence for different SiO2 samples
allows us to determine the Zhuravlev physicochemical
constants [the silanol number αOH, T, the degree of cov-
erage θOH, T of the SiO2 surface by OH groups at differ-
ent heat treatment temperature TPHT (°C)], which are
widely used in the world scientific literature. The
dependence of the silanol number αOH, T was obtained
using 100 samples of amorphous silicas of nine differ-
ent types, whose specific surfaces SKr (determined by
the BET method from the data on the low-temperature
krypton adsorption) and diameters of accessible pores
mH2O
δOH ∆mH2O2 6.02 103
×⋅⋅()/18SBET
().=
Table 1. Sizes, specific surfaces, and volumes of pores in
samples of finely dispersed geothermal silica
Temperature, K 77.2
Pressure P0, mmHg 747.17
Sample weight, g 0.13
Volume 1, cm317.45
Volume 2, cm354.32
SS, m2/g 263.53
SBET, m2/g 274.64
SMP, m2/g 26.33
SAC, m2/g 260.25
SDC, m2/g 333.52
VS, cm3/g 0.871
VMP, cm3/g 0.00827
VAC, cm3/g 1.078
VDC, cm3/g 1.088
dBET, nm 12.692
dA, nm 16.575
dD, nm 13.058

GLASS PHYSICS AND CHEMISTRY Vol. 31 No. 5 2005
TEMPERATURE DEPENDENCE OF THE CONCENTRATION 667
varied over very wide ranges from 9.5 to 945 m2/g and
from ~1 to 1000 nm and more, respectively.
Although the quantities SKr and d (without regard for
ultramicropores) for different SiO2 samples differ sub-
stantially, their silanol numbers αOH, T at a specific tem-
perature of preliminary heat treatment are close to each
other and decrease in a similar manner under close
heating conditions. The silanol number αOH, T decreases
rapidly in the range 190–400°C (portion AB in Fig. 7)
and then changes more slowly in the range from 400 to
~780°C (portion BC in Fig. 7), in which the curve
becomes more flattened. The dashed lines bound the
region of scatter in the experimental data observed for
different SiO2 samples over the entire temperature
range from 190 to ~780°C. Table 4 presents the Zhurav-
lev physicochemical constants [4] (corresponding to
the solid lines in Fig. 7), i.e., the most probable silanol
numbers αOH, T or the concentrations of surface silanol
groups (Table 4, column 2) at fixed temperatures TPHT
(°C) of preliminary heat treatment (Table 4, column 1).
The corresponding averaged degrees of surface cover-
age θOH, T by OH groups are also listed in Table 4 (col-
umn 3). The physicochemical constants αOH, T and
Table 2. Volumes and specific surfaces of pores as a function of their diameter for the geothermal silica sample according to
the adsorption analysis data
Pore diameter dp,
nm Mean diameter,
nm Porosity, g/cm3Total porosity,
g/cm3Specific surface
of pores, m2/g Total specific sur-
face of pores, m2/g
333.0–125.1 150.03 0.0238 0.0238 0.635 0.635
125.1–88.9 100.89 0.0333 0.0571 1.321 1.956
88.9–72.7 79.1 0.0284 0.0856 1.438 3.394
72.7–40.0 47.2 0.1539 0.2395 13.03 16.42
40.0–26.5 30.4 0.1669 0.4065 21.94 38.37
26.5–20.6 22.7 0.1303 0.5368 22.90 61.27
20.6–16.7 18.2 0.1182 0.6550 25.93 87.20
16.7–14.0 15.1 0.0960 0.7510 25.35 112.55
14.0–11.6 12.6 0.1005 0.8516 31.89 144.45
11.6–10.3 10.89 0.0550 0.9066 20.23 164.68
10.3–8.36 9.11 0.0764 0.9831 33.57 198.25
8.36–7.00 7.55 0.0425 1.0257 22.55 220.80
7.00–5.97 6.40 0.0243 1.0501 15.25 236.06
5.97–5.18 5.52 0.0141 1.0642 10.24 246.310
5.18–4.54 4.81 0.0079 1.0722 6.624 252.93
4.54–4.02 4.24 0.0039 1.0761 3.760 256.69
4.02–3.58 3.77 0.0011 1.0773 1.226 257.92
3.58–3.20 3.36 0.000061 1.0774 0.072 257.99
3.20–1.96 2.01 0.000059 1.0774 0.118 258.11
1.96–1.86 1.91 0.00046 1.0779 0.963 259.07
1.86–1.76 1.81 0.00053 1.0784 1.178 260.25
Table 3. Concentrations of OH groups on the surface and in the bulk of hydrothermally treated silica sample AK1b (as a func-
tion of temperature)
Number of OH groups
per unit specific sur-
face of silica, OH/nm2
Temperature of preliminary heat treatment, °C
200 300 400 500 600 700 800 900
δOH 8.29 6.71 4.92 3.33 2.23 1.49 0.89 0.40
αOH 4.90 3.56 2.33 1.84 1.52 1.30 0.70 0.40
γOH 3.39 3.15 2.59 1.49 0.71 0.19 0.19 0.0
Note: δOH is the total concentration of silanol groups (according to the water loss), αOH is the concentration of silanol groups on the SiO2
surface, and γOH is the concentration of silanol groups inside the skeleton and micropores of the sample.

668
GLASS PHYSICS AND CHEMISTRY Vol. 31 No. 5 2005
POTAPOV, ZHURAVLEV
θOH, T are universal constants for any amorphous silicas
irrespective of their origin and structural characteristics
(Table 4, columns 1–3; Fig. 7, portions AB, BC) if they
in the initial state have a completely hydroxylated sur-
face (Fig. 7, point A). In this case, a possible presence
of silanol groups in the bulk and ultramicropores is
ignored.
The dependences αOH, T = f(T, C) and θOH, T = g(T, C)
in range II (Fig. 7) have two portions with different
slopes in range IIa from 190 to ~400°C (straight solid
line) and range IIb' (solid line approximated by a power
law). Portion AB (Fig. 7) can be approximated by the
linear relationship with the use of reference point A
(αOH, T = 4.9, Table 4); that is,
αOH, T (OH/nm2) = –0.0122 · T(°C) + 7.218, (4)
whose approximation reliability R2 = 0.8768 is close to
unity.
Portion BC (Fig. 7) can be approximated by the
power relationship with the use of reference point B
(αOH, T = 2.33, Table 4); that is,
αOH, T (OH/nm2) = 1155.6 · T(°C)–1.0367, (5)
whose approximation reliability is R2 = 0.8516.
The condensation of silanols
(6)
has been studied in detail by Zhuravlev with colleagues
and also in works of other authors. This reaction occurs
at temperatures corresponding to ranges IIa, IIb', and
≡Si–OH()≡Si–OH()+
≡Si–O–Si≡()H2O↑+
IIb'', even though the activation energies ED of thermal
desorption in these ranges differ substantially [4]. In the
case when the degree of surface coverage by silanol
groups is high 1 ≥ θOH, T > 0.5 (range IIa, Table 4), the
activation energy ED of thermal desorption can be rep-
resented by the empirical expression [4]
(7)
where the activation energy ED varies weakly in the nar-
row range 19–25 kcal/mol. To put it differently, the
activation energy ED almost does not depend on the
concentration of silanol groups and is predominantly
governed by the disturbances of surface OH groups
linked together through hydrogen bonds. Note that
these disturbances are suppressed upon disappearance
of this type of vicinal silanol groups at ~400°C (at θOH, T
~0.5). Therefore, at temperatures in range IIa (portion
AB in Fig. 7), there are hydrogen bonds (lateral interac-
tions) between neighboring vicinal OH groups when
their degree of coverage is high.
If the degree of surface coverage by OH groups
(θOH, T < 0.5, range IIb'), the main contribution to por-
tion BC is made by free single OH groups, free geminal
OH groups, and SiOSi bridges (Figs. 2, 7). In ranges
IIb' and IIb'', the activation energy ED increases drasti-
cally from 25 to 50 kcal/mol and higher with a decrease
in the silanol number αOH, T. When there are only free
OH groups surrounded by SiOSi bridges, these bridges
can occupy relatively large surface areas due to the
high-temperature activation of silica. Under these con-
ditions, the main mechanism responsible for the trans-
fer of OH groups necessary for the condensation reac-
tion (6) can be represented by disordered migration of
protons along the surface (activated surface diffusion of
OH groups). The final stage involves the release of the
water molecule owing to the interaction of two OH
groups, which accidentally come close together to a
distance of ~0.3 nm (characteristic hydrogen bond
length). At a low concentration of OH groups (Fig. 7,
portions BC, DE), the condensation reaction (6) is lim-
ited by proton diffusion along the SiO2 surface.
At temperatures corresponding to portion DE
(Fig. 7), geminal groups are completely absent and the
condensation reaction is limited by the interaction of
widely spaced free single OH groups. In this high-tem-
perature range (Fig. 7, portion DE), amorphous silica
can transform partially or completely into a crystalline
silica modification. A similar transformation of amor-
phous sample AK1b (prepared by precipitation of the
hydrothermal solution) into the crystalline phase with a
cristobalite structure at 1000°C was observed in the
present work (Figs. 3a, 3b). Therefore, portion DE in
Fig. 7 is difficult to approximate with a sufficient reli-
ability, and only averaged silanol numbers αOH, T are
presented in Table 4.
Under the assumption that, according to the above
results, the maximum concentration of surface OH
ED31.4 12.3θOH T,,–=
200 400 600 800 1000 1200
T, °C
1
2
3
4
5
6
0
αOH(OH/nm2)
IIb' IIb''
A
B
C
DE
5.7
4.9
4.2
190 °C
IIa
Fig. 7. Dependence of the silanol number αOH = αOH, T on
the temperature TPHT of preliminary heat treatment for 16
different SiO2 samples. αOH, T = 4.9 OH/nm2 (left upper
point) corresponds to the surface hydroxylated to a maxi-
mum degree. Dashed lines bound the region of the experi-
mental data. Range II at TPHT > 190°C is divided into sub-
ranges IIa (190–400°C) and IIb' (400–780°C) in which por-
tions AB and BC have different slopes. For explanation of
portion DE in subrange IIb'' (800–1200°C), see the text.

GLASS PHYSICS AND CHEMISTRY Vol. 31 No. 5 2005
TEMPERATURE DEPENDENCE OF THE CONCENTRATION 669
groups in silicas of different types at a temperature of
190° (in the range 180–200°C) is equal to 4.9 OH/nm2,
the concentration γOH (OH/nm2) of internal silanol
groups OH (internal water) per unit specific surface in
sample AK1b can be determined as the difference
(8)
The results of calculations from relationship (8) are
given in Table 3. These data represent the temperature
dependences of the concentrations of OH groups on the
surface and in the bulk of silica sample AKb1. The
dependences of the concentrations δOH, αOH, and γOH on
the temperature T (°C) of preliminary heat treatment
are plotted in Fig. 8. At a temperature of 200°C, the
concentrations of surface and internal OH groups in sil-
ica precipitated from the hydrothermal solution are
comparable in magnitude. In the range 200–400°C,
internal water is slowly removed and the concentration
γOH decreases insignificantly with an increase in the
temperature. Since the removal rate of internal water is
relatively low, the concentration of internal OH groups
becomes equal to the concentration of surface OH
groups at temperatures in the range 375–425°C and
somewhat exceeds the latter concentration at a temper-
ature of approximately 400°C, as can be judged from
the intersection of the dependences γOH(T) and αOH(T)
in Fig. 8.
At temperatures higher than 400°C, the removal rate
of internal water becomes higher: the concentration of
internal OH groups at 600°C is lower than that of sur-
face OH groups by a factor of more than two. The
curves δOH(T) and αOH(T) in Fig. 8 intersect at a temper-
ature of approximately 900°C. This suggests that inter-
nal OH groups exist in silica sample AKb1 at high tem-
peratures. Internal OH groups are completely removed
at temperatures of 900–1000°C, when the concentra-
tion of surface OH groups becomes considerably lower
than an initial concentration of 4.6–4.9 OH/nm2. In the
temperature range 400–800°C, the experimental con-
centrations γOH (OH/nm2), which correspond to the
water content in the bulk of the silica sample precipi-
tated from the hydrothermal solution, can be approxi-
mated by the relationship
(9)
The data obtained indicate that the mechanisms of
water removal from the surface and bulk of the sample
differ fundamentally. As the temperature T increases,
the silanol number αOH(T) decreases rapidly in the
range 200–400°C and changes more slowly at temper-
atures above 400°C because of the disappearance of
surface vicinal silanol groups. By contrast, the concen-
tration γOH(T) decreases slowly in the range 200–400°C
and the removal rate of internal OH groups increases in
the temperature range 400–600°C. In our opinion, this
difference can be explained by the fact that, first, con-
γOH T() δ
OH T() α
OH T().–=
γOH
ln T() 0.943 0.0065 T26732
–().–=
densation (6) of silanol groups in the case of internal
water proceeds in the bulk rather than on the surface
and, second, the removal of internal water requires the
transfer of water molecules from the bulk of particles to
their surface. The transfer of H2O molecules can be pro-
vided by diffusion through the bulk of particles or ultra-
micropores. The diffusion rate increases with an
Table 4. Concentrations of surface silanol groups αOH,T and
degrees of surface coverage θOH,T by OH groups (Zhuravlev
physicochemical constants [4]) as a function of the tempera-
ture T of preliminary heat treatment under vacuum for differ-
ent samples of amorphous silicas
Temperature of
preliminary heat
treatment under
vacuum T, °C
Concentration
of surface silanol
groups αOH,T,
OH/nm2
Degree of cover-
age of the silica
surface by OH
groups θOH,T
Portion AB (approximated by the linear equation, Fig. 7)
190 4.90 1.00
225 4.47 0.91
250 4.17 0.85
275 3.86 0.79
300 3.56 0.73
325 3.25 0.66
350 2.95 0.60
375 2.64 0.54
400 2.33 0.48
Portion BC (approximated by the power equation, Fig. 7)
425 2.18 0.44
450 2.05 0.42
475 1.94 0.40
500 1.84 0.38
525 1.75 0.36
550 1.67 0.34
575 1.59 0.32
600 1.52 0.31
625 1.46 0.30
650 1.40 0.29
675 1.35 0.28
700 1.30 0.27
725 1.25 0.26
750 1.21 0.25
775 1.17 0.24
Portion DE (from 800 to 1200°C, see Fig. 7 and text)
800 0.70 0.14
900 0.40 0.08
1000 0.25 0.05
1100 0.15 0.03
1200 0.0 0.0

670
GLASS PHYSICS AND CHEMISTRY Vol. 31 No. 5 2005
POTAPOV, ZHURAVLEV
increase in the temperature. This leads to an increase in
the slope of the curve γOH(T) in the range 400–600°C.
CONCLUSIONS
A comparison of the temperature dependence of the
concentration δOH(T) determined from the thermogravi-
metric data for a silica sample precipitated from a
hydrothermal solution with the Zhuravlev physico-
chemical constants αOH(T) demonstrated that the con-
centration of internal OH groups at a temperature of
200°C is considerable and comparable to the concen-
tration of surface OH groups. Thus, the performed
investigation revealed a new type of amorphous silica
with a high concentration of internal silanol groups.
This type is represented by silica samples prepared by
coagulation and precipitation of colloidal silica parti-
cles from the hydrothermal solution or silica formed
through deposition of colloidal particles from the flow
of a hydrothermal solution to walls of carrying chan-
nels.
The conclusion was drawn that a considerable con-
centration of internal water in silica precipitated from
the hydrothermal solution is explained by two factors:
(1) the mechanism of formation of colloidal particles
via polycondensation of orthosilicic acid molecules and
the presence of noncompensated regions in the inor-
ganic polymer network and (2) the effect of the aqueous
solution at elevated temperatures and pressures (equiv-
alent to hydrothermal treatment resulting in dissolution
and reprecipitation of silica) and water diffusion.
The analysis of the dependences αOH(T) and γOH(T)
for silica precipitated from the hydrothermal solution
indicated that the mechanisms of removal of internal
and surface water differ fundamentally. The removal
rate of internal water is low in the temperature range
200–400°C and increases at temperatures of 400–
600°C. This difference can be associated with the fact
that internal water is removed as a result of condensa-
tion of silanol groups in the bulk of particles or ultrami-
cropores and the condensation products are transferred
to the particle surface due to molecular diffusion whose
rate increases with an increase in the temperature.
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400
1
600 800 1000 1200200
0
2
3
4
5
6
7
8
9
T, °C
Concentration of hydroxyl groups, nm–2
1
2
3
Fig. 8. Temperature dependences of the concentrations of
hydroxyl groups (1) δOH, (2) αOH, and (3) γOH (see Table 3).
SPELL: OK