Silanol surface: energy

The surface of silica is covered by a layer of acidic silanol and siloxane groups. This highly polar and hydrophilic character of the filler surface results in a low compatibihty with the rather apolar polymer. Besides, highly attractive forces between silica particles result in strong agglomeration forces. The formation of a hydrophobic shell around the silica particle by the sUica-sUane reaction prevents the formation of a filler-filler network by reduction of the specific surface energy [3]. 

Other evidence for the presence of calcium on the surface of tobermorite comes from work on the surface energy of tobermorite. The surface energy of calcium hydroxide is 1180 ergs per sq. cm. (6), that of a pure silanol surface is 129 (7), and that of tobermorite is 386 (8), which is close to the geometric mean of the two other figures. 

Perhaps, the most easy and common way to change the surface properties of a solid is to submit it to a heat treatment. Silicas, for example, are heat treated so as to enhance their ability to adsorb water. Upon heat treatment, surface hydroxyls condense. The variation of the number and nature of surface hydroxyls or silanol groups may be evaluated using a series of techniques [6]. However, it is only recently that the concomitant variation of surface energy characteristics was evidenced by IGC [7]. 

The solubility of antibiotic chloramphenicol (2,2-dichloro-N-[l,3-dihydroxy-l-(4-nitrophenyl) propan-2-yl]acetamide) in water is relatively low ( 2.5 g/L at 25 C). Therefore, to prepare chloramphenicol/silica composites, the impregnation method was used (Krupska et al. 2006). The LT H NMR spectroscopy study of these composites showed that the free surface energy Ys of interfacial water decreases with increasing amount of chloramphenicol in the composition (Figure 1.165, curve 1). This can be interpreted as displacanent of water from the silica surface by the drug molecules. However, despite the maximal amount of chloramphenicol (1 mmol/g) greater than the content of surface silanols (Cqh 0.6-0.7 mmol/g for A-300 samples), the perturbation... 

FIGURE 1.165 Changes in (a) the free surface energy of nanosilica A-300 impregned by chloramphenicol ( H NMR), and (b) perturbation degree of surface silanols due to the chloramphenicol adsorption (FTIR). 

On this basis the porosity and surface composition of a number of silicas and zeolites were varied systematically to maximize retention of the isothizolinone structures. For the sake of clarity, data is represented here for only four silicas (Table 1) and three zeolites (Table 2). Silicas 1 and 3 differ in their pore dimensions, these being ca. 20 A and 180 A respectively. Silicas 2 and 4, their counterparts, have been calcined to optimise the number and distribution of isolated silanol sites. Zeolites 1 and 2 are the Na- and H- forms of zeolite-Y respectively. Zeolite 3 is the H-Y zeolite after subjecting to steam calcination, thereby substantially increasing the proportion of Si Al in the structure. The minimum pore dimensions of these materials were around 15 A, selected on the basis that energy-minimized structures obtained by molecular modelling predict the widest dimension of the bulkiest biocide (OIT) to be ca. 13 A, thereby allowing entry to the pore network. 

---