Nonpolar Adsorbents

THEORETICAL CALCULATION OF HEAT OF ADSORPTION Nonpolar Adsorbents  

For a nonpolar adsorbent there are no dipole or quadrupole contributions to the adsorption energy and if the small contribution from polarization 

Such calculations for the rare gases on graphite have been carried out by many authors. A good review has been given by Ross and Olivier. The results of calculations carried out according to several different approximations are summarized in Table 2.3. It is clear that the more accurate calcula- 

FIGURE 2.2. Schematic diagram showing the potential for a probe molecule approaching the surface of a regular crystalline solid. To calculate the profile it is necessary to sum the pairwise interactions of the probe molecule with each atom of the solid. (Only the first three atomic layers of the solid are indicated.) 

TABLE 2.2. Parameters used in Calculation of Dispersion-Repulsion Potentials of Inert Gases on Graphite  

The adsorption of surfactants in fuel oil onto pulverized coal has been studied in connection with the development of coal-oil mixtures (COM), i.e., stable dispersions of finely pulverized coal in fuel oil. The stabilization of such dispersions by a cationic surfactant has been shown (Kosman, 1982) to involve adsorption of the cationic via its postively charged head group onto nucleophilic sites on the coal, with its hydrocarbon group oriented toward the oil phase. The adsorption of alkylaromatics on carbon black from w-heptane indicates adsorption in an orientation parallel to the interface, with the alkyl chains remaining mobile on the surface (van der Waarden, 1951). Increased length of the alkyl chains increases the degree of dispersion of the carbon. 

Adsorption of sodium bis(2-ethy hexyl)sulfosuccinate from benzene solution onto carbon blacks follows the Langmuir equation and depends on the amount of oxygen on the surface. No adsorption onto heat-treated hydrophobic carbon (Graphon) could be detected (Abram, 1962). 

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FiaaHy, an analysis of the energies of adsorption on many practical polar and nonpolar adsorbents has shown not only that the magnitude of the Op term depends directly upon the polarizabiUty a, but also that the sum of all of the nonspecific terms taken together, ie, + Lp, iacreases... 

The relative strengths of adsorption of these compounds foUow the same order, as expected for a nonpolar adsorbent, except that was adsorbed... 

Nonplant cost, 9 527 Nonpoint contamination source, 13 310 Nonpolar adsorbents, 1 674 for gas adsorption, 1 632 Nonpolar solvents, VDC polymer degradation in, 25 717-718 Nonporous dense membranes, 15 799 Nonporous silicone tubing, flow through, 15 722, 723... 

The form of the isotherms of the mixtures is largely independent of the cation distribution within the cage, i.e., whether the cation-poor or cation-rich model is used. This result is somewhat surprising, especially in view of the different adsorbate structures predicted by single-component isotherms (118-120). Only nonpolar adsorbates were considered in this study and the insensitivity to cation arrangement may well change if one component possesses a permanent dipole. These simulations were based on simple spherical molecules, but the competition for pore space as it depends on size, shape, and polarizability may be extended to other adsorbates. Indeed, Santilli et al. (129) observed experimentally that a branched hydrocarbon adsorbs in preference to a linear one at low loading. 

P Gimsing, E Nexo, E Hippe. Determination of cobalamins in biological material. II. The cobalamins in human plasma and erythrocytes after desalting on nonpolar adsorbent material, and separation by one-dimensional thin-layer chromatography. Anal Biochem 129 296-304, 1983. 

Structure Macroporous polystyrene-divinylbenzene nonpolar adsorbent, 62-177 pm particle size Analytical Properties Used mainly in preparative-scale HPLC stable over entire pH range (1-13) sometimes difficult to achieve column-to-column reproducibility due to packing the irregular particles relatively lower efficiency than alkyl bonded phases particles tend to swell as the organic content of the mobile phase increases Reference 1... 

For organic compounds consisting of hydrocarbon chains, solubility in water decreases with an increase in chain length of the homologous series (i.e., Traube s rule), because the compounds become more hydrophobic and nonpolar. More hydro-phobic adsorbates are expelled from water and thus allow an increasing number of water-water bond to be reformed. A nonpolar adsorbate will be strongly adsorbed from a polar solvent by a nonpolar adsorbent but will not be adsorbed much on a polar adsorbent in a nonpolar solvent. Therefore, an increase in the polarity of an adsorbate decreases its adsorption on activated carbon, which is a relatively polar adsorbent, in water. 

The London dispersive component of the surface free energy, y, of a solid may be shown to be a predominant property for the prediction of behavior of nonpolar adsorbents such as polyolefins or of practically nonpolar adsorbents like some carbon materials (natural graphites and carbon fibers). In this section, we propose a simple approach for the determination of the ys using nonpolar probes such as n-alkanes in inverse GC at infinite dilution. We also discuss the evaluation of the London dispersive component of the surface energy (or enthalpy, / ), starting from the variation of the adsorption characteristics, of a series of long-chain n-alkanes molecules, with temperature. 

Another major drawback of classical extractions is that additional clean-up procedures are frequently required before chromatographic analyses. Solid phase extraction (SPE) avoids the emulsion problems often encountered in liquid-liquid extraction. A wide range of adsorbents are commercially available and may be divided into three classes polar, ion-exchange, and nonpolar adsorbents. Solid-supported liquid-liquid extraction on Extrelut columns is frequently reported for efficient cleanup of crude tropane alkaloid mixtures. Basifled aqueous solutions of alkaloids maybe transferred to Extrelut columns and the bases recovered in dichloromethane-isopropanol mixture [13]. 

Tlie relative strengths of adsorption of these compounds follow the same order, as expected for a nonpolar adsorbent, except that C was adsorbed more strongly than C3Hg. This result indicates that the surface is weaHy polar and that specific (dipole—field and quadrupole—field gradient) contributions to the adsorption potential alter the expected order slightly. This situation may also be due to chemisorbed oxygen species on the surface. 

Almost 30 years ago, Colin and Guiochon mentioned in an excellent review [10] that there are essentially three possible ways to model separation mechanism. The first one is analyte partitioning between mobile and stationary phases, the second one is the adsorption of the analyte on the surface of nonpolar adsorbent, and the third one has been suggested by Knox and Pryde [11], where they assume the preferential adsorption of the organic mobile-phase modifier on the adsorbent surface followed by the analyte partitioning into this adsorbed layer. 

Individual components of multicomponent mixtures compete for the limited space on the adsorbent. Equilibrium curves of binary mixtures, when plotted as x vs. y diagrams, resemble those of vapor-liquid mixtures, either for gases (Fig. 15.5) or liquids (Fig. 15.6). The shapes of adsorption curves of binary mixtures, Figure 15.7, are varied the total adsorptions of the components of the pairs of Figure 15.7 would be more nearly constant over the whole range of compositions in terms of liquid volume fractions rather than the mol fractions shown. Fig 15.8 shows the variation of isosteric parts of adsorption between polar and nonpolar adsorbates. 

The previous chapter discussed the solvent and its interaction with the solute. To complete the chromatographic system the adsorbent has to be selected. As mentioned in Chapter 3.2.1 one has to distinguish between enantioselective and non-enantioselective adsorbents. Both groups of adsorbents are classified into polar, semi-polar and nonpolar adsorbents (see Tab. 4.4). This classification is based on the surface chemistry of the packing material. Interaction between mobile phase and adsorbent characterizes the phase system, which is distinguished between normal phase (NP) chromatography and reversed phase (RP) chromatography. This differentiation is historic and appointed by the ratio of the polarity of the adsorbent and the mobile phase. 

Reversed phase chromatography A mode of adsorption chromatography in which the mobile phase is a polar solvent and the stationary phase is a nonpolar adsorbent (e.g., acetonitrile-water on an ODS-silica packing). 

The F fi and 6f q terms are specific contributions, which are significant when adsorbate molecules possess permanent dipole and quadmpole moments. In the absence of these moments, these terms are zero, as is tme also if the adsorbent surface has no electric fields, a completely nonpolar adsorbent. 

For the adsorption of nonpolar molecules onto nonpolar adsorbent surfaces, it is widely accepted that dispersion forces contribute essentially all the adsorption energy. A good example is provided by the adsorption... 

The preceding discussion emphasizes the applicability of Eq. (6-6) for adsorption onto polar adsorbents, particularly adsorbent samples deactivated by varying amounts of water. However, the derivation of Eq. (6-6) does not preclude its application to nonpolar adsorbents. Claesson has in fact observed 14) the existence of a similar relationship for adsorption onto various charcoals. For the homologous carboxylic acids as samples and ethanol as solvent, the Langmuir isotherm [Eq. (3.6-)] was found to describe the adsorption of each acid. The derived values of the Langmuir coefficient k [or K° in Eq. (3-6)] could be expressed as a function of the number n of carbon atoms in the sample molecule ... 

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