Water multilayer adsorption

In the present study we try to obtain the isotherm equation in the form of a sum of the three terms Langmuir s, Henry s and multilayer adsorption, because it is the most convenient and is easily physically interpreted but, using more a realistic assumption. Namely, we take the partition functions as in the case of the isotherm of d Arcy and Watt [20], but assume that the value of V for the multilayer adsorption appearing in the (5) is equal to the sum of the number of adsorbed water molecules on the Langmuir s and Henry s sites ... 

Multilayer Adsorption of Water. As the amount of water in the clay increases over that needed for a one- or two-layer hydrate, the study of the properties of the water becomes experimentally more difficult. This is important because it is only at water contents in excess of the two-layer hydrate that a conflict arises between the short-range and long-range interaction models. In support of the short-range model, two studies are noteworthy. A small angle... 

True Type in isotherms are confined to a few systems in which the overall adsorbent -adsorbate interactions are weak in comparison with relatively strong adsorbate-adsorbate interactions. The monolayer density tends to be unevenly distributed on the adsorbent surface with a relatively high concentration of molecules located on the most active areas. As the pressure is raised the average monolayer concentration is increased, but before it can become close-packed over the complete surface, the monolayer coverage is overtaken by a form of co-operative multilayer adsorption in which molecules are clustered around the most favourable sites. This results in the isotherm curvature remaining convex to the pjp° axis with C(BET) < 2. In an extreme case (e.g. water/graphite) the adsorption may be quite difficult to detect, even at p/p° 0.8. 

To apply any gas adsorption technique for the determination of surface area, it must be assumed that the mode of monolayer and/or multilayer adsorption is not oversensitive to differences in surface structure. This requirement is more likely to be satisfied if the specific contribution to the adsorbent-adsorbate interaction is not too great. For this reason, water, alcohols and other highly polar molecules are not recommended for the determination of the overall area. 

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... 

All the techniques discussed so far refer to clean surfaces or surfaces with adsorbed molecules. When thicker adsorbed layers are present on the surface, the properties of these layers start to resemble those of the corresponding bulk phases. For Instance, for thin water layers on solid surfaces the dielectric permittivity (bulk water. A more or less gradual transition takes place towards wetting films to which we shall return in Volume III and. as far as multilayer adsorption is concerned, in sec. 1.5 g, h. 

To unravel the detailed mechanism, substrate adsorption, quenching, inhibition and kinetic studies were conducted for the ZnS-catalyzed photodehydrodimeriza-tion of 2,5-DHF [107, 148]. A plot of the amount of 2,5-DHF adsorbed ( eq) against the residual concentration in solution (cgq) exhibits saturation plateaus at eq(max) of 2.8 X 10 and 65 X 10 mol g . The first plateau is due to the formation of a mixed solvent-solute surface monolayer and the second corresponds to multilayer adsorption. Assuming that the formation of the monolayer can be described by competitive adsorption between water and 2,5-DHF, the data can be analyzed according to Hiemenz (Eq. 30) [149] ... 

Although the sorption isotherm is fundamental to the characterization of moisture interaction with water, it is generally not possible to make any judgments about the effect of water on the substrate from the isotherm alone. For example, it is not possible to determine if an increase in moisture content is due to multilayer adsorption, swelling of the substrate, or some combination of the two. 

Isotherm I is typical of adsorption in micropores, e.g., adsorption on molecular sieves and activated carbons. Isotherm II represents multilayer physisorption on a flat surface (valid for many nonporous substances). Isotherms III and V are characteristics of weak gas-solid interactions, e.g., water adsorption on gold. Isotherm IV is frequently observed in the study of practical heterogeneous catalysts. Its shape is characteristic of multilayer adsorption accompanied by capillary condensation in mesopores. When the surface of a nonporous adsorbent is energetically uniform the isotherm may have a step-like shape (Isotherm VI). A good example of such behaviour is the adsorption isotherm of Rr at 90 K on graphite [5]. 

Figure 3.3. Sorption equations fitted to experimental data for the sorption of water by wood at 40°C (Simpson, 1980). The Langmuir isotherm is parabolic, corresponding to the formation of a BET monolayer. Multilayer adsorption describes sorption behaviour better but sorption at the highest moisture contents, where capillary condensation occurs, is underestimated.

Pashley, R.M. Kitchener, J.A. Surface forces in adsorption multilayers of water on quartz. J. Colloid Interface Sci. 1979, 71, 491. 

The main differences between adsorption from the gas phase and that from liquid phase are as follows [3]. First, adsorption from solution is essentially an exchange process, and hence, molecules adsorb not only because they are attracted by solids but also because the solution may reject them. A typical illustration is that the attachment of hydrophobic molecules on hydrophobic adsorbents from aqueous solutions is mainly driven by their aversion to the water and not by their attraction to the surface. Second, isotherms from solution may exhibit nonideality, not only because of lateral interactions among adsorbed molecules but also because of nonideality in the solution. Third, multilayer adsorption from solution is less common than from the gas phase, because of the stronger screening interaction forces in condensed fluids. 

It is common in soil chemistry for adsorption of organics from solution to be of more interest than adsorption from vapor. This usually does not involve multilayer adsorption because of the tendency of water molecules to compete effectively for adsorption sites, at least if physical adsorption is the main process. In such cases, the Langmuir equation may be more accurate in describing adsorption results than the BET equation. The Langmuir equation is readily converted from the linear form used in gas... 

In the present section we discuss the mobile case making the above assumption, and consider pressures low enough so that multilayer adsorption is not important. (There are of course some systems which never form a monolayer in the usual sense but start on multilayers practically from the beginning, as in the adsorption of water vapor on graphite or other systems with BET constant c very small.) Also, we assume for simplicity a uniform surface. 

Cationic surfactants adsorb strongly on clay surfaces by cation exchange. The fatty tails of these adsorbed surfactants impart oil-wetness to the clay surfaces and shield the clays from direct contact with water. This shielding has an obvious stabilizing effect however, this change in wettability often results in undesirable side effects, such as a decrease in oil relative permeability. Moreover, because of the possibility of multilayered adsorption (formation of surface micelles), a high surfactant concentration is required to satisfy the cation exchange capacity of the clays, which can make such treatments rather expensive. 

For this reason, we consider it hardly possible to cite all of the publications. Let us focus only on the following examples. Hydroxamic acids have already been for a long time subject of the classical analytical chemistry. In [71], the possibility of using these compounds in flotation of rare-earth minerals is shown. It has been concluded that on a mineral surface cerium chelates are formed. Besides, chemisorption is accompanied by a physical multilayer adsorption of hydroxamic acid derivatives formed by reaction with cations in the water phase. A number of chelate-forming compounds including hydroxamic acids has been tested in flotation of niobium ores [72]. The best results are obtained when using alkyl phosphonic acids. Chemisorption mechanism and the structure of the surface compounds are established by spectroscopic methods. 

Halacheva, S.S., Penfold, J., Thomas, R.K., Webster, J.R.P., 2013. Solution pH and oligoamine molecular weight dependence of the transition from monolayer to multilayer adsorption at the air-water interface from sodium dodecyl sulfate/ohgoamine mixtures. Langmuir 29, 5832-5840. 

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