Chromatography & Separation
Organofunctional silanes from abcr – information and application areas
Organofunctional silanes are often used in chromatography (separation technique) for separating substance mixtures. These hybrid molecules combine the functionality of a reactive organic group with the inorganic functionality in a single molecule.
Organofunctional silanes have two reactive centres. One of these is characterised by at least one hydrolytically stable carbon-silicon bond. Organic remnants such as alkyl, aryl, perfluorophenyl alkyl, epoxy, aldehyde or anhydride groups can thereby be introduced.
The other reactive centres at the silicon atoms consist hydrolysable groups. These are mainly chloro, alkoxy and amino groups, which can be easily hydrolysed into silanols. Moreover, silicon can covalently bond to inorganic surfaces via a siloxane bond. The hydrophobic nature of the silicon-carbon bond provides improved hydrolysis stability at the boundary between the siloxane bond and the substrate under extreme pH conditions. This results in a longer lifespan due to the improved protection of the mechanical properties of the silica material. A catalyst is generally required in order to initiate the hydrolysis reaction. A mild acid or base is typically used. Chlorsilanes and aminosilanes react autocatalytically as the hydrolysable groups create hydrochloric acid or amine in situ.
Trifunctional, difunctional and monofunctional silanes from abcr
- Trifunctional silanes such as trimethoxysilanes initially hydrolyse. They then condense with the surface hydroxyl groups while forming hydrolytically stable covalent bonds. Ultimately, they cross-link and form a multi-layered silsesquioxan layer. This multilayer consists of an inorganic intermediate phase. It may also diffuse in the porous matrix in order to create an interpenetrating network within the inorganic substrate. This network leads to a hydrolytically stable silicon dioxide carrier for the liquid chromatography.
- Difunctional silanes such as dimethoxysilanes also initially hydrolyse and then condense with surface hydroxyl groups. Difunctional silanes can also form polymeric materials, which can adhere to various types of surfaces. Therefore, difunctional silanes can be used in order to produce bound phases for liquid chromatography. They are also used for the production of siloxane polymers for capillary columns, which are used in gas chromatography.
- Monofunctional silanes form self-assembled monolayers (SAM) on the surface of the inorganic substrate, which offers advantages compared to tri and difunctional silanes as the film thickness can be easily controlled.
Endcapping of easily convertible silanol groups that remain on the inorganic surfaces is usually achieved with trimethysilylation reagents. A tailored solution of trimethylchlorsilane and hexamethyldisilazane is more effective as both reagents by themselves. Dimethylaminotrimethylsilane is a highly effective silanol scavenger due to its extreme reactivity with hydroxyl groups.