Reversible adsorption of water

The adsorption of water on a fully hydroxylated silica involves hydrogen bonding but is essentially physical in nature and is completely reversible in the low pressure range the isotherm is of Type II on a nonporous sample (Fig. 5.17(a)), and of Type IV, with no low-pressure hysteresis, on a porous sample (Fig. 5.18). 

XPS was used to determine the surface composition of the anodized aluminum substrate during exposure to warm, moist environments. The information obtained was used to construct surface behavior diagrams that showed that hydration of the surface involved three steps [38]. Step one, which was reversible, consisted of adsorption of water onto the AIPO4 monolayer. The second step, which was rate-controlling, involves dissolution of the phosphate followed by rapid hydration... 

Nonpolar Parameters. In a reverse osmosis system involving cellulose acetate membranes and aqueous solutions of hydrocarbon solutes, the adsorption of water and that of solute on the polar and nonpolar sites of the membrane surface respectively may be expected to take place essentially independently. Further, since the polymer-solute interaction forces are attractive in nature for the above case, the mobility of the solute molecules through the membrane pore is retarded, and they also tend to agglomerate... 

Solid-Phase Extractions Using XAD Resins. The Amberlite XAD series (Rohm and Haas Co., Philadelphia, PA, USA) have been most often used for isolation of marine DOM by SPE. XAD resins are nonionic macroporous copolymers that differ in pore size, surface area, and polarity. Their generally large specific surface areas and more-or-less reversible adsorption of organic solutes from aqueous solution have made them well-suited for isolation of selected fractions of DOM from natural waters. Even though XAD resins have been used far more often to... 

The dissociative adsorption of water proceeds on a surface M-O pair with the formation of two neighboring OH groups on the surface. It was assumed that the energy of the reversible dissociative adsorption of water depends on the chemical environment of the corresponding surface lattice sites... 

Substances that are used as coatings in TLC include silica gel, alumina, cellulose, and reversed-phase packings. Separations occur because of adsorption of the solutes from the mobile phase onto the surface of the thin layer. However, adsorption of water from the air or solvent components from the mobile phase can give rise to partition or liquid-liquid chromatography. Specially coated plates are available that permit ion-exchange or reversed-phase separations. 

A pioneering investigation of the adsorption of water vapour by Saran charcoal (an ultramicroporous carbon) was carried out by Dacey et al., (1958). The isotherm at 55°C had the well-defined Type V character with a steep riser at p/p° 0.5-0.6. There was a small amount of low-pressure hysteresis, but the isotherm appeared to be completely reversible at p/p° > 0.55. 

The latter authors found that a reversible water isothenn was obtained after the low-temperature (i.e. 40°C) evacuation of a carbon cloth, which had been activated by oxidative HNO, treatment. The molecular sieve character of this material was reduced by evacuation at 400°C and this also led to the appearance of hysteresis in the water vapour isothenn. Barton and Koresh (1983) conclude that such hysteresis is mainly due to the concentration of surface oxides which dictate the adsorption value at which the change from cluster adsorption to a continuous adsorbed phase takes place . The relationship between the adsorption of water and the surface concentration of chemisorbed oxygen was first established by Walker and Janov (1968). Bansal et al. (1978a,b) also investigated the influence of the surface oxygen on the adsorption of water they concluded that at p/p° < 0.5 the level of water uptake is determined by the concentration of surface oxygen-containing structures. 

Another theory of the reason for increased friction in the presence of moisture was proposed by Gao et al . They found that in a humid environment molybdenum disulphide films were more readily thinned by sliding contact, due to increased ease of interlamellar slip. They suggested that adsorption of water softened the films, and that resulting increased deformation by plowing in sliding contact led to a poorly oriented film and thus to increased friction. However, they considered that this was a short-term reversible effect which was not in conflict with theories of chemical breakdown. Gao et al also poiinted out the possibility that an increase In friction is caused by capillary pressure effects of moisture at asperity contacts. 

The effect of moisture on the coefficient of friction is generally reversible, as shown in Figure 7.5, and it is accepted that the effect is due to adsorption of water vapours ". The situation is complicated to some extent by the influence of moisture on oxidation, and the effect of surface oxide in promoting adsorption of water. 

A priori, one can imagine that the reverse of Eqs. (12) and (13), oxidative desorption, would lead to desorption of hydrogen and carbon monoxide. Alternatively, desorption of water or carbon dioxide as shown at the right of Eqs. (12) and (13) would lead to permanent reduction and to the formation of Cr2+ of low coordination number. It is quite possible that such reductive adsorption followed by loss of water or carbon dioxide at high temperatures accounts for the reduction of chromia which was mentioned in Section IV. Adsorption of water followed by oxidative desorption may account for the liberation of hydrogen observed when chromia reduced at 500° is treated with water (39). 

The dehydration of thin ciystals of potassim aim [103] in dry air (323 to 343 K) showed different rates of reaction following nucleation of different surfaces. This anisotropy was attributed to the variation in density of packing of water molecules with crystallographic direction. At low pressure [104] the adsorption of water vapour was reversible, but at larger values of KHjO) multilayers were formed and uptake of water was controlled by difhision into the bulk of the crystal ( , < 2 kJ mol ). 

Zhdanov (Institute of Silicate Chemistry, the U.S.S.R. Academy of Sciences, Leningrad) (154) showed (1949) that the adsorption of water vapor by SiC>2 (porous glasses, silica gels) strongly depends on the temperature of the preliminary thermal treatment of the adsorbent. Calcination of 300-500 °C resulted in a sharp decrease in the adsorption of H2O at low values of pressure over initial pressure p/p0 (<0.3), and the adsorption isotherms were found to be irreversible. On the other hand, the adsorption isotherms of water on silica subjected to calcination in vacuo at <500 °C (but after the sample was kept in contact with water vapor or liquid water at room temperature) again became reversible that is, the adsorption activity of Si02 was restored. 

After calcination and hydration, the Al I signal in SAPO-5 has increased significantly and the Al distribution in calcined hydrated SAPO-5 is Al V (58%), Al (9%) and AlVI (33%). Meinhold and Tapp reported that in calcined AIPO4-5 up to 40% of the AlIV can be converted reversibly into Al upon adsorption of water [15]. At the magnetic field of 4.7 T used in that work [15], Al and Al signals were not resolved. In calcined hydrated SAPO-5, a comparable share of the Al atoms is found now to be coordinated to one or two water molecules (Fig.4). 

It has been proved (1) that the adsorption of HzS introducing hydrosulfurizatlon proceeds similarly to the adsorption of water (figs. Ic and d) both HF and LF bands are generated on Cd-Y and La-Y. However, HzS adsorption on the catalytically most active Cd-Y (1) is larger than on La-Y showing low activity in hydrosulfurizatlon the situation is reversed compared to water adsorption. 

The Solid Adsorbent. In characterizing the layer on which adsorption of water takes place reversibly, we make the following observations ... 

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