Hysteresis desorption/adsorption

Fig. XVll-19. Adsorption of CH4 on MgO(lOO) at 77.35 K. The vertical line locates each vertical step corresponds to the condensation of a monolayer. There was no hysteresis. Desorption points are shown as . (From Ref. 110.)...

Adsorption of water by cellulose displays hysteresis. The adsorption isotherm is not identical to the desorption isotherm and the amount of adsorbed water in equilibrium with the atmosphere at a particular relative humidity is higher during desorption from a higher humidity than during adsorption from a lower humidity. A plot of the adsorption/desorption isotherm is shown in Figure 5.4. 

In a similar study (Comans et al., 1990), the reversibility of Cs+ sorption on illite was studied by examining the hysteresis between adsorption and desorption isotherms and the isotopic exchangeability of sorbed Cs+. Apparent reversibility was found to be influenced by slow sorption kinetics and by the nature of the competing cation. Cs+ migrates slowly to energetically favorable interlayer sites from which it is not easily released. 

The adsorption isotherm of N, on FSM-16 at 77 K had an explicit hysteresis. As to the adsorption hysteresis of N-, on regular mesoporous silica, the dependencies of adsorption hysteresis on the pore width and adsorbate were observed the adsorption hysteresis can be observed for pores of w 4.0nm. The reason has been studied by several approaches [5-8]. The adsorption isotherm of acetonitrile on FSM-16 at 303K is shown in Fig. 1. The adsorption isotherm has a clear hysteresis the adsorption and desorption branches close at PIP, = 0.38. The presence of the adsorption hysteresis coincides with the anticipation of the classical capillary condensation theory for the cylindrical pores whose both ends are open. The value of the BET monolayer capacity, nm, for acetonitrile was 3.9 mmol g. By assuming the surface area from the nitrogen isotherm to be available for the adsorption of acetonitrile, the apparent molecular area, am, of adsorbed acetonitrile can be obtained from nm. The value of am for adsorbed acetonitrile (0.35 nnr) was quite different from the value (0.22 nm2) from the liquid density under the assumption of the close packing. Acetonitrile molecules on the mesopore surface are packed more loosely than the close packing. The later IR data will show that acetonitrile molecules are adsorbed on the surface hydroxyls in... 

Now the kinetic concept of surface fluctuations and relaxation enters into the theory of adsorption. A nontrivial dependence is predicted between the adsorbed quantity and the rate of pressure or concentration change of the adsorbent. Also predicted is the phenomenon of hysteresis during adsorption and desorption over a time comparable to the relaxation time of the surface. 

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. 

Kommalapati RR,Valsaraj KT, Constant WD (2000) Soil-water partition coefficients, adsorption/desorption hysteresis, desorption kinetics and bioavailability of chlorinated organic compounds at the PPI site. Submitted to Hazardous Substance Research Center, Louisiana State University, Baton Rouge, LA... 

Similar simulations for finite gold nanocrystals with 561 atoms led to a similar hysteresis between adsorption and desorption. Moreover, the adsorption starts at a lower thiol concentration for the nanocrystals than for the macroscopic crystal surface. Adding the solvent led to significant changes. There results a competitive adsorption between the thiols and the solvent and the interactions between the tails are modified. Thus, as the authors conclude, phenomena that are observed in vacuum may be different from those observed in a solution. 

The problem of adsorption hysteresis remains enigmatic after more than fifty years of active use of adsorption method for pore size characterization in mesoporous solids [1-3]. Which branch of the hysteresis loop, adsorption or desorption, should be used for calculations This problem has two aspects. The first is practical pore size distributions calculated from the adsorption and desorption branches are substantially diflferent, and the users of adsorption instruments want to have clear instructions in which situations this or that branch of the isotherm must be employed. The second is fundamental as for now, no theory exists, which can provide a quantitatively accurate description of capillary condensation hysteresis in nanopores. A better understanding of this phenomenon would shed light on peculiarities of phase transitions in confined fluids. 

The occurrence of hysteresis in adsorption phenomena, caused by capillary condensation, has led to the application of the Kelvin equation for the desorption branch of a complete adsorption isotherm and thus to a complete distribution curve of the widths of the various pores as a function of their volumes (49). The results are mostly expressed in the form of radii of cylindrical pores. The method may be applied for radii between 20 and 300 A. The figures obtained have to be corrected for the thickness of the multilayer adsorption on the surface of the nonfilled capillaries (50), and various calculation methods have recently been published (51) which need not be discussed here, since Wheeler (5 ) gave an excellent review recently. 

Cohan s quantitative analysis in 1938 was based on the suggestion of Foster (1932, 1934) that the hysteresis in adsorption is due to the delay in forming a meniscus in the capillary. For adsorption, this is occurred by radial filling, rather than vertical filling as in the case of desorption. When condensation of the first layer occurs, the effective radius r decreases, causing further condensation at a fixed reduced pressure P/Pq. This means that a pore of radius r, corresponding to the threshold reduced pressure P/Pq, will be filled instantaneously. 

FIGURE 2.27 Adsorption-desorption hysteresis in adsorption of water vapor on sugar charcoal outgassed at different temperatures. (Source Puri, B.R. and Myer, Y.P., J. Sci. and Ind. Res., India, 16B, 52, 1957. With permission.)... 

Hysteresis between adsorption and desorption has been observed frequently, but attempts at rational explanations are very rare [48], Once the biopolymer has been adsorbed onto the surface, desorption cannot be achieved under similar conditions. Molecular rearrangement is probably responsible for this behavior. 

The adsorption/desorption isotherms of O2 on Co(salen) are shown in Figure 10.16. These isotherms display a very noticeable and interesting hysteresis. The adsorption isotherm shows that very little O2 is adsorbed until the O2 pressure reaches a threshold at approximately 0.2 atm. The adsorption isotherm then sharply rises to nearly the full O2-binding capacity of the complex. A very low pressure was then required to release the bound oxygen. The... 

Figure 4.7. The phenomenon of hysteresis in adsorption where the adsorption and desorption...

FIGURE 4.11 Hysteresis between adsorption and desorption isotherms. 

Structure of the rehydrated crystal [27"] was the same as that of 27. The dehydrated compound exhibits N2 and H2 sorption properties without any hysteresis between adsorption and desorption processes. 

Abstract. A model of the conformational transitions of the nucleic acid molecule during the water adsorption-desorption cycle is proposed. The nucleic acid-water system is considered as an open system. The model describes the transitions between three main conformations of wet nucleic acid samples A-, B- and unordered forms. The analysis of kinetic equations shows the non-trivial bifurcation behaviour of the system which leads to the multistability. This fact allows one to explain the hysteresis phenomena observed experimentally in the nucleic acid-water system. The problem of self-organization in the nucleic acid-water system is of great importance for revealing physical mechanisms of the functioning of nucleic acids and for many specific practical fields. 

The adsorption-desorption hysteresis does not disappear or decrease during at least a week of exposure of the NA sample to a r.h. of 56%, this value being chosen because the adsorption hysteresis is the greatest at this r.h. The hysteresis lifetime is great enough to consider the hysteresis as a permanent phenomenon for the processes of the cellular regulation. 

A characteristic feature of a Type IV isotherm is its hysteresis loop. The exact shape of the loop varies from one adsorption system to another, but, as indicated in Fig. 3.1, the amount adsorbed is always greater at any given relative pressure along the desorption branch FJD than along the adsorption branch DEF. The loop is reproducible provided that the desorption run is started from a point beyond F which marks the upper limit of the loop. 

Thus, as pointed out by Cohan who first suggested this model, condensation and evaporation occur at difi erent relative pressures and there is hysteresis. The value of r calculated by the standard Kelvin equation (3.20) for a given uptake, will be equal to the core radius r,. if the desorption branch of the hysteresis loop is used, but equal to twice the core radius if the adsorption branch is used. The two values of should, of course, be the same in practice this is rarely found to be so. 

Before proceeding to detail, however, it is necessary to consider the question as to which branch of the hysteresis loop—the adsorption or the desorption branch—should be used. Though the mode of calculation is... 

Fig. 3.19 Contrast between the pore size distribution curves based on the adsorption and the desorption branch of the hysteresis loop respectively.

Fig. 3.28 The Kiselev method for calculation of specific surface from the Type IV isotherm of a compact of alumina powder prepared at 64 ton in". (a) Plot of log, (p7p) against n (showing the upper (n,) and lower (n,) limits of the hysteresis loop) for (i) the desorption branch, and (ii) the adsorption branch of the loop. Values of. 4(des) and /4(ads) are obtained from the area under curves (i) or (ii) respectively, between the limits II, and n,. (6) The relevant part of the isotherm.

Fig. 4.25 Adsorption isotherms showing low-pressure hysteresis, (a) Carbon tetrachloride at 20°C on unactivated polyacrylonitrile carbon Curves A and B are the desorption branches of the isotherms of the sample after heat treatment at 900°C and 2700°C respectively Curve C is the common adsorption branch (b) water at 22°C on stannic oxide gel heated to SOO C (c) krypton at 77-4 K on exfoliated graphite (d) ethyl chloride at 6°C on porous glass. (Redrawn from the diagrams in the original papers, with omission of experimental points.)...

Low-pressure hysteresis is not confined to Type I isotherms, however, and is frequently superimposed on the conventional hysteresis loop of the Type IV isotherm. In the region below the shoulder of the hysteresis loop the desorption branch runs parallel to the adsorption curve, as in Fig. 4.26, and in Fig. 4.2S(fi) and (d). It is usually found that the low-pressure hysteresis does not appear unless the desorption run commences from a relative pressure which is above some threshold value. In the study of butane adsorbed on powdered graphite referred to in Fig. 3.23, for example, the isotherm was reversible so long as the relative pressure was confined to the branch below the shoulder F. 

Fig. 4.26 Low-pressure hysteresis in the adsorption isotherm of water at 298 K on a partially dehydroxy la ted silica gel. O, first adsorption run (outgassing at 200°C) . first desorption A, second adsorption run (outgassing at 200°C) A. second desorption (after reaching p/p = 0-31) X, third adsorption run (outgassing at 25 C).

Fig. 4.29 Adsorption isotherms of water vapour on caldte, after being balt-milted for different periods (A, B, C) and on precipitated calcium carbonate (D). Period of milling (A) 1000h (B) ISOh (C) 22h outgassing temperature 2S°C. Isotherms A, B and C (but not D) all showed extensive low-pressure hysteresis, but for clarity the desorption branch is omitted. The amount adsorbed is referred to 1 m of BET-nitrogen area. ...

A new classification of hysteresis loops, as recommended in the lUPAC manual, consists of the four types shown in the Figure below. To avoid confusion with the original de Boer classification (p. 117), the characteristic types are now designated HI, H2, H3 and H4 but it is evident that the first three types correspond to types A, E and B, respectively, in the original classification. It will be noted that HI and H4 represent extreme types in the former the adsorption and desorption branches are almost vertical and nearly parallel over an appreciable range of gas uptake, whereas in the latter they are nearly horizontal and parallel over a wide range of relative pressure. Types H2 and H3 may be regarded as intermediate between the two extremes. 

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. 

The channels in zeoHtes are only a few molecular diameters in size, and overlapping potential fields from opposite walls result in a flat adsorption isotherm, which is characterized by a long horizontal section as the relative pressure approaches unity (Fig. 6). The adsorption isotherms do not exhibit hysteresis as do those in many other microporous adsorbents. Adsorption and desorption are reversible, and the contour of the desorption isotherm foUows that of adsorption. 

Physical and ionic adsorption may be either monolayer or multilayer (12). Capillary stmctures in which the diameters of the capillaries are small, ie, one to two molecular diameters, exhibit a marked hysteresis effect on desorption. Sorbed surfactant solutes do not necessarily cover ah. of a sohd iaterface and their presence does not preclude adsorption of solvent molecules. The strength of surfactant sorption generally foUows the order cationic > anionic > nonionic. Surfaces to which this rule apphes include metals, glass, plastics, textiles (13), paper, and many minerals. The pH is an important modifying factor in the adsorption of all ionic surfactants but especially for amphoteric surfactants which are least soluble at their isoelectric point. The speed and degree of adsorption are increased by the presence of dissolved inorganic salts in surfactant solutions (14). 

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