1. Sol-Gel Chemistry
Chimie douce

2. diatoms are making silica glasses from solute silica in water
Chemists at the school of diatoms
diatoms are making silica glasses
at room temperature
from solute silica in water
Chimie douce

3. Silica from the soils is dissolved by water and goes to the sea
Si(OH)4
Fe2O3
SiO H2O Si(OH)4
silicic acid Si(OH)4 Si OH HO
≈ mg/l

4. Silicic acid is a weak acid
Proton exchange between Si(OH)4 and the aqueous solvant
protonation or deprotonation depending on pH
Silicic acid is a weak acid Si OH HO

5. - Silicic acid is a weak acid OH OH OH Si Si Si O HO O
Proton exchange between Si(OH)4 and the aqueous solvant
protonation or deprotonation depending on pH
Silicic acid is a weak acid
pH ≈ 3
OH- - 2- Si OH O Si OH O Si OH HO
deprotonation

6. - Silicic acid is a weak acid + OH Si H2O HO O
Proton exchange between Si(OH)4 and the aqueous solvant
protonation or deprotonation depending on pH H+
pH ≈ 3
OH- Si OH HO O
H2O - 2- +
Point of Zero Charge
SiO2
precipitation of silica
Silica is soluble
at high pH
silicates

7. Precipitation of silica
[Si(OH)3(OH2)] [Si(OH)4] [SiO(OH)3] [SiO2(OH)2]2- , pH
Precipitation of silica
Si(OH) 4
SiO(OH) 3 -
SiO 2
(OH) 2- 6 8 10 12 pH 20 40 60 80
100%

8. Precipitation of silica via the acidification
Na2O.SiO2
water glass H+
Precipitation of silica via the acidification
of an aqueous solution of silicate
Precipitated silica
Industrial product : charge, chromatography, ….
Silica gardens

9. Aqueous solution of Na2SiO3
Silicate gardens
Aqueous solution of Na2SiO3
pH ≈ 12
Metal salt
CuSO4
FeCl3
Ni(NO3)2

10. 10
Magic Rocks

12. Biogenic synthesis of silica by diatoms
Solute silica = Si(OH)4
silicic acid OH HO Si
≈ mg/l
Biogenic synthesis of silica by diatoms
Si(OH)4
SiO H2O Si(OH)4 7
Si(OH) SiO H2O
Condensation
Si - OH + HO - Si Si - O - Si + H2O

13. Synthesis of silica by chemists
R = CH3, C2H5, ...
Synthesis of silica by chemists
Silicon alkoxide = Si(OR)4 OR RO Si
Molecular precursor
Si - OR + HO-H Si-OH + ROH
Hydrolysis
alkoxide
water OH HO Si
Condensation
Si(OR)4 + 2H2O Si(OH)4 + 4ROH
Si - OH + HO - Si Si - O - Si + H2O

14. Polycondensation ≠ precipitation
monomer
dimer
trimer
tetramer
particle
Si-OH + HO-Si
Si-O-Si +
H2O
M+ + X- MX

15. inorganic polymerization
silicate
silica
[SiO4]
SiO2
inorganic polymerization
drying
molecules
oligomers
colloïds
powder mm nm
10 nm
Colloidal silica particles

16. Two basic reactions Hydrolysis Condensation
Si - OR + HOH Si - OH + ROH
Condensation
Si - OH + HO - Si Si - O - Si + H2O
Si - OR + HO - Si Si - O - Si + ROH

17. Hydrolysis of silicon alkoxides Si(OR)4
Si - OR + HO - H Si - OH + ROH
Nucleophilic substitution SN2
coordination 5 O d- Si OR RO d+ H OH
+ ROH

18. Polycondensation of silicon alkoxides Si(OR)4
Si - OR + HO - Si Si - O - Si + ROH
Nucleophilic substitution SN2 Si O OH d- OR d+
coordination 5
+ ROH H

19. The chemical reactivity of silicon alkoxides is very low
Si(OR) H2O Si(OH) ROH SiO H2O
hydrolysis
condensation
small positive charge d+ of the cation
base catalysis
acid catalysis
PZC
electronegativity
coordination expansion difficult
acid and base catalysis
V depends on the pH of water

20. Silica gel formation no catalyst ≈ 103 hours acid or base ≈ 102 hours20
Silica gel formation
Si(OEt) H2O SiO2 + 4EtOH
Catalyst pH Tg (h)
nothing HF
HCl
AcOH 3,7 72 NH
no catalyst ≈ 103 hours
acid or base ≈ 102 hours
bio-silicification ≈ 1 hour
c ≈ 1 mole/l
c ≈ 10-3 mole/l

21. Acid catalysis (pH < 3)
RO - Si - OR OR
ROH d-
protonation of Si-OH or Si-OR
that become better leaving groups
H+ > H2O
Base catalysis (pH > 3)
RO - Si - OR OR d+
OH-
Si-O-
OH- and Si-O- better nucleophile
than H2O or Si-OH
catalysis does not only speed up the reactions
it also controls the shape of the silica particles

22. Acid catalysis pH < 3 Partial charges dSi dOH A +0.50 -0.06HO OH A C B H+
Partial charges
dSi dOH A B C
chain polymers
H+ toward the most negative Si-OHd-

23. Base catalysis pH > 3 Partial charges OH- dSi dOH A +0.50 -0.06HO OH A C B
Partial charges
OH-
dSi dOH A B C
OH-
branched polymers
OH- toward the most positive Sid+ - OH

24. Catalysis catalysis speeds up the reactions
And controls the shape of the silica particles
RO - Si - O - Si - O - Si - OR O Si OR RO H+
SiO-
Acid catalysis
Base catalysis
end groups
midle Si
chain polymers
branched polymers

25. nanoparticles
fibres

27. Catalysis controls the shape of silica particles
Si(OR)4 + 4H2O Si(OH)4 + 4ROH SiO2 + 2H2O
hydrolysis
condensation
Acid catalysis (pH < 3)
Sol
(1-2 nm)
Gel
pH > 3
( nm)
pH < 3
fast hydrolysis
chain polymers
microporous gels (pores < 20Å)
Base catalysis (pH >3)
fast condensation
spherical particles (Stöber silica)
mesoporous gels (pores > 20Å)

28. Colloidal silica in diatoms
Girdle bands
Raphe
20 m
500 nm
150 nm
pH ≈ 5
Silica walls are build up from ca. 5nm particles to give ca. 40nm diameter particles that are organised within the frustule.

29. Stöber silica
monodispersed silica colloids

30. hydrated silica - SiO2,nH2O30
hydrated silica - SiO2,nH2O
Si(OH)4 SiO H2O
Si-OH

31. water- silica interface
1. Adsorption - dissociation
- Si - O - Si - O - Si -
H H O H
2. Acid ionisation
Si - OH + H2O
Si - O- + H3O+

32. Some definitions Colloid = small solid particle (diameter < 0,1 mm)
Sol or colloidal solution = suspension of colloidal particles in a solvent
gravity
Brownian motion
Brownian motion > gravity

33. interactions between particles increase with concentration
Sols and gels
interactions between particles increase with concentration
Percolation
sol-gel transition
Sol = solid colloidal particles dispersed in a solvent
Gel = solvent trapped within a particleframework

34. colloidal solutions are not stable
Small particles tend to aggregate
collision aggregation flocculation

35. ≠ water- silica interface - 3. Electrostatic stabilisation
the surface is negatively charged - ≠

36. Stabilisation by surface charges = peptisation
Stabilisation of sols +
Stabilisation by surface charges = peptisation + H+
Electrostatic repulsion
Stabilisation by steric hindrance
Grafted polymers

37. Transition metal alkoxides are highly reactive
toward hydrolysis and condensation Si Ti
electronegativity

38. Coordination expansion is easy
Si(OPri)4
Si4+ = 0,40 Å
c = 1,74
Ti(OPri)4
Ti4+ = 0,64Å
c = 1,32 Ti Si
SiO2
TiO2
[SiO4]
[TiO6]
SiO2 gelation takes several days
Fast precipitation of TiO2
Speed up gelation via catalysis
Slow down the reaction via complexation

39. coordination saturation slowly hydrolyzable complexing ligands

40. Biogenic Silica Questions and Answers Genetics40
Biogenic Silica
Questions and Answers
Genetics
which genes are involved in the formation of bio-silica ?
Biology
which proteins control the formation of silica and how ?
Chemistry
can we mimic nature and make silica in similar conditions ?

41. The biologist approach
1. Extraction and characterisation of proteins associated with biosilica
Diatom frustules
Silaffins
N. Kröger et M. Sumper : Regensburg -Germany
Glycine
Lysine
Proline
Serine ….
Sponge spicules
Silicatein
D. Morse - Santa Barbara - USA
2. Check their activity toward the condensation of silica

42. Diatom frustules Silaffins Manfred Sumper
N. Kröger et M. Sumper : Regensburg -Germany
Manfred Sumper
Silaffins

43. cationic polypeptides interactions with negatively charged silica
Bio-synthesis of silica by diatoms
silaffin
dissolution of silica frustules in HF
Silaffins
Proteins involved in the formation of the silica shell
cationic polypeptides
interactions with negatively charged silica
N. Kröger, M. Sumper, J. Bio. Chem. 276 (2001) 26066

44. Silaffin = cationic polypeptide
two ‘lysine’ groups linked to long chain polyamines
Catalytic activity due to these lysines

45. Precipitation of silica with silaffins
N. Kröger, M. Sumper, J. Bio. Chem. 276 (2001) 26066
Coprecipitation of silaffins with silica
SiO2/ silaffin ≈ 12
1A1
pH = 6,4
silaffins catalysts for
the condensation of silicic acid
1A2
Spherical nanoparticles

46. Sponge spicules
D. Morse - G. Stucky - Santa Barbara - USA
Silicatein

47. around organic filaments that behave as templates and catalysts
1. Sponges spicules
D. Morse et al. PNAS 95 (1998) 6234
Spicules are formed
around organic filaments that behave as
templates and catalysts
Spicules of Tehya aurantia HF

48. Silicatein Strong relation of amino-acid sequence between
disulfur bridges
serine
histidine
active site
Strong relation of amino-acid sequence between
Silicatein and Cathepsin (hydrolase)

49. 2 of the 3 amino-acids of the active site are the same
Silicatein and Cathepsine L
2 of the 3 amino-acids of the active site are the same
Serine-26 and Histidin-165

50. Formation of silica from TEOS
after
G. Stucky, D. Morse, PNAS 96 (1999) 361
Silicatein filament
before
precipitation of silica
Cellulose filament
No reaction with TEOS 50

51. nucleophilic activation
Catalytic mechanism
Role of the serine-histidine couple
Serine-26
nucleophilic substitution
Histidine-165
nucleophilic activation
pentavalent Si

53. -O-Si- The chemist approach catalytic role of amino-acids silica
[Si(OH)4]0 + [SiO(OH)3] (HO)3Si-O-Si(OH)3 + OH-
Species in aqueous solutions at pH ≈ 7
amino acids
-OOC NH3+
Interactions between silica species and amino-acids
Electrostatic interactions
Hydrogen bonds
-NH3+
-O-Si-
-COO-
HO-Si-

54. dilute aqueous solution
Precipitation of silica in the presence of amino-acids and peptides
dilute aqueous solution
of silica
pH ≈ 7
peptides + H 3 N
COO - NH
Lysine
Arginine 2 HO
Serine
Chemical titration number of Si(OH)4 monomers

55. pH
5.4
6.3
7.2
8.3

56. Nanoparticles of silica precipitated
from silicic acid in the presence of polyamines
M. Sumper et al. Nano Letters 2 (2002) 91
penta propylene hexamine

57. Amino-acids and peptides+ H 3 N
COO - NH
Lysine
Arginine 2 HO
Serine
Chain length
Side groups
Small effect
Precipitation speed increases with ‘n’
Poly-Lysine

58. -O-Si-(OH)3 Silica condensation in the presence of peptides -COO-+ H 3 N
COO - NH
Lysine
-COO-
-NH3+
-O-Si-(OH)3
HO-Si(OH)3
[Si(OH)4]0 and [SiO(OH)3]-
Silica precursors are attracted by the peptide chain
They come close together and can react