Stöber process

It was pioneering[3] when it was reported by Werner Stöber and his team in 1968,[1] and remains today the most widely used wet chemistry synthetic approach to silica nanoparticles.

[3] It is an example of a sol-gel process wherein a molecular precursor (typically tetraethylorthosilicate) is first reacted with water in an alcoholic solution, the resulting molecules then joining together to build larger structures.

[5][6] The newly acquired understanding has enabled researchers to exert a high degree of control over particle size and distribution and to fine-tune the physical properties of the resulting material in order to suit intended applications.

[7] Development work has also been undertaken for larger pore structures such as macroporous monoliths,[10] shell-core particles based on polystyrene,[11] cyclen,[12] or polyamines,[13] and carbon spheres.

[14] Silica produced using the Stöber process is an ideal material to serve as a model for studying colloid phenomena[15] because of the monodispersity (uniformity) of its particle sizes.

[1] The process, an evolution and extension of research described in Gerhard Kolbe's 1956 PhD dissertation,[24] was an innovative discovery that still has wide applications more than 50 years later.

[2] In 1999 Cédric Boissière and his team developed a two-step process whereby the hydrolysis at low pH (1 – 4) is completed before the condensation reaction is initiated by the addition of sodium fluoride (NaF).

[8] The main advantage of sequencing the hydrolysis and condensation reactions is the ability to ensure complete homogeneity of the surfactant and the precursor TEOS mixture.

[7] The two-step Stöber process begins with a mixture of TEOS, water, alcohol, and a nonionic surfactant, to which hydrochloric acid is added to produce a microemulsion.

[8] Porosity in the modified process is controllable through the introduction of a swelling agent, the choice of temperature, and the quantity of sodium fluoride catalyst added.

[30] However, experimental evidence demonstrates that the concentration of hydrolyzed TEOS stays above that required for nucleation until late into the reaction, and the introduction of seeded growth nuclei does not match the kinetics of a monomer addition process.

The total interaction energy is dependent on three forces: electrostatic repulsion of like charges, van der Waals attraction between particles, and the effects of solvation.

This model for controlled growth aggregation fits with experimental observations from small-angle X-ray scattering techniques[33] and accurately predicts particle sizing based on initial conditions.

The one-step Stöber process may be modified to manufacture porous silica by adding a surfactant template to the reaction mixture and calcining the resulting particles.

[39] The surfactant forms micelles, small near-spherical balls with a hydrophobic interior and a hydrophilic surface, around which the silica network grows, producing particles with surfactant- and solvent-filled channels.

[40] Calcining the solid leads to removal of the surfactant and solvent molecules by combustion and/or evaporation, leaving mesopore voids throughout the structure, as seen in the illustration at right.

[37] The addition of polyethylene glycol (PEG) to the process causes silica particles to aggregate into a macroporous continuous block, allowing access to a monolithic morphology.

[14] Unlike the silica-based Stöber process, this reaction is completed at neutral pH and ammonia has a role in stabilizing the individual carbon particles by preventing self-adhesion and aggregation, as well as acting as a catalyst.

[41] One major advantage of the Stöber process is that it can produce silica particles that are nearly monodisperse,[16] and thus provides an ideal model for use in studying colloidal phenomena.

The process provides a convenient approach to preparing silica nanoparticles for applications including intracellular drug delivery[17] and biosensing.

[20] In addition to monodispersity, these materials have very large surface areas as well as uniform, tunable, and highly ordered pore structures,[20] which makes mesoporous silica uniquely attractive for these applications.Aerogels are highly porous ultralight materials in which the liquid component of a gel has been replaced with a gas,[44] and are noteworthy for being solids that are extremely effective thermal insulators[43][45] with very low density.

[45] Particulate gels prepared by the Stöber process can be dehydrated rapidly to produce highly effective silica aerogels, as well as xerogels.

Silica aerogels held 15 entries for materials properties in the Guinness World Records in 2011, including for best insulator and lowest-density solid, though aerographite took the latter title in 2012.