Silanization
Silanization is the attachment of an organosilyl group to some chemical species. Almost always, silanization is the conversion of a silanol-terminated surface to a alkylsiloxy-terminated surface. This conversion confers hydrophobicity to a previously hydrophilic surface. This process is often used to modify the surface properties of glass, silicon, alumina, quartz, and metal oxide substrates, which all have an abundance of hydroxyl groups. Silanization differs from silylation, which usually refers to attachment of organosilicon groups to molecular substrates.
Mechanism
Silanization mechanisms vary with substrate and with silanization reagent. In the usual circumstance, surface MOH groups react as nucleophiles with silyl chlorides or silyl alkoxides. The stoichiometry for these reactions are shown:M is typically Si, but could be many other elements.
The process is assumed to follow the pathways that apply to silylation of molecular substrates, such as alcohols.
Properties of organofunctional alkoxysilanes
Silanizing reagents often have the formula 3-nnSiR' or, where R = methyl or ethyl and R' is a long alkyl chain or a functionalized alkyl group.| Silane coupling agent | Structural formula | Comment | ||
| octadecyltrichlorosilane diethoxymethylsilane dimethylethoxysilane trimethoxysilane | ||||
| N--3-aminopropyltrimethoxysilane | diamino-terminated | |||
| bistetrasulfide | crosslinking group for silica-rubber composites | |||
| 3-mercaptopropyltrimethoxysilane | thiol-terrminated -dimethylethoxysilane | epoxide-containing |
The nature of the organic group on the silanization agent strongly influences the properties of the surface. Simple alkyl-containing silanizing groups confer hydrophobicity.
When side chains of the silane compound are amines or thiols, the surfaces assume the properties of those functional groups. These surfaces are susceptible to further reactions characteristic of the appended functional groups. Grafting can be performed, for example.
Applications
Reversed-phase chromatography
A popular stationary phase is an octadecyl carbon chain -bonded silica. Such materials are produced by reaction of silica gel with trimethoxyoctadecylsilane. The individual links involve silanol groups displacing the methoxy groups, forming an Si-O-Si bonds. This reaction changes the hydrophilic silanol groups into hydrophobic coatings. Other silanizing groups install C8-bonded silica, pure silica, cyano-bonded silica and phenyl-bonded silica. Note that C18, C8 and phenyl are dedicated reversed-phase resins, while cyano columns can be used in a reversed-phase mode depending on analyte and mobile phase conditions.Glassware
Silanization of glassware is a common application that increases the hydrophobicity of a glass container. Thus treated, the glassware produces a flat meniscus and allowing for more complete transfer of aqueous solutions.Silanization of glassware is used in cell culturing to minimize adherence of cells to flask walls. Additionally, the silanization process is also used in biomedical fields for a wide variety of purposes, including anchoring DNA to substrates.
Silanization of glassware can be achieved by dipping into a solution of 5-10% dimethyldiethoxysilane followed by heating.
Silanization is also used for DNA chips. Nucleic acids do not bond to untreated glass surfaces. Silanization can be providing a better bonding site for the nucleic acids onto the chip. A common silane used to treat glass surfaces for this application is trimethoxysilane, which increases the number of reactive thiol groups on the surface The nucleic acids can bond to these available thiol groups on the surface of the glass DNA chip after silanization occurs.