Capillary condensation association effects

Henry s law corresponds physically to the situation in which the adsorbed phase is so dilute that there is neither competition for surface sites nor any significant interaction between adsorbed molecules. At higher concentrations both of these effects become important and the form of the isotherm becomes more complex. The isotherms have been classified into five different types (9) (Eig. 4). Isotherms for a microporous adsorbent are generally of type I the more complex forms are associated with multilayer adsorption and capillary condensation. 

In fact, most mesoporous adsorbents possess complex networks of pores of different size. It is therefore unlikely that the condensation-evaporation processes can occur independently in each pore. The complexity of capillary condensation in porous materials is illustrated by the recent Monte Carlo computer simulation studies of Page and Monson (1996) and Gelb and Gubbins (1998). The well-defined hysteresis loops observed in the simulation results of both studies were attributed to the presence of thermodynamically metastable states and not to kinetic effects. However, it appears that the extent of die hysteresis was associated with the overall heterogeneity of the adsorbent structure and not simply due to capillary condensation within individual pores. 

At present, it must be recognized that adsorption hysteresis may be generated in a number of different ways. In the context of the assessment of mesoporosity, we have seen that there are two major contributing factors (a) on the adsorption branch, the development of a metastable multilayer and the associated delay in capillary condensation (b) on the desorption branch, the entrapment of condensate through the effect of network-percolation. 

It is striking that the high compaction pressure, which was sufficient to convert assemblages of spheroidal particles into well-defined mesopore structures (Gregg, 1968), had relatively little effect on the course of the adsorption isotherm. Although the desorption curve was displaced in the multilayer range, the isotherm remained pseudo-Type II (now termed Type lib). We conclude that the resulting hysteresis loop is associated with both the development of a pore network and the delayed capillary condensation on the surface of the platelets. 

Pores with different sizes show characteristic physical adsorption effects as manifested in the isotherm. The isotherm shows the relationship between the amount of a given gas taken np or released by a solid as a function of the gas pressnre nnder a constant temperature. The type-I isotherm shows a steep increase at very low pressmes and a long satnration platean and is characteristic of microporous materials. The type-IV isotherm exhibits a steep iucrease at high relative pressme and, in many cases, a hysteresis loop, which is associated with capillary condensation in mesopores. 

The presence of adsorption hysteresis cannot be reconciled with the laws of classical thermodynamics. It is evident that there are various forms of adsorption hysteresis [7], which require different explanations [7, 39, 40]. In the capillary condensation range, well-defined hysteresis loops are generally associated with delayed condensation or percolation [11] through pore networks or ink bottles [38, 39]. In the case of activated carbons, delayed condensation is likely to be the most important mechanism [11], but we cannot rule out the other effects. 

Bound moisture is associated with the hygroscopic nature of the woody components. There are some uncertainties about the limits of hygroscopic behavior, particularly with woods of high extractives content but it is useful to define a maximum sorptive moisture content, called the fiber saturation point (FSP). If the capillary condensation effects in pores greater than 0.1 xm in equivalent cylindrical diameter are ignored, FSP of the wood may be defined as the equilibrium moisture content (EMC) in an environment of 99% relative humidity. This yields a value of 30 to 32%i for most commercial species (Keey et al., 2000) at room temperature. FSP falls with increasing temperature. For a softwood such as Sitka spruce (Picea sitchensis), FSP falls from about 31%i at 25° C to 23% at 100°C (Stamm, 1964). 

The time of wetness is obviously strongly dependent on the critical relative humidity. Apart from the primary critical humidity, associated with clean surfaces, secondary and even tertiary critical humidity levels may be created by hygroscopic corrosion products and capillary condensation of moisture in corrosion products, respectively. A capillary condensation mechanism may also account for electrolyte formation in microscopic surface cracks and the metal surface-dust particle interface. Other sources of surface electrolyte include chemical condensation (by chlorides, sulfates, and carbonates), adsorbed molecular water layers, and direct moisture precipitation (ocean spray, dew, rain). The effects of rain on atmospheric corrosion damage are somewhat ambiguous. While providing electrolyte for corrosion reactions, rain can act in a beneficial manner by washing away or diluting harmful corrosive surface species. 

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. 

Adsorption hysteresis is often associated with porous solids, so we must examine porosity for an understanding of the origin of this effect. As a first approximation, we may imagine a pore to be a cylindrical capillary of radius r. As just noted, r will be very small. The surface of any liquid condensed in this capillary will be described by a radius of curvature related to r. According to the Laplace equation (Equation (6.29)), the pressure difference across a curved interface increases as the radius of curvature decreases. This means that vapor will condense... 

Effective inhibition of evaporation is associated with a sharp decrease of the coefficient of condensation ao by the formation of a monolayer of CA molecules. On the basis of Eq. (18), the values of Oo might be found using measured rates of evaporation and — /> ) values, where po is the vapor pressure in the surrounding medium. The diffusion resistance to the vapor flux inside the capillary between the meniscus and the entrance may, in this case, be neglected. For different regions of the graphs presented in Fig. 14, values of o range from 1.8 x 10 to 2.7 x 10 In another experiment with a capillary ofthe radius 8.7 irm(z = Avam,plp = 0.54) almost the same value, oq = 3 x 10 was calculated. 

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