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1119
ISSN 0036-0244, Russian Journal of Physical Chemistry, 2006, Vol. 80, No. 7, pp. 1119–1128. © Pleiades Publishing, Inc., 2006.
Original Russian Text © L.T. Zhuravlev, V.V. Potapov, 2006, published in Zhurnal Fizicheskoi Khimii, 2006, Vol. 80, No. 7, pp. 1272–1282.
In the present work, we report the results of studying
the physicochemical characteristics of silica precipi-
tated from a high-temperature hydrothermal solution
[1–3]. Using the Zhuravlev model [4], we determined
the content of sorbed water and the distribution of sil-
anol OH groups between the surface and the bulk of the
silica sample. The formation of a colloid dispersion of
silica in a hydrothermal solution occurs via a sequence
of physicochemical processes. The initial concentration
of silica depends on the temperature at which the chem-
ical equilibrium between the water and the alumosili-
cate minerals of the rocks of the high-temperature
hydrothermal deposit [1–3]. At
250–350°ë
, the overall
content
c
t
of silica in water nearly coincides with the
solubility of quartz (500–700 mg/kg), with the silica
being present in the solution mainly in the form of
H
4
SiO
4
molecules (orthosilicic acid).
After ascending filtration in strata or after appearing
at the surface in supplying wells of a hydrothermal
power station, the solution becomes supersaturated with
respect to the solubility
c
e
of amorphous silica [3] due to
a decrease in the pressure and temperature and an
increase in the silica concentration due to evaporation.
The total concentration
c
t
of silica in the solution is typi-
cally 700–1500 mg/kg [3]. The supersaturation of the
solution, which is equal to the difference between the
concentration of orthosilicic acid
c
s
and the solubility
c
e
,
(
c
s
–
c
e
), is the driving force of the polycondensation of
orthosilicic acid, a process that involves the formation of
siloxane bonds and partial dehydration [5]:
(1)OH–Si–OH OH–Si–OH OH–Si–O–Si–OH H2O++
OH
––
OH
OH
––
OH
OH
––
OH
OH
––
OH
or
(2)
As a result of nucleation and polycondensation,
hydrated colloid silica particles
n
SiO
2
·
m
H
2
O are
formed. The dissociation of surface silanol groups
SiOH with the abstraction of H
+
imparts a negative
charge to the surface. Electrostatic repulsion prevents
particles from coagulating, thereby making colloid sil-
ica stable in the hydrothermal solution.
SimOm1– OH()
2m2+ SinOn1– OH()
2n2+
+
Simn+Omn1–+ OH2n2m2++ H2O.+
Studying the physicochemical characteristics of col-
loid silica in a hydrothermal solution after its precipita-
tion from the solution is important for developing mod-
els of the formation of hydrothermal minerals [6, 7],
including ores, and for creating technologies of extrac-
tion and utilization of silica, technologies that can
enhance the efficiency of geothermal power plants and
combined heat and power plants [8].
The silica samples were obtained by freezing-out of
dispersed solutions from wells of the Mutnovskii geo-
thermal field. As solution droplets were frozen on a
snow surface, colloid silica particles concentrated
Density of Silanol Groups on the Surface of Silica Precipitated
from a Hydrothermal Solution
L. T. Zhuravlev
a
and V. V. Potapov
b
a
Institute of Physical Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 117915 Russia
b
Research Geotechnological Center, Far East Division, Russian Academy of Sciences, Petropavlovsk-Kamchatskii, Russia
E-mail: vadim_p@inbox.ru
Received August 31, 2005
Abstract
—The physicochemical properties of amorphous silica precipitated from a hydrothermal solution were
studied. Low-temperature nitrogen adsorption in conjunction with the BET method was used to determine the spe-
cific surface area of this silica. Based on thermogravimetry data, the total content of water was estimated. A com-
parison of the thermogravimetry data with the Zhuravlev physicochemical constants made it possible to determine
the temperature dependences of the concentration of surface and internal silanols over a temperature range of from
200 to
1200°ë
. A new type of amorphous silica with enhanced internal water content was revealed. The distinc-
tions between the mechanisms of the removal of surface and internal water were established.
DOI:
10.1134/S0036024406070211
PHYSICAL CHEMISTRY
OF SURFACE PHENOMENA
1120
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY
Vol. 80
No. 7
2006
ZHURAVLEV, POTAPOV
between ice crystals; as a result, the distances between
them decreased, a factor favorable for their coagula-
tion. After being heated to
105°ë
, the gellike mass
formed from the mixture of snow and deposited silica
transformed into a fine power. The deposited silica was
removed from the surface of the plate and dried using
hydrothermal heat. The density of the gellike silica col-
lected from the surface of the snow was 2.0 g/cm
3
. After
drying at
110°ë
for 12–16 h, the material transformed
into a fine powder with a density of from 0.22 to
0.24 g/cm
3
.
Amorphous silicas with various degrees of hydrox-
ylation are widely used in science and technology [4].
Generally, surface silanol groups (
≡
Si
–
OH) are formed
via two thermodynamically favorable processes [4].
First, such groups are formed during synthesis, for
example, the polycondensation polymerization of
Si
(
OH
)
4
(Fig. 1a), when a supersaturated solution of
orthosilicic acid transforms into polysilicic acids, with
the subsequent formation of SiO
2
sols and gels contain-
ing surface OH groups. When dried, the final product,
xerogel, retains surface silanols, at least partially. Sec-
ond, silanols can be produced by rehydroxylation of
thermally dehydroxylated silica during its treatment
with water of an aqueous solution (Fig. 1b).
The types of groups at the surface and in the bulk of
silica are displayed in Fig. 2 [4, 9]: free single (isolated)
surface silanols (
≡
SiOH; type Q
3
); free germinal (iso-
lated) surface groups (
=
Si
(
OH
)
2
, silanediols; type Q
2
);
vicinal bridge silanols (i.e., single silanols, single ger-
minal silanols, or combinations thereof bound with one
another by hydrogen bonds;
≡
Si
–
O
–
Si
≡
, siloxane
bridges with the O atom at the surface (type Q
4
); and
internal silanols (located in the skeleton and/or in ultra-
micropores of SiO
2
). Thus, amorphous silicas, both
treated over a wide temperature range and untreated,
contain only two types of OH groups, single and germi-
nal, which, in turn, can be subdivided into isolated,
free, and H-bonded vicinal ones [4, 9].
The properties of disperse amorphous silica, as an
adsorbent, are determined by its porous structure and
the chemical activity of the surface, with the latter
being dependent on the concentration of OH groups
(i.e., the total concentration of all silanols and the con-
centrations of their varieties), on the temperature and
energy distribution of the silanols, and on whether
siloxane (SiOSi) bridges are present. At the same time,
≡
Si
–
OH groups can be located at the surface (surface
silanols) or in the bulk of amorphous silica particles
H
HH
H
HH
HH
HH
H
HH
H
H
H
H
H
H
HH
H
H
H
H
H
H
H
H
OO
O
OO
OO
O
O
OO
OOO
O
O
OO
OO
O
OO
O
OO
O
O
O
O
OO
O
O
O
O
O
Si Si Si
Si Si
Si
Si
Si
Si Si
Si
SiSi
Si
Si
Si
Si
Si
Si
Si Si
Si
Si
+ H
2
O
(‡)
(b)
Fig. 1.
Scheme of the formation of the structure of a silica surface (
≡
Si
–
OH silanol groups): (a) condensation polymerization and
(b) rehydroxylation.
H
Si Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
H
H
H
H
H
H
H
OO
O
O
O
O
O
O
O
O
OSi–OH
(Q
3
)(Q
2
)
(Q)
(Q
4
)
(Q)
Internal
silanols
Fig. 2.
Types of silanol groups and siloxane bridges at the
surface of an amorphous silica and internal OH groups:
vicinal (Q), germinal (Q
2
), and isolated (Q
3
) silanols;
Q
4
denotes surface siloxanes.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY
Vol. 80
No. 7
2006
DENSITY OF SILANOL GROUPS ON THE SURFACE OF SILICA PRECIPITATED 1121
and/or in ultramicropores in them (less than 1 nm in
diameter), which are accessible for small molecule, for
example, water (internal silanols). As discussed in
detail in [4], OH groups at the surface of large transport
pores readily participate in deuteroexchange with
heavy water D
2
O in the gaseous or liquid state at room
temperature, but the water bound in the SiO
2
skeleton
remains virtually intact, while the isotope exchange
with the silanols located in ultramicropores occurs very
slowly.
Internal silanols, i.e., OH groups covalently bound
to internal atoms of silica particles (Fig. 2), i.e., OH
groups not involved in surface processes, are typical of
amorphous silicas. Internal silanol groups (
≡
Si
–
OH)
are not considered when it is necessary to calculate the
surface concentration of OH groups (the silanol number
α
éç
, i.e., the number of OH groups per nm
2
) and to
study the active behavior of only surface silanols, since
such silanols play the main role in various processes at
the surface.
Silanols in the skeleton and ultramicropores of silica
can be formed by various mechanisms:
(1) Some silanols in silica obtained by condensation
of low-molecular-weight polysilicic acids can remain in
the network of the inorganic polymer (due to incomplete
polycondensation) if suitable partners are absent during
the synthesis. In addition, silicas prepared from sodium
silicate can contain a certain amount of OH groups in the
bulk, trapped during the aggregation of small particles
and the subsequent aging of the SiO
2
gel.
(2) According to [5], when colloid particles are
gradually grown in an alkaline solution (pH ~ 9),
sodium ions can be adsorbed on particles concurrently
with SiO
2
deposition, a factor favorable for the trapping
of silanols in the silica structure.
(3) Large spherical particles in pyrogenic silica
(10
−
20 nm in diameter) are formed through the aggre-
gation of primary elementary globules (1–2 nm in size)
produced by hydrolysis at elevated temperatures (in
flames). Since primary particles contain a certain
amount of surface silanols, such OH groups may occur
trapped in large globules of the final product.
(4) According to [10], the existence of internal sil-
anols can be explained by the diffusion of
ç
2
é
mole-
cules into the bulk of the solid SiO
2
structure (up to 15 nm
in depth) at elevated temperatures.
(5) One of the most widespread methods that pro-
duces silanols in the skeleton and in ultramicropores of
SiO
2
is hydrothermal treatment (HTT). Hydrothermal
treatment of amorphous silica (at elevated temperatures
of water and aqueous solutions brought in contact)
involves complicated processes of dissolution and
reprecipitation of silica and the diffusion of water in the
solid phase, processes that lead to the formation of sec-
ondary (geometrically modified) silicas with a different
pore structure, which is capable of retaining silanols or
bound water inside particles and in ultramicropores.
(5a) Using HTT in an autoclave at the stage of SiO
2
hydrogels, the authors of [11] obtained a number of sil-
ica gels with a mesoporous structure and dense particle
packing, but without ultramicropores.
(5b) Using HTT at the stage of SiO
2
xerogels, the
authors of [12] obtained a number of silica gels with
various porous structures of particles (globular, inter-
mediate, and spongy). The silica gels contained silanols
at the surface, inside particles, and in ultramicropores.
(5c) Using HTT of pyrogenic silicas (aerosils) in an
autoclave, the authors of [12] obtained a number of sil-
ica gels with various porous structures, with silanols
being located inside silica particles and at their surface.
(5d) In [13], various treatments and HTT (a pro-
longed boiling in water) of initial porous glasses
yielded geometrically modified porous glasses with
various porous structures. The modified glasses con-
tained OH groups at the surface, inside particles, and in
ultramicropores.
Note that internal silanols are expected to be present
in various amorphous silicas that form precipitates at
the walls of wells, pipelines, and hot-water-carrying
units of geothermal heat and power plants in Russia,
New Zealand, Japan, United States, the Philippines,
Mexico, Iceland, Italy, etc.
Table 1 lists data on the chemical composition of sil-
ica sample AK1b, which were obtained in our experi-
ments. The mass fraction of silicon dioxide in the sam-
ple (after subtracting the mass losses during drying at
105°ë
and calcining at
1000°ë
) was within 95.00–
97.69 and even to 99.02 wt %; the total mass fraction of
calcium, aluminum, and iron was below 0.6%.
The silica samples precipitated by freezing-out of
hydrothermal solutions had an amorphous structure
(Fig. 3a). The XRD patterns of the samples exhibited a
well-pronounced halo with a maximum within 0.387–
0.400 nm. After calcination at
1000°ë
, the amorphous
silicas transformed into crystalline cristobalite (Fig. 3b).
The IR spectra of the precipitates were recorded on
a Vector 22/N (Bruker) FTIR spectrometer within 250–
4250 cm
–1
. The 250–1200-cm
–1
range featured three
Table 1.
Chemical composition
m
i
(wt %) of silica sample AK1b of finely dispersed silica precipitated by freezing-out
Substance
m
i
Substance
m
i
Substance miSubstance mi
SiO281.13 Al2O30.41 FeO 0.09 K2O 0.29
TiO20.02 Fe2O30.07 Na2O 0.60 P2O50.06
Note: The mass losses associated with drying at 110°C and calcination at 1000°C were 10.93 and 6.03 wt %; MnO, MgO and CaO were
not detected.
1122
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 7 2006
ZHURAVLEV, POTAPOV
peaks (two moderate-intensity peaks at 500 and 750–
850 cm–1 and a strong peak at 1096–1104 cm–1), which
corresponded to the vibrations of the Si–O–Si bonds of
the SiO4 tetrahedron (Fig. 4). The 1200–4000-cm–1
range featured two moderate-intensity peaks at 1600–
1640 and 2344–2368 cm–1 and an intense peak at 3440–
3480 cm–1 (belonging to the vibrations of hydroxy
groups). Note, however, that the 3750-cm–1 band, typi-
cal of free ≡Si–OH groups, was undetectable, probably
because of the screening by vicinal silanols and/or
adsorbed water. The intensity of the IR bands and the
positions of the two prime bands, at 1096–1104 and
3440–3480 cm–1, are typical of various forms of amor-
phous silicas.
Figure 5 shows the results of a thermal analysis (on
a Perkin Elmer Pyris Diamond TG/DTA instrument of
silica sample AK1b) of silica sample AK1b. Table 2
lists the corresponding TGA data. The measurements
were performed in air at a heating rare of 20 K/min.
The reflection coefficient of the surface within
400.0–760.0 nm was measured on an MSFU-K spec-
trophotometric microscope. The reflection coefficient
(whiteness) of hydrothermal silica was within from 91–
95 till 94–98%. It was found that the reflection coeffi-
cient increased with the wavelength (Table 3).
The surface area and volume of the pores in the sil-
ica samples was measured by low-temperature nitrogen
adsorption on an ASAP-2010N porosimeter (Micro-
metrics, United States). The mechanism is based on
measuring the adsorption–desorption of nitrogen at the
liquid-nitrogen temperature [14]. Monitoring the mass
of a disperse sample made it possible to determine the
amount of nitrogen adsorbed at a given relative pres-
sure of nitrogen p/p0 (p and p0 are, respectively, the cur-
rent pressure and saturation vapor pressure of nitrogen
at the temperature at which the experiment was per-
formed) in the ampoule with the sample. The volume of
adsorbed nitrogen V was first measured at increasing
p/p0 values (from 0.01 to 1.0), to plot the adsorption
curve, and then at decreasing p/p0 values (from 1.0 to
841
10
020 30 40 50 60
θ, deg
(b)
137
0
(‡)
IX
Fig. 3. Data of an XRD analysis of the silica samples (a) before and (b) after calcination at 1000°ë.
0.2
10000 2000 3000 4000
ν, Òm–1
0.4
0.6
0.8
1.0
472
800
1104
1632 2368
3440
Fig. 4. IR spectrum of the silica sample.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 7 2006
DENSITY OF SILANOL GROUPS ON THE SURFACE OF SILICA PRECIPITATED 1123
0.04), to plot the desorption curve. The V(p/p0) depen-
dence for silica sample AK1b is displayed in Fig. 6.
The isotherms obtained belong to type IV of adsorp-
tion isotherms [14]. Such isotherms first run convex,
pass through an inflection point (at p/p0 = 0.3–0.5),
become concave, and again convex near p/p0 = 1.0
(Fig. 6). The specific surface area was determined by
the Brunauer–Emmett–Teller (BET) and Brunauer–
Deming–Helsey (BDH) methods. The specific surface
area was determined from measurements within p/p0 =
0–1.0 by using the BET equation for polymolecular
vapor adsorption [14].
Based on the classical theory of adsorption and des-
orption processes [14] and the dependence V(p/p0) we
calculated the differential pore volume Vp and pore sur-
face area sp distributions over the diameter dp within a
particular range and the integral pore volume and the
integral pore surface area for pores with diameters from
1.7 nm to a given value of dp. The dVp/d and
dsp/d distributions passed through a maximum at
dp = 18.0 and 11.7 nm, respectively.
The characteristics of pores in disperse hydrother-
mal silica as determined by the adsorption method are
given below: T = 77.2 ä, p0 = 747.17, sample weight is
0.13 g, volume is 17.54 Òm3, volume 2 is 54.32 Òm3, ss =
263.53 m2/g, sBET = 274.64 m2/g, sMP = 26.33 m2/g,
sAC = 260.25 m2/g, sDC = 333.52 m2/g, VS = 0.871 Òm3/g,
dp
log
dp
log
VMP = 0.00827 Òm3/g, VAC = 1.078 Òm3/g, VDC =
1.088 m3/g, dBET = 12.692 nm, dA = 16.575 nm, and
dD = 13.058 nm.
Here, volumes 1 and 2 are the volumes of the
ampoule at room and liquid-nitrogen temperatures
(these quantities are measured by the instrument auto-
matically and are then used in determining the coeffi-
cients of the equations); sS is the specific pore surface
area determined at p/p0 = 0.200; sBET is the total specific
pore surface area determined by the BET method; sMP
is the specific surface area of pores with a diameter of
~1.7 nm; sAC is the total specific pore surface area deter-
mined by the BDH method from the adsorption curve
for pores with a diameter of from 1.7 to 300.0 nm; sDC
is the specific pore surface area determined by the BDH
method from the desorption curve for pores with a
diameter of from 1.7 to 300.0 nm; VS is the total specific
volume of pores with a diameter of less than 40.0 nm,
which was measured at a relatively nitrogen pressure of
p/p0 = 0.950; VMP is the specific volume of micropores
with a diameter of ~1.7 nm; VAC is the total specific
pore volume determined by the BDH method from the
adsorption curve for pores with a diameter of from 1.7
to 300.0 nm; dBET is the mean pore diameter defined as
4VS/sBET; dA is the mean diameter 4VAC/sAC; and dD is
the mean diameter 4VDC/sDC.
–100
200 400 600 800 1000
t, °C
0
100
200
mV
68°C
1
2
3
200
100
0
–100
–200
–300
mg/min 100
96
92
88
wt, %
DTG
DTA
TG
Fig. 5. Results of the thermal analysis of the silica sample
in various modes: (1) TG, (2) DTA, and (3) DTG.
Table 2. Mass of silica sample AK1b m (wt %) as a function of the temperature during thermogravimetric analysis
t, °Cmt, °Cmt, °Cmt, °Cm
22.6 100 300 92.10 600 90.09 900 89.27
100 94.65 400 91.30 700 89.76 1000 89.09
200 92.81 500 90.58 800 89.49 1100 88.61
100
0.20 0.4 0.6 0.8 1.0
p/p0
200
300
400
500
600
700
VN, Òm3/g
1
2
Fig. 6. Curves of (1) adsorption and (2) desorption from the
silica sample.
1124
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 7 2006
ZHURAVLEV, POTAPOV
As can be seen from the above characteristics, the
specific surface area of silica, porosity, and mean pore
diameter for sample AK1b (prepared from a solution
extracted from a Mutnovskii field well) are 300 m2/g,
1.1 g/cm3, and 12.7–16.6 nm, respectively. As can be
seen, the hydrothermal silica samples are characterized
by moderate values of the specific surface area and vol-
ume of pores in are moderate. The ratio of the surface
area of micropores to the total surface area of pores and
that of the volume of micropores to the total volume of
pores were found to be 0.09–0.107 and 0.005–0.0085,
respectively (Table 2).
Table 4 lists the distributions of the pore surface area
and volume of pores over the pore diameter; as can be
seen, these distributions are rather narrow. The differ-
ential distribution of the specific pore volume over the
pore diameter passes through a maximum at dp = 33 nm,
while the distribution of the specific pore surface area,
while the distribution of the specific surface area over
the pore diameter exhibits two maxima, at 13 and 9 nm.
The volume of pores with diameters within 5.18–20.61,
5.18–26.47, and 5.18–40.0 nm constitutes 71.1, 79.8,
and 88.3%, respectively, of the total pore volume. The
surface area of pores with diameters within 5.18–26.47
and 5.18–40.0 nm constitutes, respectively, 60.9 and
76.4%. These characteristics of pores ensure a rather
high reactivity of the deposited material and a rapid dis-
solution of it during technological processes.
Based on the specific surface area sBET (m2/g) of sil-
ica and the mass loss associated with the removal of
water and OH groups during thermogravimetric analy-
sis ∆ (wt %), one can determine the total concen-
tration δOH (OH/nm2) of all silanol groups at the surface
mH2O
Table 3. Dependence of the reflection coefficient k on the wavelength λ (nm)
λKλKλKλK
480 0.9402 540 0.95394 600 0.97425 660 0.97182
500 0.9531 560 0.96534 620 0.97911 680 0.97274
520 0.95029 580 0.96592 640 0.97679 700 0.98265
Table 4. Dependences of the specific volume V (g/cm3) and surface area s (m2/g) on the mean pore diameter dp for a hydro-
thermal silica sample as determined by adsorption analysis
dp, nm , nm VV
Σss
Σ
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
dp
m
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 7 2006
DENSITY OF SILANOL GROUPS ON THE SURFACE OF SILICA PRECIPITATED 1125
and in the bulk of silica per specific surface area of sam-
ple AK1b:
δOH = ∆ × 2 × 6.02 × 103/18sBET. (3)
Assuming that the temperature at which the silanol
groups are completely removed from the surface of
AK1b equals 1000°ë and using the data of Table 2, we
calculated δOH (at the surface and in the bulk) per unit
surface area of the sample at various temperatures
(Table 5).
Various aspects of the dehydroxylation and dehy-
droxylation of the surface of amorphous silica have
been considered in many studies, since the chemistry of
such a surface, determined largely by the concentration,
distribution, and reactivity of surface ≡Si–OH groups,
is of considerable theoretical and practical importance.
Figure 7 and Table 5 present the silanol number αéç
for the surface as a function of the thermoevacuation
temperature t (°ë), which was determined by the deute-
roexchange method (the contributions from free iso-
lated, free germinal, and vicinal OH groups were
included). This dependence for various SiO2 samples
makes it possible to determine the Zhuravlev physico-
chemical constants (the silanol number αéç and the
surface coverage of θéç on a SiO2 surface at various
fixed temperatures), which are widely used in studies
on the subject. To determine the temperature depen-
dence of αéç, we used 100 samples of amorphous sili-
cas with significantly different specific surface areas sKr
determined by low-temperature krypton adsorption
(from 9.5 to 945 m2/g) and accessible pore diameters
(from ~1.0 to 1000 nm or even higher).
Despite significant differences in the values of sKr
and d (without the contribution from ultramicropores)
for various SiO2 samples, the values of αéç at a given
pretreatment temperature are similar, as is the decrease
of αéç with increasing temperature under similar con-
ditions of heating. The value of αéç decreases rapidly
as the temperature increases from 190 to 400°ë (seg-
ment AB of the plot in Fig. 7) and, then, from 400 to
~780°ë more slowly (segment BC).
mH2O
Table 6 lists the Zhuravlev physicochemical con-
stants [4] (shown by the bold lines in Fig. 7), i.e., the set
of the most probable values of αéç or the concentra-
tions of the silanols at the surface at fixed pretreatment
temperatures. The corresponding coverages θéç of the
surface by OH groups are also presented. These physi-
cochemical constants, αéç and θOH, are universal for
amorphous silicas irrespective of their origin and struc-
tural characteristics (Table 6 and Fig. 7, segments AB
and BC) if the SiO2 surface in the initial state was com-
pletely hydroxylated (Fig. 7, point A) (whether silanols
are present in the bulk and in ultramicropores is not
taken into account).
The αéç = f(t) and θOH = g(t) dependences exhibit
two characteristic segments with markedly different
slopes within range II in Fig. 7: the dependence can be
described by a straight line (bold line) within sub-
range IIa (190–400°ë),
αéç, OH/nm2 = –0.0122t + 7.218 (4)
Table 5. Distribution of OH groups between the surface and the bulk of silica sample AK1b subjected to hydrothermal treat-
ment
t, °CδOH αOH γOH t, °CδOH αOH γOH
200 8.29 4.90 3.39 600 2.23 1.52 0.71
300 6.71 3.56 3.15 700 1.49 1.30 0.19
400 4.92 2.33 2.59 800 0.89 0.70 0.19
500 3.33 1.84 1.49 900 0.40 0.40 0.0
Note: t is the temperature of thermoevacuation of sample AK1b; δOH is the total water loss for sample AK1b measured by calcining it at
elevated temperatures (thermogravimetric analysis) and expressed as the number of OH groups per unit area of SiO2 surface; αOH
is the mean total concentration of silanols at the SiO2 surface as a function of the pretreatment temperature, as determined by Zhurav-
lev by using the deuteroexchange method (the Zhuravlev physicochemical constants at various temperatures, constants universal for
amorphous silicas) [4]; γOH is the content of internal silanols (internal bound water) located in the skeleton and ultramicropores of
sample AK1b, defined as the difference between δOH – αOH at a given temperature (Eq. (8)) (this quantity can also be expressed as
the number of OH groups per unit area of SiO2 surface (γOH, OH/nm2).
C
DE
B
A
4.9
5.7 IIa IIb' IIb''
4.2
190°C
6
4
2
0 200 400 600 800 1000
t, °C
αOH, OH/nm2
Fig. 7. Dependence of the silanol number αéç of the pre-
treatment temperature for 16 various SiO2 samples. The
band limited the dashed lines show the range of experimen-
tal values. Range II consists of subranges IIa (segment AB,
190–400°C), IIb' (BC, 400–780°ë), and IIb" (DE, 800–
1200°ë); for details, see the text.
1126
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 7 2006
ZHURAVLEV, POTAPOV
(passes through point A (αOH = 4.9, Table 6); the corre-
lation coefficient equals R2 = 0.8768) and by a power
function within subrange IIb' (400–780°C) (αOH = 2.33,
Table 6),
αéç, (OH/nm2) = 1155.6t–1.0367 (R2 = 0.8516). (5)
The condensation of silanols,
(≡Si–OH) + (≡Si–OH) (≡Si–O–Si≡) + H2O↑, (6)
has been extensively studied extensively studied. This
reaction is characteristic of subrange IIa, as well as sub-
ranges IIb' and IIb'', although the values of the activa-
tion energy of desorption in these subranges differ from
each other significantly [4]. At high surface coverages
of silanols (1 ≥ θéç > 0.5; subrange IIa in Fig. 7 and
Table 5), the following empirical equation is valid [4]:
ED = 31.4 – 12.3θOH, (7)
where ED varies insignificantly, within 19–25 kcal/mol.
In other words, the activation energy ED is nearly inde-
pendent of the concentration of silanols, being largely
determined by perturbations arising at a surface con-
taining H-bonded OH groups; such perturbations disap-
pear as the vicinal silanols are removed at ~400°ë (at
θéç ~ 0.5). Thus, when present in a high concentration
(within subrange IIa (segment AB in Fig. 7)), neighbor-
ing vicinal OH groups interact with one another (lateral
interactions).
At low OH-group surface coverages (θéç < 0.5;
subrange IIb'), free single and free germinal OH
groups, as well as SiOSi bridges play the primary role
(Figs. 2 and 7). In subranges IIb' and IIb'', the activation
energy ED increases sharply, from 25 to 50 kcal/mol
and higher, as αéç decreases. If there are only free OH
groups surrounded by SiOSi bridges, the latter can
cover significant areas due to the high-temperature acti-
vation of SiO2. Under such conditions, the main mech-
anism of the transfer of OH groups during the conden-
sation of silanols (reaction (6)) may be stochastic
migration of protons over the surface (the process of
activated surface diffusion of OH groups). At the final
stage, as two OH groups randomly come within ~0.3 nm
(a typical length of a hydrogen bond) of each other, they
interact to yield a water molecule. At low concentra-
tions of OH groups (segments BC and DE in Fig. 7), the
diffusion of protons over the SiO2 surface limits the
process of condensation as a whole (reaction (6)).
Segment DE (Fig. 7) corresponds to a situation
where the surface contains no germinal silanols, and,
hence, the reaction of condensation occurs only due to
the interaction of sparsely distributed single OH
groups. Within this high-temperature subrange (seg-
ment DE in Fig. 7), amorphous silica can completely or
partially crystallize, as was demonstrated in the present
work for amorphous sample AK1b (obtained by precip-
itation from a hydrothermal solution), which trans-
formed into cristobalite at 1000°ë (Figs. 3a, 3b). Since
it is difficult to approximate segment DE in Fig. 7, we
limited ourselves to calculating the mean values of αéç
(Table 6).
Thus, based on the above results, we found that,
at180–200°ë, the ultimate surface concentration of OH
groups at a silica surface can be as high as 4.9 OH/nm2;
the concentration of internal silanol OH group (internal
water) per unit surface area of sample AK1b can be cal-
culated by the formula
γOH(T) = δOH(T) – αOH(T). (8)
Table 6. Surface concentration of silanols αOH and OH
group surface area coverage θOH as functions of the temper-
ature of thermoevacuation of various amorphous silicas (the
Zhuravlev physicochemical constants [4])
t, °CαOH θOH
Segment AB
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
Segment BC
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
Segment DE
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
Note: Segments AB and BC (Fig. 7) were approximated by a linear
and a power equation, respectively.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 7 2006
DENSITY OF SILANOL GROUPS ON THE SURFACE OF SILICA PRECIPITATED 1127
The calculation results are listed in Table 5. These
results show how the distribution of OH groups
between the surface and bulk of silica sample AK1b
depends on the temperature. The temperature depen-
dences of δOH, αOH, and γOH are displayed in Fig. 8. At
200°ë, the contents of surface and internal silanol OH
groups are comparable. Within 200–400°ë, internal
water is removed slowly, and, therefore, γOH decreases
insignificantly with increasing temperature. Since the
rate of the removal of internal water is relatively low,
the content of internal OH groups coincides with that of
surface OH groups at 375 and 425°ë, being somewhat
higher at the middle of this range, ~400°ë (as can be
seen from the crossing of the γ(T) and αOH(T) curves in
Fig. 8.
Above 400°ë, the rate of internal water removal
increases; as a result, at 600°ë, the concentration of
internal OH groups is two times that of surface OH
groups. The δOH(T) and αOH(T) curves intersect at a
temperature of ~900°ë, a feature indicative of the exist-
ence of internal OH groups in silica AK1b at elevated
temperatures. Hydroxy groups disappear completely at
900–1000°ë, temperatures at which the concentration
of surface OH groups is substantially lower than the ini-
tial one (4.6–4.9 nm–2). Within 400–800°ë, the experi-
mental values of γOH (nm–2), which correspond to the
content of internal water in the silica sample, precipi-
tated from a hydrothermal solution can be approxi-
mated by the formula
lnγOH = 0.943 – 0.0065(T2 – 6732). (9)
The results obtained suggest that the mechanisms
of the removal of internal and surface water differ sig-
nificantly. Within 200–400°ë, αOH(T) decreases rap-
idly with increasing T; at temperatures above 400°ë,
this decrease slows down due to the disappearance of
surface vicinal silanol groups. By contrast, γOH
decreases slowly within 200–400°ë, while the rate of
the removal of internal OH groups increases as the
temperature rises from 400 to 600°ë. In our opinion,
this distinction can be explained by two factors: (1) in
this case, the internal condensation of silanol groups
(reaction (6)) occurs in the bulk rather than on the sur-
face and (2) internal water is removed by the transport
of water molecules (through the solid material or
ultramicropores) from the bulk to the surface. The dif-
fusion rate increases with the temperature, giving rise
to an increase in the slope of the γOH(T) curve within
400–600°ë.
A comparison of the thermogravimetry data for the
silica sample precipitated from the hydrothermal solu-
tion, presented as the temperature dependence of
δOH(T) with the Zhuravlev physicochemical constant
αOH(T) showed that this sample contained at 200°C a
significant amount of internal OH groups, comparable
to that of surface OH groups. Thus, we revealed a new
type of amorphous silica, silica with a considerable
amount of internal silanols. Samples of such silica were
prepared by coagulation and precipitation of colloid sil-
ica particles from a hydrothermal solution or collected
from the walls of ducts through which a hydrothermal
solution flowed.
In our opinion, the presence of a significant
amount of internal water in silica deposited from a
hydrothermal solution can be explained by (1) the
mechanism of the formation of colloid particle dur-
ing the polycondensation of orthosilicic acid, (2) the
action of the aqueous solution at elevated tempera-
tures and pressures (the effect of which is similar to
hydrothermal treatment, accompanied by the disso-
lution and reprecipitation of silica), and (3) water
diffusion.
A comparison of the αOH(T) and γOH(T) depen-
dence for silica precipitated from a hydrothermal
solution led us to conclude that the mechanism of the
removal of internal and surface water differ drasti-
cally: within 200–400°ë, internal water is removed
slowly, with the rate of this process increasing at 400–
600°ë. This distinction can be explained by the fact
that internal water is removed via the condensation of
silanol group in the bulk of the particles and/or in
ultramicropores while the transport of the condensa-
tion products to the surface of the particles occurs by
the diffusion of molecules, the rate of which increases
with the temperature.
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ZHURAVLEV, POTAPOV
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