Physical adsorption multilayer

Chemisorption is by its very nature hmited to less than monolayer coverage of the surface whereas, in physical adsorption, multilayer adsorption is common. In a microporous solid the ultimate capacity for physical adsorption corresponds to the specific micropore volume, which is generally much larger than the monolayer coverage. The economic viability of an adsorption... 

In the second picture, an interfacial layer or region persists over several molecular diameters due to a more slowly decaying interaction potential with the solid (note Section X-7C). This situation would then be more like the physical adsorption of vapors (see Chapter XVII), which become multilayer near the saturation vapor pressure (e.g.. Fig. X-15). Adsorption from solution, from this point of view, corresponds to a partition between bulk and interfacial phases here the Polanyi potential concept may be used (see Sections X-7C, XI-1 A, and XVII-7). 

The immediate site of the adsorbent-adsorbate interaction is presumably that between adjacent atoms of the respective species. This is certainly true in chemisorption, where actual chemical bond formation is the rule, and is largely true in the case of physical adsorption, with the possible exception of multilayer formation, which can be viewed as a consequence of weak, long-range force helds. Another possible exception would be the case of molecules where some electron delocalization is present, as with aromatic ring systems. 

Because of their prevalence in physical adsorption studies on high-energy, powdered solids, type II isotherms are of considerable practical importance. Bmnauer, Emmett, and Teller (BET) [39] showed how to extent Langmuir s approach to multilayer adsorption, and their equation has come to be known as the BET equation. The derivation that follows is the traditional one, based on a detailed balancing of forward and reverse rates. 

In considering isotherm models for chemisorption, it is important to remember the types of systems that are involved. As pointed out, conditions are generally such that physical adsorption is not important, nor is multilayer adsorption, in determining the equilibrium state, although the former especially can play a role in the kinetics of chemisorption. 

Surface areas are deterrnined routinely and exactiy from measurements of the amount of physically adsorbed, physisorbed, nitrogen. Physical adsorption is a process akin to condensation the adsorbed molecules interact weakly with the surface and multilayers form. The standard interpretation of nitrogen adsorption data is based on the BET model (45), which accounts for multilayer adsorption. From a measured adsorption isotherm and the known area of an adsorbed N2 molecule, taken to be 0.162 nm, the surface area of the soHd is calculated (see Adsorption). 

Adsorption of dispersants at the soHd—Hquid interface from solution is normally measured by changes in the concentration of the dispersant after adsorption has occurred, and plotted as an adsorption isotherm. A classification system of adsorption isotherms has been developed to identify the mechanisms that may be operating, such as monolayer vs multilayer adsorption, and chemisorption vs physical adsorption (8). For moderate to high mol wt polymeric dispersants, the low energy (equiUbrium) configurations of the adsorbed layer are typically about 3—30 nm thick. Normally, the adsorption is monolayer, since the thickness of the first layer significantly reduces attraction for a second layer, unless the polymer is very low mol wt or adsorbs by being nearly immiscible with the solvent. 

Of special interest in liquid dispersions are the surface-active agents that tend to accumulate at air/ liquid, liquid/liquid, and/or solid/liquid interfaces. Surfactants can arrange themselves to form a coherent film surrounding the dispersed droplets (in emulsions) or suspended particles (in suspensions). This process is an oriented physical adsorption. Adsorption at the interface tends to increase with increasing thermodynamic activity of the surfactant in solution until a complete monolayer is formed at the interface or until the active sites are saturated with surfactant molecules. Also, a multilayer of adsorbed surfactant molecules may occur, resulting in more complex adsorption isotherms. 

Type II adsorption, by contrast, is widely observed with physical adsorption and is interpreted to mean multilayer adsorption. 

As noted above, the range of pressures over which gas adsorption studies are conducted extends from zero to the normal vapor pressure of the adsorbed species p0. An adsorbed layer on a small particle may readily be seen as a potential nucleation center for phase separation at p0. Thus at the upper limit of the pressure range, adsorption and liquefaction appear to converge. At very low pressures it is plausible to restrict the adsorbed molecules to a mono-layer. At the upper limit, however, the imminence of liquefaction suggests that the adsorbed molecules may be more than one layer thick. There is a good deal of evidence supporting the idea that multilayer adsorption is a very common form of physical adsorption on nonporous solids. In this section we are primarily concerned with an adsorption isotherm derived by Brunauer, Emmett, and Teller in 1938 the theory and final equation are invariably known by the initials of the authors BET. 

Polar molecules like II2O show apparent polymerization to an extent quite impossible in the gas phase at low pressures. The dipole field interaction, which is of the order of 1 ev., results in an artificial multilayer physical adsorption at pressures and temperatures where ordinarily only a minute fraction of the first layer would exist. Since multilayer adsorption is quite liquid-like, the high degree of polymerization can be explained. It is interesting to note that at low fields individual peaks show some substructure, which could be due to alignment differences at the time of ionization or could correspond to ionization from different layers within the adsorbate. It is hoped to study physical adsorption near the condensation point at low pressure with nonpolar rare gas atoms to see if layer structure can be elucidated in this way. 

Capillary Condensation. In a porous adsorbent the region of multilayer physical adsorption merges gradually with the capillary condensation regime, leading to upward curvature of the equilibrium isotherm at higher relative pressure. In the capillary condensation region the intrinsic selectivity of the adsorbent is lost. 

The attainment of physical adsorption equilibrium is usually rapid, since there is no activation energy involved, and (apart from complications introduced by capillary condensation) the process is readily reversible. Multilayer physical adsorption is possible, and at... 

Type II isotherms (e.g. nitrogen on silica gel at 77 K) are frequently encountered, and represent multilayer physical adsorption on non-porous solids. They are often referred to as sigmoid isotherms. For such solids, point B represents the formation of an adsorbed monolayer. Physical adsorption on microporous solids can also result in type II isotherms. In this case, point B represents both monolayer adsorption on the surface as a whole and condensation in the fine pores. The remainder of the curve represents multilayer adsorption as for non-porous solids. 

Values reflect multilayer physical adsorption on a fairly uniform solid surface... 

The foregoing represent the classic models for physical adsorption, that of Langmuir (monomolecular adsorption and constant AH is, independent of the extent of surface coverage) and BET (multilayer adsorption and several AHacjs components). Many other models are available as well [13,15,30], These models require a knowledge of the area which each molecule occupies on the surface. 

In adsorption, we call the gas or solution solute the adsorbate and the solid the adsorbent. Monolayer adsorption involves up to one layer of adsorbate on the adsorbent, whereas multilayer adsorption involves more than one layer of adsorbate. Adsorption may be either physical adsorption (physisorption), where the adsorbate is bound to the surface by relatively weak physical forces (AT/desorp < 40 kJ/mol) or chemical adsorption (chemisorption), where the binding forces are stronger (A//desorb > 40 kJ/mol).6 Because chemical adsorption involves chemical-type bonds between adsorbate and adsorbent, it is limited to the first monolayer on the surface. Physical adsorption can involve multiple layers and physical adsorption can occur on top of chemisorbed layer. 

As adsorbate, the polysaccharide dct is sensitive to the DP and polydis-persity (Cohen Stuart et al., 1982). Higher DP polysaccharides are less kinetically active, are therefore slower to accumulate than lower DP polysaccharides because of the time taken for surface orientation, and are thus more inclined to stay adsorbed longer and reach higher concentrations. Agitation increases the rate of physical adsorption. From the foregoing discussion on polysaccharide dispersibility, it is safe to conclude that multilayer adsorption is antecedent to polysaccharide phase inversion and in some instances to sol-gel transition. 

The importance of surface area in colloidal chemistry has spurred many attempts to develop a method of its accurate measurement from physical adsorption processes. All of the methods so far are empirical and attended with difficulty involving surface nonuniformity, polymolecularity, conformational shifts, and multilayer adsorption. Polysaccharide surfaces are seldom... 

The type II is very common in the case of physical adsorption and undoubtedly corresponds to multilayer formation. For many years it was the practice to take point B at the knee of the curve, as the point of completion of mono layer consistent with those found using adsorbate that give type I isotherms. 

Chemisorption or specific adsorption involves greater forces of attraction than physical adsorption. As hydrogen bonding or n rbital interactions are utilised, the adsorbed species lose their hydrated spheres and can approach the surface as close as the ionic radius. Whereas multilayer adsorption is possible in physical adsorption, chemisorption is necessarily limited to monolayer coverage. 

This experiment is concerned with the multilayer physical adsorption of a gas (the adsorbate) on a high-area solid (the adsorbent). Since such adsorption is caused by forces very similar to those that cause the condensation of a gas to a bulk liquid, appreciable adsorption occurs only at temperatures near the boiling point of the adsorbate. The adsorption of N2 gas on a high-area solid will be studied at 77.4 K (the boihng point of liquid nitrogen), and the surface area of the solid will be obtained. 

Brunauer, Emmett, and Teller were the first to propose a theory for multilayer adsorption (BET theory). Since the behavior of adsorbed molecules is even more difficult to describe in detail than that of molecules in the liquid state, the BET theory contains some rather drastic assumptions. In spite of this, it is still a generally useful theory of physical adsorption. The BET theory gives a correct semiquantitative description of the shape of the isotherm and provides a good means of evaluating (which is then used to estimate the surface area of the soM). 

A. Dabrowskl, M. Jaroniec, Adv. Colloid Interface Set 27 (1987) 211, Theoretical Foundations of Physical Adsorption from Binary Non-electrolytic Liquid Mixtures on Solid Surfaces Present and Future (emphasis on adsorption models covering heterogeneity, adsorptlves of different sizes and multilayers). 

The type I isotherm corresponds to the Langmuir case when adsorption is confined to a monolayer. The multilayer physical adsorption of gases by nonporous solids, in a vast majority of cases, gives rise to a type II isotherm, which can be described by the Brunauer, Emmet, and Teller (BET) equation (6,51). 

The complexities of solid surfaces and onr inability to characterize exactly their interactions with adsorbed molecules hmits our understanding of the adsorption process. It does not, however, prevent development of an exact thennodynamic description of adsorption equilibrium, applicable alike to physical adsorption and chemisorption and equally to monolayer and multilayer adsorption. The thermodynamic frame work is independent of any particular theoretical or empirical description of material behavior. However, in application such a description is essential, and meaningful results require appropriate models of behavior. 

Apart from the LB technique, there are several alternative ways of forming ordered layer structures of amphiphilic molecules on solid supports which, are sometimes less cumbersome or quicker. One of these is the self-assembly technique, in which ordered monolayers or multilayers are formed by chemisorption or physical adsorption. Reviews of this and other techniques are given in Refs... 

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