Trichlorosilane layers

Fig. 63. Force-distance curve on Si(lll) covered with a self-assembled octadecyl-trichlorosilane layer.

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobihty of individual molecules is restricted. This lack of mobihty is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibihty problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quahty. Alkyl silanes remain, however, ideal materials for surface modification and functionalization apphcations, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

More complicated surface structures can be produced by changing the functionality of the silylating agent and the conditions under which the reaction is carried out. The use of di- or trichlorosilanes in the presence of moisture can produce a crosslinked polymeric layer at the silica surface, as shown in Fig. 3.2a (if). Monomeric bonded phases are preferred, as their structure is better defined and they are easier to manufacture reproducibly than the polymeric materials. 

It is seen that the trichlorosilane reacts with the silanol groups to form siloxane bridges. Subsequently the residual chlorines are hydrolyzed. Under carefiiUy controlled reaction conditions it is possible to obtain a product in which the hydrocarbonaceous layer at the surface is similar to that in a corresponding monomeric bonded phase. However, the hydrolysis of chlorines that did not react with surface silanbis may result in a silanol concentration at the surface that is higher than that in the silica gel proper used as the starting material for the reaction with alkyltri-chlorosilanes. 

Trichlorosilane derivatives of large dye molecules are difficult to purify and owing to moisture sensitivity are hard to handle. Their organic solutions tend to become turbid rather quickly owing to the formation of insoluble polymers. Thus, solutions must be replaced frequendy. An exception may be the combination of self-assembly and surface chemical reaction (186-189,202). On the other hand, co-substituted alkyltrichlorosilane derivatives are easy to synthesize the purify. These could be used for the engineering of surface free energy through the control of chemical functionalities in their SAMs, or as active layers for attachment of biomolecules in biosensors. 

Polymerization is not possible in the complete absence of water or when reactions are carried out using monochlorosilanes. However, trichlorosilanes are attractive because it is possible to increase the strength of the adsorbed silane layer through cross-linking between adjacent molecules. The other approach, the exclusion of trace quantities of water, especially in solution, is extremely difficult and costly. 

The gas-phase modification of the CSC technique will be exemplified by means of a case study, aiming at the creation of a Si3N4 layer on the silica surface, using trichlorosilane (HSiCl3) and ammonia (NH3) as the cycling gases. 

Allyl trichlorosilane (0.47 mmol) was added to a solution of the METHOX catalyst (0.02 mmol), diisopropylethylamine (2 mmol) and aldehyde (0.4 mmol) in acetonitrile (2 mL) under nitrogen at — 40 °C. The mixture was stirred at the same temperature for 18 h, after which the reaction was quenched with aqueous saturated NaHC03 (1 mL). The aqueous layer was extracted with ethyl acetate (3 x 10 mL) and the combined organic extracts were washed with brine and dried over Na2S04. The solvent was removed in vacuo and the residue purified by FC on a silica gel column (15 cm x 1 cm) with a petroleum ether-ethyl acetate mixture (95 5) to produce (S)-(—)-l-phenyl-but-3-en-l-ol (95%, 96% ee). 

The production of trichlorosilane in the fluidised layer by continuous technique. 

Trichlorosilane can be obtained by direct synthesis from free silicon and anhydrous hydrogen chloride in the fluidised layer ... 

Trichlorosilane is synthesised in fluidised layer reactors, similar to the apparatuses for the direct synthesis of alkyl- and arylchlorosilanes. For example, it can be a vertical steel cylindrical apparatus with a gas distribution device in the form of a conical bottom. The upper (expanded) part of the tower (expander) separates small particles of silicon carried from the fluidised layer by gas flow. The expander has filters from porous metal inside. (Steel 3). The reactor and expander are electrically heated. Trichlorosilane can also be synthesised in vertical section reactors. 

Preparation of a multilayer is possible using the self-assembled mono-layer method and appropriate molecular design (Fig. 4.37). In this method, a trichlorosilane amphiphile with a long chain terminated in a double bond was used as the monolayer component. This compound was hydrolyzed to silanol to provide a self-assembled monolayer that covalently immobihzed to... 

Wagner and Pines [27] carried out the hydrolysis of trichlorosilane on the previously wetted silica surface with a specific surface area of about 300 m /g and obtained samples containing from 7.4 to 16.7 % wt. of HSi03/2. The surface of the silica modified in this way exhibits properties inherent in polyhydrosiloxane, namely hydrophobic nature, ability to evolve hydrogen under the action of an alkali, to reduce silver ions and other ions, and to add alkenes at high temperatures (about 450°C). Thus, in the case of the silica surface with a deposited layer of polyhydrosiloxane the chemisorption of ethylene, pentene, octene, cyclohexene, and butadiene has been carried out. 

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