Physisorbed water

Content of chemisorbed and physisorbed water on a silica gel surface ... 

The BET monolayer capacity N, calculated from the first water isotherm included both physisorbed and chemisorbed water, whereas that from the second isotherm iV, included only the physisorbed water. Thus the difference (iV, - N,) gave the amount of chemisorbed water taken up as hydroxyl groups during the isotherm determination. N, + iV ) was therefore the total concentration of hydroxyl groups on the surface when the second water isotherm was being measured. 

Thus in Fig. 5.22 the first outgassing at 25°C will have removed physisorbed water only, so that curve (1) is the isotherm of physical adsorption on the fully hydroxylated material. The 300°C outgassing, on the other hand, will have removed all the ligand water and the majority of the hydroxyl groups when isotherm (4) is determined, therefore, the Ti ions will chemisorb ligand water at low relative pressure, but the number of hydroxyl groups reformed will be very small. 

Water intrusion-extrusion isotherms performed at room temperature on hydrophobic pure silica chabazite show that the water-Si-CHA system displays a real spring behavior. However, Pressure/Volume differences are observed between the first and the second cycle indicating that some water molecules interact with the inorganic framework after the first intrusion. 29Si and especially H solid state NMR and powder X-ray diffraction demonstrated the creation of new defect sites upon the intrusion-extrusion of water and the existence of two kinds of water molecules trapped in the super-cage of the Si-CHA a first layer of water strongly hydrogen bonded with the silanols of the framework and a subsequent layer of liquid-like physisorbed water molecules in interaction with the first water layer. 

The H MAS NMR spectra of the starting Si-CHA material before and after four water intrusion-extrusion cycles are displayed in Figure 2. The samples were dehydrated at 110°C under reduced pressure during 12h in order to remove as much as possible the H signal from physisorbed water molecules. Note the good resolution of the spectra. Thus, thanks to decomposition with DMfit software [2], 6 resonances at 0.9, 1.3, 1.9, 2.2, 3.2 and 4.7 ppm are observed. According to their chemical shifts they can be assigned to... 

It is worth to note that XRD and H DQMAS NMR data are in excellent agreement, offering a detailed view of the water behavior upon intrusion. Indeed, both techniques evidenced two populations of water a first layer of water strongly connected to the framework and a subsequent layer of liquid-like physisorbed water in interaction with the water of the first layer. 

What is the state of the moisture associated with the solid (i.e., bulk water, water of hydration, physisorbed water, etc.) ... 

ZnxZryFz. A fluoro complex of zirconium, ZrF62, and an ammine complex of zinc, ZnfNFF), have been used to grow ZnxZryFz films on silicon. The Zn/Zr/F ratio was 0.08/1/0.32, and the films contained hydroxyl groups and physisorbed water.128... 

Physiological functions, of vitamins, 25 784 Physiology, of ascorbic acid, 25 766-773 Physisorbed water, 23 71-72 Physostigmine, 2 817-818 Phytane, 18 592... 

On Au(lll) with o =10, a single H2O desorption peak is observed at 168 K, as shown in the right panel of Figure 6. This peak is due to physisorbed water on the oxygen monolayer. This coverage of oxygen blocks the high temperature desorption state, which indicates that the direct adsorption of water to the Au(lll) surface is blocked. 

The surface chemical groups present on the adherend surfaces consist of species chemically bonded to the surface. A real surface exposed to the environment contains not only the surface chemical groups but also physisorbed water, carbon monoxide and carbon dioxide molecules. The amount adsorbed depends on the surface free energy of the adherend and can range in thickness from portions of a monolayer to multilayers of material. 

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 terms dehydration and dehydroxylation are often confused in literature, which is not surprising, since it is very difficult to separate the two processes. Dehydration is the loss of physisorbed water as a function of increasing temperature, whereas dehydroxylation stands for the condensation of hydroxylgroups to form siloxane bonds (reaction (A)). 

Bermudez20 showed by NMR techniques that drying at 373 K not only removes physisorbed water, but also a dehydroxylation of some silanol groups was observed. 

The presence of significant quantities of physisorbed water promotes the chemisorption of alkoxysilanes, especially of the ones with two or three alkoxy groups. Engelhardt and Orth90 reported that chemisorption of n-octyltriethoxysilane from a dry toluene solution on a dehydrated silica amounts 0.30 fimol/m2. However, if a toluene solution containing traces of water is used, the amount of chemisorbed silane increases up to 1.25 /tmol/m2. 

Interaction of H20 with the surface species containing Si-O-Al structure leads to the formation of Si-OH and Al-OH groups, interacting with each other and with the hydroxyls generated by reaction (P). Further adsorption then leads to ordinary physisorbed water and to adsorbed water, that is coordinated to the surface Al-ions. 

In summary, the first step to functionalize a support is to thermally treat it, in order to remove physisorbed water and dehydroxylate the surface [11], The second step is the anchoring process, which is the chemical reaction of condensation between the chemical precursor and the -OH surface group [s-OH] [12],... 

Prior to the reaction, the inorganic oxide support is thermally pretreated to remove physisorbed water and thus avoid spurious reactions which would not lead to anchoring/grafting. The support can be also dchy-droxylated to various extents to control the amount and dispersion of the anchored/graftcd species. Different types of hydroxy groups with different reactivities are present on supports. The structure of support surfaces and their reactivity have been extensively described in several reviews ([12-18] and references therein). 

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