Adsorbed layer hysteresis

Figures 9.12d and 12e illustrate another model for adsorption hysteresis that considers multilayer adsorption explicitly. During adsorption the capillary is viewed as a cylinder of radius (r - t), with t the thickness of the adsorbed layer at that pressure. This is represented by Figure 9.12d. For such a surface the Kelvin equation becomes...

Pores, thus explaining why for some counter-ions a low-pressure hysteresis loop is observed, and not for others. This change of shape may have the effect of disrupting the reorganisation of the adsorbed layer. 

A complication arises because isotherms display hysteresis as shown in Fig. 7.17. Much has been written on the origin of these hysteresis cur xs, which provide information about the shape of pores. There are two extremes. Cylindrical or slit-shaped pores (Fig. 7.18a) give moderate hysteresis, such as Fig. 7.17a. The adsorption branch of the isotherm results from adsorbed layers on the cylindrical walls, which thicken until a miniscus forms. Applicability of the Kelvin equation to such a microscopic system is doubtful. Upon desorption, however, evaporation occurs at the larger miniscus and the Kelvin equation is valid. In this case, pore size distributions from the desorption branch arc recommended as more reliable. The other extreme... 

In the discussion of the mesopore shape, the contact angle, is assumed to be zero (uniform adsorbed film formation). The lower hysteresis loop of file same adsorbate encloses at a common relative pressure depending to the stability of the adsorbed layer regardless of the different adsorbents due to the so called tensile strength effect. This tensile strength effect is not sufficiently considered for analysis of mesopore structures. The Kelvin equation provides the relationship between the pore radius and the amount of adsorption at a relative pressure. Many researchers developed a method for the calculation of the pore size distribution on the basis of the Kelvin equation with a correction term for the thickness of the multilayer adsorbed film. 

Unlike physical adsorption, chemisorption involves very specific interactions between the solid surface and the adsorbing molecules, as illustrated by the much higher heats of adsorption. Another important result of that specificity is that chemisorption is by nature limited to the formation of a monomolecu-lar adsorbed layer. Chemisorption processes will generally have some activation energy and may therefore be much slower than physical adsorption. They may also exhibit hysteresis that is, they may not be readily reversible. 

A comparison of Equation 2.115 and Equation 2.116 shows that p becomes equal to p when y = 2D (i.e., when the radius of the adsorbed layer is equal to twice the thickness of the adsorbed film). As the smallest possible value of D is the diameter of the adsorbate molecule, it means that hysterens cannot occur in pores narrower than four molecular diameters i.e., in cylindrical pores, hysteresis starts when r becomes greater than 2D. 

Below the critical temperature of the adsorbate, adsorption is generally multilayer in type, and the presence of pores may have the effect not only of limiting the possible number of layers of adsorbate (see Eq. XVII-65) but also of introducing capillary condensation phenomena. A wide range of porous adsorbents is now involved and usually having a broad distribution of pore sizes and shapes, unlike the zeolites. The most general characteristic of such adsorption systems is that of hysteresis as illustrated in Fig. XVII-27 and, more gener-... 

The properties of both organic matter and clay minerals may affect the release of contaminants from adsorbed surfaces. Zhang et al. (1990) report that desorption (in aqueous solution) of acetonitrille solvent from homoionic montmorillonite clays is reversible, and hysteresis appears to exist except for K+-montmorillonite. This behavior suggests that desorption may be affected by the fundamental difference in the swelling of the various homoionic montmorillonites, when acetonitrile is present in the water solution. During adsorption, it was observed that the presence of acetonitrile affects the swelling of different homoionic clays. At a concentration of 0.5 M acetonitrile in solution, the layers of K+-montmorillonite do not expand as they would in pure water, while the layers of Ca +- and Mg +-montmorillonite expand beyond a partially collapsed state. The behaviors of K+-, Ca +-, and Mg +-montmorillonite are different from the behavior of the these clays in pure water. Na+-montmorillonite is not affected by acetonitrile presence in an aqueous solution. 

An example of the adsorption to one such material is shown in Fig. 9.16. The siliceous material, called MCM-41, contains cylindrical pores [397], With increasing pressure first a layer is adsorbed to the surface. Up until a pressure of P/Po 0.45 is reached, this could be described by a BET adsorption isotherm equation. Then capillary condensation sets in. At a pressure of P/Po 0.75, all pores are filled. This leads to a very much reduced accessible surface and practically to saturation. When reducing the pressure the pores remain filled until the pressure is reduced to P/Pq rs 0.6. The hysteresis between adsorption and desorption is obvious. At P/Po 0.45 all pores are empty and are only coated with roughly a monolayer. Adsorption and desorption isotherms are indistinguishable again below P/Po 0.45. 

Figure 7.42 Types of gas sorption isotherm - microporous solids are characterised by a type I isotherm. Type II corresponds to macroporous materials with point B being the point at which monolayer coverage is complete. Type III is similar to type II but with adsorbate-adsorbate interactions playing an important role. Type IV corresponds to mesoporous industrial materials with the hysteresis arising from capillary condensation. The limiting adsorption at high P/P0 is a characteristic feature. Type V is uncommon. It is related to type III with weak adsorbent-adsorbate interactions. Type VI represents multilayer adsorption onto a uniform, non-porous surface with each step size representing the layer capacity (reproduced by permission of IUPAC).

The term adsorption may also be used to denote the process in which adsorptive molecules are transferred to, and accumulate in, the interfacial layer. Its counterpart, desorption, denotes the converse process, in which the amount adsorbed decreases. Adsorption and desorption are often used adjectivally to indicate the direction from which experimentally determined adsorption values have been approached, e.g. the adsorption curve (or point) and the desorption curve (or point). Adsorption hysteresis arises when the adsorption and desorption curves do not coincide. 

In monolayer adsorption all the adsorbed molecules are in contact with the surface layer of the adsorbent. In multilayer adsorption the adsorption space accommodates more than one layer of molecules so that not all adsorbed molecules are in direct contact with the surface layer of the adsorbent. In capillary condensation the residual pore space which remains after multilayer adsorption has occurred is filled with condensate separated from the gas phase by menisci. Capillary condensation is often accompanied by hysteresis. The term capillary condensation should not be used to describe micropore filling because this process does not involve the formation of liquid menisci. 

The application of the CPSM model in the interpratation a variety of nitrogen sorption hysteresis loops validated a slightly modified Halsey correlation for the statistical adsorbed gas layer thickness [8,9]. Thus ... 

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