Sonogelation

The initial silica sol consisted of 11 ml of tetramethoxy silane (TMOS), 5 mL of distilled water, and 1 drop of 12 N HC1. A monophasic solution was prepared by the sonogel method 9). Under continuous sonication, a total of 30 ml of tetraethoxy silane (TEOS), 7.5 mL of distilled H2O, and 11 mL of TMOS were addded in a dropwise fashion to form the precursor solution. The final silica sol was composed of a 1 1 5 mole ratio of TEOS TMOS H2O. The acid catalyst was present as a... 

Sonogels Gel exposed to ultrasound in the 20-kHz range prior to autoclave treatment Assists in the formation of multicomponent gels... 

Ballarin et al. used sonogel-carbon composite electrodes containing CoPc for the oxidation of cysteine The catalytic process is mediated by the generation of the Co Pc species. Oxidation of sulfide ion was catalyzed by microcrystals of CoPc immobilized at a surface of paraffin impregnated graphite electrode. Filanovsky reported the use of a carbon electrode prepared by hand pressing carbon powder mixed with CoPc and a polymer solution (poly(methyl methacrylate)) in dichloromethane, followed by evaporating the dichloro-methane, for the analysis of mercaptoethanol in petroleum and cysteine in human urine with detection limits of 8 x 10 M for the former and 2 x 10 M for the latter . 

Ultrasound is used for the synthesis of almost all transition metal oxides. Sonogels of zirconia, the representative of ferroelastics, were obtained through the sonication of zirconium tetrapropoxide in acetic acid. Time of gelling is reduced when sonicated and the sonogel contains nanosized particles (10-15 nm) [259]. 

D. R. Vollet, D. A. Donatti, and A. Ibanez Ruiz, A SAXS Study of Kinetics of Aggregation of TEOS-Derived Sonogels at Different Temperatures, J. Non-CrysL Solids, 288, pp. 81-87, 2001. 

De la Rosa-Fox N, Esquivias L, Craievich AE, Zarzycki J (1990) Stmctural study of silica sonogels. J Non-... 

Vollet DR, Ntmes LM, Donatti DA, Ibanez Ruiz A, Maceti H (2008) Structural characteristics of silica sonogels prepared with different proportions of TEOS and TMOS. J Non-Cryst Solids 354 1467-1474 Livage J, Herrry M, Sanchez C (1988) Sol-Gel Chemistry of Transition Metal Oxides. Prog. Solid State Chem.l8 259-341... 

Figure 20.2. Equilibrium diagram of TE0S/H20/Et0H system showing the immiscibihty area below the dotted line. Sonogels for n = water/TEOS molar ratio = = 5 is indicated. The reaction pathway of the heterogeneous...

Figure 20.2 shows the immiscibUity area in the phase diagram of the r f S /H20/Et0H system. The sonogel method permits to obtain homogeneous mixtures below the dotted line. 

An alternative experimental setup involves the use of an ultrasonic bath [50]. In these cases, a much lower ultrasonic intensity is supplied to the reactants and, consequently, the resulting samples do not offer the characteristic sonogel features. 

Figure 20.4. Evolution of the gelation time for water/TEOS = 4 pure sonogels as a function of the ultrasrmic energy dose. Inset Evolution of the gelation time for water/TEOS = 4 pure sonogels as a function of l/T for different ultrasonic energy dose. Adapted with permission from [17].

It is possible to apply the sonogel approach in the field of OIHM based on ormosils (Chap. 13). This approach to these materials incorporates an organic phase, very commonly PDMS, in the inorganic precursor (TEOS or TMOS) sol in combination with sonication of the precursors, producing a sono-ormosil (also called hard ormosil) after gelation [41, 57-59]. The processing and properties of sono-ormosils and derived materials have been recently reviewed [60, 61]. 

Donatti, Vollet, and their collaborators carried out a systematic study [66-69] of wet sonogels prepared in excess of water, by previous pH adjusting with different amounts of dUuted NH4OH. 

Figure 20.9. Schematic illustration of the proposed model that grows during the transition from sol to gel. The agglomeration of these elementary units (left) gives rise to the particulate and microporous structure of sonogel (right). Adapted with permission from [26].

The sonogels are autoclave dried in supercritical conditions of ethanol (Chap. 2). In this section, the effect of drying on the structure is discussed in terms of WAXS, N2 physisorption, and SAXS. 

WAXS studies on the structure of sono-aerogels established that the atomic sonogel stmcture is very cross-linked. Figure 20.10 represents the sonogel and silica glass reduced RDF,... 

Figure 20.10. Reduced RDF of a silica sonogel (black line). The arrow indicates where the curve oscillations start to differ from those of the silica glass RDF (clear line), included for comparison. Adapted with permission from [70].

Figure 20.10 shows that beyond r 1 nm the correspondence between sonogel and silica bulk glass RDF maxima deviates sharply [70]. This is evidence for sonogels being formed by monosized elementary particles of 1 nm radius. 

Table 20.3. Structural parameters deduced from silica glass, silica gels, and silica sonogel...

The atomic arrangements around Ti atoms in titania-doped (5 mol%) silica sono-aerogel was deduced from Ti K-edge XAS [74]. The calculated PRDF is shown in Figure 20.11. The sonogel presents two well-defined peaks, corresponding to Ti-O and Ti-cation distances (Table 20.4). The network is formed up to the second neighboring level. 

Figure 20.11. PRDF Ti for silica aerogels doped with titania at 5 mol% sonogel (continuous thin line), classic gel (dashed line) and anatase (continuous bold line). Adapted with permission from [32].

The sonogel route produces network aerogel structure close to that one of a bulk glass. 

The MAS-NMR results (Figure 20.12) indicate that the sonogel structure (Sn, n = Rw) is highly cross-linked. The spectra are interpreted by means of the Q" components (see Sect. 20.1.6). The chemical shift (i5) gives a measure of the deviation of the chemical environment of each Si atom. 

The absence of the component indicates a polymeric formation. However, the existence of noncomplete Si-O-Si bridges in the gels indicates the existence of some -OH surface coverage given by the number of OH per Si atom ( oH/ si)- It decreases when the sonogel was prepared with a water concentration much greater or much less than the... 

A number of -OH radicals could be buried in the sonogel structure [22]. 

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