Adsorption-desorption hysteresis experiments

The conducted experiments showed that isotherms of adsorption and desorption do not always coincide. The difference between adsorption and desorption isotherm curves is called adsorption-desorption hysteresis. It turned out that the desorption rates, as a rule, are lower than adsorption rates. Moreover, it was established that the adsorption and desorption processes often have the rates too low for equilibrium to be reached during experiment. 

Rajniak, P., and Yang, R.T., A simple model and experiments for adsorption-desorption hysteresis Water vapor on silica gel, AlChE J., 39(5), 774-786 (1993). 

As a result of computer experiments with model porous networks, the factors ( other than pore shape and size distribution ) determining the form of the adsorption-desorption hysteresis loop have been elucidated. 

Pore size distributions are determined from the hysteresis loop in gas adsorption/desorption isotherms and from calorimetric measurements by the shift in the melting (or freezing) peak for a phase transition of water inside the pores. The determination of the fractional rejection properties is done by permeation experiments of a macromolecular solute with a broad molecular weight distribution (MWD). The MWD of permeate and feed are compared and translated into a fractional rejection curve. The comparison of results obtained from these three independent methods for some characteristic membranes gives an indication of the strength and weakness of each of the methods studied. 

An adsorption/desorption isotherm below Tec, exhibits a hysteresis loop. In between condensation and evaporation metastable pressures, there is an equilibrium pressure for which both the gas and liquid phases coexist. In the case of cylindrical pores of any dimensions, there are theoretical arguments to indicate that no first order transition can exist because at the critical point, the correlation length can diverge only in the direction of the pore axis a cylindrical pore can be considered as a one-dimensional system whatever its diameter. However, DFT [4], simulations [5] and experiments suggest that fluids in both confining geometries (slit and cylinders with size of several nm) behave similarly as far as condensation and evaporation are concerned the hysteresis loop shrinks as temperature increases and eventually disappears. We note that in a van der Waals picture of gas-liquid transitions in such simple systems, the temperature of hysteresis disappearance is the capillary critical temperature, i.e.. Tec-... 

The effect of compression on the structure of MCM-41 with the template within the pores was examined by N2 adsorption. Fig. 6 shows the N2 adsorption/desorption isotherms for the as-synthesized MCM-41 before and after compression. The nitrogen adsorption/desorption isotherm for as-synthesized sample and samples after hydrostatic pressure experiment exhibit over the relative pressine p/po = 0.9 hysteresis loop which is characteristic of the c illary condensation between silica particles. Some parameters characterizing porosity of the investigated samples are collected in Table 1. Two samples with the template do not exhibit porosity of MCM-41 silica. Only for MCM-41 after compression in argon the specific surface area and pore volume are little higher in comparison to the as-synthesized sample. 

Lin et al. [24] studied adsorption-desorption isotherm hysteresis exhibited by /-lactoglo-bufin A on a weakly hydrophobic surface. They found that the desorption isotherm at pH 6.0 overlapped with the adsorption isotherm and that the adsorption-desorption process of /-lactoglobulin A under this condition could be characterized by a fully reversible Langmuir model. The desorption isotherm at pH 4.5, however, did not coincide with the adsorption isotherm, giving rise to hysteresis. This would suggest that protein adsorption experiments carried out under mild conditions of pH at relatively hydrophilic siufaces might be treated with the assumption that reversible equihbrium exists between the bulk and interface. 

Nearly all of the data are collected at room temperature, and there is no accepted method for correcting them to other temperatures. Far fewer data have been collected for sorption of anions than for cations. The theory does not account for the kinetics of sorption reactions nor the hysteresis commonly observed between the adsorption and desorption of a strongly bound ion. Finally, much work remains to be done before the results of laboratory experiments performed on simple mineral-water systems can be applied to the study of complex soils. 

The two main assumptions underlying the derivation of Eq. (5) are (1) thermodynamic equilibrium and (2) conditions of constant temperature and pressure. These assumptions, especially assumption number 1, however, are often violated in food systems. Most foods are nonequilibrium systems. The complex nature of food systems (i.e., multicomponent and multiphase) lends itself readily to conditions of nonequilibrium. Many food systems, such as baked products, are not in equilibrium because they experience various physical, chemical, and microbiological changes over time. Other food products, such as butter (a water-in-oil emulsion) and mayonnaise (an oil-in-water emulsion), are produced as nonequilibrium systems, stabilized by the use of emulsifying agents. Some food products violate the assumption of equilibrium because they exhibit hysteresis (the final c/w value is dependent on the path taken, e.g., desorption or adsorption) or delayed crystallization (i.e., lactose crystallization in ice cream and powdered milk). In the case of hysteresis, the final c/w value should be independent of the path taken and should only be dependent on temperature, pressure, and composition (i.e.,... 

A difference in the rate of adsorption and desorption of Cr(VI) by alluvium was also observed in a batch experiment (Fig. 8.44b). On the basis of these two experiments, Stollenwerk and Grove (1985) concluded that the quantity of Cr(VI) adsorbed by alluvium is a function of its concentration as well as of the type and concentration of other anions in solution. The Cr(VI) adsorbed through nonspecific processes is desorbed readily by a Cr-free solution. Stronger bonds that are formed between Cr(VI) and alluvium during specific adsorption result in very slow release of this fraction. The Cr(Vl) desorption from the alluvium material illustrates the hysteresis process that results from chemical transformation of a portion of contaminant retained in the subsurface. 

Adsorption and desorption of liquid molecules at the spreading or receding liquid are accompanied by the dissipation of energy and are thus one source of hysteresis [254,255], At this point we would like to point out the similarity between contact angle and adhesion experiments. Adhesion is dominated by the solid-solid attraction, while contact angles reflect the solid-liquid attraction. 

Adsorption to porous materials is often characterized by hysteresis in the adsorption behavior. Such a hysteresis is observed when, after the adsorption process, a desorption experiment is done in which the pressure is progressively reduced from its maximum value and the desorption isotherm is measured. During the desorption process, the liquid phase vaporizes from the pores. The desorption isotherm does not precisely track the adsorption isotherm, but lies above it. Moreover isotherms often flatten out with high P/Po values, because filling up of pores decreases the available surface area. 

Figure 63.1a shows the equilibrium moisture content (dry basis) at the different a for the desorption experiments (obtained from samples dried by hot air) and also for the adsorption experiments (obtained from the freeze-dried samples). Both isotherms are very similar and no hysteresis phenomenon was observed. Hysteresis is justified by assuming that during... 

Hysteresis loops are possible (54) by following metastable but accessible parts of the loop in adsorption or desorption, or both. This predicts that the desorption branch will always lie above the adsorption branch, in agreement with experiment. 

For DVS experiments, the sample was placed into a glass pan and the vapour concentration was stepped up by 2.5% increments until 40%, 5% until 70% and 10% until 90%. Hexane and Dichloromethane have been used as probe molecules. The measurements were carried out at 25 C and two adsorption and desorption cycles have been recorded to check for hysteresis and reproducibility of data. Nitrogen was used as a carrier gas. Samples were equilibrated in-situ for 30 min in a pure gas stream prior to the experiment. 

Figure 7.5a shows molecular beam results on the temperature dependence of the steady-state rate of CO oxidation over a Pd(l 1 0) surface. At 300 K the reaction is Umited by oxygen adsorption because the surface is covered with COads In the temperature interval between 370 and 650 K, however, the rate for CO2 production increases rapidly, presumably because of desorption of some of the CO, which reduce the COads coverage on the surface. A bistability is seen in this temperature region, as indicated by the hysteresis in CO2 formation rate seen between experiments with increase and decrease of temperature (Figure 7.5a). As the temperature is increased, the transfer from the CO layer to the Oads layer is delayed, while when the temperature is decreased, the reverse is true. Local single oscillations are also seen for the CO2 rate at 372 and 382 K in the...

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