Overview
The Stöber process is a chemical process used to prepare silica () particles
One-step process
and condensation of TEOS in the Stöber process The Stöber process is a sol-gel approach to preparing monodisperse (uniform) spherical silica () materials that was developed by a team led by Werner Stöber and reported in 1968. The process, an evolution and extension of research described in Gerhard Kolbe's 1956 PhD dissertation, was an innovative discovery that still has wide applications more than 50 years later. : Si(OEt)4 + H2O -> Si(OEt)3OH + EtOH : Si(OEt)4 + 2H2O -> Si(OEt)2(OH)2 + 2EtOH The reaction produces ethanol and a mixture of ethoxysilanols (such as , , and even ), which can then condense with either TEOS or another silanol with loss of alcohol or water: Larger particles are formed when the concentrations of water and ammonia are raised, but with a consequent broadening of the particle-size distribution.
The initial concentration of TEOS is inversely proportional to the size of the resulting particles; thus, higher concentrations on average lead to smaller particles due to the greater number of nucleation sites, but with a greater spread of sizes. Particles with irregular shapes can result when the initial precursor concentration is too high. The process is temperature-dependent, with cooling (and hence slower reaction rates) leading to a monotonic increase in average particle size, but control distribution cannot be maintained at overly low temperatures.
Two-step process
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). The two-step procedure includes the addition of a nonionic surfactant template to ultimately produce mesoporous silica particles. 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. Consequently, the diameter and shape of the product particles as well as the pore size are determined solely by the reaction kinetics and the quantity of sodium fluoride introduced; higher relative fluoride levels produces a greater number of nucleation sites and hence smaller particles.
This solution is allowed to stand until hydrolysis is complete, much like in the one-step Stöber process but with the hydrochloric acid replacing the ammonia as catalyst. Sodium fluoride is added to the resulting homogeneous solution, initiating the condensation reaction by acting as nucleation seed.
Kinetics
The LaMer model for the kinetics of the formation of hydrosols is widely applicable for production of monodisperse systems, and it was originally hypothesized that the Stöber process followed this monomer addition model. This model includes a rapid burst of nucleation forming all of the particle growth sites, then proceeds with hydrolysis as the rate-limiting step for condensation of triethylsilanol monomers to the nucleation sites. The production of monodisperse particle sizes is attributed to monomer addition happening at a slower rate on larger particles as a consequence of diffusion-limited mass transfer of TEOS. 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.
Consequently, the LaMer model has been rejected in favour of a kinetic model based around growth via particle aggregation. Under an aggregation-based model, nucleation sites are continually being generated and absorbed where the merging leads to particle growth. The generation of the nucleation sites and the interaction energy between merging particles dictates the overall kinetics of the reaction. The generation of the nucleation sites follows the equation below: and accurately predicts particle sizing based on initial conditions. In addition, experimental data from techniques including microgravity analysis and variable pH analysis agree with predictions from the aggregate growth model.
Morphological variations
Several different structural and compositional motifs can be prepared using the Stöber process by the addition of chemical compounds to the reaction mixture. These additives can interact with the silica through chemical and/or physical means either during or after the reaction, leading to substantial changes in morphology of the silica particles.
Mesoporous silica
image of a nanoparticle of mesoporous silica 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. Surfactants that have been used include cetrimonium bromide, cetyltrimethylammonium chloride, and glycerol. 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. Varying the surfactant concentration allows control over the diameter and volume of pores, and thus of the surface area of the product material. Increasing the amount of surfactant leads to increases in total pore volume and hence particle surface area, but with individual pore diameters remaining unchanged.
Altering the pore diameter can be achieved by varying the amount of ammonia used relative to surfactant concentration; additional ammonia leads to pores with greater diameters, but with a corresponding decrease in total pore volume and particle surface area. The time allowed for the reaction to proceed also influences porosity, with greater reaction times leading to increases in total pore volume and particle surface area. Longer reaction times also lead to increases in overall silica particle size and related decreases in the uniformity of the size distribution.
Macroporous monolith
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. PEG polymers with allyl or silyl end groups with a molecular weight of greater than 2000 g⋅mol −1 are required. The Stöber process is initiated under neutral pH conditions, so that the PEG polymers will congregate around the outside of the growing particles, providing stabilization. Once the aggregates are sufficiently large, the PEG-stabilized particles will contact and irreversibly fuse together by "sticky aggregation" between the PEG chains. This continues until complete flocculation of all the particles has occurred and the monolith has been formed, at which point the monolith may be calcined and the PEG removed, resulting in a macroporous silica monolith.
Both particle size and sticky aggregation can be controlled by varying the molecular weight and concentration of PEG.
Shell-core particles
Several additives, including polystyrene, cyclen, and polyamines, to the Stöber process allow the creation of shell-core silica particles. Two configurations of the shell-core morphology have been described. One is a silica core with an outer shell of an alternative material such as polystyrene. The second is a silica shell with a morphologically different core such as a polyamine. The creation of the polystrene/silica core composite particles begins with creation of the silica cores via the one-step Stöber process. Once formed, the particles are treated with oleic acid, which is proposed to react with the surface silanol groups. Styrene is polymerized around the fatty-acid-modified silica cores. By virtue of size distribution of the silica cores, the styrene polymerizes around them evenly resulting composite particles are similarly sized.
The silica shell particles created with cyclen and other polyamine ligands are created in a much different fashion. The polyamines are added to the Stöber reaction in the initial steps along with the TEOS precursor. These ligands interact with the TEOS precursor, resulting in an increase in the speed of hydrolysis; however, as a result they get incorporated into the resulting silica colloids. The ligands have several nitrogen sites that contain lone pairs of electrons that interact with the hydrolyzed end groups of TEOS. Consequently, the silica condense around the ligands encapsulating them. Subsequently, the silica/ligand capsules stick together to create larger particles. Once all of the ligand has been consumed by the reaction the remaining TEOS aggregates around the outside of the silica/ligand nanoparticles, creating a solid silica outer shell.
The resultant particle has a solid silica shell and an internal core of silica-wrapped ligands. The sizes of the particles cores and shells can be controlled through selection of the shape of the ligands along with the initial concentrations added to the reaction.
Carbon spheres
A Stöber-like process has been used to produce monodisperse carbon spheres using resorcinol-formaldehyde resin in place of a silica precursor. The modified process allows production of carbon spheres with smooth surfaces and a diameter ranging from 200 to 1000 nm.
Advantages and applications
, a "solid blue smoke", which feels like very light-weight styrofoam to the touch One major advantage of the Stöber process is that it can produce silica particles that are nearly monodisperse, 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 and biosensing. The mesoporous silica nanoparticles prepared by modified Stöber processes have applications in the field of catalysis and liquid chromatography.
In addition to monodispersity, these materials have very large surface areas as well as uniform, tunable, and highly ordered pore structures, which makes mesoporous silica uniquely attractive for these applications. meteoroid particle captured by the EURECA spacecraft launched by STS-46 in a block of aerogel silica aerogel is a thermal insulator that is sufficiently insulating to protect crayons from a Bunsen burner flame.
Aerogels
Aerogels are highly porous ultralight materials in which the liquid component of a gel has been replaced with a gas, and are noteworthy for being solids that are extremely effective thermal insulators Aerogels can be prepared in a variety of ways, and though most have been based on silica, The prime disadvantage of a silica-based aerogel is its fragility, though NASA has used them for insulation on Mars rovers, the Mars Pathfinder and they have been used commercially for insulating blankets and between glass panes for translucent day-lighting panels. Particulate gels prepared by the Stöber process can be dehydrated rapidly to produce highly effective silica aerogels, as well as xerogels.
They key step is the use of supercritical fluid extraction to remove water from the gel while maintaining the gel structure, which is typically done with supercritical carbon dioxide, This aerogel has a surface area of 700 m 2 ⋅g −1 and a density of 0.040 g⋅cm −3 ; Aerographene, with a density of just 13% of that of room temperature air and less dense than helium gas, became the lowest-density solid yet developed in 2013. Stöber-like methods have been applied in the preparation of aerogels in non-silica systems. NASA has developed silica aerogels with a polymer coating to reinforce the structure, producing a material roughly two orders of magnitude stronger for the same density, and also polymer aerogels, which are flexible and can be formed into a bendable thin film.
Colloidal silica
Colloidal silica is widely used in metal casting.
Synthetic opals
Stöber process may be used to produce spherical particles to grow lustrous opal mineraloids.