Desorption, Exchange, Hysteresis

The adsorption of macromolecules is rarely an equilibrium process. Just as the properties of synthetic polymers are often dependent on non-equilibrium processes and relaxation phenomena30), so do the properties of adsorbed proteins depend on time, metastable states, and hysteresis processes. 

As the adsorption site density (Fig. 12) or total free energy of adsorption increases, one moves from the realm of reversibility to that of irreversibility. As proteins can undergo conformational and orientational changes on a surface, they can optimize their interfacial interactions so as to provide the maximum free energy of adsorption. Such conformational alterations are relatively slow and hence very time-dependent. 

Soderquist and Walton721 showed that the desorption rate is a function of residence time. The adsorption process can be characterized in three stages  

Short times where adsorption is reversible presumably little or no time is available for conformational changes. 

At longer times where slow conformational changes occur, the process is semi-reversible and desorption occurs very slowly. 

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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. 

Fig. 4 Nitrogen adsorption-desorption isotherms at 77 K of heat-treated NH4 -exchanged MSU-Ge-2 (solid circles, adsorption data open circles, desorption data). The hysteresis observed at P/Po > 0.8 is due to the voids between the agglomerated particles. (Inset) BJH pore size distribution calculated from the adsorption branch of the isotherm...

Examples of K+ and NH4 adsorption-desorption plots for the two-cation system (K-Ca NH4-Ca) and the three-cation system (K-NH4-Ca) are shown in Figure 4.42. These data show that there is a significant hysteresis or relaxation effect in the 24-hr desorption process. This hysteresis-relaxation effect in the case of K-Ca exchange is more pronounced without added NH4 than with added NH4 in the desorbing solution. 

Evidence for the growth of sulfide particles within the zeolite lattice to sizes considerably exceeding the dimension of the supercage can be obtained from adsorption isotherms of cyclopentane. The creation of mesopores due to a local destruction of the zeolite lattice around the growing particles results in a hysteresis loop in the adsorption -desorption cycle. This is depicted in Fig. 2 for a Cd-X sample with a degree of ion-exchange of a =20. ... 

Porous silicas are usually mesoporous materials and they can be made with a variety of pore dimensions. In particular, silica glasses can be made with well-defined pore diameters, typically in the range 30-250 A, using sol-gel methods. Such a system provides a good model for testing the models of relaxation behaviour of fluids in porous solids. It is normally found that the two-site fast-exchange model for relaxation described above for macroporous systems is still valid. For instance, H and relaxation times have been measured during both adsorption and desorption of water in a porous silica. Despite hysteresis in the observed adsorption isotherms, it was found that the relaxation times depended solely on water content.For deuterated water in some porous silicas, multicomponent relaxation behaviour for T2 and Tip has been observed, and this has been attributed to the fractal nature of the pore structure. 

The changes in the porosity are well evidenced in Fig. 1 by the isotherms measured on the ion-exchanged samples. The isotherm for initial aluminosilicate sample SiA120 is also present for comparison purpose. As is shown, both adsorption and desorption curves for sample without metal have two characteristic features of a type I isotherm, a steep initial portion and a well-defined horizontal plateau at larger relative pressures. In the case of ion-exchanged samples a hysteresis loop can be seen at moderate relative pressures. The appearance of this adsorption phenomenon is seen in the region of greater pore diameters. 

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