Chlorosilanes, reaction with surface silanol

The increased reactivity of alkoxysilanes in the reaction with isolated silanol groups can be explained by the higher proton affinity of the oxygen atom of the Si-O-Si group, compared to the Cl-atom in the Si-Cl group. This is evidenced by the observation that the physisorption of an alkoxysilane on a silica surface results in a free hydroxyl band shift of approximately 300 cm 1, whereas the physisorption of a chlorosilane induces a shift of only 100 cm 1.89... 

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. 

The reaction of (alkyl)chlorosilanes with a silica surface has been discussed and reviewed in great detail in literature [10], Although 5 different reactions are possible with di-, tri- or tetrachlorosilanes, basically two important surface species are created. The first is a monodentate silyl group, created by the monomolecular reaction of 1 silanol with 1 chlorosilane, according to reaction (A) (cfr. Figure 2). The second surface specie is a bidentate silyl group, created either by a bimolecular reaction (B) or by a consecutive reaction (C). We have reported previously [11] that the surface of MCM-48, prepared by the gemini 16-12-16 surfactant, possesses 0.9 OH/nm2. 

One possible strategy is to use a base to promote the reaction of chlorosilanes with the surface silanols. 

Thus, in a typical base-promoted silanization on silica, it is more likely that both polymerization and surface reaction occur to some extent. Both mechanisms can account for polymerization. The intermediate formed by attachment of the amine to the chlorosilane could react with nucleophiles (i.e., molecular water) other than the surface silanols. In the mechanism described by Blitz et al. the chlorosilane (either attached or in solution) could be hydrolyzed to the trisilanol by molecular water and the trisilanol offers an additional source of silanols for base attachment and subsequent polymerization. Polymerization often results in a thick silane layer on the surface that in many cases is undesirable. 

Once a silica is chosen, a reaction must be run to convert the silanols to surface-attached alkyl groups. This reaction may be accomplished as shown in Figure 6-2b with a mono-, di-, or trimethoxysilane. Each reaction results in a different nonpolar surface, but all are called a C]8 (or ODS) column. When a chlorosilane is used, reaction with a monochlorosilane is preferred to produce a consistent reaction without polymerization. However, not all... 

The silanization reaction in liquid phase (usually in dry toluene) is technologically more convenient, although it is practically impossible to ensure the complete absence of water in a reaction mixture that leads to hydrolysis of chlorosilanes. Using deuterium exchange, Roumeliotis and Unger [55] analyzed surface silanol concentration before and after the modification with different reagents (Table 3-2). 

One approach to solving the problem of residual silanol interactions has involved improvements in the synthetic procedures for the production of hydrocarbonaceous stationary phases. One synthetic approach for the elimination of residual silanol groups involves the reaction of the bonded phase with a small silylating reagent such as trimethylchlorosilane which is presumed to have easier access to silanol groups than bulkier, long-chained chlorosilanes. An alternative, synthetic approach involves surface polymerization of the stationary phase, which is believed to reduce the accessibility of surface silanol groups to polar analytes in the mobile phase. Stationary phases produced by the former method are often referred to as end-capped and stationary phases produced by the latter method are sometimes called base-deactivated. ... 

In the first approach, the reaction is carried out under conditions that exclude, as far as possible, water from the reaction mixture. Typical reaction conditions might involve heating dry silica under reflux with, for example, octadecyl-trichlorosilane in toluene. In the absence of moisture, no hydrolysis of the Si—Cl bonds in the chlorosilane takes place and therefore no polymerisation of the silane occurs. Bonding takes place by elimination of HC1 between the organosilane and one or more of the surface silanol groups. After removal of any excess of silane, the product is hydrolysed to convert unreacted Si—Cl groups to silanol groups as depicted below ... 

Silica particles have surface silanol groups, — SiOH. These are used for chemical bonding of stationary phases by silination reactions with chlorosilanes ... 

Most applications of liquid column chromatography are now made on silica which has been chemically modified (bonded phase chromatography). The modification is made by chemical reaction between the silanol groups and a chlorosilane compound. The carbon radicals of the chlorosilane compound determines the nature of the final column material. Using silanes containing alkyl carbon chains with 8-22 carbon atoms gives the particles hydrophobic surfaces, but more polar surfaces may be obtained by incorporation of alcohol, amino, cyano or other groups in the alkyl chain. 

In a typical reaction, a chlorosilane is reacted with the silanols on the silica surface in the presence of a basic scavenger of the hydrochloric acid (3,4) ... 

Whereas the siloxane groups are generally chemically inert, the reactivity of the silanol groups allows chemical surface modification [28,29]. Thus, the reaction with organosilanes [30-34], silicone fluids or chlorosilanes [35-38] leads to hydrophobic silicas. 

It is not entirely clear what is accomplished by acid treatment. It is generally believed that some species, perhaps iron, is removed from the support. Regarding reaction at silanol groups, it has been suggested (29) that when treated with DMCS (dimethyldi-chlorosilane) there is a reaction between the surface hydroxy groups ... 

Alkoxysilanes are easily prepared by the reaction of chlorosilanes with alcohols. When alkoxysilanes contact inorganic materials, they are hydrolyzed by the moisture absorbed on the surface to give the silanol (R Si(OH)4 ). Further, the... 

Contrarily, the sol-gel process is a wet chemical approach carried out mostly at ambient temperature for the production of nanopartides. As such, the formed particles contain many silanol groups on their surface, which can be easily functionalized, for example, with various chlorosilanes or alkoxysilanes [9]. In addition, the silica-based sol-gel process often produces stable suspensions that can be stored for a long time [10]. The precursors of the sol-gel materials can be easily prepared and purified therefore, very pure silica particles are produced, which is important for applications in electronics and the biomedical field. Due to its ability to control the particle size, size distribution, and morphology through systematic monitoring of reaction parameters, the sol-gel process is ideally suited for many different applications. 

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