Sorption desorption curves

A procedure for characterizing the rates of the volume change of gels has not been uniformly adopted. Often, the kinetics are simply presented as empirical sorption/desorption curves without quantitative analysis. In other cases, only the time required for a sample of given dimensions to reach a certain percentage of equilibrium is cited. One means of reducing sorption/desorption curves to empirical parameters is to fit the first 60% of the sorption curve to the empirical expression [119,141]... 

Normally, the moisture sorption-desorption profile of the compound is investigated. This can reveal a range of phenomena associated with the solid. For example, on reducing the RH from a high level, hysteresis (separation of the sorption-desorption curves) may be observed. There are two types of hysteresis loops an open hyteresis loop, where the final moisture content is higher than the starting moisture content due to so-called ink-bottle pores, where condensed moisture is trapped in pores with a narrow neck, and the closed hysteresis loop may be closed due to compounds having capillary pore sizes. 

Figure 2. Sorption-desorption curves obtained after drying an acid Nafion sample at different temperatures the humidity fP/Po ...

Water-Clay Ratio Affects. The strength of sorption is indicated by the ease or diflSculty of desorption, the equilibrium concentration left in solution, and the eflEect of dilution (Figure 2). After sorption of the 2,4-D, resuspension of the filtered clay with its sorbed 2,4-D did not result in desorption of as much 2,4-D as predicted from the sorption curve. If the sorption was non-specific and only physical, the desorption curve should follow the sorption curve. The 2,4-D, once sorbed, is removed with difficulty, resulting in a hysteresis sorption—desorption curve. This desorption curve is above the sorption curve on a X/m vs. Ce plot. 

Figure 21.4 SO sorption/desorption curves of pure TMGL and SILP. Different ratios of IL and SiOj were applied. Absorption conditions 20 °C, 1 bar, 100 ml min". Desorption conditions 90 °C, 0.1 bar. (Source Zhang et al. [5]. Reproduced with permission of American Chemical Society.)...

Sorbed pesticides are not available for transport, but if water having lower pesticide concentration moves through the soil layer, pesticide is desorbed from the soil surface until a new equiUbrium is reached. Thus, the kinetics of sorption and desorption relative to the water conductivity rates determine the actual rate of pesticide transport. At high rates of water flow, chances are greater that sorption and desorption reactions may not reach equihbrium (64). NonequiUbrium models may describe sorption and desorption better under these circumstances. The prediction of herbicide concentration in the soil solution is further compHcated by hysteresis in the sorption—desorption isotherms. Both sorption and dispersion contribute to the substantial retention of herbicide found behind the initial front in typical breakthrough curves and to the depth distribution of residues. 

Various anomalous diffusional behaviors have been observed and documented for both sorption and permeation experiments. Detailed discussions of these anomalies can be found elsewhere [12,35,36], Here only a brief summary of major findings is given. First, for the sorption anomaly, it has been observed that the reduced sorption curve has a distinctive thickness dependence. In this case, the reduced absorption and desorption curves obtained at various thick-... 

This solution is valid for the initially linear portion of the sorption (or desorption) curve when MtIM is plotted against the square root of time. These equations also demonstrate that for Fickian processes the sorption time scales with the square of the dimension. Thus, to confirm Fickian diffusion rigorously, a plot of MJM vs. Vt/T should be made for samples of different thicknesses a single master curve should be obtained. If the data for samples of different thicknesses do not overlap despite transport exponents of 0.5, the transport is designated pseudo-Fickian. ... 

Figure 3 Adsorption and desorption curves for the sorption of radon on coconut based charcoal. (Jebackumar, 1985).

Duplessix et al. used water vapor pressure isotherm (i.e., water uptake vs external relative humidity) data combined with simultaneous isotherm differential microcalorimeter analysis to determine the average heat of absorption per water molecule for 1200 EW acid form samples. Hysteresis was seen between sorption and subsequent desorption curves at 25 °C, and nonzero water content remained at zero relative humidity, indicating the presence of tightly... 

The sorption-desorption isotherm for the O-octylated extract is noticably different. The shape of the sorption curve is concave upward and the hysteresis effect is much less pronounced. Nearly all of the benzene (97% wt) could be desorbed under vacuum at 30" C. Clearly, this material is responding much differently than the untreated and O-methylated extracts. 

A low temperature nitrogen sorption was carried out on an automated physisorption instrument (ASAP 2000, Micromeritics Instrument Corporation). Before the measurement, the sample was degassed at 350 C for 4-5 h until the vacuum of system was better than 0.67 Pa. The data for micropore were obtained from t-plot, and those for mesopore and distribution of mesopore were calculated by BJH method (using desorption curve). The single point total pore volume at high relative pressure was taken as the total volume. 

Figure 7-13 Sorption or desorption curves in the valid range of Eq. (7-66) for different L-values.

The resulting curve is termed the absorption (or desorption) curve, or more generally the sorption curve. However, in order to interpret correctly experimental results in terms of Eq. (1) it is necessary to plot M lf), the weight of sorbed penetrant per unit volume of the swelling or deswelling film, against This requirement arises from the fact that... 

Because methanol is not actually a solvent for PVA, some interesting sorption/desorption cycling experiments can be conveniently run on PVA films without disrupting the film integrity. As noted above, methanol produces a slow increase of shear stiffness (Adh) when layered on PVA. The time scale and curve shape are suggestive of a diffusion process (liquid into polymer). By removing the liquid layer, we should permit methanol to evaporate from the film and thus reverse the effect. The... 

Water sorption isotherms for hep show marked hysteresis. Powers and Brownyard (P20) found that, while it was difficult to obtain reproducible desorption curves, the low-pressure part of the water vapour resorption curve varied little with w/c ratio, between different Portland cements, or, if allowance was made for the contents of unreacted cement, with the degree of hydration. This was their main direct evidence for the conclusion (Section 8.2.1) that the properties of the hydration product considered in their model were essentially independent of these variables. However, the water sorption iostherms obtained by different investigators have varied considerably (e.g. Refs P20 and S79), and it is not clear to what extent the above conclusion would stand had different desorption conditions been used. 

The mechanical behavior of the dry-to-wet and wet-to-dry cycles complement each other. In the first sorption curve, blocking of the network structure was explained by sorption of a small quantity of moisture Into free volume of the near saturated matrix. If this small quantity of moisture Is allowed to escape, the network takes on mechanical behavior of the unblocked plasticized state. This transition occurs rapidly during the Initial stages of drying. The peak tan 6 value Is representative of plasticized state properties. As further desorption takes place, mechanical properties approach the Initial dry state values. A qualitative description of the diffusion-mechanical mechanisms associated with each segment of the sorption-desorption cycle is outlined in Figure 7. 

It is likely, therefore, that pores analyzed by the nitrogen sorption/desorption method are Interstices in the subsurface strata. In the membranes presented here, these strata consisted of agglomerated spherical nodules. It is worth noting that curve maxima in Figures 4b,d are in the unexpected order A>B>C. 

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