Silica gel, adsorption

Though possessing a large capacity for adsorption, silica gel in its simplest state, as described previously, is used less and less for analysis since its qualities change with time, resulting in a lack of reproducibility of separations. For many applications it must be, at least, rehydrated (3—8 per cent water) in order to be deactivated. 

Strychnos alkaloids Strychnine and brucine Adsorption Silica gel Diethyl ether/ methanol (90 10)... 

Miscellaneous alkaloid separations Dansyl derivatives of cephaeline, emetine, ephedrine and morphine obtained by direct deriva-tization of syrups and aqueous slurries of capsules having a complex excipient and drug composition Adsorption Silica gel Di-isopropyl ether/isopropanol/ cone, ammonia (48 2 0.3)... 

But Maier and coworkers could not obtain positive results for stereoselective adsorption. Silica gel was imprinted with chiral 2-(4-MeOPh)-3,3-dimethylbutan-l-ol, but after its removal from the matrix the resulting imprinted silica gel was unable to resolve the same racemic mixture of the... 

C. antarctica Adsorption Silica Gel Soybean Methanol 94 Wang et ak, 2006... 

In LPLC, a mobile phase is allowed to flow through a densely packed sorbent. The separation mechanism is adsorption or size-exclusion depending on the choice of packing material for the stationary phase (adsorption silica gel, bonded-phase silica gel, alumina, polystyrene size-exclusion polyacrylamide, carbohydrates). This is almost similar to... 

Carbon tetrachloride Liquid adsorption, silica gel, oligomers 3169 ... 

HFLC, liquid adsorption, silica gel Liqmd adsorption, silica,... 

Reversed phase, silica-phenyl, siliea-ODS, gradient Normal phase, silica-amino, siliea-CN, gradient Liquid adsorption, silica gel, gradient... 

Adsorption may in principle occur at all surfaces its magnitude is particularly noticeable when porous solids, which have a high surface area, such as silica gel or charcoal are contacted with gases or liquids. Adsorption processes may involve either simple uni-molecular adsorbate layers or multilayers the forces which bind the adsorbate to the surface may be physical or chemical in nature. 

Adsorbents such as some silica gels and types of carbons and zeolites have pores of the order of molecular dimensions, that is, from several up to 10-15 A in diameter. Adsorption in such pores is not readily treated as a capillary condensation phenomenon—in fact, there is typically no hysteresis loop. What happens physically is that as multilayer adsorption develops, the pore becomes filled by a meeting of the adsorbed films from opposing walls. Pores showing this type of adsorption behavior have come to be called micropores—a conventional definition is that micropore diameters are of width not exceeding 20 A (larger pores are called mesopores), see Ref. 221a. 

Fig. XVII-30. Adsorption of Na on a silica gel at 77.3 K, expressed as a u-/ plot, illustrating a method for micropore analysis. (From Ref. 230.)...

Preferential adsorption of one of the components may be used for the same purpose. Charcoal or silica gel may be employed to adsorb one of the constituents of an azeotrope in preference to the other. If the adsorbate is readily recoverable, the process will have practical applications. 

Column [10], adsorption values from isotherm for silica gel GS50 (desorption branch). 

Evidence of a different kind is furnished by the fact that the Gurvitsch rule (p. 113) is often obeyed by systems showing Type I isotherms " the amounts of different adsorptives taken up by a given adsorbent, when expressed as a volume of liquid, agree within a few per cent. The order of agreement is illustrated by the typical examples in Table 4.1 for the adsorption of n-alkanes on ammonium phosphomolybdate, and in Table 4.2 which refers to a variety of adsorptives on a silica gel. It must be admitted, however, that there are cases where considerable deviations from the Gurvitsch mle are found, even though the isotherms are of Type 1. Thus, in Table 4.3 the variation in values of the saturation uptake is far outside... 

Fig. 4.26 Low-pressure hysteresis in the adsorption isotherm of water at 298 K on a partially dehydroxy la ted silica gel. O, first adsorption run (outgassing at 200°C) . first desorption A, second adsorption run (outgassing at 200°C) A. second desorption (after reaching p/p = 0-31) X, third adsorption run (outgassing at 25 C).

Fig. 5.7 Adsorption isotherms of n-p>entane vapour on A), quartz and pyrex glass and (B), wide-pored silica gel. O,, quartz pyrex glass A, A wide-pored silica gel. Solid symbols denote desorption. (Courtesy...

Fig. 5.8 Adsorption isotherms at 25°C of benzene and cyclohexane on a mesoporous silica gel. Curve (A), benzene curve (B), cyclohexane. Solid symbols denote desorption.

An outstanding feature of the adsorption of water vapour on silica is its sensitivity to the course and subsequent treatment of the silica sample, in particular the temperature to which it has been heated. Figure 5.15 shows the strong dependence of the isotherm for a particular silica gel on the temperature of its heat treatment the isotherm is progressively lowered as the temperature increases, especially above 400°C, and the shape changes from Type II for the lower temperatures to Type III for 600°C, 800°C and 1000°C. 

Fig. 5.18 Adsorption isotherm of water vapour at 25°C on microporous silica gel E outgassed at 25°C. O. Adsorption,. desorption.

Fig. 5.19 Adsorption of water vapour on a silica gel which had been heated at 900°C. (The water content, calculated from the loss in weight at 1000°C, was 0-2%.) First run O, adsorption ( ), desorption. Second run , adsorption desorption.

In the first step, in which the molecules of the fluid come in contact with the adsorbent, an equihbrium is established between the adsorbed fluid and the fluid remaining in the fluid phase. Figures 25-7 through 25-9 show several experimental equihbrium adsorption isotherms for a number of components adsorbed on various adsorbents. Consider Fig. 25-7, in which the concentration of adsorbed gas on the solid is plotted against the equilibrium partial pressure p of the vapor or gas at constant temperature. At 40° C, for example, pure propane vapor at a pressure of 550 mm Hg is in equilibrium with an adsorbate concentration at point P of 0.04 lb adsorbed propane per pound of silica gel. Increasing the pressure of the propane will cause... 

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