Xerogels surface

Figure 5-3 Schematic illustration of various binding modes of phenyl phosphonoic acid to guanidine-functionalized xerogel surface (upper), and solid-state51P MAS NMR (lower) of crystalline phenylphosphonoic acid (a) and silica xerogels imprinted with a 2 1 (b) and 1 1 (c) ratios of 1-trimeth-oxysilylpropyl-3-guanidium chloride.

Application of Metal Microprobe Method for Characterizing the Structure of a Xerogel Surface... 

The idea that the specific surface area of xerogels might be a characteristic of the particle size is rather common. This indeed was confirmed by adsorption and electron microscopy studies, which showed comparable particle sizes obtained by these two different methods. However, this agreement is not universal there are cases when adsorption and microscopy studies yield different results. One such example is the treatment of silicic acid hydrogel with an acid at pH 1.9. Under these conditions, the adsorption experiments revealed a substantial decrease in the xerogel surface area, while the electron microscopy indicated a decrease in the particle size. Since the observed decrease in the surface area was accompanied by a significant decrease in porosity, the observed discrepancies were explained by the inaccessibility of the surface in the vicinity of particle-particle contact to the adsorbate molecules. 

For the corresponding correlations polymer - metal containing phase the dimensions, shape and energy characteristics of nanoreactors are found with the help of AFM [4,10], Depending on a metal participating in coordination, the structure and relief of xerogel surface change. 

Type IV isotherms are often found with inorganic oxide xerogels and other porous solids. With certain qualifications, which will be discussed in this chapter, it is possible to analyse Type IV isotherms (notably those of nitrogen at 77 K) so as to obtain a reasonable estimate of the specific surface and an approximate assessment of the pore size distribution. 

The hydrogel is allowed to stand for a few days during which time a process called sinerisis takes place. During sinerisis the condensation of the primary particles, one with another, continues and the gel shrinks further, accompanied by the elimination of more saline solution that exudes from the gel. After three or four days, sinerisis is complete and the gel becomes firm and can now be washed free of residual electrolytes with water. The washed product is finally heated to 120°C to complete the condensation of the surface silanol groups between the particles, and a hard xerogel is formed. It is this xerogel that is used as the LC stationary phase and for bonded phase synthesis. It is not intended to discuss the production of silica gel in detail and those interested are referred to "Silica Gel and Bonded Phases", published by Wiley (1). 

Silica is the support of choice for catalysts used in processes operated at relatively low temperatures (below about 300 °C), such as hydrogenations, polymerizations or some oxidations. Its properties, such as pore size, particle size and surface area are easy to adjust to meet the specific requirements of particular applications. Compared with alumina, silica possesses lower thermal stability, and its propensity to form volatile hydroxides in steam at elevated temperatures also limits its applicability as a support. Most silica supports are made by one of two different preparation routes sol-gel precipitation to produce silica xerogels and flame hydrolysis to give so-called fumed silica. 

The second preparation route uses flame hydrolysis, a versatile way to produce all kinds of oxides with high specific surface areas. The advantages of fumed silica over xerogels are the better mechanical properties and higher purity of the former. 

The solid-state decomposition of OV[OSi(O Bu)3]3 occurs with a precipitous weight loss at ca. 200 °C (as observed by TGA) and a final ceramic yield that is 10% less than the expected ceramic yield [79]. This discrepancy results from volatihzation and loss of HOSi(O Bu)3. However, solution thermolyses of OV[OSi(O Bu)3]3 in n-octane produce xerogels with an approximate composition of V2O5 6Si02 (after drying) with a quantitative ceramic yield (i.e., with no loss of HOSi(0 Bu)3) that have a BET surface area of 320 m g ... 

---