Stöber process

The Stöber process is used in materials science to prepare particles of silica (SiO
₂) of a controllable and uniform size. A team led by Werner Stöber first reported the process as a development of earlier PhD research of Kolbe. It is an example of a sol-gel process where a molecular precursor (typically tetraethylorthosilicate) is first reacted with water in the presence of an alcohol like ethanol and the resulting molecules joined together to build larger structures. The reaction produces silica particles with diameter ranging from 50 to 2000 nm depending on conditions. The process has been actively researched since its 1968 discovery, including to understand its kinetics and mechanism, and a particle aggregation model is a better fit for experimental data than the initially-hypothesized monomer addition model. Consequently, it is possible to exert a high degree of control over the particle size and distribution, allowing the fine-tuning of physical properties of the resulting material to best suit intended applications.

In 1999, a two-stage modification was reported which allows the controlled formation of silica particles with small holes. The process is undertaken at low pH in the presence of a surface-active molecule, with the hydrolysis step being completed with the formation of a microemulsion before adding sodium fluoride to start the condensation process. The non-ionic surfactant is burned away to produce empty pores, increasing the area and altering the characteristics surface of the resulting particles. Consequently, there is much greater control over the physical properties of the resulting material. Development work has also been undertaken for larger pore structures like macroporous monoliths, shell-core particles based on polystyrene, cyclen,, or polyamines, and carbon spheres.

One-step process

Simplified representation of the hydrolysis and condensation of TEOS in the Stöber process

The Stöber process is a sol-gel approach to preparing monodisperse spherical silica (SiO
₂) materials by a team led by Werner Stöber and reported in 1968, as a continuation of the research from Gerhard Kolbe's 1956 Ph.D. dissertation. This method uses tetraethyl orthosilicate (Si(OEt)
₄, TEOS) as a precursor, which is hydrolyzed in alcohol (typically methanol or ethanol) in the presence of ammonia as a catalyst:

${\ce {{Si(OEt)4}+{H2O}->{Si(OEt)3(OH)}+{EtOH}}}$

The reaction produces triethoxysilanol which can then undergo a condensation reaction with either TEOS or another silanol, with ethanol as a by-product:

${\ce {2{Si(OEt)3(OH)}->{(EtO)3Si-O-Si(OEt)3}+{H2O}}}$

Further hydrolysis of the ethoxy groups and subsequent condensation leads to crosslinking. It is a one-step process as the hydrolysis and condensation reactions occur together in a single reaction vessel.

The process affords macroscopic particles of granular silica with diameters ranging from 50 to 2000 nm; particle sizes are fairly uniform with the distribution determined by the choice of conditions such as reactant concentrations, catalysts, and temperature. 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; higher concentrations thus lead to smaller particles on average 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-dependant, with cooling (and hence slower reaction rates) leading to a monotonic increase in average particle size, but control over size 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 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 from 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 greater nucleation sites and hence smaller particles. Decoupling the hydrolysis and condensation process affords a level of product control that is substantial superior to that afforded by the one-step Stöber process, with particle size controlled nearly completely by the sodium fluoride to TEOS ratio.

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. 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 homogenous solution, initiating the condensation reaction by acting as nucleation seed. The silica particles are collected by filtration and calcined to remove the nonionic surfactant template by combustion, resulting in the mesoporous silica product.

Choices of conditions for the process allows control of pore sizes, particle diameter, and their distributions, as in the case of the one-step aproach. 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. A swelling agent (such as mesitylene) causes increases in volume and hence in pore size, often by solvent absorption, but is limited by the solubility of the agent in the system. Pore size varies directly with temperature, bound by the lower out of the surfactant cloud point and the boiling point of water. Sodium fluoride concentration produces direct but non-linear changes in porosity with the effect decreasing as the added fluoride concentration tends to an upper limit.

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. Experimental evidence demonstrates that the concentration of hydrolyzed TEOS stays above that required for nucleation until late into the reaction, however, and the introduction of seeded growth nuclei does 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.

An aggregation based model is based on the generation of nucleation sites that then aggregate together to create larger particles. Throughout the most of the reaction new nucleation sites are continually created and then are absorbed into the larger particles. The general equation that governs the population balance is:

${\frac {\partial n(v,t)}{\partial t}}=J\delta (v-v^{*})+{\frac {1}{2}}\int _{v^{*}}^{v}q(v-v^{'},v^{'})n(v-v^{'},t)n(v',t)dv^{'}-n(v,t)\int _{v^{*}}^{\infty }q(v,v^{'})n(v^{'})n(v^{'},t)dv^{'}$

Where n(v,t) is the number density of particles of volume between v and v+dv, q(v,v') is the aggregation kernel for particles of volume v and v', J is the nucleation rate, and v^* is the nucleus volume.

The generation of the nucleation sites follows the equation below:

$J=g_{s}(e^{-k_{1}t}-e^{-k_{2}t})$

Where k₁ and k₂ are rate constants based on the concentrations of H₂O and NH₃ and g_s is the normalization factor from a mass balance of the silica.

The following formula for the aggregation kernel q(v,v') is used to describe the DLVO interactions:

$q(v,v^{'})=q_{B}(v,v^{'})/W(v,v^{'})$

Where q_B is the Brownian aggregation kernel defined as:

$q_{B}(v,v^{'})={\frac {3kT}{3\mu }}(v^{\frac {1}{3}}+v^{'{\frac {1}{3}}})(v^{-{\frac {1}{3}}}+v^{'-{\frac {1}{3}}})$

Where k is Boltzmann's constant, T is the temperature and μ is the medium viscosity. The stability factor W(v,v') is defined as:

$W(v,v^{'})=2\int _{0}^{\infty }{\frac {exp[V_{T}(x;v,v^{'})/kT]}{(x+2)^{2}}}dx$

Where x is the distance between the particle surfaces and V_T is the total interaction energy.

$V_{T}=V_{vdw}+V_{elec}+V_{solvation}$

The total interaction energy is the sum of all the forces, including electrostatic repulsion forces, the van der Waals attraction forces, and the solvation repulsion forces. The van der Waals attraction forces are governed by the following equation:

$V_{vdw}=-{\frac {A_{H}}{6}}\left[{\frac {2a_{1}a_{2}}{R^{2}-(a_{1}+a_{2})^{2}}}+{\frac {2a_{1}a_{2}}{R^{2}-(a_{1}-a_{2})^{2}}}+ln{\frac {R^{2}-(a_{1}+a_{2})^{2}}{R^{2}-(a_{1}-a_{2})^{2}}}\right]$

Where A_H is the Hamaker constant, R is the distance between the centers of the two particles and a₁, a₂ are the radii of the two particles.

For electrostatic repulsion force the equation is as follows:

$V_{elec}=4\pi \varepsilon a_{1}a_{2}Y_{1}Y_{2}\left({\frac {kT}{e}}\right)^{2}{\frac {exp(-\kappa x)}{x+a_{1}+a_{2}}}$

Where a₁ and a₂ are the radii of the two particles, κ is the inverse Debye length for a 1:1 electrolyte, ε is the dielectric constant of the medium and Y_i is as follows:

$Y_{i}={\frac {8tanh(e\varphi _{0}/4kT)}{1+\left[1-{\frac {2\kappa a_{i}+1}{(\kappa a_{i}+1)^{2}}}tanh^{2}(e\varphi _{0}/4kT)\right]^{\frac {1}{2}}}}$

Where e is the electronic charge, $Phi;₀ is the surface potential and Κ is the inverse Debye length for a 1:1 electrolyte.

The final component of the total interaction energy is the solvation repulsion which is as follows:

$V_{solvation}=\pi A_{s}L\left[{\frac {2a_{1}a_{2}}{a_{1}+a_{2}}}\right]exp[-(R-a_{1}-a_{2})/L]$

Where A_s is the pre-exponential factor (1.5x10^-3 J/m²) and L is the decay length (1x10^-9m)

The results of the above model for controlled growth aggregation are in line with experimental observations by small-angle X-ray scattering (SAXS) techniques and accurately predict particle sizing based on initial concentrations of reactants. In addition to this, other experimental techniques including microgravity analysis and variable pH analysis have agreed with the aggregate growth model of silica particle formation.

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 additive can interact with the silica through either chemical and/or physical means either during or after the reaction, leading to substantial changes in morphology of the silica particles.

Mesoporous silica

TEM image of a nanoparticle of mesoporous silica

The one-step Stöber process may be modified to manufacture porous silica by adding a surfactant tenplate 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 which are incorporated into the silica particles during formation. Combustion of these surfactant molecules leaves voids characteristic of a mesoporous structure, as seen in the illustration at right.

Variation of 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

Addition of polyethylene glycol (PEG) to the process leads 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. A macroporous silica monolith results, and 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 and results 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, which interact with the hydrolyzed end groups of TEOS. As a result the silica condense around the ligands encapsulating them. Subsquently, 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. Unlike the silica-based Stöber process, this reaction is completed at neutral pH and that ammonia has a role in stabilizing the individual carbon particles by preventing self-adhesion and aggregation, as well as acting as a catalyst.

Advantages

One major advantage of the Stöber process is than it can produce silica particles which are nearly monodisperse, provide ideal models for studying colloidal phenomena. The process provides a convenient approach to preparing silica nanoparticles for applications including intracellular drug delivery and biosensing.

Particulate gels prepared by the Stöber process can be dehydrated rapidly to produce highly effective thermal insulators known as aerogels, and can also make xerogels with enormously high surface areas.