Absorption versus adsorption

Figure 2.19. Schematic representation of absorptive versus adsorptive extraction and adsorption in small versus large pores. (Reprinted with permission from Ref. 51. Copyright 2000 Elsevier Science.)...

Figure 7 Schematic representation of absorptive versus adsorptive extraction, and adsorption in small versus large pores.

Fig. 10-6. Localization function/(Ql) versus adsorption energy Ql of localizing group k (absorption of aromatic substituents k on alumina). Reprinted from the Journal of Chromatography (21) by permission of Elsevier Publishing Company.

In adsorptive stripping voltammetry the deposition step occurs without electrolysis. Instead, the analyte adsorbs to the electrode s surface. During deposition the electrode is maintained at a potential that enhances adsorption. For example, adsorption of a neutral molecule on a Hg drop is enhanced if the electrode is held at -0.4 V versus the SCE, a potential at which the surface charge of mercury is approximately zero. When deposition is complete the potential is scanned in an anodic or cathodic direction depending on whether we wish to oxidize or reduce the analyte. Examples of compounds that have been analyzed by absorptive stripping voltammetry also are listed in Table 11.11. 

Adsorption and phase formation of uracil on massive Au[ (lll)-(110)] singlecrystal and Au (111 - 20 nm) film electrodes in 0.1 M IT2SO4 has been studied in electrochemical measurements and applying ATR surface-enhanced infrared reflection absorption spectroscopy [299]. At E < 0.15 V (versus trapped hydrogen electrode), uracil molecules are disordered and planar oriented. Close to the pzc, a 2D condensed physisorbed film of planar-oriented molecules interconnected by directional hydrogen bonds, is formed. 

R. E. Shirey and R. F. Mindrup, SPME-Adsorption versus Absorption Which Fiber Is Best for Your Application product literature, T400011, Sigma-Aldrich Co., 1999. 

Possible advantages of intraperitoneal versus subcutaneous insulin include the avoidance of erratic absorption (both rate and extent of absorption), convenience, avoidance of subcutaneous injection site-related complications, and prevention of peripheral hyperinsulinemia. Insulin appears to be cleared into the systemic compartment by an active transport process, or via the peritoneal lymphatics. A number of studies have demonstrated the bioavailability of intraperitoneal insufin to be about 25% to 30%, although none clearly compares the clinical effectiveness of intraperitoneal versus subcutaneous insufin in diabetes control. Insufin requirements for PD patients may be greater than in hemodialysis patients because of the continued absorption of dextrose from the peritoneal cavity. Furthermore, because of adsorption of insufin to the polyvinyl chloride bag and administration set, the intraperitoneal dose of insufin often needs to be two to three times the subcutaneous maintenance dose. 

Yang, Y., S.B. Hawthorne, D.J. Miller, Y. Liu, and M.L. Lee. 1998. Adsorption versus absorption of polychlorinated biphenyls onto solid-phase microextraction coatings. Anal. Chem. 70 1866-1869. 

The systems discussed above are, in many ways, ideal in that adsorption is very site-specific or limited to the surface layer. Many systems are known to absorb as well as adsorb. This effect is sometimes reflected in sticking probability versus coverage profiles. These may show an increase in s because of a sudden freeing of surface sites due to the absorption process. One example is 02 on A1 111] [434]. However, the adsorbed species may form at all coverages and the s versus N profiles look like those of a typical mobile precursor-trapping model. Fromm [435] has proposed a model to fit this absorption—adsorption mechanism. His... 

Morton JD, Semrau JD, Hayes KF (2001) An X-ray absorption spectroscopy study of the structure and reversibility of copper adsorbed to montmorillonite clay. Geochim Cosmocliim Acta 65 2709-2722 Muller B, Sigg L (1992) Adsorption of lead(II) on the goethite surface Voltammetric evaluation of surface complexation parameters. J Coll Interf Sci 148 517-532 Neder RB, Burghammer M, Grasl T, Schulz H, Bram A, Fiedler S (1999) Refinement of the kaolinite structure from single-crystal synchrotron data. Clays Clay Miner 47 487-494 Needleman HL (1983) Low level lead exposure and neuropsychological performance In Lead versus health - Sources and effects of low level lead exposure. Rutter M, Russell JR (eds) Wiley, Chichester, p 229-248... 

J. V. Sinisterra, J. M. Marinas, and A. Llobura [Can. J. Chem., 61, 230 (1983)] employed an extended form of the Hammett equation to correlate data for esterification of ethanol with substituted benzoic acids in the gas phase over a solid AIPO4 catalyst. The focus of their efforts was correlation of values of the adsorption equilibrium constants and apparent rate constants. Use the information tabulated below to prepare a Hammett plot of experimental absorption equilibrium constants versus the corresponding values of the a parameters for the substituted benzoic acids indicated. Employ the a values in Table 7.2 of the text. In principle the intercept of the best straight line through these data should correspond to the adsorption equilibrium constant for benzoic acid. By how much does this value differ from the experimental value below What value of the parameter p characterizes the chemisorption of substituted benzoic acids ... 

Adsorption can be most simply defined as the preferential concentration (i.e., location) of one component of a system at an interface, where the local (i.e., interfacial) concentration of one or more components of one or both phases is different from those in the bulk phases. Adsorption should be clearly differentiated from absorption, in which physical penetration of one phase into another is involved, although the two may operate concurrently. Adsorption can occur at any type of interface, although the distinct characteristics of solid versus liquid interfaces make the analysis of each case somewhat different. For that reason, the discussion of each situation is best presented in the context of specific interfaces. In many practical systems, all four of the principle interfaces may be present, leading to complex situations that make complete analysis very difficult or impossible. 

The heat of absorption, AH, can be determined by measuring at different temperatures and by plotting the In K versus the inverse absolute temperature. The slope of the straight line is —AH. Figure 5 shows the experimental plots obtained in the case of isoflurane and halothane, the calculated heats of adsorption being —20.0 and -24.2 kj/mol, respectively. A negative AH, typical of volatile compounds, reflects the exothermic nature of the adsorption process efficient sampling occurs at lower temperatures. 

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