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{{Use dmy dates|date=January 2017}}
The '''Stöber process''' is a chemical process used to prepare [[silica]] ({{chem|SiO|2}}) particles<ref name ="Stober" /> of controllable and [[monodisperse|uniform size]]<ref name="Bogush" /> for applications in [[materials science]]. It was pioneering<ref name = WideUse /> when it was reported by Werner Stöber and his team in 1968,<ref name="Stober" /> and remains today the most widely used [[wet chemistry]] synthetic approach to silica [[nanoparticles]].<ref name = WideUse /> It is an example of a [[sol-gel process]] wherein a molecular [[precursor (chemistry)|precursor]] (typically [[tetraethylorthosilicate]]) is first [[hydrolysis|reacted with water]] in an alcoholic solution, the resulting molecules then [[condensation|joining together]] to build larger structures. The reaction produces silica particles with diameters ranging from 50 to 2000&nbsp;[[nanometre|nm]], depending on conditions. The process has been actively researched since its discovery, including efforts to understand its [[chemical kinetics|kinetics]] and [[reaction mechanism|mechanism]]{{snd}}a [[particle aggregation]] model was found to be a better fit for the experimental data<ref name="Kinetics" /> than the initially hypothesized LaMer model.<ref name = LaMer /><ref name="MonomerAddition" /> 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.

In 1999 a two-stage modification was reported<ref name="Boissiere1999" /> that allowed the controlled formation of [[mesoporous silica|silica particles with small holes]].<ref name="Boissiere2000" /> The process is undertaken at low pH in the presence of a [[surfactant|surface-active molecule]]. The hydrolysis step is completed with the formation of a [[microemulsion]]<ref name="Prouzet" /> before adding [[sodium fluoride]] to [[nucleation]] the condensation process. The [[non-ionic surfactant]] is [[calcination|burned away]] to produce empty pores, increasing the surface area and altering the [[morphology (materials science)|surface characteristics]] of the resulting particles, allowing for much greater control over the physical properties of the material.<ref name="Boissiere1999" /> Development work has also been undertaken for larger pore structures such as [[macropore|macroporous]] monoliths,<ref name="Cademartiri"/> shell-core particles based on [[polystyrene]],<ref name="Polystyrene" /> [[cyclen]],<ref name="Cyclen" /> or [[polyamines]],<ref name="Polyamine" /> and carbon spheres.<ref name="Carbon1" />

Silica produced using the Stöber process is an ideal material to serve as a model for studying [[colloid]] phenomena<ref name = Berg /> because of the [[monodisperse|monodispersity]] (uniformity) of its particle sizes.<ref name = monodisperse /> Nanoparticles prepared using the Stöber process have found applications including in the [[drug delivery|delivery of medication]]s to [[intracellular|within cellular structures]]<ref name = DrugDelivery /> and in the preparation of [[biosensor]]s.<ref name = biosensor /> Porous silica Stöber materials have applications in [[catalysis]]<ref name="MesoApplications" /> and [[liquid chromatography]]<ref name="PinnavaiaSayari2000" /> due to their high [[surface area]] and their uniform, tunable, and highly ordered pore structures. Highly effective [[thermal insulator]]s known as [[aerogel]]s can also be prepared using Stöber methods,<ref name = Berg /> and Stöber techniques have been applied to prepare non-silica aerogel systems.<ref name = non-silicaStober /> Applying [[supercritical drying]] techniques, a Stöber silica aerogel with a [[specific surface area]] of 700&nbsp;m<sup>2</sup>⋅g<sup>−1</sup> and a density of 0.040&nbsp;g⋅cm<sup>−3</sup> can be prepared.<ref name = Steiner /> [[NASA]] has prepared silica aerogels with a Stöber-process approach for both the [[Mars Pathfinder|Mars ''Pathfinder'']] and ''[[Stardust (spacecraft)|Stardust]]'' missions.<ref name = Stardust>{{cite web|url = http://stardust.jpl.nasa.gov/tech/aerogel.html|title = Aerogel|date = 31 March 2005|access-date = 11 December 2016|website = [[NASA]] [[Stardust (spacecraft)|Stardust mission]]|publisher = [[Jet Propulsion Laboratory]], [[NASA]]|quote = Aerogel is made by high temperature and pressure-critical-point drying of a gel composed of colloidal silica structural units filled with solvents. Aerogel was prepared and flight qualified at the Jet Propulsion Laboratory (JPL). JPL also produced aerogel for the Mars Pathfinder and Stardust missions.}}</ref>

== One-step process ==
[[File:SolGelCartoon.png|thumb|upright=2|425px|Simplified representation of the [[hydrolysis]] and [[condensation reaction|condensation]] of [[tetraethoxysilane|TEOS]] in the Stöber process]]
The Stöber process is a [[sol-gel]] approach to preparing [[monodisperse]] (uniform) spherical [[silica]] ({{chem|SiO|2}}) materials that was developed by a team led by Werner Stöber and reported in 1968.<ref name="Stober">{{cite journal|last1=Stöber|first1=Werner|last2=Fink|first2=Arthur|last3=Bohn|first3=Ernst|title=Controlled growth of monodisperse silica spheres in the micron size range|journal=Journal of Colloid and Interface Science|date=January 1968|volume=26|issue=1|pages=62–69|doi=10.1016/0021-9797(68)90272-5|bibcode=1968JCIS...26...62S}}</ref> The process, an evolution and extension of research described in Gerhard Kolbe's 1956 PhD dissertation,<ref name="Kolbe">{{cite thesis|last = Kolbe|first = Gerhard|title = Das Komplexchemische Verhalten der Kieselsäure|type = Ph.D.|publisher = [[Friedrich-Schiller-Universität Jena]]|year = 1956|language = German}}</ref> was an innovative discovery that still has wide applications more than 50 years later.<ref name = WideUse /> Silica [[precursor (chemistry)|precursor]] [[tetraethyl orthosilicate]] ({{chem|Si(O[[Ethyl group|Et]])|4}}, TEOS) is [[hydrolysis|hydrolyzed]] in alcohol (typically [[methanol]] or [[ethanol]]) in the presence of [[ammonia]]{{Clarify|reason=Isn't that an [[ammonia solution]]?|date=April 2022}} as a [[catalyst]]:<ref name="Stober" /><ref name="Blaaderen">{{cite journal|last1=Van Blaaderen|first1=A|last2=Van Geest|first2=J|last3=Vrij|first3=A|title=Monodisperse colloidal silica spheres from tetraalkoxysilanes: Particle formation and growth mechanism|journal=Journal of Colloid and Interface Science|date=December 1992|volume=154|issue=2|pages=481–501|doi=10.1016/0021-9797(92)90163-G|bibcode=1992JCIS..154..481V|citeseerx=10.1.1.531.1922}}</ref>
: <chem>Si(OEt)4 + H2O -> Si(OEt)3OH + EtOH</chem>
: <chem>Si(OEt)4 + 2H2O -> Si(OEt)2(OH)2 + 2EtOH</chem>
The reaction produces ethanol and a mixture of ethoxy[[silanol]]s (such as {{chem|Si(OEt)|3|OH}}, {{chem|Si(OEt)|2|(OH)|2}}, and even {{chem|link=silicic acid|Si(OH)|4}}), which can then [[condensation reaction|condense]] with either TEOS or another silanol with loss of alcohol or water:<ref name="Blaaderen" />
: <chem>2Si(OEt)3OH -> (EtO)3Si-O-Si(OEt)3 + H2O</chem>
: <chem>Si(OEt)3OH + Si(OEt)4 -> (EtO)3Si-O-Si(OEt)3 + EtOH</chem>
: <chem>Si(OEt)3OH + Si(OEt)2(OH)2 -> (EtO)3Si-O-Si(OEt)2OH + H2O</chem>

Further hydrolysis of the ethoxy groups and subsequent condensation leads to [[crosslink]]ing. It is a [[One-pot synthesis|one-step process]] as the hydrolysis and condensation reactions occur together in a single reaction vessel.<ref name="Stober" />

The process affords [[microscopic scale|microscopic]] particles of [[colloidal silica]] with diameters ranging from 50 to 2000&nbsp;[[nanometer|nm]]; [[particle size]]s are fairly uniform with the [[particle-size distribution|distribution]] determined by the choice of conditions such as [[reagent|reactant]] concentrations, catalysts, and temperature.<ref name="Bogush">{{cite journal|last1=Bogush|first1=G.H.|last2=Tracy|first2=M.A.|last3=Zukoski|first3=C.F.|title=Preparation of monodisperse silica particles: Control of size and mass fraction|journal=Journal of Non-Crystalline Solids|date=August 1988|volume=104|issue=1|pages=95–106|doi=10.1016/0022-3093(88)90187-1|bibcode=1988JNCS..104...95B|citeseerx=10.1.1.471.9863}}</ref> Larger particles are formed when the concentrations of water and ammonia are raised, but with a consequent broadening of the particle-size distribution.<ref name="Van Helden">{{cite journal|last1=Van Helden|first1=A.K.|last2=Jansen|first2=J.W.|last3=Vrij|first3=A.|title=Preparation and characterization of spherical monodisperse silica dispersions in nonaqueous solvents|journal=Journal of Colloid and Interface Science|date=June 1981|volume=81|issue=2|pages=354–368|doi=10.1016/0021-9797(81)90417-3|bibcode=1981JCIS...81..354V}}</ref> 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.<ref name="Van Helden" /> The process is temperature-dependent, with cooling (and hence slower [[reaction rate]]s) leading to a monotonic increase in average particle size, but control {{Not a typo|over size}} distribution cannot be maintained at overly low temperatures.<ref name="Bogush" />

== Two-step process ==
In 1999 [[Cédric Boissière]] and his team developed a two-step process whereby the hydrolysis at low pH (1&nbsp;–&nbsp;4) is completed before the condensation reaction is initiated by the addition of [[sodium fluoride]] (NaF).<ref name="Boissiere1999">{{cite journal|last1=Boissière|first1=Cédric|last2=van der Lee|first2=Arie|last3=Mansouri|first3=Abdeslam El|last4=Larbot|first4=André|last5=Prouzet|first5=Eric|title=A double step synthesis of mesoporous micrometric spherical MSU-X silica particles|journal=Chemical Communications|date=1999|issue=20|pages=2047–2048|doi=10.1039/A906509A}}</ref> The two-step procedure includes the addition of a [[nonionic surfactant]] template to ultimately produce [[mesoporous silica]] particles.<ref name="Boissiere2000">{{cite journal|last1=Boissière|first1=Cédric|last2=Larbot|first2=André|last3=van der Lee|first3=Arie|last4=Kooyman|first4=Patricia J.|last5=Prouzet|first5=Eric|title=A New Synthesis of Mesoporous MSU-X Silica Controlled by a Two-Step Pathway|journal=Chemistry of Materials|date=October 2000|volume=12|issue=10|pages=2902–2913|doi=10.1021/cm991188s}}</ref> The main advantage of sequencing the hydrolysis and condensation reactions is the ability to ensure complete [[homogeneity (physics)|homogeneity]] of the surfactant and the precursor TEOS mixture. Consequently, the diameter and shape of the product particles as well as the [[porosity|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.<ref name="Boissiere1999" /> Decoupling the hydrolysis and condensation process affords a level of product control that is substantially superior to that afforded by the one-step Stöber process, with particle size controlled nearly completely by the sodium fluoride-to-TEOS ratio.<ref name="Boissiere1999" />

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]].<ref name="Prouzet">{{cite journal|last1=Prouzet|first1=Éric|last2=Boissière|first2=Cédric|title=A review on the synthesis, structure and applications in separation processes of mesoporous MSU-X silica obtained with the two-step process|journal=Comptes Rendus Chimie|date=March 2005|volume=8|issue=3–4|pages=579–596|doi=10.1016/j.crci.2004.09.011|url=https://comptes-rendus.academie-sciences.fr/chimie/articles/10.1016/j.crci.2004.09.011/ }}</ref> 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.<ref name="Boissiere1999" /> The silica particles are collected by filtration and [[calcined]] to remove the nonionic surfactant template by combustion, resulting in the mesoporous silica product.

The selection of conditions for the process allows for control of pore sizes, particle diameter, and their distributions, as in the case of the one-step approach.<ref name="Boissiere2000" /> 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.<ref name="Prouzet" /> Pore size varies directly with temperature,<ref name="Boissiere1999" /> 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.<ref>{{cite journal |last1=Boissière |first1=Cédric |last2=Larbot |first2=André |last3=Bourgaux |first3=Claudie |last4=Prouzet |first4=Eric |last5=Bunton |first5=Clifford A.|title=A Study of the Assembly Mechanism of the Mesoporous MSU-X Silica Two-Step Synthesis|journal=Chemistry of Materials|date=October 2001|volume=13|issue=10|pages=3580–3586|doi=10.1021/cm011031b}}</ref>

== Kinetics ==
The LaMer model for the [[chemical kinetics|kinetics]] of the formation of [[colloid|hydrosol]]s<ref name = LaMer>{{cite journal|title = Theory, Production and Mechanism of Formation of Monodispersed Hydrosols|first1 = Victor K.|last1 = LaMer|first2 = Robert H.|last2 = Dinegar|journal = [[J. Am. Chem. Soc.]]|year = 1950|volume = 72|issue = 11|pages = 4847–4854|doi = 10.1021/ja01167a001}}</ref> is widely applicable for production of monodisperse systems,<ref>{{cite encyclopedia|chapter = Nucleation and Growth of Monodispersed Particles: Mechanisms|first = Tadao|last = Sugimoto|pages = 4257–4270|year = 2006|publisher = [[CRC Press]]|encyclopedia = Encyclopedia of Surface and Colloid Science|volume =  7|edition = 2nd|editor-first = P.|editor-last = Somasundaran|isbn =  9780849395741}}</ref> and it was originally hypothesized that the Stöber process followed this [[monomer]] addition model.<ref name="MonomerAddition">{{cite journal|last1=Matsoukas|first1=T|last2=Gulari|first2=Erdogan|title=Dynamics of growth of silica particles from ammonia-catalyzed hydrolysis of tetra-ethyl-orthosilicate|journal=Journal of Colloid and Interface Science|date=July 1988|volume=124|issue=1|pages=252–261|doi=10.1016/0021-9797(88)90346-3|bibcode=1988JCIS..124..252M|hdl=2027.42/27243|hdl-access=free}}</ref> 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.<ref name="MonomerAddition2">{{cite journal|last1=Matsoukas|first1=Themis|last2=Gulari|first2=Erdogan|title=Monomer-addition growth with a slow initiation step: A growth model for silica particles from alkoxides|journal=Journal of Colloid and Interface Science|date=October 1989|volume=132|issue=1|pages=13–21|doi=10.1016/0021-9797(89)90210-5|bibcode=1989JCIS..132...13M|hdl=2027.42/27723|hdl-access=free}}</ref> 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.<ref name="MonomerAddition3">{{cite journal|last1=Matsoukas|first1=Themis|last2=Gulari|first2=Erdogan|title=Self-sharpening distributions revisited—polydispersity in growth by monomer addition|journal=Journal of Colloid and Interface Science|date=September 1991|volume=145|issue=2|pages=557–562|doi=10.1016/0021-9797(91)90385-L|bibcode=1991JCIS..145..557M}}</ref> 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]].<ref name="Kinetics">{{cite journal|last1=Bogush|first1=G.H|last2=Zukoski|first2=C.F|title=Studies of the kinetics of the precipitation of uniform silica particles through the hydrolysis and condensation of silicon alkoxides|journal=Journal of Colloid and Interface Science|date=March 1991|volume=142|issue=1|pages=1–18|doi=10.1016/0021-9797(91)90029-8|bibcode=1991JCIS..142....1B}}</ref>

Under an aggregation-based model, nucleation sites are continually being generated and absorbed where the merging leads to particle growth.<ref name="Kinetics2">{{cite journal|last1=Bogush|first1=G.H|last2=Zukoski|first2=C.F|title=Uniform silica particle precipitation: An aggregative growth model|journal=Journal of Colloid and Interface Science|date=March 1991|volume=142|issue=1|pages=19–34|doi=10.1016/0021-9797(91)90030-C|bibcode=1991JCIS..142...19B}}</ref> The generation of the nucleation sites and the interaction energy between merging particles dictates the overall kinetics of the reaction.<ref name="Kinetics3">{{cite journal|last1=Lee|first1=Kangtaek|last2=Sathyagal|first2=Arun N.|last3=McCormick|first3=Alon V.|title=A closer look at an aggregation model of the Stöber process|journal=Colloids and Surfaces A: Physicochemical and Engineering Aspects|date=December 1998|volume=144|issue=1–3|pages=115–125|doi=10.1016/S0927-7757(98)00566-4}}</ref> The generation of the nucleation sites follows the equation below:<ref name="Kinetics2"/>
: <math>J = g_s(e^{-k_1t}-e^{-k_2t})</math>
where ''J'' is the nucleation rate, ''k''<sub>1</sub> and ''k''<sub>2</sub> are rate constants based on the concentrations of H<sub>2</sub>O and NH<sub>3</sub> and ''g<sub>s</sub>'' is the normalization factor based on the amount of silica precursor. Adjusting the concentration ratios of these compounds directly influences the rate at which nucleation sites are produced.<ref name="Kinetics2"/>

Merging of nucleation sites between particles is influenced by their interaction energies. The total interaction energy is dependent on three forces: [[electrostatic]] repulsion of like charges, [[Van der Waals force|van{{nbsp}}der{{nbsp}}Waals]] attraction between particles, and the effects of [[solvation]].<ref name="Kinetics3"/> These interaction energies (equations below) describe the particle aggregation process and demonstrate why the Stöber process produces particles that are uniform in size.
: <math> V_T = V_{\text{vdw}} + V_{\text{elec}} + V_{\text{solv}}</math>

The van der Waals attraction forces are governed by the following equation:<ref name="Kinetics3"/>
: <math>V_{\text{vdw}}=-\frac{A_\text{H}}{6}\left[\frac{2a_1a_2}{R^2-(a_1+a_2)^2}+\frac{2a_1a_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]</math>
where ''A''<sub>H</sub> is the [[Hamaker constant]], ''R'' is the distance between the centers of the two particles and ''a''<sub>1</sub>, ''a''<sub>2</sub> are the radii of the two particles. For electrostatic repulsion force the equation is as follows:<ref name="Kinetics3"/>
: <math>V_{\text{elec}} = 4\pi\varepsilon a_1a_2Y_1Y_2\left(\frac{k_\text{B}T}{e}\right)^2 \frac{e^{-\kappa x}}{x+a_1+a_2}</math> &nbsp; &nbsp; &nbsp; where &nbsp; &nbsp; &nbsp; <math>Y_i = \frac{8\tanh\frac{e\varphi_0}{4k_\text{B}T}}{1+\sqrt{1-\frac{2\kappa a_i+1}{(\kappa a_i+1)^2}\tanh^2\frac{e\varphi_0}{4k_\text{B}T}}}</math>
Where ''&epsilon;'' is the [[dielectric constant]] of the medium, ''k''<sub>B</sub> is the [[Boltzmann constant]], ''e'' is the [[elementary charge]], ''T'' is the [[absolute temperature]], ''&kappa;'' is the inverse [[Debye length]] for a 1:1 electrolyte, ''x'' is the (variable) distance between the particles, and ''&phi;''<sub>0</sub> is the surface potential. The final component of the total interaction energy is the solvation repulsion which is as follows:<ref name="Kinetics3"/>
: <math>V_{\text{solv}}=\pi A_\text{s}L\left[\frac{2a_1a_2}{a_1+a_2}\right]e^\frac{-(R-a_1-a_2)}{L}</math>
where ''A''<sub>s</sub> is the pre-exponential factor (1.5&nbsp;&times;&nbsp;10<sup>−3</sup>&nbsp;J⋅m<sup>−2</sup>) and ''L'' is the decay length (1&nbsp;&times;&nbsp;10<sup>−9</sup>&nbsp;m).

This model for controlled growth aggregation fits with experimental observations from [[small-angle X-ray scattering]] techniques<ref name="SAXS">{{cite journal|last1=Boukari|first1=H.|last2=Lin|first2=J.S.|last3=Harris|first3=M.T.|title=Small-Angle X-Ray Scattering Study of the Formation of Colloidal Silica Particles from Alkoxides: Primary Particles or Not?|journal=Journal of Colloid and Interface Science|date=October 1997|volume=194|issue=2|pages=311–318|doi=10.1006/jcis.1997.5112|pmid=9398411|bibcode=1997JCIS..194..311B}}</ref> and accurately predicts particle sizing based on initial conditions. In addition, experimental data from techniques including microgravity analysis<ref>{{cite journal|last1=Smith|first1=David D.|last2=Sibille|first2=Laurent|last3=Cronise|first3=Raymond J.|last4=Hunt|first4=Arlon J.|last5=Oldenburg|first5=Steven J.|last6=Wolfe|first6=Daniel|last7=Halas|first7=Naomi J.|title=Effect of Microgravity on the Growth of Silica Nanostructures|journal=Langmuir|date=December 2000|volume=16|issue=26|pages=10055–10060|doi=10.1021/la000643s|hdl=2060/20010057257|s2cid=97817904 |hdl-access=free}}</ref> and variable pH analysis<ref>{{cite journal|last1=Vogelsberger|first1=Wolfram|last2=Seidel|first2=Andreas|last3=Breyer|first3=Tilo|title=Kinetics of Sol Particle Formation as a Function of pH Studied by Viscosity Measurements in Silica Solutions|journal=Langmuir|date=April 2002|volume=18|issue=8|pages=3027–3033|doi=10.1021/la0114878}}</ref> 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 compound]]s to the [[reaction mixture]]. These additives can interact with the silica through [[chemical reaction|chemical]] and/or [[physical change|physical]] means either during or after the reaction, leading to substantial changes in [[morphology (materials science)|morphology]] of the silica particles.

=== Mesoporous silica ===
[[File:Mesopourus silica closeup.jpg|thumb|right|[[Transmission electron microscopy|TEM]] image of a [[nanoparticle]] of [[mesoporous silica]]]]
The one-step Stöber process may be modified to manufacture [[mesoporous silica|porous silica]] by adding a surfactant template to the reaction mixture and calcining the resulting particles.<ref name="Unger">{{cite journal|last1=Grün|first1=Michael|last2=Lauer|first2=Iris|last3=Unger|first3=Klaus K.|title=The synthesis of micrometer- and submicrometer-size spheres of ordered mesoporous oxide MCM-41|journal=Advanced Materials|date=March 1997|volume=9|issue=3|pages=254–257|doi=10.1002/adma.19970090317|s2cid=135537204 |doi-access=free}}</ref> Surfactants that have been used include [[cetrimonium bromide]],<ref name="MCM41">{{cite journal|last1=Liu|first1=Shiquan|last2=Lu|first2=Lingchao|last3=Yang|first3=Zhongxi|last4=Cool|first4=Pegie|last5=Vansant|first5=Etienne F.|title=Further investigations on the modified Stöber method for spherical MCM-41|journal=Materials Chemistry and Physics|date=June 2006|volume=97|issue=2–3|pages=203–206|doi=10.1016/j.matchemphys.2005.09.003}}</ref> [[cetrimonium chloride|cetyltrimethylammonium chloride]],<ref name="Kambara">{{cite journal|last1=Kambara|first1=Kumiko|last2=Shimura|first2=Naoki|last3=Ogawa|first3=Makoto|title=Larger Scale Syntheses of Surfactant-Templated Nanoporous Silica Spherical Particles by the Stöber Method|journal=Journal of the Ceramic Society of Japan|date=2007|volume=115|issue=1341|pages=315–318|doi=10.2109/jcersj.115.315|doi-access=free}}</ref> and [[glycerol]].<ref name="Vacassy">{{cite journal|last1=Vacassy|first1=R.|last2=Flatt|first2=R.J.|last3=Hofmann|first3=H.|last4=Choi|first4=K.S.|last5=Singh|first5=R.K.|title=Synthesis of Microporous Silica Spheres|journal=Journal of Colloid and Interface Science|date=July 2000|volume=227|issue=2|pages=302–315|doi=10.1006/jcis.2000.6860|pmid=10873314|bibcode=2000JCIS..227..302V}}</ref> The surfactant forms [[micelle]]s, 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.<ref name = Micelle /> [[Calcination|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.<ref name="Unger" /><ref name = Micelle>{{cite book|chapter = Synthetic and Toxicological Characteristics of Silica Nanomaterials for Imaging and Drug Delivery Applications|page = 6{{hyphen}}4|first1 = Heather|last1 = Herd|first2 = Hamidreza|last2 = Ghandehari|chapter-url = https://books.google.com/books?id=DVVEHbIKERMC&pg=SA6-PA4|title = Nanobiomaterials Handbook|editor-first = Balaji|editor-last = Sitharaman|publisher = [[CRC Press]]|year = 2016|isbn = 9781420094671}}</ref>

Varying the surfactant concentration allows control over the diameter and volume of pores, and thus of the surface area of the product material.<ref name="MCM41" /> Increasing the amount of surfactant leads to increases in total pore volume and hence particle surface area, but with individual pore diameters remaining unchanged.<ref name="Kambara" /> 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.<ref name="MCM41" /> 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.<ref name="MCM41" />

=== Macroporous monolith ===
The addition of [[polyethylene glycol]] (PEG) to the process causes silica particles to aggregate into a [[Macroporous material|macroporous]] continuous block, allowing access to a monolithic morphology.<ref name="Cademartiri">{{cite journal|last1=Cademartiri|first1=Rebecca|last2=Brook|first2=Michael A.|last3=Pelton|first3=Robert|last4=Brennan|first4=John D.|title=Macroporous silica using a "sticky" Stöber process|journal=Journal of Materials Chemistry|date=2009|volume=19|issue=11|pages=1583|doi=10.1039/B815447C|s2cid=94969948}}</ref> PEG polymers with [[allyl]] or [[silyl]] end groups with a molecular weight of greater than 2000&nbsp;g⋅mol<sup>−1</sup> 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.<ref name="Cademartiri"/> 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]],<ref name="Polystyrene">{{cite journal|last1=Ding|first1=Xuefeng|last2=Zhao|first2=Jingzhe|last3=Liu|first3=Yanhua|last4=Zhang|first4=Hengbin|last5=Wang|first5=Zichen|title=Silica nanoparticles encapsulated by polystyrene via surface grafting and in situ emulsion polymerization|journal=Materials Letters|date=October 2004|volume=58|issue=25|pages=3126–3130|doi=10.1016/j.matlet.2004.06.003}}</ref> [[cyclen]],<ref name="Cyclen">{{cite journal|last1=Masse|first1=Sylvie|last2=Laurent|first2=Guillaume|last3=Coradin|first3=Thibaud|title=Influence of cyclic polyamines on silica formation during the Stöber process|journal=Physical Chemistry Chemical Physics|date=2009|volume=11|issue=43|pages=10204–10|doi=10.1039/B915428K|pmid=19865778|bibcode=2009PCCP...1110204M|s2cid=37342876}}</ref> and [[polyamines]],<ref name="Polyamine">{{cite journal|last1=Masse|first1=Sylvie|last2=Laurent|first2=Guillaume|last3=Chuburu|first3=Françoise|last4=Cadiou|first4=Cyril|last5=Déchamps|first5=Isabelle|last6=Coradin|first6=Thibaud|title=Modification of the Stöber Process by a Polyazamacrocycle Leading to Unusual Core−Shell Silica Nanoparticles|journal=Langmuir|date=April 2008|volume=24|issue=8|pages=4026–4031|doi=10.1021/la703828v|pmid=18303930}}</ref> 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.<ref name="Polystyrene"/> 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.<ref name="Polystyrene"/>

The silica shell particles created with cyclen and other polyamine [[ligand]]s are created in a much different fashion. The polyamines are added to the Stöber reaction in the initial steps along with the TEOS precursor.<ref name="Polyamine"/> 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]].<ref name="Cyclen"/> 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.<ref name="Cyclen"/> 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.<ref name="Polyamine"/>

== Carbon spheres ==
A Stöber-like process has been used to produce monodisperse carbon spheres using [[resorcinol]]-[[formaldehyde]] resin in place of a silica precursor.<ref name="Carbon1">{{cite journal|last1=Liu|first1=Jian|last2=Qiao|first2=Shi Zhang|last3=Liu|first3=Hao|last4=Chen|first4=Jun|last5=Orpe|first5=Ajay|last6=Zhao|first6=Dongyuan|last7=Lu|first7=Gao Qing Max|title=Extension of The Stöber Method to the Preparation of Monodisperse Resorcinol-Formaldehyde Resin Polymer and Carbon Spheres|journal=Angewandte Chemie International Edition|date=20 June 2011|volume=50|issue=26|pages=5947–5951|doi=10.1002/anie.201102011|pmid=21630403}}</ref> The modified process allows production of carbon spheres with smooth surfaces and a diameter ranging from 200 to 1000&nbsp;nm.<ref name="Carbon1"/> 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.<ref name="Carbon2">{{cite journal|last1=Lu|first1=An-Hui|last2=Hao|first2=Guang-Ping|last3=Sun|first3=Qiang|title=Can Carbon Spheres Be Created through the Stöber Method?|journal=Angewandte Chemie International Edition|date=19 September 2011|volume=50|issue=39|pages=9023–9025|doi=10.1002/anie.201103514|pmid=21919134}}</ref>

== Advantages and applications ==
[[File:Aerogel hand.jpg|right|thumb|A block of an [[aerogel]], a "solid blue smoke",<ref>{{cite web|url = http://stardust.jpl.nasa.gov/aerogel_factsheet.pdf|title = Aerogel – Mystifying Blue Smoke|publisher = [[Jet Propulsion Laboratory]], [[NASA]]|access-date = 23 November 2016}}</ref> 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,<ref name = monodisperse>{{cite book|chapter = Functionalization of Silica Nanoparticles for Corrosion Prevention of Underlying Metal|first1 = Dylan J.|last1 = Boday|first2 = Jason T.|last2 = Wertz|first3 = Joseph P.|last3 = Kuczynski|pages = 121–140|title = Nanomaterials, Polymers and Devices: Materials Functionalization and Device Fabrication|editor-first = Eric S. W.|editor-last = Kong|publisher = [[John Wiley & Sons]]|year = 2015|isbn = 9781118866955|chapter-url = https://books.google.com/books?id=djhPCAAAQBAJ&q=St%C3%B6ber+process&pg=PA131}}</ref> and thus provides an ideal model for use in studying colloidal phenomena.<ref name = Berg /> It was a pioneering discovery when first published, allowing synthesis of spherical monodisperse silica particles of controlled sizes, and in 2015 remains the most widely used [[wet chemistry]] approach to silica nanoparticles.<ref name = WideUse>{{cite book|chapter = Nanoparticles and Composites|first = Guido|last = Kicklebick|pages = 227–244|title = The Sol-Gel Handbook: Synthesis, Characterization and Applications|volume = 3|editor1-first = David|editor1-last = Levy|editor2-first = Marcos|editor2-last = Zayat|publisher = [[John Wiley & Sons]]|year = 2015|isbn = 9783527334865|chapter-url = https://books.google.com/books?id=9ZSbCgAAQBAJ&q=St%C3%B6ber+process&pg=PA229}}</ref>

The process provides a convenient approach to preparing silica nanoparticles for applications including [[intracellular]] [[drug delivery]]<ref name="DrugDelivery">{{cite book|chapter = Silica-Based Nanoparticles for Intracellular Drug Delivery|first1 = Sandrine|last1 = Quignard|first2 = Sylvie|last2 = Masse|first3 = Thibaud|last3 = Coradin| title=Intracellular Delivery | series=Fundamental Biomedical Technologies | volume=5 |pages = 333–361|doi = 10.1007/978-94-007-1248-5_12|editor-first = Ales|editor-last = Prokop|publisher = [[Springer Science & Business Media]]|year = 2011|isbn = 9789400712485|chapter-url = https://books.google.com/books?id=dhCpx2kKo1wC&q=St%C3%B6ber+process&pg=PA336}}</ref> and [[biosensor|biosensing]].<ref name="biosensor">{{cite book|doi = 10.1007/978-1-4419-9622-0_10|pages = 305–332|first1 = Huangxian|last1 = Ju|first2 = Zhang|last2 = Xueji|first3 = Joseph|last3 = Wang| title=NanoBiosensing | chapter=Biosensors Based on Sol–Gel Nanoparticle Matrices | series=Biological and Medical Physics, Biomedical Engineering |publisher = [[Springer Science & Business Media]]|year = 2011|isbn = 9781441996220|chapter-url = https://books.google.com/books?id=1r1Y4g3116IC&q=St%C3%B6ber+process&pg=PA310}}</ref> The mesoporous silica nanoparticles prepared by modified Stöber processes have applications in the field of [[catalysis]]<ref name="MesoApplications">{{cite journal|last1=Giraldo|first1=L. F.|last2=López|first2=B. L.|last3=Pérez|first3=L.|last4=Urrego|first4=S.|last5=Sierra|first5=L.|last6=Mesa|first6=M.|title=Mesoporous Silica Applications|journal=Macromolecular Symposia|date=November 2007|volume=258|issue=1|pages=129–141|doi=10.1002/masy.200751215}}</ref> and [[liquid chromatography]].<ref name="PinnavaiaSayari2000">{{cite book|last1=Pinnavaia|first1=T.J.|last2=Sayari|first2=Abdel|last3=Jaroniec|first3=M.|title=Nanoporous Materials II|url=https://books.google.com/books?id=4kzhw--AEMgC&pg=PA747|date=14 April 2000|publisher=Elsevier|isbn=978-0-08-053726-9|pages=747–55}}</ref> In addition to monodispersity, these materials have very large [[surface area]]s as well as uniform, tunable, and highly ordered pore structures,<ref name="PinnavaiaSayari2000" /> which makes mesoporous silica uniquely attractive for these applications.[[File:Particle captured in aerogel.jpg|left|thumb|A 10&nbsp;[[Micrometre|&mu;m]] meteoroid particle captured by the [[European Retrievable Carrier|EURECA]] spacecraft launched by [[STS-46]] in a block of aerogel<ref name = Stardust />]]
[[File:Aerogel crayons.jpg|right|thumb|This [[NASA]] silica [[aerogel]]<ref name = Stardust /> is a [[thermal insulator]] that is sufficiently insulating to protect crayons from a [[Bunsen burner]] flame.<ref name = NASASpinoff />]]

=== Aerogels ===
[[Aerogel]]s are highly porous [[ultralight material]]s in which the [[liquid]] component of a [[gel]] has been replaced with a [[gas]],<ref>{{cite book|year = 2007|title = Definitions of terms relating to the structure and processing of sols, gels, networks, and inorganic-organic hybrid materials (IUPAC Recommendations 2007)|url = http://goldbook.iupac.org/A00173.html|journal = [[Pure and Applied Chemistry]]|volume = 79|issue = 10|pages = 1801–1829|doi = 10.1351/goldbook.A00173|isbn = 978-0-9678550-9-7}}</ref> and are noteworthy for being solids that are extremely effective [[thermal conductivity|thermal insulators]]<ref name="NASASpinoff">{{cite web|publisher = [[NASA|NASA Spinoff Technology Transfer Program]]|url = https://spinoff.nasa.gov/Spinoff2010/cg_2.html|title = Aerogels Insulate Against Extreme Temperatures|year = 2010|access-date = 11 December 2016}}</ref><ref name="NASA" /> with very low [[density]].<ref>{{cite web|url = http://stardust.jpl.nasa.gov/news/news93.html|author = NASA|title = Guinness Records Names JPL's Aerogel World's Lightest Solid|date = 7 May 2002|publisher = [[Jet Propulsion Laboratory]]|access-date = 25 May 2009|archive-url = https://web.archive.org/web/20090525181226/http://stardust.jpl.nasa.gov/news/news93.html|archive-date = 25 May 2009|url-status = live|author-link = NASA}}</ref> Aerogels can be prepared in a variety of ways, and though most have been based on silica,<ref name="NASA" /> materials based on [[zirconia]], [[Titanium dioxide|titania]], [[cellulose]], [[polyurethane]], and [[resorcinol]]—[[formaldehyde]] systems, amongst others, have been reported and explored.<ref>{{cite book|title = Aerogels Handbook|editor1-last = Aegerter|editor1-first = Michel A.|editor2-last = Leventis|editor2-first = Nicholas|editor3-last = Koebel|editor3-first = Matthias M.|year = 2011|publisher = [[Springer Science & Business Media]]|isbn = 9781441975898|series = Advances in Sol-Gel Derived Materials and Technologies}}</ref> The prime disadvantage of a silica-based aerogel is its fragility, though [[NASA]] has used them for insulation on [[Mars rover]]s,<ref name="PolymerReinforced">{{cite book|title = Aerogels Handbook|editor1-last = Aegerter|editor1-first = Michel A.|editor2-last = Leventis|editor2-first = Nicholas|editor3-last = Koebel|editor3-first = Matthias M.|year = 2011|publisher = [[Springer Science & Business Media]]|isbn = 9781441975898|series = Advances in Sol-Gel Derived Materials and Technologies|first = Mary Ann B.|last = Meador|chapter = Improving Elastic Properties of Polymer-Reinforced Aerogels|pages = 315–334|doi = 10.1007/978-1-4419-75898-8_15|chapter-url = https://books.google.com/books?id=3FXWzJFOlnAC&q=mars+rover+silica+aerogel&pg=PA315|doi-broken-date = 1 November 2024}}</ref> the [[Mars Pathfinder|Mars ''Pathfinder'']] and they have been used commercially for insulating blankets and between glass panes for translucent day-lighting panels.<ref name="NASA">{{cite web|title = Aerogels: Thinner, Lighter, Stronger|date = 28 July 2011|access-date = 22 November 2016|first = Tori|last = Woods|publisher = [[Glenn Research Center]], [[NASA]]|url = https://www.nasa.gov/topics/technology/features/aerogels.html}}</ref> Particulate gels prepared by the Stöber process can be dehydrated rapidly to produce highly effective silica aerogels, as well as [[xerogel]]s.<ref name="Berg">{{cite book|title = An Introduction to Interfaces and Colloids: The Bridge to Nanoscience|first = John C.|last = Berg|publisher = [[World Scientific Publishing]]|year = 2009|isbn = 9789813100985|pages = 367–368, 452–454|chapter-url =  https://books.google.com/books?id=nmZIDQAAQBAJ&q=St%C3%B6ber+process&pg=PA367|chapter = Colloidal Systems: Phenomenology and Characterization|bibcode = 2009iicb.book.....B}}</ref> 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]],<ref name="NASA" /> as NASA does.<ref name="Stardust" /> The resulting aerogels are very effective thermal insulators because of their high porosity with very small pores (in the [[nanometre]] range). Conduction of heat through the gas phase is poor, and as the structure greatly inhibits movement of air molecules through the structure, heat transfer through the material is poor,<ref name="NASA" /> as can be seen in the image at right where heat from a Bunsen burner transfers so poorly that crayons resting on the aerogel do not melt.<ref name="NASASpinoff" /> Due to their low density, aerogels have also been used to capture interstellar dust particles with minimal heat changes in slowing them down (to prevent heat-induced changes in the particles) as part of the [[Stardust (spacecraft)|Stardust mission]].<ref name="Stardust" />

One method to produce a silica aerogel uses a modified Stöber process and [[supercritical drying]]. The product appears [[Transparency and translucency|translucent]] with a blue tinge as a consequence of [[Rayleigh scattering]]; when placed in front of a light source, the transmitted light becomes yellowish because the blue one has been scattered.<ref name="Steiner">{{cite web|title = Silica Aerogel (TEOS, Base-Catalyzed)|url = http://www.aerogel.org/?p=1027|last = Steiner|first = Stephen|access-date = 21 November 2016|website = aerogel.org}}</ref> This aerogel has a surface area of 700&nbsp;m<sup>2</sup>⋅g<sup>−1</sup> and a density of 0.040&nbsp;g⋅cm<sup>−3</sup>;<ref name="Steiner" /> by way of contrast, the [[density of air]] is 0.0012&nbsp;g⋅cm<sup>−3</sup> (at 15&nbsp;°C and 1&nbsp;[[Atmosphere (unit)|atm]]).

Silica aerogels held 15&nbsp;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.<ref>{{cite journal|last = Mecklenburg|first = Matthias|year = 2012|title = Aerographite: Ultra Lightweight, Flexible Nanowall, Carbon Microtube Material with Outstanding Mechanical Performance|journal = [[Advanced Materials]]|volume = 24|issue = 26|pages = 3486–3490|doi = 10.1002/adma.201200491|pmid = 22688858| s2cid=2787227 }}</ref>

[[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.<ref>{{cite web|url = http://www.zju.edu.cn/c165055/content_2285977.html|title = Ultra-light Aerogel Produced at a Zhejiang University Lab|date = 19 March 2013|publisher = [[Zhejiang University]]|access-date = 21 November 2016|url-status = dead|archive-url = https://web.archive.org/web/20130523111001/http://www.zju.edu.cn/c165055/content_2285977.html|archive-date = 23 May 2013|df = dmy-all}}</ref><ref>{{cite news|last = Whitwam|first = Ryan|date = 26 March 2013|url = http://www.geek.com/articles/chips/graphene-aerogel-is-worlds-lightest-material-20130326/|title = Graphene aerogel is world's lightest material|newspaper = [[geek.com]]|access-date = 21 November 2016|archive-date = 27 March 2013|archive-url = https://web.archive.org/web/20130327134015/http://www.geek.com/articles/chips/graphene-aerogel-is-worlds-lightest-material-20130326/|url-status = dead}}</ref> Stöber-like methods have been applied in the preparation of aerogels in non-silica systems.<ref name="non-silicaStober">{{cite journal|title = Stöber-like method to synthesize ultralight, porous, stretchable Fe<sub>2</sub>O<sub>3</sub>/graphene aerogels for excellent performance in photo-Fenton reaction and electrochemical capacitors|first1 = Bocheng|last1 = Qiu|first2 = Mingyang|last2 = Xing|first3 = Jinlong|last3 = Zhang|journal = [[J. Mater. Chem. A]]|volume = 3|issue = 24|pages = 12820–12827|doi = 10.1039/C5TA02675J|year = 2015}}</ref> NASA has developed silica aerogels with a polymer coating to reinforce the structure,<ref name="PolymerReinforced" /> 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.<ref name="NASA" />

=== 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.{{Efn|At least two researches have reported.}}<ref>{{Cite journal |last1=Gao |first1=Weihong |last2=Rigout |first2=Muriel |last3=Owens |first3=Huw |date=December 2016 |title=Facile control of silica nanoparticles using a novel solvent varying method for the fabrication of artificial opal photonic crystals |journal=Journal of Nanoparticle Research |language=en |volume=18 |issue=12 |pages=387 |doi=10.1007/s11051-016-3691-8 |issn=1388-0764 |pmc=5161756 |pmid=28042282}}</ref><ref>{{Citation |last1=E. Anachkov |first1=Svetoslav |title=Preparation of two-dimensional direct opals by controlled assembly of silica spheres |date=October 2009 |url=https://www.researchgate.net/publication/275032002 |last2=Vasileva Tsanova |first2=Penka}}</ref>

== Notes ==
{{notelist}}

== References ==
{{reflist|30em}}

{{DEFAULTSORT:Stöber process}}
[[Category:Physical chemistry]]
[[Category:Silicon]]
[[Category:Condensation reactions]]
[[Category:Silicon dioxide]]