- Email: [email protected]
Preparation of boehmite—silica colloids: Rods, spheres, needles and gels
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 80 (1993) 203-210 0927-7757/93/$06.00 0 1993 ~ Elsevier Science Publishers B.V. All rights reserved.
Preparation of boehmite-silica needles and gels
203
colloids: rods, spheres, _
Albert P. Philipse Van’t HofSLaboratory for Physical and Colloid Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands (Received
14 January
1993; accepted
27 April 1993)
Abstract A method is presented for the synthesis of amorphous silica rods with a boehmite core. The addition of boehmite needles to a mixture of tetraethoxysilane (TES), ethanol and aqueous tetramethylammonium hydroxide solution produces gels or aggregates. However, when boehmite needles are previously coated with silica in an aqueous sodium silicate solution, they serve in the TES solution as nuclei for the polymerisation of part of the hydrolysed TES into discrete, amorphous silica rods. The rods tend to stick together probably because of the increase in ionic strength caused by the TES hydrolysis. In the absence of boehmite nuclei, silica only forms discrete, non-aggregated spheres, which demonstrates that the Stdber silica sphere synthesis can be performed with a base other than ammonia. Key words: Boehmite
needles; Boehmite-silica
gels; Silica rods; Silica spheres
1. Introduction Recently, Buining et al. [ 1,2] developed a hydrothermal alkoxide synthesis of fairly monodisperse boehmite (AlOOH) needles in aqueous solution. The synthesis is a convenient one, with the special advantage that the needle length can be controlled by reagent concentrations Cl]. The thickness of the needles,
however,
does not show much
variation.
It would be of interest if the thickness could also be varied, as many properties of dispersions of rod-like particles are sensitive to the dimensions and shape of the particles (for extensive information, see Ref. 2). In order to control the thickness we considered the following idea. In an alkaline mixture of ethanol, water and tetraethoxysilane (TES), silica is formed by the hydrolysis and polymerisation of TES. If anisotropic nuclei are added in the form of boehmite needles [l], silica can polymerise onto these nuclei to form rods. The silica growth would mainly
determine the thickness of the rods, whereas the rod length roughly equals the needle length, which is determined by the boehmite synthesis. In this way one can vary independently the length and diameter of the final rods. In practice, however, seeding of the basic TES solution with boehmite needles immediately produces boehmite aggregates or gels. The main reason for this loss of colloidal stability is that the isoelectric
point (IEP) of boehmite
is about
pH 10;
therefore in the basic TES solution the needles are only weakly (positively) charged, and they stick together because of attractive van der Waals forces. But even if at high pH, the sticking of boehmite needles could be initially prevented by rapid stirring, aggregation is likely to occur when some silica has precipitated onto the positively charged needles. Silica, having an IEP of about pH 2, is negatively charged and partly coated needles will aggregate by “silica bridges”. Thus boehmite needles should be covered with
A.P. PhilipselColloids
204
Surfaces
A: Physicochem.
a (thin) silica layer before they collide by (rotational or translational) diffusion - or before they are
2. Experimental
added
2.1. Boehmite needles
option,
to the TES solution. because
tion is slow precipitation
We chose
the latter
silica formation
in the TES solu-
on the diffusion is faster in an
time scale. Silica acidified aqueous
sodium silicate solution. Therefore we investigated the possibility of a rapid coating of boehmite needles
in a dilute
(low
ionic
strength)
silicate
solution with a thin silica layer, followed by further silica growth in a basic solution of TES in ethanol. The purpose of this communication is to evaluate this method with respect to the formation of amorphous silica rods and their colloidal stability. Several methods were employed to monitor the stability and properties of (coated) boehmite. Any gel formation by sticking boehmite needles is a clear visible sign of instability. Streaming birefringence also provides a sensitive stability test. This birefringence is clearly visible in stable dispersions illuminated between crossed polarisers, but the effect disappears if the needles aggregate so they can no longer align in a flow field. The stability behaviour at various pH values and electrophoresis were used to demonstrate the presence of a silica layer on boehmite. The scope of this communication is primarily the formation of amorphous silica rods to be used for physical studies on dispersions of colloidal rods [2]. We note, however, that boehmite needles covered with silica are also interesting for other purposes, for example the preparation of porous mullite ceramics. Sintering of the thin boehmite needles (about 12 nm in diameter) covered with a thin silica layer of the correct thickness will produce mullite (3A1,0,.2SiO,). Thus one could deposit boehmite-silica needles from a dispersion into a particle packing on a porous (membrane) carrier, which can be sintered into a mullite membrane top layer. The formation of a homogeneous dense packing (and membrane structure) requires individually settling particles which do not aggregate in the dispersion. So, also from an application point of view, it is worthwhile investigating silica thickness control and the stability of the particles.
Eng. Aspects 80 (1993) 203-210
Boehmite needles ASBIPll in water were prepared according to Buining’s method [l]. A mixture of 0.146 mol of aluminium tri-set-butoxide (Fluka AG) and 0.146 mol of aluminium triisopropoxide (Janssen Chimica) in 3 1 of aqueous 0.06 M HCl solution was stirred for 2 days. The slightly turbid solution was then autoclaved for 20 h at 150 “C. The resulting boehmite dispersion (about pH 2) was dialysed in cellophane tubes against demineralised water for 1 week to remove alcohols and salts. The dialysed dispersion (about pH 6) which was the boehmite dispersion in all subsequent experiments, had a boehmite weight concentration of c = 4.96 g l- ‘, which corresponds to a particle volume fraction of 0.17%. Before dialysis, the boehmite dispersion exhibited streaming birefringence and the viscosity was low. After dialysis, the birefringence was permanent and the viscosity of the dispersion was high enough to trap air bubbles. These observations demonstrate that the boehmite needles in the dispersion are not aggregated, and that the electrical double layer repulsion between the stable particles is further increased by salt removal during dialysis [Z]. The dimensions of the boehmite particles were determined by electron microscopy. The average needle length L was also determined from the angular dependence of the static light scattering intensity of a dilute dispersion, making use of the Guinier approximation for non-aggregated monodisperse needles [3]. The result agrees well with that of electron microscopy (see Table 1). 2.2. Boehmite-silica
gels
To demonstrate the influence of a basic dispersion of negatively charged silica on the stability of the positively charged boehmite particles, the boehmite dispersion was mixed with an aqueous silica suspension (Ludox HS-40; used as received; pH 9.7; particle diameter, 12 nm according to the manufacturer (du Pont)). The mixing produced
A.P. PhilipselColloids
Surfaces
Table I Results of the characterisation Sample
ASBIPI BSilO
Electron
I
A: Physicochem.
of boehmite
microscopy
Eng. Aspects 80 (1993) 203-210
(silica) needles
results
Average rod length L
Standard deviation
Average rod diameter D
Standard deviation
(nm)
(%)
(nm)
@)
321 -
26
11.7 _
26 _
eL
205
instantaneously almost transparent gels (see also Fig. 1). The addition of aqueous ammonia or tetramethylammonium hydroxide (TMA) solutions also destabilised the boehmite system, as shown by the disappearance of streaming birefringence and the formation of floes or gels. The boehmite dispersion was no longer stable at pH 7 or above. Basic mixtures of tetraethoxysilane (TES), TMA, ethanol and boehmite dispersion yielded, over a wide composition range, aggregates or gels, the latter often being milky white because of silica polymerisation. Acid mixtures of TES, HCl, ethanol and water generally did not influence the
Fig. 1. Transmission electron microscopy micrograph of particles in the gel formed after mixing boehmite needles with oppositely charged Ludox silica spheres. The bar represents 0.1 urn.
Static light scattering results, L on
IEP
:mV)
mm)
348 + 30 _
Estimated zeta potential
9.9 1.74+0.15
+50 -30
boehmite dispersion immediately, as long as no silica was formed. But any silica polymerisation, manifesting itself by a turbidity increase, eventually led to aggregates or gels. 2.3 Boehmite-silica
needles
To precipitate a thin silica layer onto boehmite needles in water we made use of a sodium silicate solution (Na20-(Si02)3_5; ABCR-Karlsruhe; pH 12, 28 wt.% SiO,), inducing silica precipitation by lowering the pH. The following procedure, found by trial and error, yielded reproducibly stable dispersions. To 250 ml of silicate solution (0.06 wt.% SiO,), prepared by dilution of the stock solution with demineralised water, 50 ml of the dialysed boehmite dispersion were added under vigorous stirring. The pH was then reduced to pH 9 by the dropwise addition of aqueous HCl solution. The resulting dispersion BSilO, with weight concentration c= 0.8 g l- ‘, exhibited streaming birefringence. When a sample of this dispersion was dialysed against demineralised water, the birefringence effect disappeared and the dispersion had a gel-like consistency. Electrophoretic mobility measurements on needles before and after treatment with sodium silicate were performed with use of a Pen Kern System 3000. These measurements are part of a separate study on the electrokinetics of rod-like colloids [2] and will only be discussed in Section 3 in as far as they demonstrate the presence of silica on the BSilO particles. BSilO dispersion gelated when the pH was
A.P. Philipse/Colloids
206
Fig. 2. Boehmite
needles before (left) and after (right) treatment
reduced to pH 6 or below, in clear contrast with the initial boehmite needles, which are stable in this pH range. This observation also explains the gelation during the dialysis, which reduces the pH. After the addition of Ludox, the BSilO dispersion remained stable and exhibited streaming birefringence.
Surfaces
with aqueous
A: Physicochem.
silicate solution.
Eng. Aspects 80 (1993) 203-2IO
The bar represents
0.1 pm.
turbid supernatant. The sediments, which started to form only after about 15-30 min, contained mainly silica rods and some spheres, whereas supernatants contained mainly spheres and occasionally
(see Fig. 2) were
some rods (see Fig. 3). However, in the mixture with the lowest amount of TES (0.04 vol.%), only silica rods were observed on electron microscope grids, sometimes contaminated with structureless silica polymerisation products, as in Fig. 3F. We also performed experiments with ammonia as a base instead of TMA over an even wider
used in the following way as nuclei for silica polymerisation. BSilO dispersion (3 ml) was added to stirred mixtures of 100 ml of ethanol and 1 ml
range of TES concentrations. The results, however, do not differ from those obtained with TMA, and do not change any of the conclusions in
aqueous TMA (1%) solution. To each mixture a volume percentage of TES was then added in the range 0.04415%. After stirring, the mixtures were left undisturbed at room temperature. Silica growth led to the separation of the mixtures into a sediment of loose floes and a stable
Section
2.4 Mixtures of silica rods and spheres The
boehmiteesilica
needles
3.
2.5 Silica spheres Stable dispersions of silica spheres with a narrow size distribution were formed in an alkaline TES
Fig. 3. Formation of silica rods and spheres in solutions of tetraethoxysilane (TES) and tetramethylammonium hydroxide in ethanol-water, seeded with boehmiteesilica needles, which can be discerned as thin black lines in the silica rods (see the markers in E). The TES volume percentage 4 is varied (see Section 2.4): 4=3.5, particles found in sediment (A) and supernatant (B); @=0.75, sediment (C) and supernatant (D); $I = 0.04, sediment (E) and supernatant (F).
A.P. Philipse/Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 203-2t0
207
A.P. Philipse/Colloids
208
solution
without
example,
boehmiteesilica
needles.
3.8 ml of TES were added
For
to a mixture
of 100 ml ethanol, 1 ml aqueous TMA (1%) solution and 3 ml water (to replace the boehmite dispersion).
Surfaces A: Physicochem.
higher
water
and
larger spheres. It is possible
Eng. Aspects 80 (1993) 203-210
base
concentrations
to monitor
produced
silica sphere
growth
The light scattering radius R of the final spheres (Fig. 4) was about 150 nm. The preparation of silica spheres was repeated
with static light scattering and we used this procedure, described in Ref. 4, to determine the onset time of silica polymerisation in the mixture described above. The idea was to see if this onset
several
coincides
times
with less water
(2 ml) and/or
more
with the formation
of sedimenting
Aocs
base (2 ml of 1% TMA). Both dynamic and static light scattering measurements indicated non-
when boehmite-silica needles are present. Already 3 min after preparing the mixture the scattered
aggregated
intensity started to increase. After 5 min, the angular dependence of the intensity was pronounced enough to determine the silica sphere size (Fig. 4). Thus silica formation starts earlier than the formation of floes mentioned in Section 2.4.
spheres.
The observed
trend
was that
3. Results and discussion The main
results
are the following.
Boehmite
needles treated with sodium silicate (“boehmitesilica needles”, see Fig. 2) serve as nuclei for the formation of discrete silica rods in a solution of tetraethoxysilane nium hydroxide
and tetramethylammoin ethanol-water. The
nucleation and growth of separate silica spheres can be suppressed by taking a sufficiently low TES concentration (Fig. 3). However only silica spheres are formed when no boehmite-silica needles are present (Fig. 4). This sphere formation must be equivalent to the
final radius
!I
process occurring employs instead
in the Stiiber synthesis, which of TMA a volatile electrolyte,
namely
[S]. Higher
ammonia
base and water con-
centrations produce larger spheres, as is also the case for the conventional Stiiber method. Whether
150 0
(TES) (TMA)
I 0
I
5 -t
/
I
I
10
15
minutes
Fig. 4. Repeat of experiment with 4 = 3.5% (see Fig. 3) without boehmite-silica needles. Only silica spheres are formed (A), which have a detectable light scattering radius R already a few minutes after mixing the reagents at time t =0 (B).
TMA allows better control of particle size than ammonia remains to be seen. The formation of silica rods clearly benefits from the thin silica layer on the boehmite needles. Without this layer the boehmite nuclei immediately aggregate in the basic TES starting solutions, in which the needles are only weakly charged because the pH is close to the IEP of boehmite. Also, newly formed
(small)
silica particles
will lead to hetero-
A.P. Philipse/Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 203S210
flocculation, mixtures
analogous
of boehmite
Comparing
to the
micrographs
before and after sodium one
may
wonder
gel formation
in
and Ludox silica (see Fig. 1). of boehmite silicate
whether
treatment
the
needles (Fig. 2)
treatment
really
produces a surface silica coating: the needles look quite similar. The treatment as described in Section
2.3, however,
thickness
of only
precipitate
produces
2-3 nm,
(at about
a silica
provided
coating
all silicates
pH 9) onto the needles.
Any
remaining soluble silicates will of course reduce this thickness, and the layer will be hard to observe with electron microscopy. The presence of silica on the needles is nevertheless clear from the stability behaviour: the boehmite-silica needles do not aggregate above pH 6-7, in contrast to untreated boehmite which aggregates in basic solutions. This stability difference is also observed when boehmite is mixed with Ludox silica. Before treatment with sodium silicate, this mixing produces gels immediately. After treatment the needles remain in stable suspension. Electrophoresis provides a more quantitative demonstration of the silica surface coating. Boehmite needles and boehmite-silica needles migrate to the negative and positive pole respectively. (Estimated zeta potentials are given in Table 1 and are discussed in more detail elsewhere [2],) The boehmite-silica needles not only carry a net negative surface charge; they also have, according to Fig. 5, an IEP at pH 1.74( + 0.15), which 0.5 7 7
0 m
>
-0.5
ZE b 5.
T
-1
-1.5 -2 -2.5 -3
--1 ____-___---_-. .
.
.
.
i
1
0
1
l
2
3
+ Fig. 5. Electrophoretic mobility boehmite-silica needles BSilO.
.
.
4
--=__
5
6
. 7
PH p as a function
209
agrees with values reported of boehmite
needles
is about
for silica [6]. The IEP pH 9.9 [2]. We note
here that these results do not prove that the needles are completely
covered
with silica. There may be a
few bare boehmite spots left, a point to which we return later. We now discuss in more detail the results relating to Fig. 3. In the synthesis mixtures of Section 2.4, silica is generated by the base-catalysed hydrolysis and polymerisation of TES. Lowering the initial TES concentration will suppress the homogeneous nucleation of silica spheres. Indeed, at the lowest TES concentration used, most of the silica grows onto the boehmite-silica needles, whereas at higher concentrations spheres also are formed, which remain dispersed in the supernatant, while rods are preferentially present in sedimented floes (see Fig. 3). The colloidal stability of silica spheres in the alkaline ethanol solution is also confirmed by growth experiments in the absence of boehmitesilica needles (see Fig. 4) which produced nonaggregated spheres. So why would silica rods form sedimented blocs? The floes are not the result of the sedimentation of single rods. The radius of the silica spheres is generally larger than the radius of the rods, so single rods will not settle faster than single spheres. Thus rods already form aggregates in the dispersion. This aggregation occurs fairly late in the silica polymerisation process. This polymerisation starts after a few minutes (Fig. 4) whereas sedimenting floes are observed only after 15-30 min. Also the morphology of clusters in Fig. 3 suggests that the rods have grown significantly before clustering. A possible source of the aggregation of the rods are van der Waals attractions, which become stronger as the thickness of the rods increases. A simple calculation, however, shows that these attractions nevertheless must be weaker than those between bare boehmite particles in water. The attraction energy V is strongest for the parallel orientation of two rods, for which
of pH for the
V= - ALfi/24H3”
(1)
A.P. Philipse/Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 203-210
210
where
A is the Hamaker
surface-to-surface
distance
constant
and
between
rods of radius
H is the
the ethanol
solution
also contributes
contains
sodium silicate, which
to the ionic
strength
and, more-
R and length L [7]. Thus, if we compare pure boehmite needles in water (subscript bw) and pure silica rods in ethanol (subscript se), we find for the
over, which can form silicate bonds between weakly repelling rods. (This sodium silicate could not be removed by dialysis without the occurrence of
ratio of attraction
particle gelation; see Section 2.3.) To optimise the silica rod synthesis it would therefore be worthwhile growing a thicker silica
energies
Here we estimate the Hamaker constants using the Lifschitz theory [7]. For boehmite in water the constant A,, is relatively large, because the refractive index difference between boehmite and water is larger than that between silica and ethanol. For the boehmite needles, R,,=6 nm and one can easily verify that the ratio in Eq. (2) is larger than 1, even for silica rods as thick as R,, z 200 nm, so the van der Waals attractions between boehmite needles in water are stronger than those between silica rods in ethanol. Thus to stabilise the silica rods, a less strong double layer repulsion than for the boehmite needles is required. The rods may nevertheless aggregate if for some reason this repulsion is sufficiently weakened during the silica growth process. A plausible course of events after the mixing of boehmite-silica needles with a TES solution is therefore the following. During hydrolysis and polymerisation of TES in the presence of ammonia, the conductivity of the ethanol solution increases because of the formation of charged silicate species and passes a maximum, as discussed in more detail in Refs. 8 and 9. The same will happen when TMA is used as a base. At the conductivity maximum, the ionic strength is maximal and consequently the electrical double layer repulsion between colloidal particles is at its weakest. Van der Waals attractions can then induce flocculation. This aggregation will be facilitated by any bare boehmite spots (with a weakly positive or zero charge) on the surface of the boehmitesilica need!es. We should also bear in mind that
layer on boehmite needles to eliminate the possibility of bare boehmite spots, and to find a method of removing remaining sodium silicate from the dispersion of boehmiteesilica needles. This is the subject
of current
investigations.
Acknowledgements Frieda Langeveld and Petri Mast are acknowledged for performing a part of the experiments. Professor Henk Lekkerkerker is thanked for his stimulating interest in this study. This work was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organisation for the Advancement of Pure Research (NWO). References P.A. Buining, C. Pathmamanoharan, J.B.H. Jansen and H.N.W. Lekkerkerker, J. Am. Ceram. Sot., 74 (1991) 1303. P.A. Buining, Preparation and Properties of Dispersions of Colloidal Boehmite Rods; a Systems-oriented Study, Thesis, Utrecht University, 1992. The Scattering of Light and other M. Kerker, Electromagnetic Radiation, Academic Press, London, 1969. A.P. Philipse, Colloid Polym. Sci., 266 (1988) 1174. W. Stober, A. Fink and E. Bohn, J. Colloid Interface Sci., 26 (I 968) 62. R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979. J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, London, 1985. A. van Blaaderen, Colloidal Dispersions of (Organo-) Silica Spheres: Formation Mechanism, Structure and Dynamics, Thesis, Utrecht University, 1992. G.H. Bogush and C.F. Zukoski, J. Colloid Interface Sci., 142 (1) (1991) 1.
Preparation of boehmite-silica needles and gels
203
colloids: rods, spheres, _
Albert P. Philipse Van’t HofSLaboratory for Physical and Colloid Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands (Received
14 January
1993; accepted
27 April 1993)
Abstract A method is presented for the synthesis of amorphous silica rods with a boehmite core. The addition of boehmite needles to a mixture of tetraethoxysilane (TES), ethanol and aqueous tetramethylammonium hydroxide solution produces gels or aggregates. However, when boehmite needles are previously coated with silica in an aqueous sodium silicate solution, they serve in the TES solution as nuclei for the polymerisation of part of the hydrolysed TES into discrete, amorphous silica rods. The rods tend to stick together probably because of the increase in ionic strength caused by the TES hydrolysis. In the absence of boehmite nuclei, silica only forms discrete, non-aggregated spheres, which demonstrates that the Stdber silica sphere synthesis can be performed with a base other than ammonia. Key words: Boehmite
needles; Boehmite-silica
gels; Silica rods; Silica spheres
1. Introduction Recently, Buining et al. [ 1,2] developed a hydrothermal alkoxide synthesis of fairly monodisperse boehmite (AlOOH) needles in aqueous solution. The synthesis is a convenient one, with the special advantage that the needle length can be controlled by reagent concentrations Cl]. The thickness of the needles,
however,
does not show much
variation.
It would be of interest if the thickness could also be varied, as many properties of dispersions of rod-like particles are sensitive to the dimensions and shape of the particles (for extensive information, see Ref. 2). In order to control the thickness we considered the following idea. In an alkaline mixture of ethanol, water and tetraethoxysilane (TES), silica is formed by the hydrolysis and polymerisation of TES. If anisotropic nuclei are added in the form of boehmite needles [l], silica can polymerise onto these nuclei to form rods. The silica growth would mainly
determine the thickness of the rods, whereas the rod length roughly equals the needle length, which is determined by the boehmite synthesis. In this way one can vary independently the length and diameter of the final rods. In practice, however, seeding of the basic TES solution with boehmite needles immediately produces boehmite aggregates or gels. The main reason for this loss of colloidal stability is that the isoelectric
point (IEP) of boehmite
is about
pH 10;
therefore in the basic TES solution the needles are only weakly (positively) charged, and they stick together because of attractive van der Waals forces. But even if at high pH, the sticking of boehmite needles could be initially prevented by rapid stirring, aggregation is likely to occur when some silica has precipitated onto the positively charged needles. Silica, having an IEP of about pH 2, is negatively charged and partly coated needles will aggregate by “silica bridges”. Thus boehmite needles should be covered with
A.P. PhilipselColloids
204
Surfaces
A: Physicochem.
a (thin) silica layer before they collide by (rotational or translational) diffusion - or before they are
2. Experimental
added
2.1. Boehmite needles
option,
to the TES solution. because
tion is slow precipitation
We chose
the latter
silica formation
in the TES solu-
on the diffusion is faster in an
time scale. Silica acidified aqueous
sodium silicate solution. Therefore we investigated the possibility of a rapid coating of boehmite needles
in a dilute
(low
ionic
strength)
silicate
solution with a thin silica layer, followed by further silica growth in a basic solution of TES in ethanol. The purpose of this communication is to evaluate this method with respect to the formation of amorphous silica rods and their colloidal stability. Several methods were employed to monitor the stability and properties of (coated) boehmite. Any gel formation by sticking boehmite needles is a clear visible sign of instability. Streaming birefringence also provides a sensitive stability test. This birefringence is clearly visible in stable dispersions illuminated between crossed polarisers, but the effect disappears if the needles aggregate so they can no longer align in a flow field. The stability behaviour at various pH values and electrophoresis were used to demonstrate the presence of a silica layer on boehmite. The scope of this communication is primarily the formation of amorphous silica rods to be used for physical studies on dispersions of colloidal rods [2]. We note, however, that boehmite needles covered with silica are also interesting for other purposes, for example the preparation of porous mullite ceramics. Sintering of the thin boehmite needles (about 12 nm in diameter) covered with a thin silica layer of the correct thickness will produce mullite (3A1,0,.2SiO,). Thus one could deposit boehmite-silica needles from a dispersion into a particle packing on a porous (membrane) carrier, which can be sintered into a mullite membrane top layer. The formation of a homogeneous dense packing (and membrane structure) requires individually settling particles which do not aggregate in the dispersion. So, also from an application point of view, it is worthwhile investigating silica thickness control and the stability of the particles.
Eng. Aspects 80 (1993) 203-210
Boehmite needles ASBIPll in water were prepared according to Buining’s method [l]. A mixture of 0.146 mol of aluminium tri-set-butoxide (Fluka AG) and 0.146 mol of aluminium triisopropoxide (Janssen Chimica) in 3 1 of aqueous 0.06 M HCl solution was stirred for 2 days. The slightly turbid solution was then autoclaved for 20 h at 150 “C. The resulting boehmite dispersion (about pH 2) was dialysed in cellophane tubes against demineralised water for 1 week to remove alcohols and salts. The dialysed dispersion (about pH 6) which was the boehmite dispersion in all subsequent experiments, had a boehmite weight concentration of c = 4.96 g l- ‘, which corresponds to a particle volume fraction of 0.17%. Before dialysis, the boehmite dispersion exhibited streaming birefringence and the viscosity was low. After dialysis, the birefringence was permanent and the viscosity of the dispersion was high enough to trap air bubbles. These observations demonstrate that the boehmite needles in the dispersion are not aggregated, and that the electrical double layer repulsion between the stable particles is further increased by salt removal during dialysis [Z]. The dimensions of the boehmite particles were determined by electron microscopy. The average needle length L was also determined from the angular dependence of the static light scattering intensity of a dilute dispersion, making use of the Guinier approximation for non-aggregated monodisperse needles [3]. The result agrees well with that of electron microscopy (see Table 1). 2.2. Boehmite-silica
gels
To demonstrate the influence of a basic dispersion of negatively charged silica on the stability of the positively charged boehmite particles, the boehmite dispersion was mixed with an aqueous silica suspension (Ludox HS-40; used as received; pH 9.7; particle diameter, 12 nm according to the manufacturer (du Pont)). The mixing produced
A.P. PhilipselColloids
Surfaces
Table I Results of the characterisation Sample
ASBIPI BSilO
Electron
I
A: Physicochem.
of boehmite
microscopy
Eng. Aspects 80 (1993) 203-210
(silica) needles
results
Average rod length L
Standard deviation
Average rod diameter D
Standard deviation
(nm)
(%)
(nm)
@)
321 -
26
11.7 _
26 _
eL
205
instantaneously almost transparent gels (see also Fig. 1). The addition of aqueous ammonia or tetramethylammonium hydroxide (TMA) solutions also destabilised the boehmite system, as shown by the disappearance of streaming birefringence and the formation of floes or gels. The boehmite dispersion was no longer stable at pH 7 or above. Basic mixtures of tetraethoxysilane (TES), TMA, ethanol and boehmite dispersion yielded, over a wide composition range, aggregates or gels, the latter often being milky white because of silica polymerisation. Acid mixtures of TES, HCl, ethanol and water generally did not influence the
Fig. 1. Transmission electron microscopy micrograph of particles in the gel formed after mixing boehmite needles with oppositely charged Ludox silica spheres. The bar represents 0.1 urn.
Static light scattering results, L on
IEP
:mV)
mm)
348 + 30 _
Estimated zeta potential
9.9 1.74+0.15
+50 -30
boehmite dispersion immediately, as long as no silica was formed. But any silica polymerisation, manifesting itself by a turbidity increase, eventually led to aggregates or gels. 2.3 Boehmite-silica
needles
To precipitate a thin silica layer onto boehmite needles in water we made use of a sodium silicate solution (Na20-(Si02)3_5; ABCR-Karlsruhe; pH 12, 28 wt.% SiO,), inducing silica precipitation by lowering the pH. The following procedure, found by trial and error, yielded reproducibly stable dispersions. To 250 ml of silicate solution (0.06 wt.% SiO,), prepared by dilution of the stock solution with demineralised water, 50 ml of the dialysed boehmite dispersion were added under vigorous stirring. The pH was then reduced to pH 9 by the dropwise addition of aqueous HCl solution. The resulting dispersion BSilO, with weight concentration c= 0.8 g l- ‘, exhibited streaming birefringence. When a sample of this dispersion was dialysed against demineralised water, the birefringence effect disappeared and the dispersion had a gel-like consistency. Electrophoretic mobility measurements on needles before and after treatment with sodium silicate were performed with use of a Pen Kern System 3000. These measurements are part of a separate study on the electrokinetics of rod-like colloids [2] and will only be discussed in Section 3 in as far as they demonstrate the presence of silica on the BSilO particles. BSilO dispersion gelated when the pH was
A.P. Philipse/Colloids
206
Fig. 2. Boehmite
needles before (left) and after (right) treatment
reduced to pH 6 or below, in clear contrast with the initial boehmite needles, which are stable in this pH range. This observation also explains the gelation during the dialysis, which reduces the pH. After the addition of Ludox, the BSilO dispersion remained stable and exhibited streaming birefringence.
Surfaces
with aqueous
A: Physicochem.
silicate solution.
Eng. Aspects 80 (1993) 203-2IO
The bar represents
0.1 pm.
turbid supernatant. The sediments, which started to form only after about 15-30 min, contained mainly silica rods and some spheres, whereas supernatants contained mainly spheres and occasionally
(see Fig. 2) were
some rods (see Fig. 3). However, in the mixture with the lowest amount of TES (0.04 vol.%), only silica rods were observed on electron microscope grids, sometimes contaminated with structureless silica polymerisation products, as in Fig. 3F. We also performed experiments with ammonia as a base instead of TMA over an even wider
used in the following way as nuclei for silica polymerisation. BSilO dispersion (3 ml) was added to stirred mixtures of 100 ml of ethanol and 1 ml
range of TES concentrations. The results, however, do not differ from those obtained with TMA, and do not change any of the conclusions in
aqueous TMA (1%) solution. To each mixture a volume percentage of TES was then added in the range 0.04415%. After stirring, the mixtures were left undisturbed at room temperature. Silica growth led to the separation of the mixtures into a sediment of loose floes and a stable
Section
2.4 Mixtures of silica rods and spheres The
boehmiteesilica
needles
3.
2.5 Silica spheres Stable dispersions of silica spheres with a narrow size distribution were formed in an alkaline TES
Fig. 3. Formation of silica rods and spheres in solutions of tetraethoxysilane (TES) and tetramethylammonium hydroxide in ethanol-water, seeded with boehmiteesilica needles, which can be discerned as thin black lines in the silica rods (see the markers in E). The TES volume percentage 4 is varied (see Section 2.4): 4=3.5, particles found in sediment (A) and supernatant (B); @=0.75, sediment (C) and supernatant (D); $I = 0.04, sediment (E) and supernatant (F).
A.P. Philipse/Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 203-2t0
207
A.P. Philipse/Colloids
208
solution
without
example,
boehmiteesilica
needles.
3.8 ml of TES were added
For
to a mixture
of 100 ml ethanol, 1 ml aqueous TMA (1%) solution and 3 ml water (to replace the boehmite dispersion).
Surfaces A: Physicochem.
higher
water
and
larger spheres. It is possible
Eng. Aspects 80 (1993) 203-210
base
concentrations
to monitor
produced
silica sphere
growth
The light scattering radius R of the final spheres (Fig. 4) was about 150 nm. The preparation of silica spheres was repeated
with static light scattering and we used this procedure, described in Ref. 4, to determine the onset time of silica polymerisation in the mixture described above. The idea was to see if this onset
several
coincides
times
with less water
(2 ml) and/or
more
with the formation
of sedimenting
Aocs
base (2 ml of 1% TMA). Both dynamic and static light scattering measurements indicated non-
when boehmite-silica needles are present. Already 3 min after preparing the mixture the scattered
aggregated
intensity started to increase. After 5 min, the angular dependence of the intensity was pronounced enough to determine the silica sphere size (Fig. 4). Thus silica formation starts earlier than the formation of floes mentioned in Section 2.4.
spheres.
The observed
trend
was that
3. Results and discussion The main
results
are the following.
Boehmite
needles treated with sodium silicate (“boehmitesilica needles”, see Fig. 2) serve as nuclei for the formation of discrete silica rods in a solution of tetraethoxysilane nium hydroxide
and tetramethylammoin ethanol-water. The
nucleation and growth of separate silica spheres can be suppressed by taking a sufficiently low TES concentration (Fig. 3). However only silica spheres are formed when no boehmite-silica needles are present (Fig. 4). This sphere formation must be equivalent to the
final radius
!I
process occurring employs instead
in the Stiiber synthesis, which of TMA a volatile electrolyte,
namely
[S]. Higher
ammonia
base and water con-
centrations produce larger spheres, as is also the case for the conventional Stiiber method. Whether
150 0
(TES) (TMA)
I 0
I
5 -t
/
I
I
10
15
minutes
Fig. 4. Repeat of experiment with 4 = 3.5% (see Fig. 3) without boehmite-silica needles. Only silica spheres are formed (A), which have a detectable light scattering radius R already a few minutes after mixing the reagents at time t =0 (B).
TMA allows better control of particle size than ammonia remains to be seen. The formation of silica rods clearly benefits from the thin silica layer on the boehmite needles. Without this layer the boehmite nuclei immediately aggregate in the basic TES starting solutions, in which the needles are only weakly charged because the pH is close to the IEP of boehmite. Also, newly formed
(small)
silica particles
will lead to hetero-
A.P. Philipse/Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 203S210
flocculation, mixtures
analogous
of boehmite
Comparing
to the
micrographs
before and after sodium one
may
wonder
gel formation
in
and Ludox silica (see Fig. 1). of boehmite silicate
whether
treatment
the
needles (Fig. 2)
treatment
really
produces a surface silica coating: the needles look quite similar. The treatment as described in Section
2.3, however,
thickness
of only
precipitate
produces
2-3 nm,
(at about
a silica
provided
coating
all silicates
pH 9) onto the needles.
Any
remaining soluble silicates will of course reduce this thickness, and the layer will be hard to observe with electron microscopy. The presence of silica on the needles is nevertheless clear from the stability behaviour: the boehmite-silica needles do not aggregate above pH 6-7, in contrast to untreated boehmite which aggregates in basic solutions. This stability difference is also observed when boehmite is mixed with Ludox silica. Before treatment with sodium silicate, this mixing produces gels immediately. After treatment the needles remain in stable suspension. Electrophoresis provides a more quantitative demonstration of the silica surface coating. Boehmite needles and boehmite-silica needles migrate to the negative and positive pole respectively. (Estimated zeta potentials are given in Table 1 and are discussed in more detail elsewhere [2],) The boehmite-silica needles not only carry a net negative surface charge; they also have, according to Fig. 5, an IEP at pH 1.74( + 0.15), which 0.5 7 7
0 m
>
-0.5
ZE b 5.
T
-1
-1.5 -2 -2.5 -3
--1 ____-___---_-. .
.
.
.
i
1
0
1
l
2
3
+ Fig. 5. Electrophoretic mobility boehmite-silica needles BSilO.
.
.
4
--=__
5
6
. 7
PH p as a function
209
agrees with values reported of boehmite
needles
is about
for silica [6]. The IEP pH 9.9 [2]. We note
here that these results do not prove that the needles are completely
covered
with silica. There may be a
few bare boehmite spots left, a point to which we return later. We now discuss in more detail the results relating to Fig. 3. In the synthesis mixtures of Section 2.4, silica is generated by the base-catalysed hydrolysis and polymerisation of TES. Lowering the initial TES concentration will suppress the homogeneous nucleation of silica spheres. Indeed, at the lowest TES concentration used, most of the silica grows onto the boehmite-silica needles, whereas at higher concentrations spheres also are formed, which remain dispersed in the supernatant, while rods are preferentially present in sedimented floes (see Fig. 3). The colloidal stability of silica spheres in the alkaline ethanol solution is also confirmed by growth experiments in the absence of boehmitesilica needles (see Fig. 4) which produced nonaggregated spheres. So why would silica rods form sedimented blocs? The floes are not the result of the sedimentation of single rods. The radius of the silica spheres is generally larger than the radius of the rods, so single rods will not settle faster than single spheres. Thus rods already form aggregates in the dispersion. This aggregation occurs fairly late in the silica polymerisation process. This polymerisation starts after a few minutes (Fig. 4) whereas sedimenting floes are observed only after 15-30 min. Also the morphology of clusters in Fig. 3 suggests that the rods have grown significantly before clustering. A possible source of the aggregation of the rods are van der Waals attractions, which become stronger as the thickness of the rods increases. A simple calculation, however, shows that these attractions nevertheless must be weaker than those between bare boehmite particles in water. The attraction energy V is strongest for the parallel orientation of two rods, for which
of pH for the
V= - ALfi/24H3”
(1)
A.P. Philipse/Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 203-210
210
where
A is the Hamaker
surface-to-surface
distance
constant
and
between
rods of radius
H is the
the ethanol
solution
also contributes
contains
sodium silicate, which
to the ionic
strength
and, more-
R and length L [7]. Thus, if we compare pure boehmite needles in water (subscript bw) and pure silica rods in ethanol (subscript se), we find for the
over, which can form silicate bonds between weakly repelling rods. (This sodium silicate could not be removed by dialysis without the occurrence of
ratio of attraction
particle gelation; see Section 2.3.) To optimise the silica rod synthesis it would therefore be worthwhile growing a thicker silica
energies
Here we estimate the Hamaker constants using the Lifschitz theory [7]. For boehmite in water the constant A,, is relatively large, because the refractive index difference between boehmite and water is larger than that between silica and ethanol. For the boehmite needles, R,,=6 nm and one can easily verify that the ratio in Eq. (2) is larger than 1, even for silica rods as thick as R,, z 200 nm, so the van der Waals attractions between boehmite needles in water are stronger than those between silica rods in ethanol. Thus to stabilise the silica rods, a less strong double layer repulsion than for the boehmite needles is required. The rods may nevertheless aggregate if for some reason this repulsion is sufficiently weakened during the silica growth process. A plausible course of events after the mixing of boehmite-silica needles with a TES solution is therefore the following. During hydrolysis and polymerisation of TES in the presence of ammonia, the conductivity of the ethanol solution increases because of the formation of charged silicate species and passes a maximum, as discussed in more detail in Refs. 8 and 9. The same will happen when TMA is used as a base. At the conductivity maximum, the ionic strength is maximal and consequently the electrical double layer repulsion between colloidal particles is at its weakest. Van der Waals attractions can then induce flocculation. This aggregation will be facilitated by any bare boehmite spots (with a weakly positive or zero charge) on the surface of the boehmitesilica need!es. We should also bear in mind that
layer on boehmite needles to eliminate the possibility of bare boehmite spots, and to find a method of removing remaining sodium silicate from the dispersion of boehmiteesilica needles. This is the subject
of current
investigations.
Acknowledgements Frieda Langeveld and Petri Mast are acknowledged for performing a part of the experiments. Professor Henk Lekkerkerker is thanked for his stimulating interest in this study. This work was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organisation for the Advancement of Pure Research (NWO). References P.A. Buining, C. Pathmamanoharan, J.B.H. Jansen and H.N.W. Lekkerkerker, J. Am. Ceram. Sot., 74 (1991) 1303. P.A. Buining, Preparation and Properties of Dispersions of Colloidal Boehmite Rods; a Systems-oriented Study, Thesis, Utrecht University, 1992. The Scattering of Light and other M. Kerker, Electromagnetic Radiation, Academic Press, London, 1969. A.P. Philipse, Colloid Polym. Sci., 266 (1988) 1174. W. Stober, A. Fink and E. Bohn, J. Colloid Interface Sci., 26 (I 968) 62. R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979. J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, London, 1985. A. van Blaaderen, Colloidal Dispersions of (Organo-) Silica Spheres: Formation Mechanism, Structure and Dynamics, Thesis, Utrecht University, 1992. G.H. Bogush and C.F. Zukoski, J. Colloid Interface Sci., 142 (1) (1991) 1.