Mesoporous hydrate surface

FIGURE 3.29 Scanning Electron Micrographs (SEM) of methane hydrate, (a, b) Hydrate shells (c, d) mesoporous hydrate surface, (e, f) quenched hydrate, (g, h) hydrate crystal edges. (Reproduced from Stern, L., Circone, S., Kerby, S., Durham, W., in Proc. Fifth International Conference on Gas Hydrates (2005). With permission.)... 

The hydrophobicity of the trimethylsilylated Ti-MCM-41 was estimated from the weight loss of the hydrated samples at 150°C, since this weight loss is generally attributed to physisorbed water on the surface of the mesoporous solids. It was found that there is a nearly linear correlation between amount of water adsorbed on the fully hydrated trimethylsilylated catalysts and the amount of trimethylsilyl groups bounded to the surface as it is shown in figure 5. 

The use of a polymer species as a way to control diffusion to the inside of mesoporous silica was also employed by Lopez and coworkers.67 In this work the researchers polymerized iV-isopropyl acrylamide on mesoporous silica by atom transfer radical polymerization, and took advantage of the changes the polymer experiences upon thermal treatment. The authors discovered that the hybrid material could take up more fluorescein than nonfunctionalized material at temperatures above 45°C. At that temperature the polymer is in a collapsed hydrophobic state and partially covers the negatively charged surface of silica that otherwise repels the negatively charged fluorescein dye. At temperatures below 30°C the polymer exists in a hydrated state in which the chains are expanded. Interestingly, the fluorescein loaded hybrid particles were... 

Thus, the adsorption of methane onto nanosilica A-300, composed of nonporous primary nanoparticles (average diameter 8.1 nm), at standard pressure is a function of temperature and silica hydration. The silica hydration dependence is nonlinear, and maximal adsorption of methane (1.9-1.2 wt% at 200-280 K) is observed at hydration h = 0.l g/g for intact silica. Decrease (on heating) and increase (on wetting) of the silica hydration both lead to a reduction of methane adsorption. Coadsorption of methane and water leads to the appearance of a H NMR signal from WAW at 8h 1 ppm. The amount of this water correlates to concentration of adsorbed methane, because weakly associated bound water is most clustered at the surface of nanosilica composed of nonporous primary nanoparticles. The adsorption of methane on nano/mesoporous... 

Depending on the mode of carbonization of CS synthesized on the basis of a mesoporous silica gel, significant changes in the hydration properties of the surface are observed. 

In the case of the wetted starch powder (Qjj>50 wt%), WBW is practically absent (Figure 5.4 and Table 5.1, C" ) because all the water is strongly bound to the starch molecules, and then-surface area in contact with structured water is high and increases with increasing hydration degree (Table 5.1, S). It should be noted that the surface area (5 , ), the volume (Fnano), and the radius of nanopores increase with increasing hydration but the surface area of mesopores (Table 5.2, decreases with decreasing Qt, value from 71 to 50 wt%. 

Water can fill 70%-90% (dependent on filling conditions) of the total pore volume of LiChrolut EN adsorbent possessing nanopores and narrow mesopores of hydrophobic and partially hydrophilic characters. Adsorbed water is characterized by high associativity. It does not contact to the total surface area of the adsorbent. It is weakly affected by co-adsorbed polar DMSO. Weakly polar chloroform can displace a portion of adsorbed water from narrow pores into larger one or onto the outer surface of polymer particles. Methane can form the hydrate system with water adsorbed in narrow pores of LiChrolut EN adsorbent at low pressures. 

Turov, V.V, Leboda, R., Bogillo, V.I., and Skubiszewska-Zi ba, J. 1997b. Study of hydrated structures on the surface of mesoporous silicas and carbosils by H NMR spectroscopy of adsorbed water. Langmuir 13 1237-1244. 

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