Nature of the Silica Surface

The nature of the silica surface may be changed by modification with a variety of molecules, causing a still broader range of applications. A survey of these will be given in chapter 2. 

Besides boron and silicon compounds, many other hydrogen sequestering agents have been used in the last decades to modify the silica surface. The reasons for such a surface modification vary from a purely fundamental study (gaining a better insight in the nature of the silica surface) to the creation of new materials, with outstanding properties. 

On the other hand, in support of the hypothesis that the polarization of water might play an important role in the hydration force between silica surfaces, one should note that the polarization model predicts an increase in the hydration force at higher ionic strength [30], which can be indeed observed in Fig. 5, by comparing the experiments at c =0.01 M (pH=3) with those at cE= 1 M. While both the gel-induced steric repulsion and the polarization model are consistent with the present experimental data, a final decision about the microscopic origin of the hydration force in the case of silica should be postponed until more accurate data or additional information regarding the nature of the silica surface will become available. 

Luminescence probing techniques have demonstrated their versatility in characterizing the environment found on, the mobility and accessibility allowed at, and the reactivity occurring at gas-solid and liquid-solid interfaces. Pyrene can be used to monitor the interactive nature of the silica surface, and 1-AP and PCA faithfully distinguish the silanol functionality present at the silica gel surface. MCB silica gel possesses a higher relative geminal silanol concentration compared to the FS-662 silica gel ... 

This observation illustrates the nonuniform nature of the silica surface, with active centers having different adsorption potentials. 

Sample molecules which adsorb with localization (Q" lafge) do not exhibit weak localization on alumina (6). While isomers of such compounds frequently exhibit different /f" values, which in some cases resemble the differences caused by weak localization, the /C" values of localized isomers do not obey Eq. (11-3). Weak localization is also not encountered, in adsorption on silica. Aromatic hydrocarbons with marked differences in relative linearity are generally adsorbed equally strongly on silica (52,69), whereas such compounds can exhibit wide differences in relative adsorption on alumina as a result of weak localization. The difference between alumina and silica in this respect probably arises from the essentially noncrystalline nature of the silica surface and the greater area corresponding to a silica site, as in Fig. 10-4. 

Figure 19.8 Influence of the nature of the silica surface on the release of [P4444][lbu] at 20% loading in PBS (pH 7.4) non-pre-calcined silica (hydrophilic silica red) and precalcined silica (hydrophobic silica blue).

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