Physical adsorption characteristics

The vast majority of modem liquid chromatography systems involve the use of silica gel or a derivative of silica gel, such as a bonded phase, as a stationary phase. Thus, it would appear that most LC separations are carried out by liquid-solid chromatography. Owing to the adsorption of solvent on the surface of both silica and bonded phases, however, the physical chemical characteristics of the separation are more akin to a liquid-liquid distribution system than that of a liquid-solid system. As a consequence, although most modern stationary phases are in fact solids, solute distribution is usually treated theoretically as a liquid-liquid system. 

The adsorption of gases on solids can be classified into physical and chemical adsorption. Physical adsorption is accompanied by a low enthalpy of adsorption, and the adsorption is reversible. The adsorption/desorption characteristics are in these cases often described by adsorption isotherms. On the other hand, chemical adsorption or segregation involves significantly larger enthalpies and is generally irreversible at low temperatures. It is also often accompanied by reconstruction of the surface due to the formation of strong ionic or covalent bonds. 

It is often convenient to think of adsorption as occurring in three stages as the adsorbate concentration increases. Firstly, a single layer of molecules builds up over the surface of the solid. This monolayer may be chemisorbed and associated with a change in free energy which is characteristic of the forces which hold it. As the fluid concentration is further increased, layers form by physical adsorption and the number of layers which form may be limited by the size of the pores. Finally, for adsorption from the gas phase, capillary condensation may occur in which capillaries become filled with condensed adsorbate, and its partial pressure reaches a critical value relative to the size of the pore. 

The presence of pre-adsorbed polyacrylic acid significantly reduces the adsorption of sodium dodecylsulfonate on hematite from dilute acidic solutions. Nonionic polyacrylamide was found to have a much lesser effect on the adsorption of sulfonate. The isotherm for sulfonate adsorption in absence of polymer on positively charged hematite exhibits the typical three regions characteristic of physical adsorption in aqueous surfactant systems. Adsorption behavior of the sulfonate and polymer is related to electrokinetic potentials in this system. Contact angle measurements on a hematite disk in sulfonate solutions revealed that pre-adsorption of polymer resulted in reduced surface hydrophobicity. 

In Figure 11 a van t Hoff-type plot is given of the characteristic buildup time as a function of 1/T. The data clearly indicate a linear behavior in this plot with one value of the slope (AG/ ratio) for laminar flow and cinother for turbulent flow. Two lines are shown for the turbulent case, one in which only three of the data points were correlated, with what appears to be an inconsistent point left out, and one in which all four points were correlated. The effective association energies of Eq. (15), as defined by these correlations, are relatively low, ranging between 6.5 and 19 kJ/mol, which is in the range of values corresponding to physical adsorption. These results indicate that the foulant film buildup is controlled by physical rather than chemical phenomena. 

The second category, reversible or physical adsorption, exhibits characteristics that make it most suitable for surface area determinations as indicated by the following ... 

Schlier, R. E. and Farnsworth, H. E. Structure and adsorption characteristics of clean surfaces of germanium and silicon. Journal of Chemical Physics 30, 917 (1959). 

From the foregoing it is evident that the lithotypes of anthracite are different. Differences in adsorption characteristics, surface structure, physical properties, and chemical composition conclusively demonstrate that the anthracite lithotypes are unalike and are distinct entities. It is also evident that to interpret research results of anthracite coals correctly, a petrographic knowledge of the coal is necessary. Anthracite cannot be regarded as a homogeneous substance. 

The three types of adsorption are (1) physical, (2) chemical, and (3) exchange adsorption. Especially important to the success of in situ treatment by Fe° are the soil characteristics, which affect soil sorptive behavior such as mineralogy, permeability, porosity texture, surface qualities, and pH. Physical adsorption is due to van der Waal s forces between molecules where the adsorbed molecule is not fixed on the solid surface but is free to move over the surface and may condense and form several superimposed layers. An important characteristic of physical adsorption is its reversibility. On the other hand, chemical adsorption is a result of much stronger forces with a layer forming, usually of one molecule thickness, where the molecules do not move. It is normally not reversible and must be removed by heat. The exchange adsorption and ion exchange process involves adsorption by electrical attraction between the adsorbate and the surface (Rulkens, 1998). 

Flavonoids and other polyphenols can interact with lipids and proteins. The interactions with proteins could be both unspecific or specific, meanwhile the interactions with lipids seems to be rather unspecific, based essentially on physical adsorption. This physical adsorption would mostly depend on the hydrophobic/hydrophilic characteristics of the flavonoid molecule, the number of hydroxyl substituents, and the polymerization degree [Erlejman et al., 2004 Verstraeten et al., 2005, 2003, 2004]. 

Their main characteristic is that the optically active reagents are immobilized over the surface of an optical component (waveguide, metallic plate, glass, prism, etc.) or in a porous matrix in a way that they are in direct contact with the analyte in the sample solution. This immobilization can be carried out by covalent bonding [29], by mechanical interactions such as physical adsorption [30], by physical entrapment or by electrostatic interactions [31]. 

Although nonspecificity is generally considered characteristic of van der Waals adsorption, ordinarily heats of physical adsorption vary considerably with the amount of gas adsorbed in a manner characteristic of a given adsorbent. Roberts (141) has shown that interaction between adsorbed molecules can lead to a complex variation of the differential heats of adsorption with increasing coverage of the surface even when the... 

Dissociation constants, 25, 30,344 Precipitation, 342,436 Sorption on soil, 190 Physical adsorption, 167 Characteristics, 178 pATa, 25, 140... 

Pores with different sizes show characteristic physical adsorption effects as manifested in the isotherm. The isotherm shows the relationship between the amount of a given gas taken np or released by a solid as a function of the gas pressnre nnder a constant temperature. The type-I isotherm shows a steep increase at very low pressmes and a long satnration platean and is characteristic of microporous materials. The type-IV isotherm exhibits a steep iucrease at high relative pressme and, in many cases, a hysteresis loop, which is associated with capillary condensation in mesopores. 

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