Limiting adsorption volumes

In exclusion chromatography, the total volume of mobile phase in the column is the sum of the volume external to the stationary phase particles (the void volume, V0) and the volume within the pores of the particles (the interstitial volume, Vj). Large molecules that are excluded from the pores must have a retention volume VQ, small molecules that can completely permeate the porous network will have a retention volume of (Vo + Fj). Molecules of intermediate size that can enter some, but not all of the pore space will have a retention volume between VQ and (V0 + Fj). Provided that exclusion is the only separation mechanism (ie no adsorption, partition or ion-exchange), the entire sample must elute between these two volume limits. 

With monolayer adsorption, we saw how the saturation limit could be related to the specific surface area of the adsorbent. The BET equation permits us to extract from multilayer adsorption data (by means of Equation (77)) the volume of adsorbed gas that would saturate the surface if the adsorption were limited to a monolayer. Therefore Vm may be interpreted in the same manner that the limiting value of the ordinate is handled in the case of monolayer adsorption. Since it is traditional to express both V and Vm in cubic centimeters at STP per gram, we write (see Equation (7.72))... 

A Correlation of the Calculated Intracrystalline Void Volumes and Limiting Adsorption Volumes in Zeolites... 

The amount of adsorption is limited by the available surface and pore volume, and depends also on the chemical natures of the fluid and solid. The rate of adsorption also depends on the amount of exposed surface but, in addition, on the rate of diffusion to the external surface and through the pores of the solid for accessing the internal surface which comprises the bulk of the surface. Diffusion rates depend on temperature and differences in concentration or partial pressures. The smaller the particle size, the greater is the utilization of the internal surface, but also the greater the pressure drop for flow of bulk fluid through a mass of the particles. 

In Equation 6, do° is an experimentally determined adsorption value for a temperature To, and do the calculated limiting adsorption value for a pre-assigned temperature, T. In the case of zeolites, the limiting adsorption volumes. Wo, are perceptibly reduced with an increase in the size of the molecules adsorbed. Thus, Wo acquire the nature of effective values. 

The studies have shown that irrespective of the composition of the initial mixture, as a result of uniform distribution of components in the structure of samples, the shape of their adsorption isotherms is transformed gradually from the typical isotherm for one individual component to the isotherm for the second component. In this case, as the content of the second component, having a higher sorption capacity, increases, transitional isotherms accumulate features inherent in its structure the micro- and mesopore volume, limiting sorption capacity Vs, and specific surface area Ssp increase. The latter increases because of the fact that along with development of porosity of samples, their more open structure accessible to adsorbate molecules is formed. 

The curves for n-paraflSns and sulfur compounds were adapted from similar plots by Grant and Manes, who used a different definition of the molar volume. Limited data show that highly chlorinated hydrocarbons are more strongly adsorbed than the sulfur compounds, as shown in Fig. 25.4, but data for vinyl chloride fall closer to the line for paraffins, as expected. The adsorption of oxygenated species such as ketones and alcohols on BPL carbon can be estimated using the curve for sulfur compounds. 

The thickness of the adsorbed phase depends upon the number of adsorbed molecular layers and upon the orientation of adsorbed molecules. A useful approximation for most liquid-solid chromatographic systems is the assumption of monolayer adsorption. This limits the possible volume of the adsorbed phase within narrow limits and leads to a mathematical basis for correlating relative adsorption with adsorbent surface area (see Section 6-2A). 

Pulsed splitless Reduction of the residence time in the liner resulting in the decreased analyte degradation/adsorption and reduced matrix-induced response enhancement Larger injection volumes (limited by the liner size and solvent expansion volume) Potential transfer of nonvolatile matrix cx)mponents further into the column... 

Here Fads is the specific adsorptive volume adsorbed at plpo, F ,ic,o is the so-called limiting micropore volume, is the gas specific affinity coefficient that empirically includes the properties of the adsorbate, while the characteristic energy Eo is describing the adsorbent properties only. A is defined as... 

The porous structure of active carbons is the defining factor of their adsorption performance The pore diameter distribution determines the adsorption energy and therefore, the slope of adsorption isotherm, whereas in mainly microporous carbons the micropore volume limits the adsorption capacity at the higher end of t.he adsorptive concentration. Furthermore, the chemical compositor of the carbon surface influences the selectivity of adsorption, e.g. the competition of the water adsorption when working in aqueous solution. However, here only the structural properties of carbons are taken into account. 

Since the drop volume method involves creation of surface, it is frequently used as a dynamic technique to study adsorption processes occurring over intervals of seconds to minutes. A commercial instrument delivers computer-controlled drops over intervals from 0.5 sec to several hours [38, 39]. Accurate determination of the surface tension is limited to drop times of a second or greater due to hydrodynamic instabilities on the liquid bridge between the detaching and residing drops [40],... 

Since in practice the lower limit of mercury porosimetry is around 35 A, and the upper limit of the gas adsorption method is in the region 100-200 A (cf. p. 133) the two methods need to be used in conjunction if the complete curve of total pore volume against pore radius is to be obtained. 

In the irreversible limit R < 0.1), the adsorption front within the particle approaches a shock transition separating an inner core into which the adsorbate has not yet penetrated from an outer layer in which the adsorbed phase concentration is uniform at the saturation value. The dynamics of this process is described approximately by the shrinldng-core model [Yagi and Kunii, Chem. Eng. (Japan), 19, 500 (1955)]. For an infinite fluid volume, the solution is ... 

Since adsorption takes place at the interphase boundaiy, the adsorption surface area becomes an important consideration. Generally, the higher the adsorption surface area, the greater its adsorption capacity. However, the surface area has to be available in a particular pore size within the adsorbent. At low partial pressure (or concentration) a surface area in the smallest pores in which the adsorbate can enter is the most efficient. At higher pressures the larger pores become more important at very high concentrations, capiDaiy condensation will take place within the pores, and the total micropore volume becomes the limiting factor. 

Samples and reference substances should be dissolved in the same solvents to ensure that comparable substance distribution occurs in all the starting zones. In order to keep the size of the starting zones down to a minimum (diameter TLC 2 to 4 mm, HPTLC 0.5 to 1 mm) the application volumes are normally limited to a maximum of 5 xl for TLC and 500 nl for HPTLC when the samples are applied as spots. Particularly in the case of adsorption-chromatographic systems layers with concentrating zones offer another possibility of producing small starting zones. Here the applied zones are compressed to narrow bands at the solvent front before the mobile phase reaches the active chromatographic layer. 

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