Adsorption isotherms of water vapor

The hybrid mesoporous material has some silanol groups corresponding to the T2 type Si species. The peak intensity ratio of the T2/T3 signals of the hybrid mesoporous material is almost equivalent to the Q3/Q4 signals of siliceous MCM-41 material. This indicates that silanol density is almost same to the siliceous MCM-41 material. Adsorption isotherm of water vapor showed that the hybrid mesoporous material has more hydrophobic surface than siliceous MCM-41. This fact supports that the organic fragments are exposed on the surface and increased the surface hydrophobicity. 

With different methods one can recognize different hydrates of phospholipids with different properties. Solved in different organic solvents like benzene, xylene etc. egg yolk phosphatidylcholine adsorbed water in an amount of 13.6 water/mol lipid, independently of the used organic solvent165. Studying adsorption isotherms of water vapor on lipids Elworthy166 has found saturation values at 25 °C ... 

Figure 3. Adsorption isotherms of water vapor on zeolite NaX (x = 2.96) at different temperatures (7) densities of water in adsorbed state p were calculated after Ref. 7...

Figure 14. Adsorption isotherms of water vapor (at 25 °C) on titania (O) and 0.6 wt % DS-coated titania (%). The out gassing temperature was 150 °C. (Reproduced with permission from reference 42. Copyright 1986.)...

Tsunoda, R. (1999). Fractal approach to adsorption isotherms of water vapor on active carbons. J. Colloid Interface Sci., 218, 269-274. 

A. Pore size distribution determined from adsorption isotherm of water vapor on collagen fiber... 

A. Pore Size Distribution Determined from Adsorption Isotherm of Water Vapor on Collagen Fiber... 

The pore size distribution of collagen fiber is determined by using adsorption isotherms of water vapor or nitrogen gas. The preparation of the sample, adsorption procedure, and calculation method are mentioned below. 

Figure 5.6. Adsorption isotherms of water vapor on BPL carbon (bituminous coai-based, gas-phase carbon, from Caigon Corp.) at 25°C. Untreated BPL ( ) and BPL oxidized by 6 M HNO3 at time lengths in hours HNO3-O.5 ( ) HNO3-KA) HN03-2(o) HN03-4(H) HNO3 - 7(T). Data taken from Barton et ai., 1984.

Figure 5.7. Adsorption isotherms of water vapor at 20°C on activated carbon CWN-2. Sam-pie 1 untreated. Sampie 2 heat treated under vacuum at 500°C. Sampie 3 oxidized with air at 400°C (from Jankowska et ai., 1991, with permission).

Figure 1.17. Adsorption isotherms of water vapor on different porous materials taken at T = 25 °C, the saturation pressure being po = 3.1 kPa. Reprint taken from 11.39] with permission granted.

Fig 2 6 Typical examples of adsorption isotherms of water vapor on Silica gel type A and B and active alumina... 

Silica gels of two types of pore size distribution are frequently used for commercial purposes. Type A and B have diftetent shapes of adsorption isotherms of water vapor (Fig. 2.6). This difference originates from the fact that type A is controlled to form pores of 2.0/3.0nm while Type B has larger pores of about 7.0nm. Internal surface areas are about 650 m /g (Type A) and 450 m /g (Type B). 

Active alumina is also used as a drying agent and the typical adsorption isotherm of water vapor is included in Fig. 2.6. It is also employed for removal of polar gases from hydrocarbon streams. 

Fig. 10. Adsorption ( , O)"desorption (A, A) isotherms of water vapor on poly(vinylidene chloride) (PVDC) carbon before (filled symbols) and after...

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.

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... 

FIGURE 14.14 Adsorption data of water vapor on silica gel at 25°C. Isotherm fits Brunauer-Deming-Dem-ing-Teller Freundlich, Langmuir, and Redhch-Peterson equations. 

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. 

FIGURE 2.23 Adsorption-desorption isotherms of water vapor on PVDC eharcoal before and after degassing at 1000°C. (Source Bansal, R.C., Dhami, T.L., and Prakash, S., Carbon, 16, 389, 1978. With permission.)... 

Puri et al. ° determined the adsorption-desorption isotherms of water vapor on different samples of charcoals associated with different types of carbon-oxygen surface groups and observed that the presence of acidic surface groups, which are evolved as CO2 on evacuation, make the charcoal surface hydrophilic and increase the size of the hysteresis loop, the increase being proportional to the amount of acidic surface groups present (Figure 2.27). This was attributed to the chemisorption... 

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. 

Similar to other fluids, a liquid-vapor phase transition of confined water appears, for example, as a rapid change in the mass adsorbed when the pressure of external bulk water is varied. Numerous examples of the adsorption isotherms of water in various pores, obtained in experiments or in simulations, can be found in literature (some of them we consider below in Section 4.3). However, there are only a few studies... 

The adsorption and desorption isotherms of water vapor are drawn at 25°C for dealuminated HY zeolites upon framework Si/Al ratio. The isotherms are compared to that of the parent NaY zeolite. The isotherm changes in shape from the type I to the type IV with an hysteresis loop changing from the type H4 to the type H2, as increases the Si/Al ratio. The POLANYI-DUBININ theory is used to determine the micropore volume accessible to water. It decreases with increasing Si/Al ratios, down to zero at a Si/Al ratio of 35. Such a result is accounted by the adsorption on the hydrophilic centers which are the cations (H ) associated with the structural aluminium ions, each cation being coordinated by 8H2O. 

The adsorption and desorption isotherms of water vapor on a series of dealuminated HY zeolites are experimentally drawn to study the change in shape of the isotherm upon both the Si/Al ratio and the dealumination procedure. This study is realized on the one hand, to ascertain whether only the shape of the curve foreshadows the value of the Si/Al ratio and the dealumination procedure and on the other hand, to test the efficiency of the molecular H2O probe for determining the structural and secondary pore volumes. To allow for a satisfactory interpretation the adsorption and desorption isotherm of water vapor on the parent zeolite is studied in wide ranges of temperature and filling coefficient. 

Adsorption and desorption isotherms of water vapor on HY zeolites change in shape with increasing dealumination. Thus, for only steamed samples the isotherm is close to the type I whereas for zeolites submitted to an additional acid leaching, it is close to the type IV with a progressive lowering of the point B towards the pressure axis, what emphasizes a more and more hydrophobic character of adsorption. The micropore volume accessible to water and the unit cell parameter are simply correlated with the framework Si/Al ratio. The water molecule does not allow to determine quantitatively structural and secondary pore volumes, but appears to be a selective molecular probe of the structural aluminium ions. For HY zeolites and without any restraint, 8H2O are associated with such a framework aluminic site. 

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. 

Hollabaugh and Chessick (301) concluded from adsorption studies with water, m-propanol, and w-butyl chloride that the surface of rutile is covered with hydroxyl groups. After evacuation at 450°, a definite chemisorption of water vapor was observed as well as of n-propanol. The adsorption of -butyl chloride was very little influenced by the outgassing temperature of the rutile sample (90 and 450°). A type I adsorption isotherm was observed after outgassing at 450°. Apparently surface esters had formed, forming a hydrocarbonlike surface. No further vapor was physically adsorbed up to high relative pressures. 

Naono, H. Sonoda, J. Oka, K. Hakuman, M. (1993) Evaluation of microporous texture of undecomposed and decomposed p-FeOOH fine particles by means of adsorption isotherms of nitrogen gas and water vapor. Proc. IVth Int. Conf on Fundamentals of Adsorption, Kyoto 1992, 467-474 Naumann, F. (1855) Elemente der Mineralogie. 4. Auflage, Leipzig... 

Equation 10.27 is generally known as Freundlich equation. Equation 10.27 with concentration replaced by pressure was also used to describe the adsorption isotherms of gases on solids, suggesting the incorrect idea that adsorption from solution by a solid could be paralleled with gas or vapor adsorption on the same adsorbents. Whereas in some cases the restriction to dilute solutions was imposed by the solubility of solids (e.g., benzoic acid in water or stearic acid in benzene) it was not imposed on the investigation of mixtures of completely miscible liquids, e.g., acetic acid in water. 

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