Adsorption isotherm for water

Some data obtained by Nicholas et al. [150] are given in Table III-3, for the surface tension of mercury at 25°C in contact with various pressures of water vapor. Calculate the adsorption isotherm for water on mercury, and plot it as F versus P. 

Fig. 5.20 Adsorption isotherms for water vapour on x-Fe,Oj at 15°C for various outgassing temperatures. Solid points indicate second isotherm after 25°C evacuation of physically adsorbed water. (Courtesy Zettlemoyer.) Outgassing temperature,<, 25°C , I00°C O, 250°C ...

Figure 17.22. Adsorption isotherm for water vapour in air on silica gel...

Figure 11.3 Typical adsorption isotherm for water on a mineral oxide surface.

All zeolites have a highly hydrophilic surface and are very efficient desiccants. Contrary to other nonzeolitic desiccants such as silica gel and activated alumina, zeolite adsorbents have type I adsorption isotherms for water—i.e., a high water adsorption capacity at a low concentration of water. To obtain extremely dry gases and liquids, therefore, zeolite adsorbents are strongly preferred over amorphous desiccants. The 3A mo-... 

Example 9.3. Plot the estimated adsorption isotherm for water vapor on silicon oxide at 20°C. First we need to estimate the constant C. From Chapter 6 we know that it is related to the Hamaker constant AH (Eq. 6.16) C = TtpBCABl = Ah/3ttpa. Here, pA and pB are the number densities of molecules in liquid water and silicon oxide, respectively. The Hamaker constant for water interacting with silicon oxide across air is Ah = 10-20 J (Table 6.3). With a density of water of 1000 kg/m3, a molecular weight of 18 g/mol, and a molecular radius of Do 1 A we get Vr% = 0.018 kgmoU1 /(1000 kgm-3) = 18 x 10 6m3moU1 and... 

The adsorption isotherms for water with five of the samples were obtained at 25° C. by using a volumetric adsorption apparatus described previously (13). Prior to adsorption measurements these samples were also outgassed at 160° and 10-6 mm. Hg. Each adsorption isotherm consisted of 20 to 40 experimental points covering the relative pressure range 0.01 to 0.95. 

Figure 10.7. Adsorption isotherms for water on a Na-montmorillonite at three temperatures. (Adapted from P. L Hall and D. M. Astill. 1989. Adsorption of water by homoionic exchange forms of Wyoming montmorillonite. (SWy-1). Clays Clay Min. 37 355-363.)...

Figure 6.5. Absolute adsorption isotherms for water on different wide-pore amorphous silicas at room temperature (after the silicas were treated at 200 °C). The line shows the average of data from the literature (Kiselev, 1986 Zhuravlev, 1993).

In order to evaluate the behavior of the pellets towards the adsorption of polar molecules, the adsorption isotherms for water vapor in air were determined at 23 °C (Figure 5.49), and show the different chemical nature of sepiolite and activated carbon. Thus, granular sepiolite S exhibits a hydrophilic behavior, whereas the type V isotherm for the activated carbon is typical of a small interaction between the carbon surface and the water molecules at low relative humidity (RH). The water adsorption isotherms, on pellets SP30 and SC30, have an intermediate shape between samples S and P or C. Furthermore, the uptake at each RH is the additive of the humidity adsorption by the individual two components, taking into account the proportion of each one in the mixture. 

Fig. 4.29 Adsorption isotherms of water vapour on caldte, after being balt-milted for different periods (A, B, C) and on precipitated calcium carbonate (D). Period of milling (A) 1000h (B) ISOh (C) 22h outgassing temperature 2S°C. Isotherms A, B and C (but not D) all showed extensive low-pressure hysteresis, but for clarity the desorption branch is omitted. The amount adsorbed is referred to 1 m of BET-nitrogen area. ...

Beyer and Belenykaia (27) have investigated the sorption properties of DAY zeolites prepared from Y zeolite and SiCl vapors. They reported a very low adsorption capacity for water and ammonia, similar to that of the almost aluminum-free silicalite (49). The low adsorption capacity for water is indicative of a hydrophobic zeolite surface. The adsorption isotherms for n-butane, benzene and n-hexane obtained on the aluminum-deficient zeolite have a shape similar to those obtained on NaY zeolite and are characteristic for micropore structures. They show the absence of secondary pores in this DAY zeolite. 

To test this theory, the authors measured water vapor adsorption isotherms for RAMEB-treated soils. The amount of water adsorbed (kg water/kg soil) was monitored as a function of the partial pressure of water (p/po), the dose of RAMEB in the soil (0, 1, or 9%), and the type of soil. Seven soils were studied and arranged in order of increasing clay content (3, 8, 11, 16, 25, 36, and 49% clay content for SI, S2, S3,... S7, respectively). 

Effect of RAMEB on Water Vapor Adsorption on Soils. Experimental adsorption isotherms for the RAMEB-treated soils are presented in Figure 1 [see p. 127]. As pure RAMEB sorbs a very high amount of water (ca. 1 g g" at p/po = 0.99), an increase in water sorption was expected after RAMEB addition to all soils. However, the isotherms for RAMEB-treated clay-rich S6 and S7 soils showed lower adsorption than the original soils, which is illustrated for S7 soil with 49% clay. This potentially indicates that RAMEB decreases the amount of water-available surfaces in clay-rich soils, similar to what was observed for pure clay minerals (20). In sandy soils (S1-S4), the water sorption markedly increased, particularly at higher RAMEB doses, as is illustrated for S2 soil. 

Figure 1. Water vapor adsorption isotherms for RAMEB-treated soils. Soil symbols are as in Table 1. The number in parentheses following the soil symbol is the dose of RAMEB (%). 

The adsorption isotherms for metallic surfaces are reported in the literature however, an important part of the atmospheric corrosion process takes place under rust layers, which play a decisive role in the long-term course of corrosion because of its sorption capacity for water. The influence of the chloride and sulfate anions has a real effect only when the corrosion products layer is already formed. Thus, the adsorption isotherms of the steel corrosion products formed in different atmospheres were determined. 

Fig. 5 (a) shows the nitrogen adsorption isotherms of aluminum hydroxy pillared clays after heat-treatment at 300-500°C. These are of the typical Langmuir type isotherm for microporous crystals. Fig, 5 (b) shows the water adsorption isotherms on the same Al-hydroxy pillared clays [27]. Unlike the water adsorption isotherms for hydrophilic zeolites, such as zeolites X and A, apparently these isotherms cannot be explained by Langmuir nor BET adsorption equations the water adsorption in the early stages is greatly suppressed, and shows hydrophobicity. Water adsorption isotherms for several microporous crystals [20] are compared with that of the alumina pillared clay in Fig. 6. Zeolites NaX and 4A have very steep Langmuir type adsorption isotherms, while new microporous crystals such as silicalite and AlPO -S having no cations in the...

Fig. 1.17 Adsorption isotherms for various water-reducing admixtures.

Pierce and Smith (56) have observed a Type III isotherm for water vapor on a highly graphitized carbon black (graphon, 80 square meters per gram) at 28.9° C. They suggest that water vapor adsorption occurs only on the most active sites and that the entire surface is probably never covered. 

Figure 3. Water-vapor adsorption isotherms for glass fibers after the silane treatment the isotherms for the untreated fibers with 0% and 6% B,0, are included. APS - 1% solution of y-aminopropyl-silane at pH 10 0%—O,4% Oand6% B.

The adsorption isotherm for pentanol is typical for lyophobic substances, i.e., substances which do not like to stay in solution, and for weakly amphiphilic substances. They become enriched in the interface and decrease the surface tension. If water is the solvent, most organic substances show such a behaviour. The LiCl adsorption isotherm is characteristic for lyophilic substances. Most ions in water show such behaviour. 

In order to describe the influence of a substance on the surface tension, one could specify the gradient of the adsorption isotherm for c —> 0. A list of these values for some substances dissolved in water at room temperature is shown in Table 3.2. 

Figure 9.9 Left BET adsorption isotherms plotted as total number of moles adsorbed, n, divided by the number of moles in a complete monolayer, ri7non, versus the partial pressure, P, divided by the equilibrium vapor pressure, Po. Isotherms were calculated for different values of the parameter C. Right Adsorption isotherms of water on a sample of alumina (Baikowski CR 1) and silica (Aerosil 200) at 20°C (P0 = 2.7 kPa, redrawn from Ref. [379]). The BET curves were plotted using Eq. (9.37) with C = 28 (alumina) and C = 11 (silica). To convert from n/nmo to thickness, the factors 0.194 nm and 0.104 nm were used, which correspond to n-mon = 6.5 and 3.6 water molecules per nm2, respectively.

Figure 1. (a) Adsorption-desorption water isotherms at 293 K, and (b) adsorption potentials for water adsorbed onto carbon Norit at different CNaci values. 

The adsorption isotherms for many of the samples in Table I have been measured (II), allowing the integral entropies of adsorption to be evaluated according to the prescription of Jura and Hill (16). It was found (II) that the entropy of the adsorbed species relative to that of liquid water decreases regularly with decreasing specific surface area. This would be expected for a gradual decrease in periodicity in the underlying structure and thus in the adsorbed H20. 

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