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Chimie douce

Sol-Gel Chemistry. Chimie douce. Chemists at the school of diatoms. diatoms are making silica glasses at room temperature from solute silica in water. Chimie douce. SiO 2 + 2H 2 O Si(OH) 4. Fe 2 O 3. Si(OH) 4. OH. Si. HO. OH. OH.

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Chimie douce

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  1. Sol-Gel Chemistry Chimie douce

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

  3. SiO2 + 2H2O Si(OH)4 Fe2O3 Si(OH)4 OH Si HO OH OH Silica from the soils is dissolved by water and goes to the sea silicic acid Si(OH)4 ≈ 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 OH Si HO OH OH 0

  5. Silicic acid is a weak acid Proton exchange between Si(OH)4 and the aqueous solvant protonation or deprotonation depending on pH OH OH OH Si Si Si O OH O OH HO OH OH O OH pH ≈ 3 OH- 2- - 0 deprotonation

  6. OH OH OH Si Si Si O OH O OH HO OH OH O OH OH Point of Zero Charge Silica is soluble at high pH silicates Si H2O OH OH SiO2 precipitation of silica Silicic acid is a weak acid Proton exchange between Si(OH)4 and the aqueous solvant protonation or deprotonation depending on pH H+ pH ≈ 3 OH- 2- - + 0

  7. [Si(OH)3(OH2)]+ [Si(OH)4]0 [SiO(OH)3]- [SiO2(OH)2]2- 2 9,9 13 100% 80 Si(OH) 4 - SiO(OH) 3 60 2- SiO (OH) 2 2 40 20 pH 6 8 10 12 pH Precipitation of silica

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

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

  10. 10 Magic Rocks

  11. Biogenic synthesis of silica by diatoms OH SiO2 + 2H2O Si(OH)4 Si(OH)4 OH Solute silica = Si(OH)4 HO HO silicic acid 7 Si ≈ mg/l Condensation Si - OH + HO - Si Si - O - Si + H2O Si(OH)4 SiO2 + 2H2O

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

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

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

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

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

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

  18. Si(OR)4 + 2H2O Si(OH)4 + 4ROH SiO2 + 2H2O hydrolysis condensation PZC acid catalysis base catalysis The chemical reactivity of silicon alkoxides is very low small positive charge d+ of the cation electronegativity coordination expansion difficult acid and base catalysis V depends on the pH of water

  19. Si(OEt)4 + 2H2O SiO2 + 4EtOH Catalyst pH Tg (h) nothing 7 1000 HF 2 12 HCl 0 92 AcOH 3,7 72 NH3 10 107 no catalyst ≈ 103 hours acid or base ≈ 102 hours bio-silicification ≈ 1 hour c ≈ 1 mole/l c ≈ 10-3 mole/l 20 Silica gel formation

  20. ROH d- H+ OR RO - Si - OR OR OR OH- d+ RO - Si - OR Si-O- OR Acid catalysis (pH < 3) protonation of Si-OH or Si-OR that become better leaving groups H+ > H2O Base catalysis (pH > 3) 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

  21. dSidOH A +0.50 -0.06 B +0.58 +0.06 C +0.54 0.00 H O A HO OH OH H H O O C B HO A OH A B HO O OH O H O H OH H O HO H H+ H+ H+ toward the most negative Si-OHd- chain polymers Acid catalysis pH < 3 Partial charges

  22. dSidOH A +0.50 -0.06 B+0.58 +0.06 C +0.54 0.00 H O A HO OH OH H H O O C B HO A OH A B HO O OH O H O H H O H OH- toward the most positive Sid+ - OH branched polymers Base catalysis pH > 3 Partial charges OH- OH-

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

  24. fibres nanoparticles

  25. hydrolysis condensation Si(OR)4 + 4H2O Si(OH)4 + 4ROH SiO2 + 2H2O fast hydrolysis Gel chain polymers pH < 3 microporous gels (pores < 20Å) Sol (1-2 nm) fast condensation spherical particles (Stöber silica) pH > 3 mesoporous gels (pores > 20Å) Sol (10-100 nm) Catalysis controls the shape of silica particles Acid catalysis (pH < 3) Base catalysis (pH >3)

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

  27. Stöber silica monodispersed silica colloids

  28. Si(OH)4 SiO2 + 2H2O 30 hydrated silica - SiO2,nH2O Si-OH

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

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

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

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

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

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

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

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

  37. coordination saturation slowly hydrolyzable complexing ligands

  38. 40 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 ?

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

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

  41. silaffin silaffin Bio-synthesis of silica by diatoms 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

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

  43. 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

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

  45. HF 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

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

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

  48. 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

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