Silica dehydroxylated “hydrophobic

The hydrophobic character exhibited by dehydroxylated silica is not shared by the metal oxides on which detailed adsorption studies have been made, in particular the oxides of Al, Cr, Fe, Mg, Ti and Zn. With these oxides, the progressive removal of chemisorbed water leads to an increase, rather than a decrease, in the affinity for water. In recent years much attention has been devoted, notably by use of spectroscopic and adsorption techniques, to the elucidation of the mechanism of the physisorption and chemisorption of water by those oxides the following brief account brings out some of the salient features. 

It is now apparent that isolated silanols have relatively low affinity for water. Thus, the hydrophobic nature of silica is manifested after dehydroxylation when only the siloxane bridges and some isolated silanols (giving an IR band at c. 3750 cm-1) remain. On the dehydroxylated surface the net adsorption enthalpy for water is negative. In this case, the enthalpy of adsorption is lower than the normal enthalpy of condensation. Application of adsorption microcalorimetry has allowed an assessment to be made of the relative extents of the hydrophilic and hydrophobic areas of the surface (Bolis et al., 1991). On the hydrophilic surface, it appears that water is adsorbed via two hydrogen bonds to two silanols - one acting as the hydrogen donor and the other as the acceptor. In the case of the weaker attachment to the isolated OH, the attachment involves one hydrogen bond. 

The energetics of the reaction (reaction 21) for the case of the acceptor adsorption (see Table 2) are not favorable (i.e., AE > 0). Because this adsorption is precisely the type of adsorption onto siloxane bonds (see Fig. 15, below), the unfavorable AE confirms the hydrophobic nature of the dehydroxylated dis-iloxane-rich surface of silica. 

For strongly dehydroxylated silica the surface concentration of siloxane groups is high (Figure 3.19). Because of the shift in the electronic density in going from O atom to Si atom, the formation of a hydrogen bond between the oxygen on the surface of the sample and water molecules is not favored. Thus, the siloxane surface is hydrophobic. 

Fully dehydroxylated covalent surfaces have a homopolar character, with no sensible charge separation. Adsorption on covalent solids is mainly physical covalent surfaces tend to be hydrophobic, and extensive hydroxylation requires high temperatures. Hydroxyl groups present on partially dehydroxylated surfaces show very weak acidic character. They act as hydrogen bond donors with most bases and can protonate only very strong bases. A typical example is silica [35]. 

A partly hydrophobic and organophilic silica can be made very simply by a process patented by Goebel (507b). Fine silica powder is dehydrated and the surface dehydroxylated to an optimum degree by heating it for 1.5 hr in air at 470-530 C. The BET area was 297 m g" but the resulting specific hydroxylated area was 184 m g . When this powder was cooled and refluxed at 118 C in butanol at atmospheric pressure or even when simply exposed to butanol vapor in a closed container for a week, esterification occurred and the surface became about half covered with attached butoxy groups. 

In other oxides, the hydroxylation reaction is strongly limited by a high activation barrier. For instance, in the case of silica, the totally dehydroxylated surface is highly hydrophobic and hydroxylation can only be achieved under severe pre-treatments such as high temperature conditions and pretreatment with aggressive chemicals [15],... 

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