Chimie douce

Sol-Gel Chemistry

diatoms are making silica glasses

at room temperature

from solute silica in water

Chemists at the school of diatoms

Chimie douce

Si(OH)4

Fe2O3

Silica from the soils is dissolved by water and goes to the sea

silicic acid Si(OH)4

Si

OH

HO OHOH

SiO2 + 2H2O Si(OH)4

≈ mg/l

Si

OH

HO OHOH

0

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 OHOH

Si

OH

O OHOH

Si

OH

O OHO

OH-

0- 2-

pH ≈ 3

deprotonation

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 OHOH

Si

OH

O OHOH

Si

OH

O OHO

Si

OH

H2O OHOH

OH-H+

+ 0- 2-

pH ≈ 3

Point of Zero Charge

SiO2

precipitation of silica

Proton exchange between Si(OH)4 and the aqueous solvant

protonation or deprotonation depending on pH

Silica is soluble

at high pH

silicates

Silicic acid is a weak acid

Si(OH)4SiO(OH)3-

SiO2(OH)22-

6 8 10 12 pH

20

40

60

80

100%

[Si(OH)3(OH2)]+ [Si(OH)4]0 [SiO(OH)3]- [SiO2(OH)2]2-

2 9,9 13pH

Precipitation of silica

Precipitation of silica via the acidification of an aqueous solution of silicate

Silica gardens

Na2O.SiO2

water glass

H+

Precipitated silica Industrial product : charge, chromatography, ….

Silicate gardens

Aqueous solution of Na2SiO3

pH ≈ 12

Metal salt CuSO4

FeCl3 Ni(NO3)2

Magic Rocks10

Si(OH)4 SiO2 + 2H2O

Condensation

Si - OH + HO - Si Si - O - Si + H2O

Solute silica = Si(OH)4

silicic acidOH

OHHOHO

Si≈ mg/l

Biogenic synthesis of silica by diatoms

Si(OH)4SiO2 + 2H2O Si(OH)4

7

R = CH3, C2H5, ...

Synthesis of silica by chemists

Silicon alkoxide = Si(OR)4

OR

ORRORO

SiMolecular precursor

Si - OR + HO-H Si-OH + ROH

Hydrolysisalkoxide

water

OH

OHHOHO

Si

Condensation

Si(OR)4 + 2H2O Si(OH)4 + 4ROH

Si - OH + HO - Si Si - O - Si + H2O

M+ + X-

MX

Polycondensation ≠ precipitation

Si-OH + HO-Si

Si-O-Si +

H2O

monomer dimer

trimer

tetramer

particle

silicate silica

[SiO4] SiO2

inorganic polymerization

nm 10 nmmolecules oligomers colloïds powderm

Colloidal silica particles

drying

Two basic reactions

Hydrolysis Si - OR + HOH Si - OH + ROH

CondensationSi - OH + HO - Si Si - O - Si + H2O

Si - OR + HO - Si Si - O - Si + ROH

Nucleophilic substitution SN2

Hydrolysis of silicon alkoxides Si(OR)4

Si - OR + HO - H Si - OH + ROH

coordination 5

O

Si

OR

RO OROR

H H

Si

OH

RO OROR

+ ROHSi

OR

RO OROR

OH

-

H

Si

OR

RO OROR

OH

H

Nucleophilic substitution SN2

Polycondensation of silicon alkoxides Si(OR)4

Si - OR + HO - Si Si - O - Si + ROH

Si

O

O OHO

Si

OR

O OO

Si

O

OO

Si

O

O OO

coordination 5

+ ROH

Si

OR

O OO

Si

O

O OO

H

-

Si

OR

O OO

Si

O

O OO

H

base catalysisacid catalysis

PZC

acid and base catalysis

V depends on the pH of water

The chemical reactivity of silicon alkoxides is very low

Si(OR)4 + 2H2O Si(OH)4 + 4ROH SiO2 + 2H2Ohydrolysis condensation

small positive charge + of the cation

electronegativity

coordination expansion difficult

Silica gel formation

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

Acid catalysis (pH < 3)

H+

RO - Si - OR

OR

OR

ROH-

protonation of Si-OH or Si-OR that become better leaving groups

Base catalysis (pH > 3)

RO - Si - OR

OR

OR

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

Acid catalysis pH < 3

Si OH

A +0.50 -0.06

B +0.58 +0.06

C +0.54 0.00

Partial charges

chain polymers

H+ toward the most negative Si-OH

HO

HO

HOOH

HO

HO

HO

HO

OH

OH

HO

HO

A

C

B

A

A

B

HO

OH

H+ H+

OHHO

Base catalysis pH > 3

Si OH

A +0.50 -0.06

B +0.58 +0.06

C +0.54 0.00

branched polymers

OH- toward the most positive Si+ - OH

Partial chargesHO

HO

HOOH

HO

HO

HO

HO

OH

OH

HO

HO

A

C

B

A

A

B

HO

OH

OH-

OH-

Catalysis

RO - Si - O - Si - O - Si - OR

O

Si

OR

RO

Si

O

OR

H+SiO-

Acid catalysis Base catalysis

end groups midle Si

chain polymers branched polymers

catalysis speeds up the reactions

And controls the shape of the silica particles

nanoparticles

fibres

Acid catalysis (pH < 3)

fast hydrolysis

chain polymers

microporous gels (pores < 20Å)

Base catalysis (pH >3)

fast condensation

spherical particles (Stöber silica)

mesoporous gels (pores > 20Å)

Si(OR)4 + 4H2O Si(OH)4 + 4ROH SiO2 + 2H2Ohydrolysis condensation

Catalysis controls the shape of silica particles

Sol (1-2 nm)

Gel

pH > 3

Sol(10-100 nm)

pH < 3

Colloidal silica in diatoms

Girdle bands

Raphe

20 m

500 nm 150 nm

Silica walls are build up from ca. 5nm particles to give ca. 40nm diameter particles that are organised within the frustule.

pH ≈ 5

Stöber silica

monodispersed silica colloids

hydrated silica - SiO2,nH2O

Si-OH

Si(OH)4 SiO2 + 2H2O

30

- Si - O - Si - O - Si -

H HO

- Si - O - Si - O - Si -

HOH

- Si - O - Si - O - Si -

H HO

water- silica interface1. Adsorption - dissociation

2. Acid ionisation

Si - OH + H2O

Si - O- + H3O+

Some definitions

Colloid = small solid particle (diameter < 0,1 m)

Sol or colloidal solution = suspension of colloidal particles in a solvent

gravity

Brownian motion

Brownian motion > gravity

interactions between particles increase with concentration

Percolation

sol-gel transition

Sol = solid colloidal particles dispersed in a solvent

Gel = solvent trapped within a particleframework

Sols and gels

colloidal solutions are not stable

collision aggregation flocculation

Small particles tend to aggregate

water- silica interface

3. Electrostatic stabilisation the surface is negatively charged

-

-

- -

-

- -

-

- -

-

-

≠

Stabilisation by surface charges = peptisation

H+ +

+ +

++

Electrostatic repulsion

+

+ +

++

+

+ +

++

+

+ +

++

Stabilisation of sols

Stabilisation by steric hindrance

Grafted polymers

Si

Ti

Transition metal alkoxides are highly reactive toward hydrolysis and condensation

electronegativity

Si(OPri)4

Si4+ = 0,40 Å

= 1,74

Ti(OPri)4

Ti4+ = 0,64Å

= 1,32

Speed up gelation via catalysis Slow down the reaction via complexation

[SiO4] [TiO6]

SiO2 gelation takes several days Fast precipitation of TiO2

SiO2 TiO2

TiSi

Coordination expansion is easy

iOPrOPriH C3OPri OOCH CCH C3TiOPriPriOOPriOPriTiOOC3OOCOPriOPriPriOPriOPriOTiTiPriO3CHCHacetic acidacetylacetone

coordination saturationslowly hydrolyzable complexing ligands

Questions and Answers

which genes are involved in the formation of bio-silica ?

which proteins control the formation of silica and how ?

can we mimic nature and make silica in similar conditions ?

Genetics

Biology

Chemistry

Biogenic Silica40

GlycineLysineProlineSerine….

2. Check their activity toward the condensation of silica

The biologist approach

1. Extraction and characterisation of proteins associated with biosilica

Sponge spicules Silicatein

D. Morse - Santa Barbara - USA

Diatom frustules Silaffins

N. Kröger et M. Sumper : Regensburg -Germany

Diatom frustules

Silaffins

N. Kröger et M. Sumper : Regensburg -Germany

Manfred Sumper

cationic polypeptides

interactions with negatively charged silica

silaffin

silaffin

Proteins involved in the formation of the silica shell

Silaffins

dissolution of silica frustules in HF

N. Kröger, M. Sumper, J. Bio. Chem. 276 (2001) 26066

Bio-synthesis of silica by diatoms

Silaffin = cationic polypeptide

two ‘lysine’ groups linked to long chain polyamines

Catalytic activity due to these lysines

Precipitation of silica with silaffins

Spherical nanoparticles

Coprecipitation of silaffins with silica

SiO2/ silaffin ≈ 12

N. Kröger, M. Sumper, J. Bio. Chem. 276 (2001) 26066

silaffins catalysts for the condensation of silicic acid

1A1

1A2

pH = 6,4

Sponge spicules

Silicatein

D. Morse - G. Stucky - Santa Barbara - USA

Spicules are formed

around organic filaments that behave as

templates and catalysts

Spicules of Tehya aurantia

HF

1. Sponges spicules

D. Morse et al. PNAS 95 (1998) 6234

Silicatein

disulfur bridges

serine histidine

active site

Strong relation of amino-acid sequence between

Silicatein and Cathepsin (hydrolase)

Silicatein and Cathepsine L

2 of the 3 amino-acids of the active site are the same

Serine-26 and Histidin-165

Silicatein filament

precipitation of silica

Cellulose filamentNo reaction with TEOS

Formation of silica from TEOS

before

after

G. Stucky, D. Morse, PNAS 96 (1999) 361

50

Catalytic mechanism

Role of the serine-histidine couple

Serine-26

nucleophilic substitution

Histidine-165

nucleophilic activationpentavalent 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-

+H3N COO-

NH3+

LysineArginine NH3

+

COO-

NH

H2N

NH2+

HONH3

+

COO-

Serine

Precipitation of silica in the presence of amino-acids and peptides

dilute aqueous solution

of silica

pH ≈ 7

peptides

Chemical titration number of Si(OH)4 monomers

pH5.4 6.3 7.2 8.3

Nanoparticles of silica precipitated

from silicic acid in the presence of polyamines

M. Sumper et al. Nano Letters 2 (2002) 91

penta propylene hexamine

Precipitation speed increases with ‘n’

Amino-acids and peptides

Chain length

Side groups Small effect

+H3N COO-

NH3+

LysineArginine NH3

+

COO-

NH

H2N

NH2+

HONH3

+

COO-

Serine

Poly-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

Silica condensation in the presence of peptides

+H3N COO-

NH3+

Lysine