Hydrolysis and Polycondensation of Ethyl Silicates. 1. Effect of PH and Catalyst On The Hydrolysis and Polycondensation of Tetraethoxysilane (TEOS)
Hydrolysis and Polycondensation of Ethyl Silicates. 1. Effect of PH and Catalyst On The Hydrolysis and Polycondensation of Tetraethoxysilane (TEOS)
Hydrolysis and Polycondensation of Ethyl Silicates. 1. Effect of PH and Catalyst On The Hydrolysis and Polycondensation of Tetraethoxysilane (TEOS)
J. CihlZ
Technical University of Brno, Technickci 2,616 69 Brno, Czechoslovakia
Abstract
The effect of pH and of catalysts on the course of the hydrolysis and condensation of tetraethoxysilane (TEOS) in
water-ethanol solution was studied with the aid of chromatography, potentiometry and gelation tests. Strong acids (HCI,
HCIO,, HNO,, H,SO,, p-toluenesulphonic acid), weak acids (Cl,CCOOH, (COOH),, ClCH,COOH, CH,COOH,
HCOOH) and LiOH were used as catalysts. The rate of hydrolysis depended on the pH of the solution and not on the
chemical structure of the catalyst. The hydrolysis was both acid and base catalysed and its rate was at a minimum
at pH 7.0.
The rate of condensation of the reaction products of the hydrolysis of TEOS (water-ethanol solutions of ethoxyhy-
droxysiloxanes) was at a minimum at a pH of about 2.0. The condensation was both acid and base catalysed and was
markedly accelerated by both HF and H, P04.
Introduction
TABLE I
include not only water but also aliphatic alcohols, the value of pH differs little from that of pH* since
it is possible to use a cell made up of a glass and the two components of 6 (diffusion potential and
a reference (e.g. calomel) electrode [12] for the medium effect) compensate each other [14]. For
measurement of pH. To calibrate this cell, standard higher concentrations of EtOH the difference
buffers of the same solvent composition as the between pH and pH* increases and for pure EtOH
solution to be measured can be used. The measure- 6 is -2.36 units of pH.
ment procedure is practically the same as for the The working value of pH* in water-ethanol
measurement of pH in water solutions. Under solutions, which is an expression of the activity of
optimum conditions (when the composition and the hydrogen ions, can thus be obtained by
acidity of the solution measured are close to the subtracting 6, which corresponds to the ethanol
composition and acidity of the standard) the value content in the solution, from the pH value obtained
pH* is obtained, which is approximately equal to with the aid of a pH meter calibrated with use of
paf (i.e. the activity of hydrogen ions in non- aqueous standards.
aqueous media). The effect of water content on the pH of water-
Between the pH* and the pH of water-alcohol ethanol solutions of HCl is apparent from Fig. 2.
solutions, the following relationship holds: The pH of ethanol-water solutions with a con-
stant HCl concentration (cHc, = 1.5. lo-’ mall-‘)
pH*=pH-6 (11)
rise steeply over the concentration range
where pH* characterizes the activity of hydrogen O-15 vol.% HZ0 and it reaches a maximum value
ions, referred to standard conditions in a solvent in the concentration range 15530 vol.% H, 0. With
of the same composition as the solution measured, increasing water content the pH rises slightly and
pH corresponds to the value obtained from an with a water content exceeding 60 vol.% it remains
instrument calibrated with aqueous buffers, and 6 constant. This pH value practically corresponds to
represents the correction parameter. The parameter the Vahe -loge,,, (1.8 pH unit).
6 includes the so-called diffusion potential E, and In the ethanol/water/hydrolysed TEOS solution
the medium effect yu_, according to the relationship with an HCl concentration of 1.5. 1O-2 mol l-l the
behaviour of the pH over the water content ranges
6 = &I - log YH.m (12)
O-l 5 vol.% H, 0 and 60-100 vol.% H,O is practi-
For water-ethanol solutions of HCl, the depen-
dence of 6 on the EtOH content in the solution
was evaluated [ 131. The results are given in Table 2.
Within the range O-80 wt% EtOH in the solution
TABLE 2
EtOH content 6
(wt%) (pH units)
I
A TEOS/EfOH/H,O/tUI - H,O/HCI
0 0 0.5 - A EtOH/H~O/HCI - H,O/HCI
20 - 0.02 . TEOSIEfGti/H,O/HCI H,O
35 0.10
50 0.2 1 L4 -
65 0.24 0 20 40 60 80
80 water conrent Iv01 %J
0.11
90 - 0.40 Fig. 2. Dependence of pH on water content in water-ethanol
100 - 2.36 solutions of HCl.
-
244 J. CihkifjColloids Surfaces A: Physicochem. Eng. Aspects 70 (1993) 239-251
tally the same as in the preceding case. From Fig. 2 a solution of TEOS hydrolysed in the presence
it is further apparent that the dependence of pH of oxalic acid is given in Fig. 4. In the EtOH/H,O/
on the water content is the same as in the preceding hydrolysed TEOS solution which had a constant
cases if water is added to hydrolysed TEOS up to concentration of oxalic acid (cuA = 5.1 lo-’ l
a concentration of 15 vol.%. With the water con- mol l- ‘), the pH decreased continuously with
centration in the solutions under investigation increasing water content, the minimum value being
ranging from 10 to 15 vol.% H2 0 the pH was reached when the water content was above
practically constant, having a value which, after 70 vol.%. If, instead of an aqueous solution of
deducting 0.1 pH unit, corresponded to -log cuci. (COOH),, water was added to the TEOS hydroly-
The pH value of a water-ethanol solution of TEOS sate, the two relationships were identical up to a
hydrolysate containing 12.5 vol.% H, 0 reduced by water content of 20 vol.%. Similar relationships
0.1 pH unit was designated as the pH’. Similar between the pH and the water content were also
results were also obtained for other strong acids obtained for TEOS solutions hydrolysed in the
(HClO,, HNO,, HzS04 and PTSA). presence of other weak acids (H,PO,, HF,
The relationship between the water content and Cl, CCOOH, CICHz COOH, CH, COOH and
the activity of hydrogen ions in water-ethanol HCOOH).
solutions of I-ICI (expressed with the aid of pH* The pH’ values of water-ethanol solutions of
or with the aid of Eqn (11)) can be seen in Fig. 3. TEOS hydrolysates containing weak acids given
In water-ethanol solutions of ethanol/water/hy- in Table 4 need not, in contrast to strong acids,
drolysed TEOS with a constant HCl concentration, reflect the activities of hydrogen ions in the solu-
pH* is almost constant over the whole concen- tions since they can also include the effect of the
tration range of cH,o and is approximately equal water-ethanol medium on the dissociation con-
to -log cHcI. The same also holds for hydrolysed stants of the weak acids.
TEOS with a water content of up to 20 wt%. Thus
the pH’ also represents an approximate expression Hydrolysis of TEOS
for the activity of hydrogen ions in solutions of Figure 5 gives the values of time vs water
hydrolysed TEOS containing strong acids. concentration in the course of a TEOS hydrolysis
The dependence of pH on the water content in catalysed by strong acids (HCl, HNO,, HClO, or
0 20 40 b0 80
water ctmmt peaI XI
Fig. 3. Dependence of pH* on water content in water-ethanol Fig. 4. Dependence of pH on water content in water-ethanol
solutions of HCI. solutions of (COOH),.
J. Cihl6f/Colloids Surfaces A: Physicochem. Eng. Aspects 70 (1993) 239-251 245
TABLE 3
0 t&PO, @ CICH,COOH
0 ICOOH),
b HF 0 HCOOH
s 0 0,CCOOH 0 CH,COOH
TABLE 4
aPrecipitation of SiOz.
o-o
-0 0
0 20 40 60 80 m
Rezctimlm @Q
.2 /
0
-3 .
8
*-4o /
\
-5- O\_/
4’
1
V’
I I.__, Tome
lhr]
0 I 2 3 4 5 6 7
PH
Fig. 12. Dependence of water con~ntration and pH of TEOS
Fig. 10. Dependence of condensation rate constant on pH’. hydrolysates containing weak acids on time.
Discussion
H, P04. Their values of k, were about three orders
higher than kK values for reactions catalysed with
pH of wuter-ethanol solutions of TEOS
the other acids at a comparable pH value. With
hydrolysates
the exception of the above acids (HF, H3P04)
the TEOS reaction solutions hydrolysed at the
The dependence of pH on water content in
HzO/TEOS molar ratio of two and a pH of about
water-ethanol solutions of HCl (or HiClOd, HNO,
2.0 are very stable (Table 4). A long-term investiga-
and HzSOs) agreed with the results reported by
tion of their pH’, pH and water contents revealed
Bates and Schwarzenbach [14]. Analogous rela-
that while the value of pH’ remained practically
tionships were also obtained for water-ethanol
constant, the value of pH in solutions with strong
solutions of hydrolysed TEOS (Fig. 2). Ethoxyhy-
acids increased (Fig. 11) and in solutions with weak
droxysiloxanes (i.e. products of TEOS hydrolysis
acids it decreased (Fig. 12). In both cases the water
and condensation) present in these solutions thus
content was slowly increasing and led to the above
had no significant effect on the value of pH. The
shift of the pH (see Figs 2 and 4).
potentiometric method of determining pH with the
aid of glass and calomel electrodes is also suitable
for water-ethanol solutions of TEOS. Even with
these solutions, the value of pH did not correspond
to the activity of H+ ions (see Introduction). Using
4 6D @J HCIO. . 1 the correction parameter 6 (which has been pub-
5
lished for water-ethanol solutions of- HCl [ 13]),
H
93
estimates were made of the activity of H+ ions (or
UI of pH*) in TEOS hydrolysates with a low water
2
content (about 1 vol.%). The values of pH*, pH
and -log cHA (HA = HCI, HC104, HzS04,
I
HNO,) usually did not differ by more than
I I t-O.1 unit.
0 2000 4000 6000
ii [hr) The results of determining the pH in water-etha-
Fig. 11. Dependence of water concentration and pH of TEOS nol solutions of TEOS hydrolysed in the presence
hydrolysates containing strong acids on time. of weak acids were also consistent with the results
J. CihlLiF/Colloids Surfaces A: Physicochem. Eng. Aspects 70 (1993) 239-251 249
merization) is not significant. This assumption In the Introduction an opinion was expressed
holds for pH around 2.0 [l]. that the study of the dependence of the TEOS
In TEOS hydrolysates with HF and H,PO, the condensation rate (at a TEOS/H20 molar ratio of
condensation was roughly three orders faster than two) on pH is complicated by TEOS hydrolysis
in the other hydrolysates. This fact was not related and by determining the pH of hydrolysates with a
to the pH’ of the above solutions, which was low water content. Figure 9 confirms that at
comparable with the pH’ of the other hydrolysates pH’ 4.5 the rate of condensation is given by the
containing weak acids. The behaviour of HF can hydrolysis rate. If first the hydrolysis is carried out
be explained by the catalytic action of F- ions at pH around 2.0, when the condensation is at its
during the condensation reactions. The catalytic slowest, and only after that is the pH adjusted, it
action of F- ions during the condensation of water is possible to arrive at a dependence of kK on pH’
solutions of silicic acid has been proved [20]. In as shown in Fig. 10. This dependence confirms that
the case of H3P04 it could be assumed that water-ethanol solutions of ethoxyhydroxysilox-
phosphoric acid participates in the formation of anes (TEOS hydrolysates) behave in the acid region
cross bonds of the type in the same way as water solutions of silicic acid
I pi I described by Iler [S]. In both types of solution the
-Si-O-P-O- Si- (13)
condensation is at its slowest at pH around 2.0,
I I I
and its rate increases with both increasing and
between adjaczt chains and thus accelerates gela- decreasing pH. F- ions catalyse the condensation
tion. Compounds with Si-O-P bonds were in fact in both cases.
prepared by the reaction of H3P04 with SiOl but
under different reaction conditions [2 11. On analy- Conclusion
sis (by analytical electron microscopy) of a TEOS
hydrolysate gelated in the presence of H,PO, we The similarity in the acido-basic behaviour of
found that the specimen practically did not contain water-ethanol solutions of strong acids and of
any phosphorus. Theoretically there could have water-ethanol solutions of TEOS hydrolysates
been as much as 20 wt% of phosphorus. Although (ethoxyhydroxysiloxanes) has become the basis for
this finding does not exclude the formation of determining the pH’ of TEOS hydrolysates. This
Si-O-P bonds by the condensation of Si-OH and made it possible to evaluate the dependence of
P-OH bonds, especially when we realize that both the hydrolysis and condensation rates of
Si-O-P bonds are easily liable to hydrolysis [22], TEOS on the pH’ and the catalyst type. The
the speeding up of the polycondensation of TEOS hydrolysis is an equilibrium reaction catalysed by
hydrolysate owing to the formation of Si-O-P acids or bases. In the acidic region the hydrolysis
bonds is not likely to be significant. More constant decreases with increasing pH’, with the
acceptable seems to be the idea that H3P0, forms minimum value at pH’ around 7.0.
non-chemically bonded bridges between adjacent The dependence of the condensation rate of
siloxane chains, e.g. hydrogen bridges (Eqn (14)) or water-ethanol solutions of ethoxyhydroxysilox-
other donor-acceptor interactions: anes on the pH’ is governed by the same relation-
H OH H ships as the dependence of condensation rate on
I I 1 I I (14)
_II si - en ___ 0 - p - 0 ___ He - si - the pH of water solutions of silicic acid, as
I II I
0 described by Iler [S]. The condensation of ethoxy-
These bridges can draw adjacent chains closer hydroxysiloxanes is acid as well as base catalysed
to each other and sterically facilitate the condensa- and is slowest at a pH of about 2.0. The catalytic
tion between Si-OH (or SiOEt) bonds. However, action of F- ions and of phosphoric acid has also
this problem deserves further investigation. been proved. The mechanism of accelerating the
J. Cih&/Colloids Surfaces A: Physicochem. Eng. Aspects 70 (1993) 239-251 251