Water vapor adsorption capillary condensation

Experimental results of water vapor adsorption. Helium relative permeability, Pr, and water vapor permeability, Pe, for the two alumina pellets are presented in figures 6a and 6b, for water relative pressures up to unity. As the amount of water adsorbed starts to rapidly increase with P/Po, due to capillary condensation, a significant increase of its permeability may also be observed due to the resulting capillary enhancement of flow. At a certain value of P/Po where Vs is close to unity, all pores of the membrane are in the capillary condensation regime and thus follow the capillary enhanced type of flux. At this point water vapor permeability reaches its maximum value while, helium relative permeability decreases rapidly and falls to zero well below the point of saturation. This may be attributed, according to percolation theory, to the fact that in a simple cubic lattice, if -75% of the pores are blocked by capillary condensate, the system has reached its percolation threshold and helium... 

Water vapor adsorption isotherms have been obtained on cotton from room temperature up to 150°C [303,304]. Theoretical models for explaining the water vapor sorption isotherms of cellulose have been reviewed [303]. Only adsorption theories will be discussed here at ambient temperatures. The shape of the isotherm indicates that multilayer adsorption occurs and thus the Brunauer, Emmett and Teller (BET) or the Guggenheim, Anderson and deBoer (GAB) theory can be applied. In fact, the BET equation can only be applied at relative vapor pressures (RVPs) below 0.5 and after modification up to a RVP of 0.8 [305]. The GAB equation, which was not discussed in the chapter in the book Cellulose Chemistry and Its Applications [303], can be applied up to RVPs above 0.9 [306]. Initially as the RVP increases, a monomolecular layer of water forms in the cellulose. By a RVP of 0.19-0.22 the monomo-lecular layer is complete [303], and the moisture regain, when a monomolecular layer has just formed, for cotton and mercerized cotton is 3.27 and 4.56%, respectively [261,303]. By a RVP of 0.83 0.86, about three layers of water molecules are formed, and at higher RVPs it is thought that condensation occurs in the permanent capillary structure of the sample [307]. 

This principle is illustrated in Figure 10 (45). Water adsorption at low pressures is markedly reduced on a poly(vinyhdene chloride)-based activated carbon after removal of surface oxygenated groups by degassing at 1000°C. Following this treatment, water adsorption is dominated by capillary condensation in mesopores, and the si2e of the adsorption-desorption hysteresis loop increases, because the pore volume previously occupied by water at the lower pressures now remains empty until the water pressure reaches pressures 0.3 to 0.4 times the vapor pressure) at which capillary condensation can occur. 

Five common desiccant materials are used to adsorb water vapor montmorillonite clay ([(Na,Cao.5)o.33(Al,Mg)2Si40io(OH)2 H20], silica gel, molecular sieves (synthetic zeolite), calcium sulfate (CaS04), and calcium oxide (CaO). These desiccants remove water by a variety of physical and chemical methods adsorption, a process whereby a layer or layers of water molecules adhere to the surface of the desiccant capillary condensation, a procedure whereby the small pores of the desiccant become filled with water and chemical action, a procedure whereby the desiccant undergoes a chemical reaction with water. 

Table 16-4 shows the IUPAC classification of pores by size. Micropores are small enough that a molecule is attracted to both of the opposing walls forming the pore. The potential energy functions for these walls superimpose to create a deep well, and strong adsorption results. Hysteresis is generally not observed. (However, water vapor adsorbed in the micropores of activated carbon shows a large hysteresis loop, and the desorption branch is sometimes used with the Kelvin equation to determine the pore size distribution.) Capillary condensation occurs in mesopores and a hysteresis loop is typically found. Macropores form important paths for molecules to diffuse into a par-... 

Water molecules have polar ends, and readily form hydrogen bonding. As a result, several compounds interact with water molecules by surface adsorption, condensation in capillaries, bulk retention, and chemical interaction, and are called hygroscopic. At times, the interaction between the compounds and water is so strong that the interacting water vapors result in dissolving the compound. This process is called deliquescence, wherein a saturated layer of solution is formed around the... 

Figure 5 The six types of International Union for Physical and Applied Chemistry isotherms. The type I isotherm is typical of microporous solids and chemisorption isotherms. Type II is shown by finely divided nonporous solids. Types III and V are typical of vapor adsorption (i.e., water vapor on hydrophobic materials). Types V and VI feature a hysteresis loop generated by the capillary condensation of the adsorbate in the mesopores of the solid. The rare type VI, the step-like isotherm, is shown by nitrogen adsorbed on special carbon.

Owing to the method of contact-angle or surface-energy measurement, the surface of wool necessarily includes the region between cuticle cells in addition to the cuticle itself Horr has further suggested that vapor adsorption due to capillary condensation may occur at the fiber cuticle scale edges, and that the phenomenon may contribute to the above interpretation that the wool surface is not entirely methyl. Horr also found that the possible composition of the wool fiber surface may even vary depending on the liquid with which it is in contact (e.g., water or methylene iodide). 

The powder X-ray diffraction patterns of porous crystalline cellulose (PCC) -10% ethenzamide (EZ) mixtures before and after storage of the mixtures for 1 month at 40°C and 0, 40.0, and 97.0% relative humidity are shown in Fig. 3 [7]. In the freshly prepared mixture (A), X-ray diffraction peaks were observed at 20 = 14.5, 19.3, and 25.3° that were attributable to EZ crystals. Following storage at 0 and 40.0% RH (represented by patterns B and C in Fig. 3), the X-ray diffraction peaks of EZ crystals disappeared. It was found that the mixing of EZ with PCC under dry conditions led to the transformation of crystalline EZ into the amorphous state. EZ molecules would be adsorbed physically onto the pore surface of PCC. In the case of 97.0% RH (Fig. 3D), X-ray diffraction peaks of EZ crystals were still observed EZ remained in the crystalline state under this condition. Matsumura et ai. [8] reported that coexisting water vapor caused a decrease in the adsorption of methanol onto porous materials. At 97.0% RH, the maximum pore diameter for water condensation was calculated as 42 nm. All capillaries of PCC were filled with water at 97.0% RH, and molecules of EZ had little chance to adsorb onto the surface of PCC. These results indicated that the indispensable condition for amorphization of EZ by mixing with PCC was storage under dry conditions. 

Figure 4.5 shows typical plots of a type V isotherm as a function of both the pressure and its logarithm. In the lower pressure region, the graph is quite similar to that of type III. This is explained by the formation of a monolayer followed by multilayer growth. However, a saturation level is reached below the vapor pressure. This is explained on the basis of capillary condensation of the adsorbate gas in the pores of the adsorbent at a pressure below the saturation (vapor) pressure of the gas, Py, in a way similar as the case of type III isotherms. The situation is essentially the same as that shown in Figure 4.4c. A classical example is the adsorption of water on charcoal at 100°C (Coolidge 1927).

In particular, Connolly et al. (2005) designed NH capacitive sensor with 500-nm-thick porous SiC film. The response in humidity was very low for RH<50 %, which was attributed to the porous dimensions. The exact sensing mechanism is still not clear, but NH levels as low as-0.5 ppm were detected. Porous alumina (AI2O3) has also been examined as a sensing material for capacitive gas sensors and in particular for humidity measurements (Nahar and Khanna 1982 Timar-Horvath et al. 2008). The Al Og-based humidity sensor was a volume-effect device based on physical adsorption. At low humidity, the walls of the pores are lined with one-molecular-thickness liquid layer. As the humidity increases, after saturating the walls, due to a capillary condensation effect, the water starts condensing in the pores (Boucher 1976 Neimark and Ravikovitch 2001). It was established that the water molecules, even at a partial pressure higher than the saturated vapor pressure tend to condense in capillary pores with a radius below the Kelvin radius r, which is defined as function (1) (Boucher 1976) ... 

In addition to adsorption-chemisorption processes, capillary condensation phenomena are also involved when considering the fine structure of the porous matrix. According to the basic theory of adsorption on a porous matrix (Adamson and Cast 1997), when the vapor molecules are first physico-sorbed onto the porous material, capillary condensation will occur if the micropores are narrow enough. The critical size of pores for a capillary condensation effect is characterized by the Kelvin radius. In the case of water the condensation of vapor into the pores can be expressed by a simplified Kelvin equation (Ponec et al. 1974) ... 

Figure 3.3.42 Typical shapes of adsorption isotherms pA- partial pressure of adsorbate A, p - vapor pressure of liquid A, 0a = coverage relative to monolayer capacity type I Langmuir adsorption, for example, benzene on silica gel, NH3 on charcoal, or H2S on molecular sieve (Figure 3.3.40) type II multilayer BET type of adsorption, for example, water on AI2O3, N2 on silica type III multilayer adsorption, for example, bromine on silica, type IV and V multilayer adsorption and capillary condensation in pores, for example, water on silica or benzene on Fe203 (IV) and water on charcoal (V).

The whole adsorption / desorption isotherms at 300 K, 350 K, 400 K, 500 K, and 650 K are plotted in Fig 3 Oefi panel) (normalized to the sur ce area of the substrate exposed to water vapor, in pmol / m ). The result is given as a function of the chemicai potentiai of water imposed by the GCMC simulation instead of pressure to avoid the introduction of uncertainties in conversion. It is important to note that this chemical mtential does not contain the ideal rotational contribution -kT(-4.09+3/2ln1), witii T in Kelvin. The high temperature (650 K) isotherm is reversible (supercritical), whereas the low temperature cases present large hysteresis (type IV isotherm in tiie lUPAC classification). The steep rises are associated to the capillary condensation of water in tiie mesoporosity of the Vycor ass. These simulation results are in qualitative agreement with experimental data, witii an asymmetric adsorption / desorption hysteresis characteristic of disordered and intercormected pores. ... 

Water may be absorbed from the atmosphere and wet the metal smface if hygroscopic salts are deposited or formed by corrosion. This absorption will take place when the relative humidity exceeds the critical relative hmnidity. The value of the critical relative humidity is dependent on the specific metal and the specific metal contaminants. When the relative humidity exceeds the value at which the salt starts to absorb water and dissolve, the corrosion rate increases sharply. This critical relative humidity corresponds to the vapor pressure above a saturated solution of the salt present. Adsorption layers of electrolyte on the surface of the metal may also be the result of capillary condensation. 

On this basis, a rapid and nondestructive method, ellipsometric porosimetry (EP), has been developed in which adsorption-desorption isotherms are determined from the variations of film refractive index efr induced by the change of partial pressme of a solvent above a film. The setup combines a pressure-controlled chamber (conventional gas volumetric characterization devices) and a classical eUipsometer thus, HeS is determined for each vapor pressure and is a direct measme of the adsorption isotherm. A typical example is shown in Figure 33.3a for a Si02 templated with CTAB thin film (Martinez Ricci, M.L., Fuertes, M.C., Violi, I.L., Grosso, D., and Soler lUia, G.J.AA., Rational design of mesoporous films for synthesis of responsive Bragg reflectors (unpublished).). The refractive index increases from eff (630 nm) = 1.21, for a large fraction of air inside micropores and/or mesopores within the silica nanostructure, to (630 nm) = 1.37 when pores are saturated with water. The steep increase at intermediate vapor pressures is associated with the capillary condensation inside pores. The hysteresis in the desorption branch is due to the presence of water in the necks that join pores, whose effective radii are smaller than the pore radius. 

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