Sol-Gel Reactions

Sol-Gel Reactions

1. Chimie Douce methods

   • "Soft chemistry" methods are at low temps for making solid chemistry

2. Chimie Douce methods

   • "Soft Chemistry" methods are at low temps for making solid materials
   • Reactants aren't mixed on the atomic scale, but low temps can help with this. Initial low temp -> Firing at high temp.
   • Advantages
     • Higher purity
     • More uniform in texture and chemical homogeneity
   • Disadvantages
     • Reagents are expensive
     • Reaction conditions are specify to a given reaction and must be optimised

3. The most important Chimie Douce method

4. Sol => Colloidal suspension of particles in a liquid. Formed by dispersion of insoluble particles in a solvent, or reaction of precursor and solvent

5. Gel => Semi-rigid solid-solvent is entrapped in a network of colloidal/polymetric material

   • This is formed from sol by; Concentrating the solution (Evaporation), ageing or heating

6. The Sol-gels can then be supercritically extracted to form Aerogels (Low density) or evaporated to form Xerogels (High density)

7. Makes oxide materials through concentrated hydrolysis of metal alkoxides as precursors (Alumina, Titana)

8. Silica - Sol-gel reaction

   • Made for optical fibres
   • Hydrolysis of the liquid alkoxide (Si(OR)4) with water occurs in 2 steps
     • 1) Replace OR with OH, yielding Si(OH)4 and alcohol
       • Si(OMe)4 + 4H2O -> Si(OH)4 + 4MeOH
     • 2) Condensation polymerisation of Si(OH)4 forming Si-O-Si bonds
       • (RO)3Si-OH + HO-Si(OR)3 -> (RO)3Si-O-Si(OR)3
       • Condensation continues to form a sol, this is cast in a mold, and condensation continues, fibres dawn as gelation occurs.
         • Alcohol and water is removed by heating to 200-400 C
           • Gels are porous, so when hardening they yield nanoporous solids
             • The pores can be collapsed by annealing at high temps (.1200 C)

9. Mesoporous solids

   • Formed by templating surfactant phases (2nm-50nm)
   • Not naturally occuring
   • Templating => Scaffolding to direct formation of the solid. Template can be removed to yield desired product
   • Low temps mean that organic templates can be used

10. Lyotropic surfactant phases

    • Surfactants self-assemble in solution to give different structures (phases) according to structure and conc. of surfactant
    • Surfactant/Solvent systems are lyotropic surfactants that change phase due to temp. are Thermotropic