Surface modification, with chlorosilanes

Surface Modification with Chlorosilanes. Chlorosilanes are volatile and very sensitive to the presence of trace amounts of water. Vapor-phase silanization reactions with rigorous control of the absence of water show that Cl SiMe4-n-type chlorosilanes react with aerosil (nonporous microparticulate silica) as... 

For the surface modification of sihca, the reactive surface group is the silanol group which can be reacted with a mono- or polyfunctional alkoxy- or chlorosilane ... 

Comparing the Cl-contents on the silica samples, pretreated at 973 K, with and without a pre-modification with HMDS, the reaction with strained siloxane bridges seems to be quite significant. However, in a real situation, the different reactions are competitive. HMDS modification creates a surface where the reaction with siloxane bridges can occur free of competition. Therefore, these values may overestimate the degree of reaction in a real situation. The scientific relevance of these data is concealed in the unambiguous proof that (1) chlorosilanes do react with strained... 

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). 

Mesoporous silicas with various pore sizes are hydrophobic by silylation with silanes. Changes in the pore structure as a result of the silylation reactions are monitored in order to assess the distribution of the hydrophobic groups. Extensive polymerization of dimethyldi-chlorosilane causes blocking of the micropore fraction. For silica with pore sizes in the supermicroporous range (2nm), this leads to hydrophobization of almost exclusively the outer surface. While for trimethylchlorosilane a smaller number of molecules react with the surface, modification is more homogeneous and an open structure is optimally preserved. Both silanes lead to lower surface polarity and increased hydrothermal stability, i.e., preservation of the porous structure during exposure to water.12231... 

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. 

Various chlorosilanes may be used for surface modification. Data are shown in Table II.4 on the adhesion of spherical glass particles (diameter 70 /im) to a glass surface modified with different silanes. The degree of modification was monitored on the basis of the contact angle. 

Chlorosilanes are discussed, considering both those with short-chain and long-chain organic groups. Short-chain chlorosilanes are important as model compounds in the general study of silica surface reactivity in vapour phase modification. Furthermore,... 

In a number of cases, (e.g. modification of a silica surface, containing N-fiinctions), alkoxysilanes are more useful than the corresponding chlorosilanes for reacting with the silica surface. 

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. 

Chlorosilanes are used in the chemical modification (derivatization) of silica support materials, hi these cases R and R are chloride and/or a short-chain alkane (e.g., methyl, isopropyl, etc) and R" is the functional group of interest such as octadecyl, octyl, methylphenyl, etc. Monochlorosilanes are used to produce monolayer bonded phases, whereas di- and trichlorosilanes are frequently used to product polymeric layer bonded phases. The general reaction of a chlorosilane with the silica surface is (the example is for a general dimethylalkyl bonded phase). 

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