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Hydration, surface

While the system conforms to coagulation in a secondary minimum, the redispersion region is best accounted for in terms of gel formation originating from the rod-like shape of the particles and hydrated surface. During the final coagulation process additional attractive forces such as dipolar and hydrogen bonding form floes which are irreversible and denser than those formed at a lower salt concentration. [Pg.377]

Even though the vacuum-oriented surface techniques yield much useful information about the chemistry of a surface, their use is not totally without problems. Hydrated surfaces, for example, are susceptible to dehydration due to the vacuum and localized sample heating induced by x-ray and electron beams. Still, successful studies have been conducted on aquated inorganic salts (3), water on metals (3), and hydrated iron oxide minerals (4). Even aqueous solutions themselves have been studied by x-ray photoelectron spectroscopy (j>). The reader should also remember that even dry samples can sometimes undergo deterioration under the proper circumstances. In most cases, however, alterations in the sample surface can be detected by monitoring the spectra as a function of time of x-ray or electron beam exposure and by a careful, visual inspection of the sample. [Pg.390]

The soluble complexes formed by activators will desorb cation from the lime depressed pyrite surface, which will expose a fresh pyrite surface and activate pyrite flotation. Therefore, the moderately strong acids such as oxalic acid and phosphoric acid exhibit a strong activation action on lime-depressed pyrite because of their ability to decrease pulp pH and to form soluble complexes with hydrated surface cations. [Pg.163]

An improved adsorption of DNA bases has been observed at a chemically modified electrode based on a Nafion/ruthenium oxide pyrochlore (Pb2Ru2-x FhxOj-y modified GC (CME). Nafion is a polyanionic perfiuorosulfonated ionomer with selective permeability due to accumulation of large hydrophobic cations rather than small hydrophilic ones. The Nafion coating was demonstrated to improve the accumulation of DNA bases, while the ruthenium oxide pyrochlore proved to have electrocatalytic effects towards the oxidation of G and A. The inherent catalytic activity of the CME results from the Nafion-bound oxide surface being hydrated. The catalytically active centers are the hydrated surface-boimd oxy-metal groups which act as binding centers for substrates [50]. [Pg.18]

The DTS has been used as a strength criterion for meeting American Dental Association specification 27. Reported DTS values for microfilled composites range from 30-50 MPa and for the conventional and hybrid composites, from 45-75 MPa [199-204], The wide variation in the ranges of properties reported may relate not only to differences in materials, but also to test methodologies such as state of hydration, surface roughness of the samples, and strain rates [205,206]. [Pg.206]

Zellmer, L. A. White, W. B. 1985. Characterization of hydrated surface layers on nuclear waste glasses by infrared reflectance spectroscopy. In Jantzen, C. M., Stone, J. A. Ewing, R. C. (eds) Scientific Basis for Nuclear Waste Management VIII. Materials Research Society Symposia Proceedings, 44, 73-80. [Pg.410]

Fig. 6. Hydration surface dynamics of the protein (enzyme) Substilisin Carlsberg, and for comparison that of bulk water, and a probe outside the water layer... Fig. 6. Hydration surface dynamics of the protein (enzyme) Substilisin Carlsberg, and for comparison that of bulk water, and a probe outside the water layer...
Fig. 1. Ideal surface (100) plane of alumina after Peri [25]. (A) Top layer viewed per-pendicualrly to the plane (B) section through the three top layers, (a) Fully hydrated surface, (b) dehydroxylated surface. Open circles denote oxygen, filled circles hydroxyl, small black points aluminium,... Fig. 1. Ideal surface (100) plane of alumina after Peri [25]. (A) Top layer viewed per-pendicualrly to the plane (B) section through the three top layers, (a) Fully hydrated surface, (b) dehydroxylated surface. Open circles denote oxygen, filled circles hydroxyl, small black points aluminium,...
The different degrees of water inhibition on the ether and olefin formation from ethanol on alumina, and the agreement of ether/ethylene selectivity ratios found experimentally with those calculated by the Monte Carlo simulation of the hydrated surface of alumina [144],... [Pg.293]

The most that can be concluded from such experiments is that Cr03 attaches initially to the hydrated surface as chromate. Hierl and Kraus, in... [Pg.50]

Ordinarily the chromium binds to the silica by reacting with hydroxyls on a fully hydrated surface, because chromium is impregnated aqueously onto the silica and then calcined. However, a different catalyst results if the chromium attaches instead to a surface already dehydrated by calcining. A large promotional effect, particularly on the termination rate, is obtained (76). To do this the silica is first dehydrated at 900°C, for example, then impregnated with chromium anhydrously so that the surface is not rehydrated. A secondary calcining step at some lower temperature such as 300-600°C then fixes the chromium to the silica. The method is especially effective if the support also contains titania. [Pg.81]

With regard to a solubility equilibrium, the fact that vitreous silica behaves like a precipitate of polymeric silicic acid must be caused by the similarity between polymeric silicic acid and the hydrated surface of vitreous silica. Both forms can release silicic acid by hydrolysis and desorption, and likewise both forms are able to adsorb and condense silicic acid by means of silanol groups randomly distributed on their surfaces. Thus, in order to explain equal final states, the only assumption necessary is that the condensates will not attain the degree of dehydration of the bulk of the vitreous silica. The resulting equilibrium then relates to the two-phase system silicic acid—polymeric precipitate, and strictly speaking, this system is in a supersaturated state with respect to vitreous silica, which can be considered as an aged form of silica gel. [Pg.169]

In essence, aqueous titration of surface acidity is an ion-exchange process in which hydrated surface protons are replaced by other hydrated cations (e.g., Na+, NIV") during the course of the titration. The procedure is straightforward. It usually consists of the direct titration of an aqueous suspension of the sample of powdered solid with a dilute base (e.g., sodium hydroxide) to a neutral endpoint. Another commonly used procedure consists of noting the pH of an appropriate salt solution (e.g., ammonium acetate), adding the sample, and measuring the amount of di-... [Pg.99]

On the molecular level, hydrate growth can be considered to be a combination of three factors (1) the kinetics of crystal growth at the hydrate surface, (2) mass transfer of components to the growing crystal surface, and (3) heat transfer of the exothermic heat of hydrate formation away from the growing crystal surface (see Section 3.2.3 for heat transfer models). [Pg.150]

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.)... [Pg.164]

Rehder et al. (2004) measured the dissociation rates of methane and carbon dioxide hydrates in seawater during a seafloor experiment. The seafloor conditions provided constant temperature and pressure conditions, and enabled heat transfer limitations to be largely eliminated. Hydrate dissociation was caused by differences in concentration of the guest molecule in the hydrate surface and in the bulk solution. In this case, a solubility-controlled boundary layer model (mass transfer limited) was able to predict the dissociation data. The results showed that carbon dioxide hydrate dissociated much more rapidly than methane hydrate due to the higher solubility in water of carbon dioxide compared to methane. [Pg.178]

Small angle neutron scattering instruments are specifically designed to examine disordered materials, such as to determine hydration structures during hydrate formation (Koh et al., 2000 Buchanan et al., 2005 Thompson et al., 2006), or to study kinetic inhibitor adsorption onto a hydrate surface (Hutter et al., 2000 King et al., 2000). [Pg.349]

The above discussion demonstrates two principles for hydrate dissociation (1) hydrates will not occur outside the thermodynamic restrictions of the phase equilibria, that is, hydrates require the appropriate temperature, pressure, as shown in the area to the left of the lines in Figure 7.18, as well as methane and water and (2) when hydrates are dissociated, even at constant temperature as shown in Figure 7.18, heat must flow from the surrounding media to the hydrates, causing a cooling. This last point is also intuitive, because gas and water molecules from dissociated hydrates have more energy than they do in nondissociated hydrates. Thus energy must flow to the hydrate surface in order to dissociate it. [Pg.584]


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See also in sourсe #XX -- [ Pg.505 ]




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