CHAPTER 3
Polymerization of Silica
From the time of Graham ( I ) , who made an intensive study of sols and gels, many
attempts have been made to explain the behavior of silicic acid. When freshly made
by acidifying a soluble silicate or hydrolyzing the ester, silicic acid is not “colloidal,”
since i t diffuses easily through parchment or animal membranes and has a molecular
weight by freezing point depression corresponding to monomer. Soon the molecular
units become larger and pass through membranes only slowly and then not at all (2).
This could be because the monomer or other small primary particles form
aggregates, or because the individual particles increase in size and decrease in
number.
Freundlich appeared to recognize these alternatives when he wrote:
Whether it is rather a matter of polysilicic acids, which give larger micellae, being formed
from simple silicic acid, or whether the crystalloid particles originally present already consist of
polysilicic acids, but are exceedingly fine amicrons which continually increase in size-cannot
yet be said with certainty.
In his terminology, a “micella” is a colloidal particle in which foreign substances
(ions, water) are present in its structure, that is, a porous aggregate, whereas the
“amicron” is a discrete parlick too small to be seen with the ultramicroscope. He
recognized that such particles in a colloidal solution could “consist of one very large
molecule,” in other words, a single unit, not an aggregate.
Because the most obvious behavior of a silicic acid solution is that it increases in
viscosity and finally forms a gel, its polymerization was generally assumed to be an
aggregation process or a polymerization by which smaller molecular units linked
together into larger ones. The nucleation and growth of discrete particles prior t o the
stage where aggregation begins have not been recognized by many workers, who
held to the idea that Si(OH), polymerized into siloxane chains which then were
branched and cross-linked as in many organic polymers. Even now attempts are still
made to apply the idea of monomer functionality and condensation polymerization
theory of organic chemistry t o the silica system. In fact, there is n o relation or
analogy between silicic acid polymerized in an aqueous system and condensationtype organic polymers. [However, the behavior of silica in molten glasses is quite a
I72
Polymerization of Silica
173
different matter, and in that system the conventional polymer theory has been shown
by Masson (3) to be applicable.]
In 1925, Kruyt and Postma (4a) pointed out that there are two groups of silicic
acid sols. The first group has a p H of 4.5 or less, and the viscosity of the sol
increases with time. On the other hand, pure silica sols, having a p H of 7 or higher,
a r e relatively stable, the viscosity either remaining the same or decreasing with time.
This difference in behavior is explained as follows. The more alkaline sols bear a
negative charge and are thereby stabilized. However. the addition of soluble salts
lowers the charge of the particles and causes gelation or flocculation. On the acid
side, where there is essentially no charge, aggregation or flocculation occurs, causing
an increase in viscosity, and eventually gelation. Tourky (4b) also discussed the
structural differences between silicic acids in acidic and basic solutions; in acidic
solutions, fibrillar o r network structures arise through the formation of oxygen
bridges between silicic acid units.
It was Carmen (4c) who first clearly stated that silicic acid polymerizes to discrete
particles which then aggregate into chains and networks.
The formation of silica gel can be regarded as taking place in two stages. In the first, initially
formed Si(OH), condenses to form colloidal particles. In dilute solution, a further slow
increase in particle size is the only subsequent change, but at a concentration of about I percent
silica, these primary particles are able to condense together to give a very open but continuous
structure, extending throughout the medium, thus bestowing a certain degree*ofrigidity upon
it. In both stages of polymerization, the mechanism is the same, that is, condensation to form
Si-0-Si links, but in the first stage, condensation leads to particles of massive silica, while in
the second, since it is not possible to fit two particles accurately together over a common face,
the number of Si-0-Si linkages between particles is fewer in number than those within the
particles themselves. They are merely sufficient to bind adjacent particles together, in a fixed
position relative to one another, and thereby lead to a rigid, highly porous, tangled network of
branching chains. . . .
Thus three stages are actually recognized:
1. Polymerization of monomer to form particles.
2. Growth of particles.
3. Linking of particles together into branched chains, then networks, finally extending throughout the liquid medium, thickening it to a gel.
Since Carmen published this in 1940, further experimental data continue to confirm his point of view. There is general agreement that polymerization, that is, the
reactions that result in an increase in molecular weight of the silica, involves the
condensation of silanol groups:
-SOH
+ HOSi-
=
-SiOSi-
+ H,O
T h e term “polymerization” is used in its broadest sense, the mutual condensation of
S i ( 0 H ) t o give molecularly coherent units of increasing size, whether these are
Polymerization of Silica
174
spherical particles of increasing diameter or aggregates of an increasing number of
constituent particles. Formation and growth of spherical particles is one kind of
polymerization that takes place under certain conditions. Aggregation of particles to
form viscous sols and gels is another kind of polymerization occurring under other
conditions. Both types of polymerization may occur at once.
GENERAL THEORY OF POLYMERIZATION
The general theory of polymerization is first outlined. Then the details of each step
are reviewed and finally the more recent work of a number of investigators is discussed. Succeeding steps in polymerization from monomer to large particles and gels
or powders have been represented schematically by ller ( 5 ) as in Figure 3.1. This
applies to aqueous systems, in which silica is somewhat soluble. Very little is known
about the polymerization when Si(OH), is formed in nonaqueous solutions.
The individual steps are as follows. Later each step is considered in detail in the
light of individual investigations.
(a) Monosilicic acid is soluble and stable in water at 25'C for long periods of
time i f the concentration is less than about 100 ppm as SiO,. When a solution of
MONOMER
I
DIMER
i
CYCLIC
Figure 3.1. Polymerization behavior 0 1 silica. I n basic solution ( B ) particles in sol grow in sire
w i t h decrease in numbers: in acid solution or in presence of flocculating salts ( A ) , particles
aggregate into three-dimensional networks and form gels.
General Theory of Polymerization
175
monomer, Si(OH),, is formed at a concentration greater than about 100-200 ppm as
S O 2 , that is, greater than the solubility of the solid phase of amorphous silica, and
in the absence of solid phase on which the soluble silica might be deposited, then the
monomer polymerizes by condensation to form dimer and higher molecular weight
species of silicic acid.
(b) The condensation polymerization involves an ionic mechanism. Above pH 2
the rate is proportional to the concentration of O H - ion and below 2 to the H + ion.
(c) Silicic acid has a strong tendency to polymerize in such a way that in the
polymer there is a maximum of siloxane (Si-0-Si) bonds and a minimum of
uncondensed SiOH groups. Thus at the earliest stage of polymerization, condensation quickly leads to ring structures, for example, the cyclic tetramer, foll'owed by
addition of monomer to these and linking together of the cyclic polymers to larger
three-dimensional molecules. These condense internally to the most compact state
with SiOH groups remaining on the outside.
(d) The resulting spherical units are, in effect, the nuclei that develop into larger
particles. The solubility of these very small particles depends on the particle size,
that is, the radius of curvature of the surface. It also depends on the completeness of
the dehydration of the internal solid phase. If the latter is formed at ordinary
temperature it may contain uncondensed O H groups but if formed above 80"C, and
especially above pH 7, it is almost anhydrous.
(e) Because small particles are more soluble than larger ones (Chapter I ) and
since not all the small three-dimensional particles are the same size, the particles
grow in average size and diminish in numbers as the smaller ones dissolve and the
silica is deposited upon the larger ones (Ostwald ripening). However, the higher solubility of smaller particles is pronounced only when the particle size is smaller than
about 5 nm and very pronounced when it is less than 3 nm. Hence above pH 7,
where the rate of dissolution and deposition of silica is high, particle growth
continues at ordinary temperature until the particles are 5-10 nm in diameter, after
which growth is slow. However, at low pH, where the rate of polymerization and
depolymerization is slower, particle growth becomes negligible after a size of 2-4 nm
is reached. At higher temperatures, growth continues to larger sizes, especially above
pH I .
The very early formation of particles was also proposed by Vysotskii et al. ( 6 ) ,
who studied the early stages of polymerization and similarly stated that there are
two basic processes of particle growth of silica in the aqueous system:
I . Growth of particles at the expense of silicic acid in the solution from the moment
of its preparation.
2. Further growth of larger particles by deposition of silicic acid dissolving from the
smaller particles. This is a slower process and may be negligible at low pH after
the monomer has been used up.
(r) Above pH 6 or 7, and up to 10.5, where silica begins to dissolve as silicate, the
silica particles are negatively charged and repel each other. Therefore they do not
collide, so that particle growth continues without aggregation. However, if salt is
I76
Polymerization of Silica
present at a concentration greater than 0.2-0.3 N , as when sodium silicate is neutralized with acid, the charge repulsion is reduced and aggregation and gelling occur.
It is paradoxical that under some conditions, precipitation or gelling is prevented
by raising the temperature. I n this pH range a sol of 2-370 silica with a borderline
salt concentration of 0.2-0.3 N gels i f aged at ordinary temperature, However, if the
sol is first heated to 80-IOO°C the particles grow in size and decrease in number so
that aggregation and gelling are greatly retarded or even prevented permanently.
(g) At low pH the silica particles bear very little ionic charge and thus can collide
and aggregate into chains and then gel networks. If the concentration of SiO, is
more than I % such aggregation may begin as soon as the first small particles are
formed. However, at lower concentrations and at pH around 2, the monomer is
converted largely to discrete particles before they begin to aggregate. On the other
hand, at pH 5-6, monomer is converted rapidly to particles which simultaneously
aggregate and gel so that i t is not possible to separate two processes. The rate of
aggregation increases rapidly with concentration so that in any case above 1 % silica,
aggregation probably involves not only particles but also oligomers.
The process of aggregation and gelling in the silica system is unique because,
unlike other metal oxides, the solid phase remains completely amorphous and appreciably soluble in water and is generally in solubility equilibrium with the monomer.
It is essential to understand that while sol is being converted to gel, the growing
aggregates contain the same concentration ojsilica and water as in the surrounding
sol regions. These aggregates or "gel phase" cannot be seen because the density and
refractive index of the gel phase are the same as those of the remaining sol. Thus
before the sol solidifies only a slow increase in viscosity can be noted, with little
change in other properties, u p to a point where the viscosity begins to increase
rapidly and solidification occurs at the "gel point." The most common way of determining the "gel point" is to observe when the meniscus of a sol in a container no
longer remains horizontal when the container is tilted.
It may be difficult to visualize how particles in a suspension can rearrange
themselves into three-dimensional networks without changing the silica concentration. I n certain microscopic regions in the sol the particles arrange themselves in
chains, and these in turn branch and form networks. These can be isolated in sols
gelling at low pH by adding an inert miscible fluid such as water or alcohol. With a
twofold dilution, all the particles not attached to a three-dimensional network move
apart. but the rigid networks retain their structure and thus are more dense than the
medium, and so can be separated by centrifuging. In this way the percentage of
silica that has been converted to "gel phase" can be measured. Gelling occurs when
about half of the silica has entered the gel phase, which can be thought of as
spherical solidified regions in suspension which cause a rapid increase in viscosity
when the "volume fraction" reaches about 0.5.
After the gel network has been formed, the structure becomes stronger as the
necks between particles become thicker owing to solution and deposition of silica.
I n [his chapier, attention is concenirated on the details of the various processes o j
polynierization up to the point 0.f gel formation. The completion of gel formation
and subsequent changes in structure are dealt wirh in Chapter 6.
Monosilicic Acid
177
Overall Effect of pH on Gelling
Whether polysilicic acids or larger particles of colloidal silica are involved, the
general effects of pH are generally as indicated schematically in Figure 3.2. Curve
A B C represents the behavior of silica in the absence of salts. Sols have a maximum
temporary stability with longest gel time around p H 1.5-3, and a minimum stability
with rapid gelling around p H 5-6. Above about p H 7, no gel is formed since the
particles are charged and only particle growth occurs. Curve DEF represents the
general behavior when an electrolyte such as NaCl or Na,SO, is present a t a
concentration above about 0.2-0.3 N . The salt lowers the ionic charge on particles.
At low p H both sols gel, and salt has little effect. In the neutral region the pH of
minimum stability is higher ( E ) when salt is present.
MONOSILICIC ACID
Monomeric silicic acid, Si(OH), has never been isolated. It is a very weak acid and
exists only in dilute aqueous solution, since it polymerizes when it is concentrated. I t
I
I
2
4
1
I
I
6
8
IO
PH
The effect of pH on the gelling of silica sols. Curves A-C-sols in the absence o f
sodium salts; D-F: in the presence of sodium salts.
Figure 3.2.
178
Polymerization of Silica
is the soluble form of silica that is in equilibrium with the solid phases. As described
in Chapter I , it is a neutral highly hydrophilic, essentially nonionized substance that
cannot be isolated from water. In pure form, if it could be prevented from polymerizing, it might be expected to be a clear liquid resembling glycerin. The basis for
this speculation is that a very low molecular weight polysilicic acid was isolated as a
clear viscous anhydrous liquid by Robinson (7), who found that it polymerized
immediately to clear, hard silica gel when exposed to a trace of atmospheric moisture. It was highly hygroscopic, and soluble in polar organic solvents such as
alcohol, but insoluble i n hydrocarbons. In view of its unique character, a brief
description of its preparation is justified.
To a violently agitated mixture of 83.2 grams tetraethylene glycol dimethyl ether,
52 ml H,O, and 40 ml20% H2S04,was added, in a thin stream, 125 ml of a solution
of sodium silicate containing 122 g I - ' SiO, and 38 g I-' Na,O. After 10 min, 50 g
anhydrous Na,SO, was added and the mixture stirred, then allowed to stand. The
supernatant clear liquid layer of the ether contained silicic acid weighing 186 grams.
This was separated and at once subjected to vacuum distillation at ordinary
temperature to remove the water. Then the high-boiling ether was extracted with an
equal volume of benzene, leaving a viscous, water-clear, anhydrous polysilicic acid.
Sufficient H,SO, was present to make the liquid acidic, which gave it temporary
stability. It soon set to a clear hard gel, especially when exposed to moisture or
warmed.
Monomer can be removed from solution by strong-base ion-exchange resin, presumably because it is ionized to HSi0,- by the O H - ions at the resin surface, and is
then adsorbed.
Preparation
The various methods of preparing monosilicic acid may be summarized as follows.
A saturated solution of monosilicic acid, Si(OH),, containing about 0.01% SiO,, is
obtained when pure amorphous silica is equilibrated with water at room temperature. A more concentrated (supersaturated) solution can be obtained only indirectly
by liberating monosilicic acid from its compounds under carefully controlled conditions; at low temperature and low pH, dilute solutions remain supersaturated with
respect to amorphous silica for appreciable periods. For example, at pH 3 and O°C,
solutions of monosilicic acid up to 0.1 M (0.6% SiO,) can be prepared by spontaneous hydrolysis of monomeric silicon compounds, sich as silicon tetrachloride or
methyl orthosilicate, and also by reacting monomeric silicates, such as sodium or
magnesium orthosilicates or hydrated crystalline sodium metasilicate, with dilute
acid.
Dissolving Silica
Jander and Heukeshoven (8) reported that amorphous silica gel gives a true solution
of silica in water; later Jander and Jahr (9) found that the silica in solution had a diffusion coefficient of 0.53, indicating a molecular size about equivalent to Si(OH),.
Monosilicic Acid
I79
Subsequently Alexander, Heston, and Iler (IO) verified that amorphous silica
exhibits a definite equilibrium solubility in water, amounting to about 0.0 1-0.012%
SiO, in the saturated solution. Kitahara and Oshimo ( 1 1, 12) dissolved quartz under
high pressure a t 400°C to obtain a solution containing 350-400 ppm which was
quenched to obtain a solution of monomer at pH 6 which then polymerized. A 400
ppm solution of monomer was obtained by saturating water with silica gel at
95-IOO°C (13). Egorova (14) reports that i n the pH range 1.2-3.7 it remains
unpolymerized even after 2 hr.
This appears to be a convenient way to make and store a 400 ppm solution of
monomer for polymerization or deposition studies a t 25"C, where the solubility of
gel is around 100-120 pprn.
Hydrolysis of Monomeric Silicon Compounds
Dilute solutions of monomer can be obtained by hydrolysis of halides, esters, or acyl
derivatives such as silicon tetraacetate; SiCI, requires the later removal of HCI. Thus
Willstatter, Kraut, and Lobinger (15) led SiCI, vapor into water a t 0°C while adding
silver oxide to maintain p H 3 and t o precipitate the chloride. This method was also
investigated by Gruner and Elod (16). The very rapid hydrolysis of tetraacetate gave
monosilicic acid, whereas ( C H , S 0 0 ) , S i 0 S i ( C H , C 0 0 ) , gave disilicic acid according
to Schott and Fischer (17). Using a cryoscopic method, they demonstrated that in
this system a t 25°C Si(OH), was most stable a t p H 2.8 and the disilicic acid a t 3.1.
The relative rates of polymerization of monomer and dimer, each at its most stable
pH, were compared a t unspecified concentrations. The monomer was said to be
more stable than the dimer but this probably depends on the p H and the method of
preparation.
Ethyl orthosilicate has been used to prepare silicic acid that is not completely
monomeric because the two-phase hydrolysis is not instantaneous. Disilicic acid was
prepared from tetraethyl orthosilicate by the Brintzingers (18).
Methyl orthosilicate hydrolyzes rapidly to monomer. Brintzinger and Troemer
( 1 9) obtained essentially monosilicic acid by hydrolyzing methyl orthosilicate in
0.001-0.01 N l HCI. Weitz, Franck, and Schuchard (20) prepared monosilicic acid by
hydrolyzing tetramethyl silicate in 0.002 NI HCI at room temperature, checking the
molecular weight by the freezing-point method. The methyl ester gives monomer
when hydrolyzed in about IO sec in water at p H 3. Thus monosilicic acid can be
obtained as a 0.13 M solution (0.8% SO,) by hydrolyzing methyl orthosilicate in
N H,SO, or HCI solution a t 25"C, and it remains practically unchanged for
2-3 hr as described by Schwarz and Knauff (21). They hydrolyzed the ester in an
apparatus from which the methanol and some water could be vacuum distilled and
the molecular weight of the silicic acid was determined by the freezing-point method.
After 24 hr, the molecular weight corresponded t o that of dimer, but there was no
indication that it was more stable a t this point.
At silica concentrations around 0.5-1% SO,, it is impossible to preserve the
monomeric state at 25"C, even a t the pH of optimum stability. Five minutes after
the preparation of a 0.083 M solution of Si(OH), (0.5% S O ) , ) a t p H 2. some
disilicic acid is present (22).
180
Polymerization of Silica
Dissolving Mononieric Silicates in Acid
Monomeric crystalline silicates dissolve and are neutralized to liberate monosilicic
acid at about pH 2. Kraut (23) prepared monosilicic acid by dissolving sodium metasilicate hexahydrate i n various acidic solutions at low temperature. He reported that
monosilicic acid is most stable at around pH 2-3. Weitz, Franck, and Schuchard
(20) demonstrated that when Na,SiO,. 9H,O was reacted with acetic acid it liberated
Si(OH),. Also olivine (magnesium orthosilicate, Mg,SiO,) dissolved in 1.0 N HCI to
give a practically 100% yield of monosilicic acid, the solution containing 0.04%
SiO,. Thus monosilicic acid may be liberated from silicates which contain Si0,lions separated by cations. such as are present in anhydrous orthosilicates.
Alexander (24a) found that sodium metasilicate hydrolyzed to disilicate when
dissolved in water unless NaOH is added to form orthosilicate, Na,SiO,. However,
if crystalline Na,SiO,. 9H,O and strong-acid H ion-exchange resin are added
N solution of H,SO, at O°C to maintain pH 3, a 0.1 M
simultaneously to a
solution of monomer can be obtained.
Thilo, Wieker, and Stadt (24b) made monomer by dissolving Na,SiO, glass in
cold dilute acid. With glass of composition 1 SO,: 1.5 Na,O, disilicic acid was
obtained. Contrary to Schott and Fischer (17), they found disilicic acid is much
more stable than the monomer and can be made reproducibly at higher concentrations.
Coudurier, Baudru, and Donnet (25) modified Alexander’s method by maintaining
the pH at 2.5, and Okkerse (29) preferred 0.01 N HCI at pH 2 as the reaction
medium.
Silicic acid was prepared by Funk (26) by dissolving certain mineral orthosilicates
(“monosilicates” in German) in a solution of HCI in anhydrous methanol. The calcium, barium, and magnesium chlorides were soluble in methanol and the resulting
monosilicic acid was much more stable than in water. Soluble minerals were beta
and gamma Ca,SiO, (dicalcium silicate); Ca,OH(HOSiO,) (dicalcium silicate alpha
hydrate); Ca, ( H O S i 0 3 ) , . 2 H , 0 (synthetic aufwillite); BaOH(H,SiO,) .4H,O;
Mg,SiO, (synthetic forsterite); and Mg,AI,(SiO,), (garnet). The monosilicic acid
polymerizes and is precipitated when water is added. I t would seem likely that it is
present at least partially as a methyl ester. Funk and Frydrych (27) prepared solutions in which 90% of the dissolved silica was Si(OH), at concentra:ions up to I % ,
by dissolving anhydrous Ca,SiO, in dry methanolic HCI. Polymerization was very
rapid unless the solution was diluted at once to 0. I % S O , . By adding acetone, the
CaCI, was precipitated, leaving a relatively pure solution of silicic acid (28).
Characteristics of Silicic Acid
Since Si(OH), has never been isolated or even obtained in a concentrated solution
without considerable polymerization, very little is known about its physical or
chemical properties. Most measurements have therefore been made in very dilute
solutions.
Monosilicic Acid
181
Diffusion Conslant
This was measured in seawater by Wollast and Garrels (30) and found to be 1.0 *
0.05 x IO-5 cmz sec-'.
Ionization Conslants
The ionization constant of monosilicic acid has been evaluated in many ways. As
already discussed in Chapters I and 2, the pK, appears to be about 9.8 at 25°C
determined by Roller and Ervin (31) in a system involving calcium oxide, silica, and
water. More recently, careful measurements by Marsh, Klein, and Vermeulen (32)
of the equilibrium between Si(OH), and HSi(OH),- over a range of pH led to a
value of pK, = 9.9.
Measurements on a system containing extremely pure silica by Schwartz and
Muller (33) give a still more precise value of pK, = 9.91 i 0.04. They hydrolyzed
extremely pure methyl orthosilicate in water in a system which rigorously excluded
atmospheric impurities, to obtain solutions containing from 12.4 to 155 ppm SO,.
The conductivity and pH were measured with precision at 25°C using low frequency
alternating current. From the initial conductivities the value of the acidity constant
was calculated.
In 0.5 M NaCIO, solution Bilinski and Ingri (34a) found that monosilicic acid,
Si(OH),, had a first dissociation constant at 25°C corresponding to pK, = 9.46 *
0.02.
The values of the ionization constants according to Scherban (34b) are as follows:
Kl
=
[H'] [H3SiO;]
=
2 x lO-'O
[HSiO,l
K,
=
[ H + ][H,SiO:-]
[H3SiO:]
=
2 x 10-l2
K,
=
[H'] [HSi03-]
[H,SiO:-]
=
2 x IO-'*
K,
=
[H'] [SiO'-]
(HSiOS,-]
=
2 x IO-',
The increase in the first ionization constant of monosilicic acid with temperature
was measured by Seward (34c). The data were obtained in the presence of borax buffer at 0.1-0.6 M concentrations. The value of pK, ranged from 8.88 i 0.15 at 130°C
to 10.0 0.2 at 350°C.
The ionization behavior of Si(OH), and the formation of polysilicate ions i n 1-5
M NaCl solutions at silica concentrations of 0.005-0.05 m have been measured with
precision by Busey and Mesmer (34d). They found negligible formation of any complexes between monomer and the sodium ion to form NaO(OH), in solution. (The
complexing behavior of polysilicic acid is of course quite different.)
*
182
Polymerization of Silica
Increase in Ionization Constant with Polymerization
The ionization constants of disilicic and polysilicic acids, colloidal silicas, and gels
are pertinent to the polymerization of monomer and SO are considered here.
The increasing acidity of silicic acid upon polymerization was reported by Belyakov et a). (35). As the monomer polymerized, the pK, was determined by titration
and the degree of polymerization by the cryoscopic method. The maximum pK, was
reported as 10.7 for H,Si,O, but then decreases to 6.5 for high polymers. However,
it is not known whether the decrease occurs upon the formation of ring compounds
with =Si(OH), groups or when three-dimensional particles with =SiOH groups on
their surfaces have been formed.
The most doubtful point is the reported pK, of 10.7 for disilicic acid, which would
mean it is a weaker acid than monosilicic acid, of which the pK, is 9.8. Analogy
with other inorganic acids would suggest that disilicic acid should be a stronger acid
than monomer. Unfortunately the dimer is difficult if not impossible to prepare and
keep in sufficiently pure state for strength measurements, although a solution in
which probably at least 50% of the silica was dimeric was prepared by Coudurier,
Baudru, and Donnet (36).
Another indication that polysilicic acid is a stronger acid than Si(OH), is
furnished by ion-exchange studies by Strazhesko and others (37, 38). The ionization
constants of acid centers on the polysilicic acid surface are at least two or three
orders of magnitude higher than the constant of monomeric Si(OH),. Dugger et al.
(39a) estimated the acidity of the silanol groups on silica by measuring the ion
exchange of H + with 20 metal ions. By this means they showed that the first
hydrogen to leave the pure silanol surface must have a dissociation constant k, of
10-4-10-8.This is much more acidic than Si(OH),, which has a k, of
The variation of pK, of the silica surface with degree of neutralization was found
to be as shown in Figure 3.3 by Strazhesko et al. (39b), who carried out studies of
the mechanism of ion exchange on silica gel using N a + , C a t , Cs+, Caz+,SP+,and
Bal’, and also on gel in the form of divalent and trivalent metal salts.
Duffy and lngram (39c) have been able to estimate the ionization constants of a
wide variety of acids from the electro-negativity of the constituent elements and also
from the Lewis basicity or “optical basicity” from spectroscopic data. Although this
has failed when applied to monosilicic acid it might give useful data for polysilicic
acids (39d).
Allen, Matijevic, and Meites (39e) also developed an equation relating surface
change on particles of colloidal silica and pH which indicated that the pK, for the
surface entirely in the hydrogen form is 6.4 and completely in the sodium form is
9.6.
The acidity of silanol groups on the surface of silica or polysilicic acid has been
examined by Schindler and Kamber (40), who calculated the intrinsic acidity
constant, K,,,, from results of titrating silica gel at 25°C in 0. I M NaCIO, solution:
Monosilicic Acid
183
0.25
0
0 75
0 50
DEGREE OF NEUTRALIZATION
Figure 3.3 Relation between pK, of silanol groups on the surface of amorphous silica and the
degree of neutralization according to Strazhesko (39b).
With regard to the equation
log K
=
log [H+) + log
a
1-a
where a is the degree of neutralization or the fraction of silanol groups that are
ionized at a given pH.
The intrinsic acidity constant K,,, is defined as equal to K when a approaches
zero. From the data they developed the following equation:
log K
=
log K,,, -
+ aZ)
0.039 + u
1.9(a
where log K i n , = -6.81. Then
log [H-]= logK,n, -
+ a2)
0.039 + a
1.9(a
log
a
1 -u
To obtain this equation it was necessary to take into account the silicate species in
solution at equilibrium. The following ionization constants for these species in 0.5 M
NaCIO, solution at 25OC were reported by Bilinski and Ingri (34a).
Polymerization of Silica
184
This assumes the solubility of silica gel is I20 ppm.
[H') [(HO),SiOl-] [( HO), SiO-]
10-1258
=
K:
The total exchange capacity, C, defined as the maximum number of ionizable groups
under the conditions of titration, was found to be 2.43 O H groups nm-, in 0.1 N
NaCIO, solution. As a matter of interest, the authors report 3.4 O H nm-, in I M
NaCIO, and 5.83 in 3 M NaCIO, solution.
This approach to the acidity of the silica surface is quite different from that of
Yates (41, 5), which was developed as a modification of the conventional equation
applicable to organic polyacids:
pH
=
pK - k , log,, a N - k , log,,
;t -9
where R is the ratio of the molar concentrations CSIOz/CN.,O
(where Na,O is the titratable alkali), A is the specific surface area of silica in square meters per gram, and
a and N are the activity and normality of sodium salt in the system. Based on Bolt's
data (421, the constants were found to be pK = 12.08, k, = 0.74, k , = 3.47, and k ,
= 2430.
In this formula there is no assumption made as to the fraction of silanol groups
that are ionizable, but the number of charges per unit area can be calculated from
CNs10and A . As an example, a point was taken on Schindler and Kamber's curve
for silica gel of specific surface area of 372 m z g - ' . At a = 0.1, log K was - 8 . 3 .
From their equations a pH of 7.35 was calculated. This is a reference to the "a" of
page 183.
Then taking the Yates equation with pH = 7.35 and A = 372, a value of R was
found to be 240 S O , : Na,O. Thus for 60 x 240 grams of SiO, there are present 2 x
6 x IOzs Na* counterions and ionized O H groups. Then the number of ionized sites
per square nanometer is
12 x 1 0 2 3
= 0.22
60 x 240 x 372 x 10"
Schindler assumed there are 2.43 ionizable O H groups nm-, so a = 0.22/2.43 =
0.09. This is very close to the value of 0.10 for the point originally selected. In other
words, the two approaches appear to lead to similar results.
Monosilicic Acid
185
The acid dissociation constant of the O H groups on polymeric silica has also been
shown to be about lo-'.' by an entirely different method. Hair and Hertl (43)
measured the frequency shift of the infrared absorption band of phenolic hydroxyl
groups when adsorbed on the surface of silica and compared these with the shifts of
phenol in the presence of alcohols of known acidity constants. Marshall et al. (44)
concluded from a similar study that the pK, of some SiOH groups on the silica surface may be about 7.2.
Isoelectric Point
The isoelectric point (iep) of Si(OH), in solution in the absence of colloid or solid
phase has apparently not been measured but presumably it would be between pK,
and pKb, where these are the negative logarithms of the equilibrium constants for:
K,: Si(OH),
=
(HO),SiO-
Kb: Si(OH),
=
(HO),Si+
+ H+
+ OH-
The latter equation might also be written
H,O
+ Si(OH), '= (HO),SiOH: + O H -
Most measurements have involved solutions in which both Si(OH), and polymeric
silica or colloid or solid phase were present. An exception is the case where the
initial rate of polymerization of Si(OH), has been measured at different pH values.
Here the initial step is one of the following:
+ -OSi(OH), = (HO),SiOSi(OH), + O H Si(OH), + +Si(OH), = (HO),SiOSi(OH), + H +
Si(OH),
Presumably, then, the pH at which monomer reacts most slowly with itself to
form dimer might correspond to the iep of Si(OH),. Okkerse (29) measured the rate
of disappearance of molybdate-reactive silica from solution and found it to be at a
minimum between pH 2 and 3. In a study by Goto (45) on the rate of disappearance
of monomer from a solution of 2400 ppm SiO, at 25"C, a minimum at pH 2.0-2.2
was found.
SILICON.The existence of a cationic form of monomeric silica is of
CATIONIC
course implied in the assumption that Si(OH), has an iep. Colloidal particles of
silica have been shown to carry a positive charge at low pH, but direct proof that
silicon can exist as a cation has not been available. It is therefore interesting that in
very dilute solution (66 ppm) monomeric silica has been shown to react with HCI to
form the ion (H,O,Si(OH),+CI- according to Cherkinski and Knyaz'kova (46). This
was determined by the difference in precise conductivity measurements of 0.0025 N
186
Polymerization of Silica
N a O H solution titrated with 0.005 N HCI, with and without the presence of 0.001 I
M %(OH),. The chloride compound exists only in very dilute solution.
Point ofzero Charge
The point of zero charge (pzc) where the surface charge is zero and the isoelectric
point where the electrical mobility of silica particles is zero have been measured by
many methods. De Bassetti, Tschadek, and Helmy (47) measured the pzc for silica
gel by a calorimetric method, from which they concluded the value must be between
2 . 5 and 3 . However, the d a t a may not preclude a value as low a s p H 2 since the heat
of neutralization becomes exceedingly small below p H 3.
I n an extensive study of silica polymerization, De Boer, Linsen. and Okkerse (48)
found the iep to be between p H 1 and 1.5, and that condensation was slowest there,
as shown by several means including viscosity studies.
Vysotskii and Strazhesko (49) have pointed out that there has been relatively little
attention paid to the pzc or iep of silica in spite of the fact that in other colloid
systems they are key factors. These authors recalled the observation o f Freundlich
( 2 ) that whereas lyophobic colloids are least stable at the iep, the lyophilic colloid,
silica, appeared t o be the most stable. This is not quite true because colloidal silica is
permanently stable when it is negatively charged a t pH 9-10, but there is, as Freundlich recognized, a marked temporary stability maximum at the iep around pH 2
(see Figure 3.2).
Vysotskii and Strazhesko show that in the presence of a given acid such as
sulfuric, the iep is not only the point of minimum rate of gelling but also of syneresis
and is also the point at which gels of maximum strength and maximum specific surface area are obtained. All these characteristics result not only because the rate of
aggregation is a t a minimum at the iep, but also because the rate of growth of the
ultimate particles from monomer is at a minimum, so that the ultimate particles are
smallest as they form the gel.
These authors noted the relation between the p H of slowest gelling and the pK, o f
the acid used. Their data are plotted in Figures 3.4 and 3.5. Their p H values for
HNO,, H,SO,, and H,CrO, are similar to those reported by Iler (50), who also
reported a number of other very strong acids which gave maximum gel times at
about the same p H as for HCI and HNO,, for example, N H 2 S 0 3 H , HCIO,, and
CH(SO,H),. However, the point was not brought out that with weaker acids such as
acetic, although the sol may be most stable at p H 3.5, it is nevertheless far less stable than the sols made with stronger acids at p H 1.5-2.0.
Similar results were reported by Tai and Kiang (51), hydrochloric, sulfuric, and
nitric acid giving a maximum gel time at pH 2 , phosphoric a t 2 . 5 , and acetic a t 4.0.
It was proposed that the polymerization rate is proportional to
The iep appears t o be at about pH 1.5, according to ion-exchange studies by
Vysotskii and Strazhesko (52) and Kirichenko and Vysotskii (53) of ion-exchange
20
I
w
I
IO
0
0
1
2
3
4
PH
Figure 3.4. Effect of p H on gel time at 25°C of silicic acid sols of different concentrations
made from H,SO, and sodium silicate: Curve I : 1.09 M , 65.4 gI . I SiO,. Curve 2: 1.33 M , 79.8
g1-I SO2.Curve 3: 1.78 M , 106.8 g1-l SO,. [From Vysotskii and Strazhesko (49).]
+5
-5
0
2
I
pH
OF
3
4
MAXIMUM GEL TIME
Relation betw,een the pH of slowest gelling rate and pK, of the acid used for neutralizing the sodium silicate. [From Vysotskii and Strazbrsko (49).]
Figure 3.5.
I87
Polymerization of Silica
188
sorption of rubidium ions from 0.1 N RbNO, solution on silica gels pretreated a t
temperatures up t o 1ooO"C. All curves in Figure 3.6 converge to this p H a t zero
adsorption. This general approach is summarized by Klimentova, Kirichenko, and
Vysotskii (54).
In summary, iep and pzc of silica have been variously reported t o be from p H 0.5
to 3 . 7 according t o a review of the literature on this point by Parks ( 5 5 ) , who cited
12 references. However, a p H of around 2 f 0.5 appeared to be an average for
various types of silica ranging from purified ground quartz to colloidal silica. S o m e
variation may be expected, depending on whether the surface is crystalline or amorphous, possibly on particle size, and especially on the presence of impurities. The
question remains how the iep determined from maximum gel time or minimum rate
of disappearance of monomer relates to the polymerization mechanisms involved.
The relation between the isoelectric point of polysilicic acid and the stability of
sols, rate of gelling, and properties of resulting gels has been summarized by Klimentova, Kirichenko, and Vysotskii (54). This behavior can be summed up by saying that all the phenomena observed involve the formation and hydrolysis of
Si-0-Si bonds, and that the rates of these reactions depend on a catalytic effect
which is a t a minimum a t p H 1.5-2.0'in the presence of anions of strong acids and
the minimum becomes greater at higher pH in the presence of anions of weaker acids.
From the fact that the rate of disappearance of monomer by polymerization is
second order above p H 2 and third order below 2. Okkerse (29) concluded that an
anionic form of silica was involved above p H 2 and a cationic form below 2. Thus
the isoelectric point must be at p H 2. Similarly, De Boer, Linsen, and Okkerse (56)
considered that the isoelectric point is around p H 2, since the polymerization rate is
?
0
x
e
Y
Y
g
a
3
90
5:
a
W
a
v,
c
2
2
50
3
5
O
W
5
(z
a
0
a
The isoelectric point. Adsorption of
rubidium ions versus pH on silica gels preheated to
various temperatures. Curves 1-5. temperatures 300,
500, 700, 900, and IOOO"C, respectively. (From
Kirichenko and Vysotskii (53).]
Figure 3.6.
Y o
1
2
3
4
PH
5
6
7
Monosilicic Acid
189
a function of H + and O H - on each side of this point. In further work (57) they
found by electrophoresis studies that the iep was between pH 1.0 and 1.5 in a 0.5%
SiO, sol and at pH 2 when the sol was diluted to 0.26%. It was also shown that there
was a sharp minimum in the viscosity at pH 1.9.
Similar observations were made by Tai An-Pang (SS), who related gel time to the
ionization constant of silica.
The significance of the iep of silica in the silica-water system involving Si(OH),
and polymerized or solid silica surfaces is still not clear, but the preponderance of
evidence suggests that for monomeric Si(OH), the iep may be between pH 2 and 3,
and for polymeric forms between 1.5 and 2.
Stability of Monomeric Silica
As long as the concentration of Si(OH), is below the equilibrium solubility of amorphous silica, usually assumed to be about 120 ppm for silica gel but around 70-80
pprn for vitreous silica, it has been assumed that monomer would remain, as such, in
water solution at 25°C. However, such a solution is supersaturated with respect to
quartz and probably to other crystalline species(Chapter I).There is also a possibility
that a solution of monomer at a concentration of 100-150 ppm might nucleate a
particular less soluble polymeric species of lower solubility.
Such a case may be involved in the observations of Schwartz and Muller (33), who
made a highly purified solution of silicic acid from methyl orthosilicate at concentrations up to 150 ppm. Initially, conductivity measurements indicated that the silica
was monomeric, but after half an hour the conductivity, at all concentrations, slowly
decreased to about half the original value. This happened even though in half the
samples the concentrations were less than the solubility of amorphous silica. It was
assumed that the monomer polymerized slowly at pH 7 to a polymer species that is
smaller than usual colloidal dimensions, since it passed through an ultratiler, yet it
must be more insoluble than amorphous silica.
Unfortunately, this change was not followed by means of the molybdic acid
method to see whether it involved simple dimerization at pH 7, which might have
escaped the notice of previous investigators. However, if this were the case, and if
disilicic acid has a pK, of - 10.7 as reported by Belyakov et al. (35), then the conductivity would have decreased by much more than 50%. (This is discussed later in
further detail.)
Reactions of Monosilicic Acid
In view of the relatively neutral character of Si(OH), with its physical resemblance
to an organic polyol, it is not surprising that at pH 2, where it is not ionized, few if
any interactions with other substances have been observed. Its most obvious reaction
is self-polymerization to higher molecular weight polysilicic acids which are more
reactive. The interaction of polysilicic acids with other substances is considered later
in this chapter. However, there are a few reactions in which Si(OH), may take part.
Polymerization of Silica
190
These are interactions either with other acids to form anhydrides or with a few
extremely weakly basic metal cations.
Phosphoric and Boric Acids
Silica has long been known to react with anhydrous H,PO, but the wide variety of
possible compounds has not been investigated. The reaction is, in effect, a condensation, with water eliminated. For example, by heating amorphous silica with H,PO,
at a molar ratio of 1 : 2 for a week at 8O-18O0C, silicon phosphate is formed. Excess
A 10%solution
H 3 P 0 , is removed with dioxane and the product is dried at 10ODC.
can be made in water, giving a 2 . 7 % concentration of silica (59a). Silicon phosphate
has long been known but this example of a water-soluble material is mentioned
because i t probably hydrolyzes to Si(OH),.
The reactions of boric acid with silica appear to parallel those of phosphoric acid
since in dilute solution there appears to be no interaction between the acids, but on
dehydration at high temperature, Si-0-B bonds are formed in the resulting mixedoxide glass. The Si-0-P and Si-0-B bonds are hydrolyzed in aqueous solution.
Sulfuric Acid
The issuance of a series of patents involving silicon salts of sulfuric acid is surprising
since it is unexpected that a reaction product of two of the oldest known chemicals
should have escaped attention for so long. However, the existence of silicon phosphate suggests that the sulfate might also exist. Blount (59b)has disclosed the compound “silicodihydrogen sulfate,” SiO(HSO,), which was obtained by dehydrating
“dihydroxy silicon dihydrogen sulfate,” (HO),Si(HSO,),, with concentrated sulfuric
acid. It is claimed that these solid compounds are obtained by stirring powdered
Na,SiO,. 5 H 2 0 for several hours in an excess of concentrated H,SO,. Finally the
sulfate salt is hydrolyzed i n water giving a white granular “silico-formic acid” or
“monosilanol.” HSi(O)OH, and “monosilandiol,” H,Si(OH),(?). However, no
further information about properties or analysis is given. I f the products exhibited a
characteristic x-ray diffraction pattern or other identifying features their existence as
compounds would be less equivocal. I f a crystalline character is retained the compounds might be clathrates with H,SO, within the lattice or exist as a different crystal
structure, as in the case of the phosphates. On the other hand, if the powders are
amorphous then the) may be microporous silica gels with pores filled with acid; if
anhydrous, internal surface groups of = S i O S 0 3 H may be present.
Iron and Uranium
Monomeric silica does not react with most metal ions in water at low pH where
Si(OH), can exist, since for reaction to occur it is probable that some hydrolysis to a
basic metal ion must first take place.
H,O t Fe3+zFez+OH + H‘
Monosilicic Acid
191
However, very few metal ions form basic ions at the pH of 2, where monomeric
Si(OH), is most stable. Iron and uranium are the only ones which have so far been
reported. Monomeric silica reacts with uranyl ion as follows, according to Porter
and Weber (60):
UO:*
+ %(OH),
=
UO,SiO(OH);
+ H+
The equilibrium constant for monomer concentrations in the range 0.024-0.035 M
is 0.01 0.00 1.
As evidence of some chemical combination, certain forms of natural hydrated
silica gels and also laboratory-prepared gels impregnated with uranyl salts have been
observed by Iler to fluoresce with a strong greenish yellow color under ultraviolet
light.
The other known reaction of monomer with a metal cation is the case offerric
iron, reported by Weber and Stumm (61) and further examined by Porter and
Weber in regard to the effect of the degree of polymerization of silica. They
polymerized the silica at a concentration of 2280 ppm at pH 9-10 for various lengths
of time, conditions that are known to give very small spherical particles. With
increasing polymerization of silica with formation of adjacent SiOH groups that can
combine with iron, at pH 2, the number of SiOH groups combined per iron ion
increases from one on the monomer to two or three as the particles become larger,
the radius of curvature larger, and the SiOH groups closer together. The following
equation suggested by the authors does not indicate the degree of polymerization of
silica, but only the number of SiOH groups that can react with Fe3+,liberating the
corresponding number of H ions:
+
Following the absorption characteristics of the iron as it complexes, the following
values were obtained:
Mol. Wt.
60
13000
26000
120000
Qn
=
D.P.
=
D.P.
Calculated
Diameter (nm)
1
217
434
2000
2.6
3.4
5.6
-
n
-log[Qn(Si)l
1.02
1.67
1.67
1.76
2.76
4.22
4.26
4.48
equilibrium constant
degree of polymerization
In the stock solutions containing 2280 ppm of silica at pH 9-10 there must have
been an appreciable concentration of monomer in equilibrium with the polymer.
Based on the calculated particle sizes this would amount to at least 2.6, 2.2, and 1.7
Polymerization of Silica
192
m M or 156, 132, and 102 ppm as monomer, which undoubtedly also combined with
the iron but was not taken into account.
The interaction of Si(OH)r with ferric iron is evidenced by the fact that concentrations of 10-4-10-3M SiO, in water catalyze the oxidation of Fez+ to Fe3+.Schenk
(62) has derived a quantitative relation between the rate of oxidation and the
concentration of monomeric silica. Below pH 3.5 a soluble complex between
Si(OH), and Fe3+exists.
At pH 6-8, a ratio of 3 Si(OH), to 1 Fe3+ prevents precipitation of Fe(OH),.
However, in the case of AI3+, a fivefold excess of Si(OH), is required to prevent
precipitation.
Chrom iu m
It is peculiar that in view of the similarities of AI3+ and Cr3+ in their precipitation
behavior as hydrous oxides these elements are widely different in their interaction
with silica. One reason is that the chromite ion is not formed as easily as the
aluminate ion, AI0,-. The Cr5+ ion is much larger than AI3+and cannot fit into the
SiO, lattice to give stable anions like (SiAIO,)-.
T h u s the Cr3+ ions show a peculiar inertness relative to monomeric silica, in
marked contrast to the behavior of AISy.When amorphous silica was heated under
pressure with a mixture of Cr(OH), and AI(OH), for 2 days at 30O0C, only the alumina combined with silica (63). This behavior of chromium probably explains the
rarity of chromium silicate minerals.
Hexavalent chromium as H,CrO, appears to form a complex with Si(OH),,
according to Her (50). The chromate ion is unique among inorganic anions in that it
retards the polymerization of Si(OH), in the pH range from about 0.5 to 3.0. At pH
1.7, where Si(OH), is most stable, the increase in gel time of a 1 M silica sol was
linear with Cr03:Si0, ratio. I t ranged from 69 hr with no CrO, to 270 hr at a
CrO,:SiO? ratio of 0.75, and at higher ratios was then constant at 270 hr. The latter
is the gel time of a 0.5 M SiO, sol in the absence of H,CrO,.
Thus the system behaved as though HzCrO, dimerized the silica quantitatively and
the excess had no further effect:
0 -H - -0- - H -0
I
I
II
II
HO-Si-0-Cr-0-Si-OH
I
0- H - - 0 - -H-0
I
I
It appears that this “dimer” then gelled at a rate as though the concentration of
silica was only half the original. Unfortunately, the gel obtained was not examined
to see i f the CrO,Z- ion was actually bound in the structure.
No compound such as silicon chromate has been reported, but evidently silicon
can be linked through oxygen to hexavalent chromium. A chromic acid ester of a
silanol group was made by Schmidt and Schmidbaur (64), who prepared the tri-
Monosilicic Acid
193
methylsilyl ester:
0
(CH,),SiOCrOSi(CH,),
0
Aluminum
As discussed in other chapters relating to the effect of aluminum ions, there is a
peculiar affinity between the oxides of aluminum and silicon. At this point only a
few observations regarding the interaction with monomeric Si(OH), are noted.
Aluminum oxide is far less soluble than silica in water at 25"C, p H 5-8, as evidenced by early data by Okura, Goto, and Murai (65), shown in Figure 3.7.
Monomeric silica reacts with AI3+ ions and is precipitated most effectively at pH
9, according to G o t o (66). Thus with a solution containing initially 35 ppm
monomeric SiO, a t p H 9, the addition of 20-100 ppm of AI as AI3+ ions reduced the
silica concentration to a value C, such that A C = 300, where A and C are ppm of
AI3+ and SiOz, respectively. However, this probably did not represent true equilibrium.
Over a long period of time monomeric silica, (SIOH),, reacts with AIS+ ion at
25°C to form colloidal aluminum silicate of the halloysite composition:
2 Si(OH),
+ 2 AI9+ + H,O
=
A1,Siz0,(OH)4
+ 6 H+
By reacting soluble silica and alumina a t various pH values for periods up to 4 years
and measuring the concentrations of residual Si(OH), and AI3+, Hem et al. (67)
IO
a
i f 1
05
J
d
02
01
4
5
7
6
8
9
PH
Figure 3.7. Solubility of aluminum oxide in water versus pH [From Okura. Goto. and Murai
(65)l.
Polymerization o f Silica
194
measured the following constants:
[AI(OH);]' [Si(OH),j2 [H-]'
=
''
The standard free energy of the colloidal aluminum silicate was -897 i I kcal
m ole- I .
Monomeric silica is strongly adsorbed onto the surface of hydrous aluminum
oxides. There is a reaction between Si(OH), and crystalline AI(OH), by which
several reaction layers of SiO, are built up, with simultaneous decrease in p H of the
suspension (68a). Formation of the first layer is rapid, but the second and third
layers form progressively much more slowly. It would seem that diffusion of AIS+or
AI0,- from the surface of the crystal must be involved, with the formation of a
silica-rich aluminosilicate. A relatively low content of aluminum ion in the S O ,
layer greatly reduces its solubility, thus explaining the deposition of SiO, from a
solution unsaturated with respect to pure amorphous silica.
Baumann (68b) found that when different amounts of aluminum ion were added
to a solution of monomer (420 ppm SiO,), more silica remained in the molybdate
reactive state than when no aluminum was present. With no aluminum present, after
4 days there remained 130 ppm of molybdate-reactive silica a s monomer in equilibrium with 290 ppm of relatively inactive high polymer. But when aluminum was
present in the AI:Si atomic ratio of I : 7 , there remained about 200 ppm of molybdate-reactive silica. I t can be interpreted that the alumina had combined with silica
to form an aluminosilicate that later was decomposed by the strongly acidic molybdate reagent liberating additional active silica that appeared as monomer.
However, when the silica concentration was only 60 ppm, and thus below the solubility of amorphous silica, no polymerization occurs excepi when alumina is added.
I n this case when the AI:Si ratio is I : I to 1 : IO the aluminum ion brings together
monomer to form a silica-rich complex in which some of the silica is also linked
together into a state that is later less molybdate-reactive. Baumann's extensive data
deserve detailed stud).
The final reaction product at the alumina surface is halloysite. When a dilute
solution of monomeric silica is brought in contact with g a m m a alumina, it is
adsorbed at a rate strongly dependent on pH and area A . In a medium of constant
ionic strength (0.1 N NaCI) and at silica concentrations of 10-3-10-' M , Huang
(69a) found the initial adsorption to be rapid. When below pH 9 the rate is proportional to A [SiO,]' [H+J-O5 , whereas above p H 9 it is proportional to A z [Si0,]'.5
[H '1, Huang proposed that HSiO; is the major reacting species.
I t is possible that with a quaternary ammonium base and in the absence of metal
cations, aluminosilicate anions may remain in solution, for example,
(HO)3SiOAl(OH)zOSi(OH)31~,
Flanigen (69b) reported that quaternary ammonium
silicate and aluminate remained in solution until a sodium salt was added.
I n the case of pure alpha alumina, the writer has found that there is no interaction
with monomeric silica. Colloidal alpha alumina free from other forms of alumina or
Characterization of Silicic Acids
195
AI3+ ions has been prepared by treating the particles with 24% H F solution for 24 hr
to remove all other types of alumina and silica impurities, and washing with water,
then NH,OH t o remove all F - ions from the alumina surface (70a). At p H 7-8,
monomeric silica is not adsorbed, nor does it react with this form of alumina even
though the specific surface area is 24 m 2 g-’. Presumably AI3+or polybasic AI ions
are required for reaction with Si(OH),.
Divalent Cations
It is known that the ligand properties of deprotonated Si(OH), with a polyvalent
metal cation can lead to a stable complex such as [FeOSi(OH)J3+, as reported by
Weber and Stumm (61); however, much less is known about complexes of divalent
cations. Santschi and Schindler (70b) measured the stability of complexes involving
C a Z + and MEz+ a t around p H 8-9 in 1 M sodium perchlorate solution a t 25°C.
Complex formation was weak and occurs only in the presence of excess salts. In
natural waters, such complexes are not formed.
CHARACTERIZATION OF SILICIC ACIDS
It is not possible to discuss all the techniques used for measuring or characterizing
silicic and polysilicic acids and small colloidal particles, but some of the methods,
especially applicable t o following the polymerization, are reviewed.
Reaction with Molybdic Acid
The history and use of this reaction in analyzing for silica is discussed in detail in
Chapter 1 and its application in characterizing silicate ions in Chapter 2.
Further refinements and use of this indispensable reaction for studying the
polymerization of silica are now described. Most of these involve following the
course of polymerization by measuring the rate a t which the monomer, or the
monomer and dimer, disappears. This in turn involves distinguishing monomer and
dimer, which react rapidly with molybdic acid, from higher polymers that react
more slowly. The method is particularly useful because, a s described earlier, the
color-forming reaction is carried out a t a low p H where the polymerization or
depolymerization of silica is a t a minimum. Hence samples taken from rapidly
polymerizing or depolymerizing solutions a t higher or lower p H are “frozen” a t the
moment they are added to the molybdic acid reagent. The monomer and dimer react
very quickly, whereas each higher polymeric species depolymerizes a t a slower,
characteristic rate.
The structure of the silicomolybdic acid is such that within the molecule there is a
tetrahedron of four oxygen atoms in which only one silicon atom can fit (20, 24).
Thus only monosilicic acid, Si(OH),, can react directly. All polymeric species must
first depolymerize to monomer. The silicomolybdate anion SiMo,,O,,“ apparently
has a compact structure similar t o that established for basic aluminum chloride in
1%
Polymerization of Silica
Figure 3.8. Structure o f silicomolybdic acid. All corners of octahedra are occupied by
oxygen atoms. A molybdenum atom is at the center of each octahedron. (see text).
which the polybasic aluminum ion is A I 130,(OH),,(H,0),,’+, as established by
Johansson (71). The analogous structure for the silicomolybdate ion requires that all
the oxygen sites be filled by oxygen aioms (including those that in the aluminum
complex are filled by OH groups and coordinating water molecules): SiMo,,O:;.
The structure of the silicomolybdate heteropolyion is shown in Figure 3.8. The
details of the sharing of the oxygen atoms (or ions) between the MOO, octahedra
and the central SiO, tetrahedron have been described clearly by Cotton and
Wi Ik inson (72a).
Alpha and Beta Silicic Acids
Polymers of silica were classified first by Goto (45) into two types. “A” reacts
rapidly with molybdic acid and has a low degree of polymerization of less than four,
whereas “B” reacts more slowly with increasing molecular weight. It appears that
the difference is the size of the ultimate particles and thus the reaction rate varies in
proportion to the specific surface area. Other workers have variously defined the
increasing degree of polymerization, as evidenced by decreasing rate of reaction with
molybdic acid, as alpha, beta, and gamma. Usually alpha is defined as silica that
reacts almost completely in less than 5 min. Beta reacts completely in 10-30 min.
Characterization of Silicic Acids
I97
and has been classed as an oligomer or oligosilicic acid by Baumann (72b). G a m m a
then is the higher polymers that d o not react after 10-30 min; it is often referred to
simply as “higher polymers.”
Goto and Okura (72c) proposed that the monomer and dimer species which
reacted in 5 min be classed as type A . These could be removed from solution by a
strong-base anion-exchange resin. They recognized that there are different types of B
type polymer, since those formed in an acid medium depolymerized more rapidly
than those formed in a basic medium.
A method that is said to distinguish alpha and beta from gamma silicic acid was
developed by Nemodruk and Bezrogova (73a), who defined the gumma silicic acid as
that which did not react with molybdic acid reagent at 100°C in 20 min, whereas
beta reacted completely.
Measurements of Reaction Rates
A number of investigators began to use the procedure developed by Alexander (24a)
to measure the rate of reaction of specific polysilicic acids with molybdic acid. This,
in effect, was a measurement of the rate of depolymerization i n the colorimetric
reagent. It was hoped that once the reaction rates of individual polyacids were
known, the more complex reaction rate of a mixture of polymers could be
interpreted as a distribution of molecular weights.
The depolymerization of a particular species of silicic acid is a krst-order reaction
so that the species can be characterized by a specific reaction rate constant.
Since in most solutions monomer is already present along with a higher polymer
or colloid, the following equations will hold:
C,
C,
=
M,
Po
=
C,
- =
Ct
=
=
silica reacted with silico-molybdate at time t
total silica in the system at I = 0
total monomer in system a t t = 0
total polymer in system a t t = 0
M, (1 -
’)
+ P,(1 - e-‘P
I )
M, + Po
where k , and k , are the reaction velocity constants for monomer and polymer. Taking a hypothetical case where 735 ppm of monomer is in equilibrium with 7265 ppm
of cubic octamer, or 9.2% of the silica is monomeric, with the known values k , =
1.5 and k , = 0.45, the color development curves are calculated from the equation. In
Figure 3.9, curve A is the curve that results when all the silica is monomeric, B is the
curve for the above mixture, and C is the curve for higher polymer alone. It will be
noted that the amount of monomer would be difficult to estimate from this plot.
However, by plotting the log of the fraction of silica not yet reacted a t time t against
time, as in Figure 3. IO, lines are obtained for A and C and, a t longer times, also for
the mixture B. The linear part of E extrapolates a t zero time to the fraction of
higher polymer (90.8%) in the mixture.
IO
00
5
0
IO
MINUTES
Figure 3.9. Reaction o f silica S i t h molybdic acid. Calculated curves: A , monomeric silica, B .
a mixture of 9 . 2 % monomer and 90.8L?c cubic octamer; C. cubic octamer alone.
10
0
W
I-
y
0 5
U
a
0
=I
Ln
aE
0 2
LL
0
2
P
4
01
E
0 05
0
I
2
3
4
5
6
MINUTES
Figure 3.10. Reaction of silica with molybdic acid, A , monomer, B, 9 1 % monomer and
90 0% cubic octamer. C, cubic octamrr alone
I98
Characterization of Silicic Acids
199
Because of rather low precision the method is of value only for distinguishing
monomer and very low polymers from relatively high polymers, not for following
the early stages of polymerization. However, Baumann (72b) studied the early stages
by stopping the reaction by adding citric acid and reducing the yellow complex to
the more sensitive molybdenum blue.
Alexander's method (24a) was used by Thilo (73b) and several other investigators
to characterize polysilicic acids by the rates of reaction with molybdic acid, each
having a characteristic reaction velocity constant k. Their procedure, in slightly
modified form for convenience, is given in detail in Chapter 1 as a recommended
procedure.
In some cases the polysilicic acid acid must be liberated from a crystalline silicate
in acid at 2"C, or even i n methanol-HCI, to obtain a solution stabilized long
enough to take a sample for the molybdate test. The reaction of molybdic acid with
disilicic or linear trisilicic acid is rapid because these depolymerize to monomer
within a few minutes at pH 3. Schwartz and Knauf (21) prepared the pure methyl
esters of these acids and found that by the time they had completely hydrolyzed in
water in 4 and 10 min, respectively, only monomer was present in solution.
The molybdic acid was somewhat modified by Coudurier, Baudru, and Donnet (36)
for their extensive study of polymerization of disilicic acid. Two solutions of molybdic
acid were used containing 4 and 6 g I-' ammonium molybdate, respectively, both at
pH 1.4. These contain 0.0235 and 0.0352 g-atoms I - ' molybdenum. When they were
reacted with monomer at 25OC the reaction rate constants were 2.1 min-' for the
more dilute and 2.6 inin-' for the more concentrated solution. However, with higher
polymers the reaction rates were the same, thus indicating that the slow step is the
depolymerization to monomer:
ka
polymer -Si(OH),+
k,
silicomolybdic acid
However, disilicic acid also reacted at different rates with the two different
concentrations of molybdic acid, indicating that it dissociates very rapidly to
monomer. Equations were developed on the basis that polymer must first
depolymerize before reaction. Using these equations, experimental data plotted as
logarithm of unreacted silica versus time can be resolved to give the relative proportions of monomer, dimer, and polymer.
The reaction rate of molybdic acid with specific polysilicate anions has been
measured after obtaining a solution of the free polysilicic acid by dissolving waterinsoluble, but acid-soluble, crystalline silicates of known crystal structure. Wieker
(74) applied this method to a number of calcium silicates. Four different types of
silicic acid were characterized by their rates of reaction with molybdic acid, by Funk
and Frydrich (75). However, they did not use the method of Alexander. Instead, the
reagent was more concentrated containing 0.28 g-atoms I-' Mo with a H': M o ratio
of 1.5. The high concentration of molybdic acid and the relatively low acidity
accounts for the rapid reaction of this reagent with monomer and also its promotion
of the depolymerization of polymers more than twice as fast as Alexander's reagent
(75). I t will even gradually attack quartz. The reaction was followed not
Polymerization of Silica
200
colorimetrically, but by precipitating the silicomolybdate as quinoline salt and titrating the latter with base. Thus the method has the advantage of not requiring a spectrophotometer or colorimeter (for details see Chapter I ) .
As sources of the silicic acids, crystalline acid-soluble salts of monosilicic,
disilicic. and cyclic tri-, tetra-, and hexasilicic acids were dissolved rapidly in methanolic HCI, in which the silicic acids are more rapidly dissolved yet are more stable
against further polymerization than in water. The liberated silicic acids were reacted
at once w i t h molybdic acid reagent at 20°C.
For each silicic acid the reaction is first order and the constant is calculated:
where C is the fraction of unreacred silica at time 1, and K is the rate constant (C
1 .O at I = 0). Then k (min-') = 0.693 ( t h ) - l , where th = half-life.
=
Reaction Rate Constants
Values of constants for silicic acids from known crystalline silicates are given in
Table 3. I . I t is emphasized that these apply only when Funk and Frydrych's type of
reagent is used. The reaction rate decreases more rapidly than the increase in
number of siloxane bonds that must be hydrolyzed to depolymerize the polysilicic
acid to monomer. This is probably because of the greater stability of the ring structures as compared to corresponding chain polymers.
Since several investigators have used nearly the same molybdic acid reagent solution as used by Alexander (24a), a number of values for the constants can be compared for monomer and polymers. excluding those of Funk and Frydrych, who used
other reaction conditions. Each polysilicic acid i n Table 3.2 was prepared from a
particular crystalline silicate known to contain that polysilicate anion, by dissolving
it under conditions that avoided changing the structure.
The linear polysilicic acids hydrolyze rapidly to monomer according to O'Connor
(77), and the linear pentamer should have a rate constant of 0.66. that is, 90%
reacted in 3.5 min. I t appears that in the case of all linear, cyclic. or polycyclic silicic
acids where all siloxane bonds are exposed to the solution. the rate of depolymerizaTable 3.1. Reaction Rate Constants of Silicic Acids with Funk and
Frydrych's Molybdic Acid Reagent
~
Starting Silicate
Dissolved
~
Silicic Acid
TYpe
Cd*SIO,
Si(OH),
CalNa,SilO,
cd,sI,op
(HO),SiOSi(OH),
I(H 0 , 3 1 0 1 3
K,H,Si,Oi,
I( HOhSlOla
Cu,Si,O,, 6H20
(( HO),SiO],
~~~
Half-life
(secl
55
23 5
360
83 0
3600
?:
(Sec
012
0032
0019
00084
00018
K
(min
72
I 9
I I4
0 SO
0 II
'I
Characterization of Silicic Acids
20 1
Table 3.2. Reaction Rate Constants of Silicic Acids with Alexander's Molybdic Acid Reagent
Silicic Acid
Rate
Type of
Polymer
Degree of
Polymerization
I
2
Monomer
Dimer
Constant,
K (min-I)
2.3
I .7
2.05
1.87
I .5
2. I
0.9
0.9
1.09
0.82
I .oo
0.67
0.79, 0.65
0.66
0.6
0.51
4
Cyclic
5
6
Linear
Cyclic
8
Double 4-ring. cubic
0.46
0.42
High mol. wt.
High mol. wt.
0.050
Single chain
Double chain
0.015
Author
Alexander
Thilo et al.
Marsh et al.
O'Connor
Hoebbel et al.
Coudurier et al.
Alexander
Thilo et al.
O'Connor
Hoebbel et al.
Cordurier et al.
Thilo et al.
Hoebbel et al.
O'Connor
Hoebbel et al.
Wieker et al.
Hoebbel et al.
Hoebbel et ai.
Wieker
Hoebbel et al.
tion is so rapid that the rate of reaction with molybdate does not increase very
greatly with the degree of polymerization. However, with Frydrych's faster reacting
reagent, differences in depolymerization rates are more apparent.
Composition of Molybdic Acid Reagents
The compositions of the solution in which the color was actually developed are summarized in Table 3.3. One group of investigators used the Alexander composition
essentially unchanged. Others modified this for specific reasons. Except for the compositions of Funk and Frydrych and of Nemodruk, the reaction rate constants with
the various silicic acids are all about the same. The more concentrated reagents,
such as Iler's, permit the use of higher concentrations of silica. The indicated ratio
H + : Mo is not based on the H + ion concentration in the solution, but is the ratio of
acid to ammonium molybdenum used in making up the mixture. Actually, a ratio of
H + : M o of0.86 is required to neutralize the NH,' ion.
Other 0bservations
The composition of silicomolybdic acid, determined by Khomchenko et al. (78), corresponded to H,[SiMo,,O,,]~ 29H,O.
Complete conversion of silicomolybdic acid from the beta to the less intense
MOO,*- of 1.66: 1.0 according to Mars
yellow alpha form was observed at a H + :
Polymerization of Silica
202
Table 3.3.
Composition of Molybdic Acid Reagents
~
Author
H*.Mo
Mo (g-atoms I-’)
Acid
44
44
0 0221
0 0227
44
44
0 0227
0 0227
0 0235
0 28
0 0600
0 0566
0 0566
0 0226
HzSO,
HzSO,,
HCI
HCI
HZSO,
HCI
HZSO,
HzSO,
~~
Alexander
Thilo et al
Marsh et al
O’Connor
Coudurier et al
Funk and Frldrych
Govett
ller
Kautskk
Nemodruk
(PH I 4)
15
33
53
65
22
H W ,
HzSO,
HSO,
(79). I t is for this reason that a H + :Mo ratio greater than 4 is generally used to
develop the beta form.
Sugars and other polyhydroxy organic compounds interfere with the reaction of
molybdic acid with monomeric silica. This is believed to be due t o the formation of
stable complexes with the molybdic acid (80).
Goto and Okura (81) were the first to recognize that the depolymerization of
silicic acid is catalyzed by the presence of molybdic acid. Thus at pH 1-2 in the
presence of HCI alone, polysilicic acid formed monomer only very slowly, as shown
by adding molybdic acid after 50 min. The rate of formation of silicomolybdate was
then the same as when molybdate was added a t the start. However, it is not known
whether the molybdic acid is actually involved as a catalyst by direct interaction
with the polymer or whether it simply reduces the concentration of monomer in
solution to such a low level that an equilibrium between polymer and monomer is
displaced.
A peculiar phenomenon has been noted by Iler. When a small amount of N a F is
added to a polysilicic acid solution a t p H 2 it converts an equivalent amount of the
silica to SiF,*-, which, when molybdic acid is then added reacts as though it were
monomer. However, if the same amount of N a F is added wirh or after the addition
of molybdic acid reagent it does not depolymerize an equivalent amount of silica,
but instead acts as a catalyst for the depolymerization of polysilicic acids. When
N a F is added before the molybdic acid so that it is converted to SiF,Z-, then when
the latter reacts with molybdic acid, the fluoride ion combines irreversibly with
molybdenum so that is is no longer free in the system. When added later, the
molybdic acid reacts with monomer as it is developed, but does not inactivate the
fluoride, which at the low p H is probably present as H F .
Separation of Silicic Acids
Although the rate of reaction of molybdic acid with individual polysilicic acid
species obtained from crystalline silicates can be measured, the results are of no
Characterization of Silicic Acids
203
value in studying the polymerization reaction unless it can be shown which polyacids
are actually present in the polymerizing mixture. For this reason, methods of
separating the oligomers or low molecular weight species are essential. A few examples follow.
Chromatography can be used, provided conditions are chosed to minimize
polymerization or depolymerization during the procedure. Wieker and Hoebbel (22)
found that by working rapidly, monomer, dimer, and higher species can be separated
by paper chromatography in 3-4 hr using dioxane containing ( a ) 1.6 g
l-lCC1,COOH and 30 g I - ' H,O to separate monomer and lower polymers, or (b)
8.0 g I-' CCI,COOH and 90 g IL,H ,O to separate higher cyclic polymers. The paper
is dried and the separated spots are developed by spraying with 0.1 N N a O H and
aged wet for I O min to depolymerize the silica, then with 2% ammonium molybdate
in 0.3 N HCI and aged wet horizontally for 30 min, then the yellow spots are
reduced to blue with 0.1 N ascorbic acid and bleached with ammonia gas to destroy
molybdenum blue, thus leaving the spots of blue silicomolybdate.
Low molecular weight silicic acids were separated by Baumann (82) with paper
chromatography using a mixture of isopropyl alcohol, water, and acetic acid as the
moving liquid and the molybdic acid reaction to locate the separate species.
Polysilicic acids of different molecular weights can be separated and molecular
weights estimated by gel chromatography on Sephadex columns, using 0. I M NaCl
solution adjusted to p H 2 with HCI as the eluent. A blue dextran 2000 in 0.2% solution was used as a standard. Tarutani (83) made silicic acid at a concentration of 500
ppm by neutralizing the monomeric solution of sodium metasilicate with acid to p H
7. This solution was aged for various lengths of time and then acidified to p H 2 t o
stop polymerization.
Polysilicic acids of low molecular weight have been isolated as trimethylsilyl esters
and separated by thin layer and gas chromatography by Hoebbel et al. (84). Specific
polysilicate ions known to exist in certain crystals were used to make the corresponding trimethylsilyl derivatives to use as standards. This method makes it
possible to separate these derivatives and characterize them further by gas
chromatography and mass spectroscopy. The sources of individual silicic acids and
their chromatographic constants are listed in Table 3.4. The derivatives were
separated, using a mixture of Merck alumina G and Merck silica gel G as adsorbent
and n-heptane as solvent. Programmed temperature chromatography was also used
(84).
Particle Size and Surface Area by Titration
At a relatively early stage in the polymerization it is possible to characterize the
polymeric silica, or silica particles in terms of the specific area of the silica-water
interface. This is done by measuring the adsorption of hydroxyl ions in the p H range
4.00-9.00 (Beckman Type E electrode) in a nearly saturated salt solution which
permits the surface charge denstiy to approach a maximum. This method was
developed by Sears ( 8 5 ) t o determine the specific surface areas of colloidal particles
and gels. Then it was found that if carried out rapidly it could give reproducible
Next Page
Polymerization of Silica
204
Table 3.4. Sources of Individual Silicic Acids and Chromatographic Constants of their
Trimethylsilyl Derivatives
Chromatographic
Constants
Source
Silicic Acid
Na,H,SiO,' BH,O
Ca,SiO,
Si(OH),
Si(OH),
I .25
0.56
1.35
0.60
1.51
0.68
0.71"
0.31"
I .oo
0.38
0.26
0.44
0.16
0.11
0.26
0.1 I
OH
Na,Cd,( Si,O,,)
I
(HO),SiOSiOSi(OH),
1
OH
Si,O,CI,,
OHOH
[( HO),SiO],
(cyclic trimer)
[( HO),SiO], (cyclic tetramer)
Tricycloheptasilicic acid
Cubic octasilicic acid
IHOSiO, J8
Cubic octasilicic acid
IHOSiO, ,II
The constants for the "cyclic trimer." as compared to those of the cyclic tetramer. suggest
that it is more stable and less reactive even though the trimer ring should be under greater
strain
Source. Hoebbel et al. (84)
a
results on sols of particles only 3-4 nm in diameter with a specific surface approaching 1000 mz g-I.
However, in sols of such small particle size, there is an appreciable concentration
of monomer at equilibrium. Also, in alkaline sols at pH 9-10.5, there is an appreciable amount of ionic silica which is converted to monomer before the titration. Since
monomer reacts with base at pH 9 it is therefore necessary to correct the titration
for the effect of soluble silica in order to obtain a reliable value for the specific surface area of the polymer. The term "soluble silica" is used to include the ionic silica
and dimer which react with alkali-like monomer.
The soluble silica can, of course, be removed a t pH 2 either by washing the silica
in a filter or ultrafilter or by centrifuging. Also the sample can be adjusted to pH 8
and let stand a few hours until the soluble silica has been polymerized upon the
colloidal material.