Bulk silica gels

Table 3.1 lists commercially available bulk silica gels for the manual preparation of PLC layers snitable for straight phase chromatography. These bulk silica gels are produced by different manufacturers, and in some cases they are offered with binding additives and fluorescent indicators. The data summarized in this table are traceable to product information from the concerned manufacturers. 

Fig. 8-4 Molecular imprinting in bulk silica gel a sol-gel hydrolysis and polycondensation (pH 2.0), b capping of surface silanols...

Despite the fact that the surface is part of a given compound sample, its chemical properties can be very different firom those observed in the bulk. Silica gel is a very good example whereas the bulk is constituted by silicon atoms bonded only to oxygen, OH groups can be found in the surface [1], which are responsible for its most usefiil characteristic, namely, its reactivity toward organic species, producing organofunctionahzed surfaces. 

TABLE 6-2. METHODS FOR MAKING LARGE-SIZE CRACKLESS BULK SILICA GELS AND GLASSES ... 

Recently, Kirkbir et al. (1998) showed that drying at subcritical pressure of almost half of the critical pressure is enough for making the pore size and pore volrnne the same as those realized under supercritical pressure. This means that fractureless formation of bulk silica gels and glasses are possible by reduced pressure. 

TABLE 6-4. COMPARISON BETWEEN THE ALKOXIDE METHOD AND AEROGEL METHOD IN DRYING FOR PREPARATION OF BULK SILICA GELS AND GLASSES BASED ON THE WORK OF GRANDI ET AL. (2003) ... 

The silica gel surface is extremely polar and, as a result, must often be deactivated with a polar solvent such as ethyl acetate, propanol or even methanol. The bulk solvent is usually an n-alkane such as n-heptane and the moderators (the name given to the deactivating agents) are usually added at concentrations ranging from 0.5 to 5% v/v. Silica gel is very effective for separating polarizable materials such as the aromatic hydrocarbons, nitro hydrocarbons (aliphatic and aromatic), aliphatic ethers, aromatic esters, etc. When separating polarizable substances as opposed to substances with permanent dipoles, mixtures of an aliphatic hydrocarbon with a chlorinated hydrocarbon such as chlorobutane or methylene dichloride are often used as the mobile... 

Chapter 3 through Chapter 8 deal with the basic aspects of the practical uses of PLC. Chapter 3 describes sorbent materials and precoated layers for normal or straight phase (adsorption) chromatography (silica gel and aluminum oxide 60) and partition chromatography (silica gel, aluminum oxide 150, and cellulose), and precoated layers for reversed-phase chromatography (RP-18 or C-18). Properties of the bulk sorbents and precoated layers, a survey of commercial products, and examples of substance classes that can be separated are given. 

Aluminum oxides, similar to silica gels, are available as bulk materials and as precoated plates, to be used not only for straight phase adsorption chromatography, but also for partition PLC (see Table 3.3 and Table 3.4). In particular, the aluminum oxide type 150 (i.e., mean pore diameter 150 A [15 tun]) is suitable for partition chromatographic purposes. 

Silica gel treated with EDTA/NaOH CH2Cl2-MeOH-Water (58 35 7) UV 365 nm Bulk drug... 

The chiral recognition ability of the insoluble (+)-l was estimated by HPLC using a column packed with small particles of l.25 However, this column showed a poor efficiency because of a low theoretical plate number. This defect was overcome by coating soluble poly(TrMA) with a DP of 50 on macroporous silica gel.26 The 1-coated silica gel had higher resistance against compression and longer lifetime than the CSP of insoluble 1. Moreover, the two 1-based CSPs show quite different chiral recognition for several race-mates, which may be attributed to the different orientation of 1 in bulk and on the surface of the silica gel.27... 

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