Gel Formation
Bressanone Sept. 2006
Gel Formation
1
Outline
Sol-Gel Processing
Chart
Gel
Sol
Solvent
Evaporation
Powders
Supercritical
Extraction
Gelation
Evaporation
Xerogel
Xerogel Film
Aerogel
Dry Heat
Heat
Dense Ceramic Film
Bressanone Sept. 2006
Dense Glass
Gel Formation
2
1
Outline
Silica-Based Gels
hydrolysis and condensation
gelation
network vs. cluster formation
two-component systems
Metal Oxide-Based Gels
difference between Si(OR)4 and M(OR)n
network vs. cluster formation
Non-hydrolytic Sol-Gel Processes
Bressanone Sept. 2006
Gel Formation
3
Hydrolysis and Condensation of Silicon Compounds
Generation of reactive species: hydrolysis
≡Si-OR + H2O
≡Si-OH + ROH
Network formation: condensation
2 ≡Si-OH
≡Si-O-Si≡ + H2O
≡Si-OH + RO-Si≡
Note:
≡Si-O-Si≡ + ROH
•
•
•
•
•
•
•
•
Kind of catalyst (pH)
Kind of molecular precursor
Water:OR-ratio (Rw)
Solvent
Temperature
Additives (e.g. electrolytes,
surfactants, complexing agents)
Absolute concentration of the
precursors
Relative concentration of the
precursors in multi-component
systems
colloidal silica dispersions (water glass “solutions”) already
contain Si-OH species
Bressanone Sept. 2006
Gel Formation
4
2
Reaction Parameters: Catalyst
Acidic conditions:
Si
OX + H+
Si
O+
H
X
Y−OH +
Si
O+
H
Y−O −Si
+ HOX + H+
X
Alkaline conditions (similar F-):
OY
Si-OX + YO−
−
Si
YO- Si
+ XO-
OX
hydrolysis reaction:
X = R,
condensation reaction:
X = R or H, Y = Si
alcohol exchange:
X = R,
Bressanone Sept. 2006
Y=H
Y = R'
Gel Formation
5
Electrostatic Stabilization of Colloids
M-O-
+ H
M-OH
M O
H
Point of zero charge (PZC):
for SiO2 ≈1.5 – 4.5
Gelation by pH change
Bressanone Sept. 2006
Gel Formation
6
3
pH Dependence of Hydrolysis and Condensation
Bressanone Sept. 2006
Gel Formation
7
Geland
Formation:
Sol-Gelof
Processing
Hydrolysis
Condensation
Silicon Alkoxides
OR
Si
RO
RO
OR
H2 O
cat.
Hydrolysis
Si
OH
OR
OH
RO
RO
Si
RO
RO
OH
OR
HO
HO
OH
Si
OH
RO
RO
Si
O
OH
OR
Si
Si
OR
OR
O
OR
OR
Si
hydrolysis and condensation
occur concomitantly !
Condensation
OR
RO
RO
RO
O
OR
OR
RO
RO
RO
RO
OR
Si
Si
O
OR
OR
OR
OR
Si
Si
O
OR
OR
Si
RO
RO
OH
Si
OR
Si
RO
O
OR
Si
OR
OR
O
Si
OR
OR
O
OH
HO
HO
Si
OR
OR
OH
RO
RO
OH
O
O
O
RO
RO
Si
Si
O
Si
OR
Si
OR
OR
RO
RO
O
RO
O
RO
RO
HO
Si
OR
Si
Si
O
Si
RO
OR
OR
O
OR
RO
Si
Si
O
OH
RO
Si
OR
OR
ORO
Si
O
RO
Si
OROR
Bressanone Sept. 2006
Gel Formation
8
4
Reaction Parameters: Influence of the Substituents
Steric and electronic influence of the OR group:
Si(OMe)4 > Si(OEt)4 > Si(OnPr)4 > Si(OiPr)4
Reaction rate
Electronic influence of organic substituents:
≡Si−R > ≡Si−OR > ≡Si−OH > ≡Si−O−Si≡
Electron density at silicon
⇒ acid catalysis:
base catalysis:
reaction rates decrease from left to right
reaction rates increase from left to right
Influence of the (CH2)n chain length
(RO)3SiCH2OC(O)C(Me)C=CH2 reacts ~60 times faster than
(RO)3Si(CH2)3OC(O)C(Me)C=CH2
Bressanone Sept. 2006
Gel Formation
9
Reaction Parameters: Influence of the Substituents
Electrophilicity of silicon
Consequences (I)
Reactivity in acidic medium
Si
OR
Si
RO
OR
>
<
Si
RO
OR
OR
OR
>
<
OR
Si
RO
OH
OH
Si
RO
OR
OR
OH
Si
>
<
OR
OH
RO
O
OH
OH
>
<
Si
RO
OR
OH
OH
>
<
Si
HO
OH
OH
Reactivity in basic medium
Bressanone Sept. 2006
Gel Formation
10
5
Reaction Parameters: Influence of the Substituents
Consequences (II)
Acidity of Si-OH groups
highest
lowest
OH
HO
Si
OR
O
RO
Si
O
OH
Si
O
Si
O
O
O
HO
Si
O
OR
O
Si
OH
O
O
O
NB: this is one of the reasons why the PZC changes with the degree of condensation.
Bressanone Sept. 2006
Gel Formation
11
Formation:
Growth Models
pHGel
Influence
on Network
Structure
Si
Consequences (IIIa)
OX + H+
Si
H
X
X = R, H
Y−OH +
O+
Si
O+
Si
H
Y−O −Si
+ HOX
X
Acidic conditions: Reaction at the least condensed Si atom
OH
HO
Si
RO
Si
O
O
O
Si
Si
O
O
HO
Condensation reaction
proceeds preferentially at
terminal Si-atoms
OR
O
Si
OH
O
O
OH
OR
Si
O
O
Polymeric network
O
Bressanone Sept. 2006
Gel Formation
12
6
Formation:
Growth Models
pHGel
Influence
on Network
Structure
Consequences (IIIb)
OY
Si-OX + YO−
Y = H,
−
Si
Si
YO- Si
+ XO-
OX
Alkaline conditions: Nucleophilic attack at the most condensed Si atoms
OH
HO
Si
O
RO
Si
O
O
O
Si
Si
O
O
HO
Condensation reaction
proceeds preferentially at
central Si-atoms
OR
Si
OH
O
O
OH
OR
Si
O
O
O
Colloidal network
Bressanone Sept. 2006
Gel Formation
13
GelInfluence
Formation:
Processing
pH
onSol-Gel
Network
Structure
Monomer
Dimer
Cyclic
Particle
Acidic
conditions
Basic
conditions
1 nm
5 nm
10 nm
30 nm
100 nm
Three-dimensional
gel networks
Bressanone Sept. 2006
Sols
Gel Formation
14
7
Gel Formation:
Processing
StöberSol-Gel
Process
O
EtOH H2O NH3
O
Si
O
O
or
O
O Si
O
O
Hydrolysis/
Condensation
EtOH
Monodisperse
particles
MeOH
Monodisperse silica spheres
(diameter of about 0.2 μm)
Bressanone Sept. 2006
Gel Formation
15
Reaction Parameters: Influence of the Substituents
Consequences (IV)
From 29Si NMR
(ethanolic solutions,
H2O/OEt = 1,
pH = 2)
The order is reverse
in base-catalyzed
systems
Bressanone Sept. 2006
Gel Formation
16
8
Other Important Reaction Parameters
The alkoxy group / H2O ratio (Rw)
Theoretically
Rw = 2 for
Rw = 1 for
Si(OR)4 → SiO2
Si(OR)4 → Si(OH)4 (without condensation)
In practice
•
lower Rw (more water) favors the formation of Si-OH over Si−O−Si
•
even with Rw >2 only mixtures of [SiOx(OH)y(OR)z]n (2x+y+z = 4) are
obtained
The solvent
•
•
Homogenization of the reaction mixture
Polar and especially protic solvents stabilize polar species such as
[Si(OR)x(OH)y]n
Bressanone Sept. 2006
Gel Formation
17
Outline
Silica-Based Gels
hydrolysis and condensation
gelation
network vs. cluster formation
two-component systems
Metal Oxide-Based Gels
difference between Si(OR)4 and M(OR)n
network vs. cluster formation
Non-hydrolytic Sol-Gel Processes
Bressanone Sept. 2006
Gel Formation
18
9
Gelation
Particle
Acidic
conditions
Basic
conditions
1 nm
5 nm
10 nm
30 nm
100 nm
Three-dimensional
gel networks
Sols
HO
HO
OH
OH
Si
O
HO
HO
O
O
Si
O
Si
O
O
Si
HO
Si
Si
O
OH
Si
O
O
OH
Si
O
HO
Si
O
Si
Si
O
O
O
O
Si
O
Si
O
HO
Si
Si
O
O
O
HO
Si
O
O
Si
OH
HO
OH
Bressanone Sept. 2006
OH
Gel Formation
19
Site Percolation on a Square Lattice
p = fraction of filled spheres
s = cluster size
Bressanone Sept. 2006
Gel Formation
20
10
Gel Kinetic
Formation:
Growth
Models
Growth
Models
Ballistic
Diffusion-limited
Cluster-Cluster
Monomer-Cluster
Reaction-limited
Bressanone Sept. 2006
Gel Formation
21
Gelation is not a Thermodynamic Event
viscosity
Viskosität
Konzentration
Si-O-Si
Si–O–Si
concentration
Gelpunkt
gel
point
Bressanone Sept. 2006
Gel Formation
Zeit
time
22
11
Consequences of Introducing Organic Substituents
• Reduced degree of crosslinking of the inorganic network
• Polarity changes (changes in hydrogen bonding)
• Reactivity change of the remaining alkoxide groups
(electronic and steric effect of the organic substituents)
These effects are an
inevitable consequence
of the organic modification
Bressanone Sept. 2006
Gel Formation
23
Outline
Silica-Based Gels
hydrolysis and condensation
gelation
network vs. cluster formation
two-component systems
Metal Oxide-Based Gels
difference between Si(OR)4 and M(OR)n
network vs. cluster formation
Non-hydrolytic Sol-Gel Processes
Bressanone Sept. 2006
Gel Formation
24
12
Silsesquioxanes (X = R) and Spherosilicates (X = O-)
Si(OR)4 + H2O (NR4OH) ⎯→ [SinO2.5n]nRSiX3 (X = Cl, OR') + H2O (organic solvent) ⎯→ RnSinO1.5n
X
X
X
O
X
O
O
X
Si
O
O
Si
Si
X
X
Si
Si
O O
Si
Si
X
Si
O
O
Si
X
Si
Si
Si
O
X
O O
Si
X
X
Si
X
O
O
Si
Si
X
OO
Si
O
Si
O
O
O
X
X
Si
X
X
Bressanone Sept. 2006
Si Si
OO
O
Si
X
X
O
O
Si
O
X
O
O
X
X
Si
O
O
O
O X
O
O
O
O
Gel Formation
25
Modification of Spherosilicates (X = O-)
-
O-
O
Si
-
O
Si
O
O
-O
O Si O
O Si O Si
O
O
Si
-
O
Si
-
O
O
Si
Si
O
Si
O
O
O
O
Si
Si
O
O
O
O
O-
O-
O
+ ClSiMe2(CH2CH=CH2)
O
O
Si
+ ClSiHMe2
Si
Si
O
O
Si
Si
O
Si
O
Si
8
Si
O
O
O
Si
Si
O
O
O
Si
O
O
Si
O
Si
Bressanone Sept. 2006
O
O
O
+8
Si
H
8
Si
O
Si
O
Si
O
O
O
Si
Si
O
O
O
O
Si
O
R O
O
Si
Si
O
O
8
Si
Gel Formation
26
13
Modification of Silsesquioxanes
Si O Si
O
O
O
O
Si O Si
Si O Si
O
O
O
Si O Si
HNO3
8
Si O Si
O
O
O
O
Si O Si
Si O Si
O
O
O
Si O Si
Pd/C
NO2
8
Si O Si
O
O
O
O
O Si
Si O Si
O
O
O
Si O Si
Si
NH2
8
Si O Si
O
O
O
O
O Si
Si O Si
O
O
O
Si O Si
Si
Si
OH
OH
+ CH2=CHCH2SiCl3
O
OH
O
O Si
Si O Si
O
O
O
Si O Si
Si
+ metal compounds
Bressanone Sept. 2006
Si O MLn
O
O
O
O
Si O Si
Si O Si
O
O
O
Si O Si
Gel Formation
27
Gel Permeation Chromatography on Hydrolyzed R‘Si(OR)3
Hydrolysis of
(RO)3Si
O
(Rw = 1.5; no catalyst)
O
Bressanone Sept. 2006
Gel Formation
28
14
Gel Permeation Chromatography on Hydrolyzed R‘Si(OR)3
Hydrolysis of
(RO)3Si
O
(Rw = 1.5)
O
Bressanone Sept. 2006
Gel Formation
29
Outline
Silica-Based Gels
hydrolysis and condensation
gelation
network vs. cluster formation
two-component systems
Metal Oxide-Based Gels
difference between Si(OR)4 and M(OR)n
network vs. cluster formation
Non-hydrolytic Sol-Gel Processes
Bressanone Sept. 2006
Gel Formation
30
15
Two-Component Systems
For example:
• Alkoxides of different metals (Si(OR)4 and Ti(OR)4)
• Substituted and unsubstituted alkoxides (Si(OR)4 and R’Si(OR)3)
Problem: Different hydolysis / condensation rates ⇒ phase separations
Possible solutions
• Pre-hydrolysis of faster reacting component
• Moderation of the reactivity of the faster reacting component
• Single-source precursours
Bressanone Sept. 2006
Gel Formation
31
Gel Two-Component
Formation: Sol-GelSystems
Processing
H2O
base
catalyst
SiO2
CH3Si(OCH3)3 + Si(OCH3)4
(1:4)
in NH4OH
Bressanone Sept. 2006
Gel Formation
32
16
Two-Component Systems
first stage
O
Ti(O Pr)3
A
O
A
BB
A BO B A
A
BB
A
A A
A
A
A
A BB
BO B A B B
B B A BO B A
BB
A
A
A
A
A
BB
BO B
BB
A
i
Bressanone Sept. 2006
A
A
(MeO)3Si
A
Gel Formation
33
Outline
Silica-Based Gels
hydrolysis and condensation
gelation
network vs. cluster formation
two-component systems
Metal Oxide-Based Gels
difference between Si(OR)4 and M(OR)n
network vs. cluster formation
Non-hydrolytic Sol-Gel Processes
Bressanone Sept. 2006
Gel Formation
34
17
Comparison of Reactivity of M(OR)x and Si(OR)4
Si(OiPr)4 <<< Sn(OiPr)4, Ti(OiPr)4 < Zr(OiPr)4 < Ce(OiPr)4
1. Metal alkoxides are stronger Lewis acids than silicon alkoxides
⇒ Nucleophilic attack is facilitated ⇒ strong increase of the hydrolysis rates.
Hydrolysis rate of Ti(OR)4 ≈ 105 times faster than that of Si(OR)4.
2. The preferred coordination number of metals is higher than their valency
Most metals have several stable coordination numbers, or the expansion of
the coordination sphere in transition states is easier.
!
Sol-gel processing of metal alkoxides does not require a
catalyst
Moderation of the reactivity of metal alkoxides
“Chemical additives” = replacement of one or more OR ligands by groups which are
less easily hydrolyzed and additionally block coordination sites at the metal.
Bressanone Sept. 2006
Gel Formation
35
Gelation of Metal Alkoxide Systems
Gelification diagrams of Zr(OPr)4
top: acetylacetone
bottom: ethyl acetoacetate
O
O
O
+ Zr(OPr) 4
R
Zr(OPr)
O
4-n
n
R = CH 3, OCH 2 CH 3
Bressanone Sept. 2006
Gel Formation
36
18
Gels from Metal Compounds: Role of the Counter-Ion
Strong metal-anion interactions (anion coordination) may influence sol–
gel processing
• The hydrolysis and condensation reactions may proceed differently when
different salts of the same metal are employed.
• Strong coordination of a counter-ion blocks coordination sites and leads to a
smaller degree of condensation (fewer M−O−M links per metal atom can be
formed).
• The counterions may (partially) stay in the material. Their (complete) removal may
be difficult and may require special post-synthesis procedures.
• Coordinated counter-ions can direct the site of nucleophilic attack (cis or trans to
the coordinated X, for example) during hydrolysis and condensation ⇒ influence on
the microstructure and morphology of the gels or precipitates.
• Non-coordinated counterions may influence the electrostatic double layer of the
sol particles and hence their aggregation.
Bressanone Sept. 2006
Gel Formation
37
Polycations and Polyanions
Examples:
[V10O28]6-
[M13O4(OH)24(H2O)12]7+ (M = Al, Ga, Fe)
“Al13”
[H2W12O42]10-
Bressanone Sept. 2006
Gel Formation
[Al13]2[Al4(OH)8(H2O)6]18+
38
19
Carboxylate-Substituted Metal Oxide Clusters
Ta4O4(OEt)8(OMc)4 *
Ti2Zr6O6(OMc)20
Ti2HfZr5O6(OMc)20
M6(OH)4O4(OMc)12
(M = Zr, Hf)
Ti4Y2O4(OMc)14(HOCH2CHOMe)2
Ti6O4(OEt)8(OMc)8
Ti4Zr2O4(OBu)6(OMc)10
[Zr6(OH)4O4(OAcr)12]2
OMc = methacrylate
Bressanone Sept. 2006
Gel Formation
39
Outline
Silica-Based Gels
hydrolysis and condensation
gelation
network vs. cluster formation
two-component systems
Metal Oxide-Based Gels
difference between Si(OR)4 and M(OR)n
network vs. cluster formation
Non-hydrolytic Sol-Gel Processes
Bressanone Sept. 2006
Gel Formation
40
20
Non-hydrolytic sol-gel
processing
Non-Hydrolytic
Sol-Gel
Processes
Reaction of a metal halide or semi-metal halide with a metal alkoxide under
non-aqueous conditions to form an inorganic oxide
M O R
M OR +
M X
M O M
+
R X
M X
Exchange
M X
+
M OR
X = Cl, acetate
The metal alkoxide may be generated by reaction of the metal halide with an ether
or an alcohol)
Bressanone Sept. 2006
Gel Formation
41
Non-hydrolytic
sol-gel
processing
Non-Hydrolytic
Sol-Gel
Processes:
Example I
Titania-Silica Gels
TiCl4 + Si(OiPr)4 ⎯→ SiO2/TiO2 + iPrCl
Ti(OiPr)4 + SiCl4 ⎯→ SiO2/TiO2 + iPrCl
Ti(OiPr)4 + Si(OOCCH3)4 ⎯→ SiO2/TiO2 + CH3COOiPr
TiCl4 + SiCl4 + iPr2O ⎯→ SiO2/TiO2 + iPrCl
( ≡M-Cl + R-O-R ⎯→ ≡M-OR + RCl)
Bressanone Sept. 2006
Gel Formation
42
21
Non-hydrolytic
sol-gel
processing
Non-Hydrolytic
Sol-Gel
Processes:
Example II
Organically Modified Silicas
Cl
CH3
H3C Si Cl + HO C CH3
Cl
CH3
Cl
CH3
H3C Si O C CH3 + HCl
Cl
CH3
Cl
Cl
CH3
CH3
H3C Si O Si O C CH3 + Cl C CH3
CH3
Cl CH3
CH3
CH3 CH3
Cl
CH3
H3C Si O C CH3 + Cl Si O C CH3
Cl
CH3
Cl
CH3
Cl
Cl
CH3
H3C Si O Si CH3 + Cl C CH3
Cl
CH3
Cl
H3C
Cl
Cl
H3C Si Cl + H3C C O Si CH3
H3C
Cl
Cl
Alkyl-silicon bonds are stable under non-hydrolytic conditions!
Bressanone Sept. 2006
Gel Formation
43
Non-hydrolytic
sol-gel processingSol-Gel Processes
Comparison of Hydrolytic
and Non-Hydrolytic
Hydrolytic
Non-hydrolytic
Facile, low-temperature process
Facile, low-temperature process
Homogenizing solvent may be
necessary
Potentially solvent free
Use of water
Reactive reagents (inert-gas
synthesis)
Water, alcohol are formed during
the reactions
Alkyl halides are formed
Good for ionic and O-containing
species
Potential problems with Ocontaining species
Limited compatibility of
hydrophobic species
Potentially phase separation in
two-component systems
No problem with hydrophobic
components
No phase separation
Well-established
New technology, good for watersensitive species
Bressanone Sept. 2006
Gel Formation
44
22
Non-hydrolytic sol-gel
processing
Non-Hydrolytic
Sol-Gel
Processes
Silsesquicarbodiimides
n MeSiCl3 + 1.5n Me3Si-N=C=N-SiMe3
pyridine
[MeSi(N=C=N)1.5]n + 3n Me3SiCl
Gel
Substitution
MeSiCl3 + Me3Si-N=C=N-SiMe3
MeCl2Si-N=C=N-SiMe3 + Me3SiCl
Condensation
MeCl2Si-N=C=N-SiMe3 + Cl Si
MeCl2Si-N=C=N-SiMe3
Bressanone Sept. 2006
MeCl2Si-N=C=N-Si
+ Me3SiCl
MeCl2Si-N=C=N-SiCl2Me + Me3Si-N=C=N-SiMe3
Gel Formation
45
Special Case: Formic Acid Route
Very rapid („schnell gels“) !
Bressanone Sept. 2006
Gel Formation
46
23