Silica multilayers

The dense silica (DS) process involves the exposure of titania particles to aqueous silica solutions of increasing silica concentration. The process is examined in this chapter by relating silica adsorption on titania surfaces to solution pH and concentration and to the various monomeric, multimeric, and polymeric species present in aqueous solutions of silica. Microelectrophoresis and gas adsorption studies reveal that adsorption of monomeric silica occurs via hydrated cation sites that constitute only approximately 40% of titania surfaces. These anchoring sites provide a base for complete surface coverage and buildup of silica multilayers (coatings), a buildup that occurs when the silica concentration is increased sufficiently at the chosen pH (around 10 in the DS process) to induce polymerization. 

Prediction of Interference Enhanced Raman Scattering in Anatase-Silica Multilayer Films... 

Adsorption may in principle occur at all surfaces its magnitude is particularly noticeable when porous solids, which have a high surface area, such as silica gel or charcoal are contacted with gases or liquids. Adsorption processes may involve either simple uni-molecular adsorbate layers or multilayers the forces which bind the adsorbate to the surface may be physical or chemical in nature. 

Adsorbents such as some silica gels and types of carbons and zeolites have pores of the order of molecular dimensions, that is, from several up to 10-15 A in diameter. Adsorption in such pores is not readily treated as a capillary condensation phenomenon—in fact, there is typically no hysteresis loop. What happens physically is that as multilayer adsorption develops, the pore becomes filled by a meeting of the adsorbed films from opposing walls. Pores showing this type of adsorption behavior have come to be called micropores—a conventional definition is that micropore diameters are of width not exceeding 20 A (larger pores are called mesopores), see Ref. 221a. 

More advanced insulations are also under development. These insulations, sometimes called superinsulations, have R that exceed 20 fthh-°F/Btu-m. This can be accomplished with encapsulated fine powders in an evacuated space. Superinsulations have been used commercially in the walls of refrigerators and freezers. The encapsulating film, which is usually plastic film, metallized film, or a combination, provides a barrier to the inward diffusion of air and water that would result in loss of the vacuum. The effective life of such insulations depends on the effectiveness of the encapsulating material. A number of powders, including silica, milled perlite, and calcium silicate powder, have been used as filler in evacuated superinsulations. In general, the smaller the particle size, the more effective and durable the insulation packet. Evacuated multilayer reflective insulations have been used in space applications in past years. 

To meet the 2001 U.S. energy standards and the 2003 phase-out of HCFCs, there is a great incentive to develop a significantly better thermal insulation. The most dramatic approach would use vacuum panels for insulating the cabinet. A number of U.S. and Japanese manufacturers have developed such panels and placed these kinds of refrigerators in homes. The panels consist of multilayer plastic envelopes filled with precipitated (fumed) silica. The claimed thermal conductivity is one-fourth that of polyurethane foam. The two major obstacles are cost and the maintenance of vacuum for twenty years. 

Burrell and Hurtubise (.32) investigated calibration curves extended well beyond the normal linear range for RTF and RTF of benzoCf)quino-line adsorbed on a silica gel chromatoplate under neutral and acidic conditions. As the benzoCf)quinoline concentration increased, the RTF curves leveled off, whereas the RTF curves passed through a maximum and then decreased. The extended calibration curves along with fluorescence and phosphorescence spectra and phosphorescence lifetimes for benzoCf)quinoline revealed differences in the RTF and RTF phenomena. For example, it was determined that RTF could arise from molecules adsorbed on the surface and in multilayers of molecules, whereas phosphorescence was only generated from molecules adsorbed on the surface of the chromatoplate and not in the multilayers. ... 

Certain azacrown-appended cholesterol derivatives can form unique vesicular or lamellar structures in the absence and the presence of metal salts in aqueous solution.187-190 These superstructures created from the azacrown-appended cholesterol derivatives are useful as a template for the transcription into the silica structure. Azacrown-appended cholesterol gelator creates the novel multilayered spherical structure in acetic acid, whereas this gelator results in the fluffy globular aggregates in the presence of Pd(N03)2.191... 

Caruso, R.A., Susha, A. and Caruso, F. (2001) Multilayered titania, silica, and laponite nanopartides coating on polystyrene colloidal templates and resulting inorganic hollow spheres. Chemistry of Materials, 13, 400—409. 

In a follow-on experiment, we formed a LbL shell on halloysite consisting of multilayers of the polycation PEI and anionic silica of 7 and 10 nm diameters (realizing by this positive/negative component alternation). Figure 14.11 shows the... 

Fig. 14.17 SEM image of (halloysite/PEI)7 multilayers (A) cross-section view, (B) top view, and (C) cross-section view of the (halloysite/PEl)2 + (PEI/silica/PEI/halloysite)3 multilayer. The sample is coated with 2 nm Pt, SEM Hitachi S-900, Japan, voltage 25 kV.

Encapsulation via the layer-by-layer assembly of multilayered polyelectrolyte (PE) or PE/nanoparticle nanocomposite thin shells of catalase in bimodal mesoporous silica spheres is also described by Wang and Caruso [198]. The use of a bimodal mesoporous structure allows faster immobilization rates and greater enzyme immobilization capacity (20-40 wt%) in comparison with a monomodal structure. The activity of the encapsulated catalase was retained (70 % after 25 successive batch reactions) and its stability enhanced. 

BDE 17, 28, 47, 66, 85, 99, 100, 153, 154 183 Palm oil Palm oil Extraction by dialysis in hexane using a semi-permeable membrane. Purification thorugh multilayer column filled with neutral silica, silica modified with sulfuric acid (44%, w/w), and silica modified with KOH. Gas Chromatography (VF-5MS Factor Four, Varian) IT-MS 0.07-1.3 pg (instrumental limit of detection) [42]... 

Cleanup of sediment samples. The extracted sediment samples were cleaned up using a multilayer column. The multilayer glass column consisted of the following materials (from top to bottom) 1 cm water-free sodium sulfate, 1 g silica, 7 g 44% sulfuric acid on silica, 1 g silica, 2 g 33% sodium hydroxide on silica, 1 g silica, 1.5 g 10% silver nitrate on silica, and a small piece of silanized glass wool. After addition of each layer, the column was compacted by tapping... 

---