Water adsorbed structure

Similar, very detailed studies were made by Ebert [112] on water adsorbed on alumina with similar conclusions. Water adsorbed on zeolites showed a dielectric constant of only 14-21, indicating greatly reduced mobility of the water dipoles [113]. Similar results were found for ammonia adsorbed in Vycor glass [114]. Klier and Zettlemoyer [114a] have reviewed a number of aspects of the molecular structure and dynamics of water at the surface of an inorganic material. 

A large amount of water is added to the dehydrated material in order to cause it to swell the swollen structure is preserved when the material is frozen and subsequently dried in vacuo (in the frozen state) to a low moisture content. Some leaching occurs during the treatment with water and this, undoubtedly, further contributes to the increase in the porosity of the solid. Drying of the lyophilized substance can.be completed in a relatively short time in a vacuum oven at an elevated temperature, or at room temperature in the presence of an efficient water adsorbent. 

Study of hydrated kaolinites shows that water molecules adsorbed on a phyllosilicate surface occupy two different structural sites. One type of water, "hole" water, is keyed into the ditrigonal holes of the silicate layer, while the other type of water, "associated" water, is situated between and is hydrogen bonded to the hole water molecules. In contrast, hole water is hydrogen bonded to the silicate layer and is less mobile than associated water. At low temperatures, all water molecules form an ordered structure reminiscent of ice as the temperature increases, the associated water disorders progressively, culminating in a rapid change in heat capacity near 270 K. To the extent that the kao-linite surfaces resemble other silicate surfaces, hydrated kaolinites are useful models for water adsorbed on silicate minerals. 

Water on Hallovsite. Central to the controversy is the observation that clay crystals present a planar array of oxygens (and hydroxyls in the case of kaolinite) which have hexagonal (or nearly) symmetry with a periodicity similar to that found in the crystal structure of ice. Because of this geometric similarity, it has frequently been assumed that water adsorbed on a clay surface will preferentially adopt an ice-like configuration. When looked at in detail, it is difficult to find unequivocal evidence to support this. 

Halloysite-10A represents a structure with few if any interlayer cations, allowing one to investigate the relatively simple case of water interacting with a clay surface. Similarly, ice-like models have been proposed for water adsorbed on smectite and vermie-ulite surfaces (2, 12, 12). These represent cases of charged clay layers with adsorbed exchangeable cations. 

In general, the 2 1 clays are not very simple systems in which to study the interaction of water and surfaces. They have complex and variable compositions and their structures are poorly understood. Water occurs in several different environments zeolitic water in the interlayer regions, water adsorbed on the external surfaces of the crystallites, water coordinating the exchangeable cations, and, often, as pore water filling voids between the crystallites. Thus, there are many variables and the effects of each on the properties of water are difficult to separate. 

Based on the study of expanding clay minerals, two models of water adsorbed on silicate surfaces have been proposed. One states that only a few layers (<5) of water are perturbed by the silicate surface, the other concludes that many layers (perhaps 10 times that number) are involved. The complexity of the interactions which occur between water molecules, surface adsorbed ions, and the atoms of the silicate mineral make it very difficult to unequivocally determine which is the correct view. Both models agree that the first few water layers are most perturbed, yet neither has presented a clear picture of the structure of the adsorbed water, nor is much known about the bonding of the water molecules to the silicate surface and to each other. 

Clarke, N.S. Hall, P.G. (1992) Adsorption of water vapour by iron oxides. 3. Inelastic incoherent neutron scattering from water adsorbed on magnetite Evidence for an icelike structure. Langmuir 8 645-649... 

On hydrophilic surfaces, such as PVA or poly(HEMA), OH-groups of the materials are incorporated in the network structure of adsorbed water molecules (see Sect. 4.4). In consequence, the absolute value of Wj(3 — Wi1 is considered to become still smaller, where - owing to the stabilization of water molecules on the hydrophilic surface - the water-removing-process (reverse reaction of Eq. (2.6)) proceeds slowly. Many experiments were carried out with water-adsorbed hydrophilic surfaces, the behavior of which was time-dependent. In a similar way, the water removal from the proteins [Eq. (2.9)] is also considered to proceed slowly. Thus, we must be careful in considering experimental results in comparison with the data in Tables 3, 4 and 5. 

Figure 3. Overtone and combination band spectrum oj ethylene and water adsorbed on Mn"A zeolite. The ethylene bands lie close to the gaseous (V -f- vn), (vt + vs), 2vlu and 2vs vibrational modes, indicating that the ethylene molecule has retained its chemical composition and structural integrity (-, 1) MnA 4- ethylene (-------------------,2) MnA hydrated CtHt (g) bands.

Sposito, G. and Prost, R. (1982) Structure of water adsorbed on smectites, Chemical Reviews 82 Suppl. 6, 553-572... 

The structural and spectral complexity of clay minerals is sufficient to consider a single mineral as a multicomponent mixture in itself. Detectible by near infrared spectroscopy are adsorbed water and structural hydroxyls (25.) exchangeable and structural transition metal cations (2fL and this work), adsorbed species including atmospheric gases (22), organic materials (2) accessory minerals (2SL) and, possibly, trapped hole centers (0 -centers). Thus it is of interest to adapt NIRA to studies of mineral surface activity. We have done this by examination of a small set of highly homologous clays in which laboratory control of only one variable at a time could be accurately achieved and independently confirmed. 

The important role of the exchangeable cation in determining the structure of adsorbed water on smectites was discussed in the review of experimental studies of the structure of water adsorbed on smectites [32], It was concluded that the spatial arrangement of the adsorbed water molecules indeed derives mainly from the solvation of exchangeable cations. Despite the great... 

In order to shed light on the mechanisms by which a given additive can increase the enzyme activity in organic solvent, we have been mainly focusing on how the additive affects the enzyme conformation and the amount of water adsorbed on the enzyme after lyophilization. We monitored these effects on the protein by means of FT-IR spectroscopy, a spectroscopic technique that allows conformational comparisons between enzyme samples in different physical states and in the presence of additives. In particular, the band that proteins show in the 1600-1700/cm region (amide I band) is composed of several overlapping components that can be assigned to different secondary structure conformations [21]. 

The adsorption of CO is probably the most extensively investigated surface process. CO is a reactant in many catalytic processes (methanol synthesis and methanation, Fischer-Tropsch synthesis, water gas shift, CO oxidation for pollution control, etc. (1,3-5,249,250)), and CO has long been used as a probe molecule to titrate the number of exposed metal atoms and determine the types of adsorption sites in catalysts (27,251). However, even for the simplest elementary step of these reactions, CO adsorption, the relevance of surface science results for heterogeneous catalysis has been questioned (43,44). Are CO adsorbate structures produced under typical UHV conditions (i.e., by exposure of a few Langmuirs (1 L = 10 Torrs) at 100—200 K) at all representative of CO structures present under reaction conditions How good are extrapolations over 10 or more orders of magnitude in pressure Such questions are justified, because there are several scenarios that may account for differences between UHV and high-pressure conditions. Apart from pressure, attention must also be paid to the temperature. 

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