Hydroxylated silica surface: ionization

This strategy consists in the initial modification of the silica surface with organosilanes having suitable anchoring groups, which are either reactive themselves or can be additionally activated for the final attachment of the chiral selector. The choice of the proper silane will depend on the presence of suitable functional groups on the chiral entity to be fixed to the matrix. As macrocyclic antibiotics contain hydroxyl, amine, and carboxylic acid functionalities, they can be linked to the silica surface in a variety of different ways [7, 55]. The obvious drawback of the stepwise assemblage of chiral selectors on the silica surface is the eventual formation of additional polar or ionizable sites on the matrix, which may cause unselective retention of chiral analytes. 

Figure 2. Dependencies of shift of valence vibration band of silica surface hydroxyl groups at adsorption of small organic bases (a) and organosilicon compounds (b) on the maximum negative charge of the bases (a) and on the ionization potential of organosilicon compounds (b).

The surface of porous silica is covered by hydroxyl groups called surface silanols (Si-OH) [1,4]. Silanol groups are responsible for the polarity of the silica surface. They can ionize, the silanol pKa being 9.8. They are responsible for the silica dissolution in basic (pH>8) solutions. Naked silica packings are too polar to be used in RPLC. They are used in normal phase LC with apolar mobile phases. To obtain less polar packings for RPLC, it is necessary to derivatize the polar silanol groups. 

Adsorption from Solution. The adsorption of methyl red dye from an organic solvent has been used as an alternative to nitrogen adsorption, but is more time-consuming (177, 178). The dye is adsorbed only on a silica surface that is fully hydroxylated, that is. has been generated in water and dried at moderate temperature. Adsorption of a fluorescent dye such as Rhodamine B was used by Kautsky and Michel (179). When adsorbed as a cation, the dye fluoresced rose-red, but when not ionized was bluish red. It might be useful for determining surface area of fully hydroxylated silica as present in a sol since it can be adsorbed from water. 

As will be further discussed in Chapter 6, the negative charge on the surface of amorphous silica can be represented in a number of ways. One view is that the silica structure is sufficiently open that a hydroxyl ion can occupy a space in the surface, so that the coordination number of each adjacent silicon atom is increased by a fractional amount (see Figure 1.11). Another representation involves ionization of surface silanol groups ... 

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