Adsorption isotherm silica

Fig. 230 Adsorption of nitrogen at 77 K on a silica powder a) adsorption isotherms b) /-plot. Broken line, uncompacted powder continuous line, power compacted at 2-00 x 10 N m (130 ton in ). (—>—) adsorption (—<-) desorption. / is the ratio of the amount adsorbed on the powder to the amount adsorbed on the compact at the same relative...

Fig. 3.22 Adsorption isotherms of nitrogen at 77 K on silica powder and its compacts. (A) uncompressed (B) 10 ton in (C) 40 ton in" (D) 50 ton in (E) 100 ton in . Open symbols represent adsorption, solid symbols desorption. (Courtesy Ramsay.)...

Fig. 3.26 Comparison plots for compacts of silica and magnesia. In each case the adsorption of nitrogen at 78 K on the compact is plotted against that on the uncompacted powder, (a) and (b), comparison plot and adsorption isotherm for silica powder compacted at 130 ton in (c) and (d), comparison plot and adsorption isotherm for precipitated magnesia compacted at 10 ton in. Note that the upward sweep of the comparison plot commences at a relative pressure below the inception of the loop.

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.

Fig. 5.9 Adsorption isotherms of carbon tetrachloride at 20 C on various samples of silica. (A) Fransil (nonporous particles) (B) TK 800 (nonporous particles) (C) a mesoporous gel (D) a microporous gel.

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

The effect of these factors on the adsorption isotherm may be elucidated by reference to specific examples. In the case of the isotherm of Fig. 5.17(a), the nonporous silica had not been re-heated after preparation, but had been exposed to near-saturated water vapour to ensure complete hydroxylation. The isotherm is of Type II and is completely reversible. On the sample outgassed at 1000°C (Fig. 5.17(h)) the isotherm is quite different the adsorption branch is very close to Type III, and there is extrensive hysteresis extending over the whole isotherm, with considerable retention of adsorbate on outgassing at 25°C at the end of the run. 

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... 

Figure 3 Adsorption isotherms at room temperature on 0.1% Rh on silica (a) CO isotherm (b) CO isotherm after pumping out sample at end of isotherm (a) for 1 min (c) difference between (a) and (b).

A solvent can be adsorbed from a solvent mixture on the surface of silica gel according to the Langmuir adsorption isotherm as previously discussed. 

Examples of mono-layer adsorption isotherms obtained for chloroform and butyl chloride are shown in Figure 5. The adsorption isotherms of the more polar solvents, ethyl acetate, isopropanol and tetrahydro-furan from -heptane solutions on silica gel were examined by Scott and Kucera [4]. Somewhat surprisingly, it was found that the experimental results for the more polar solvents did not fit the simple mono-layer... 

Figure 7. The Bi-layer Adsorption Isotherm of Ethyl Acetate on Silica Gel...

Scott and Kucera [4] carried out some experiments that were designed to confirm that the two types of solute/stationary phase interaction, sorption and displacement, did, in fact, occur in chromatographic systems. They dispersed about 10 g of silica gel in a solvent mixture made up of 0.35 %w/v of ethyl acetate in n-heptane. It is seen from the adsorption isotherms shown in Figure 8 that at an ethyl acetate concentration of 0.35%w/v more than 95% of the first layer of ethyl acetate has been formed on the silica gel. In addition, at this solvent composition, very little of the second layer was formed. Consequently, this concentration was chosen to ensure that if significant amounts of ethyl acetate were displaced by the solute, it would be derived from the first layer on the silica and not the less strongly held second layer. 

It is seen that at high concentrations (a) becomes unity and the surface is completely covered with the more strongly adsorbed solvent. The adsorption isotherm of chloroform on silica gel, determined by Scott and Kucera (5) is shown in figure 1. It is seen that the monolayer of chloroform collects on the surface continuously until the chloroform content of the mobile phase is about 50%. At this concentration the monolayer appears complete. Thus, between 0 and 50% chloroform in the n-heptane, the interactions between the solute and the chloroform in the mobile phase are continuously increasing. 

The more dispersive solvent from an aqueous solvent mixture is adsorbed onto the surface of a reverse phase according to Langmuir equation and an example of the adsorption isotherms of the lower series of aliphatic alcohols onto the surface of a reverse phase (9) is shown in figure 9. It is seen that the alcohol with the longest chain, and thus the most dispersive in character, is avidly adsorbed onto the highly dispersive stationary phase, much like the polar ethyl acetate is adsorbed onto the highly polar surface of silica gel. It is also seen that... 

Solutes will interact with the reverse phase surface in much the same way as they do with the silica gel surface. There will be basically two forms of interaction, by sorption and by displacement. Sorption interaction has been experimentally confirmed by Scott and Kucera (10) by measuring the adsorption isotherm of acetophenone on the reverse phase RP18 from a 40%w/v acetonitrile mixture in water. The authors noted that there was no change in the acetonitrile concentration, as the solute was adsorbed. Displacement interactions, although certain to occur, do not appear to have been experimentally demonstrated to date. 

As shown in Figure 3.5.3, the relaxation time versus pressure curves are dramatically different from those obtained using CF4 at a temperature well above its critical point. Indeed, while the overall form of the Tx curves for CF4 in fumed silica was similar to that of the bulk gas, the shape of the Ti plots for c-C4F8 in Vycor more closely resembles that of an adsorption isotherm (Ta of CF4 in Vycor is largely invariant with pressure, as gas-wall collisions in this material are more frequent than gas-gas collisions). This is not surprising given that we expect the behavior of this gas at 291 K to be shifted towards the adsorbed phase. The highest pressure... 

Fig. 4.3 Adsorption isotherms of lysozyme at pH 10.5 on mesoporous silica materials (a) Cn2-MCM-41 (b) Cie-MCM-41 (MCM-41) (c) SBA-15. Reprinted with permission from [119],...

Fig. 6. Plateau-values, I"P1 /mg m 2, of adsorption isotherms of lysozyme (LSZ), ribonuclease (RNase), a -lactalbumin (aLA), calcium-depleted (X -lactalbumin (aLA(-Ca )) and bovine serum albumin (BSA) on hydrophobic polystyrene (PS) and hydrophilic hematite (a — Fe203) and silica (Si02) surfaces. An indication of the charge density of the surface is given by the zeta-potential, C, and of the proteins by + and signs. Ionic strength 0.05 M T = 25°C. (Derived from Currie et al. 2003).

Figure 9 shows adsorption isotherms for this protein on the various sorbents. As is usually observed for proteins at surfaces, the adsorption affinity is higher for the hydrophobic surfaces than for the hydrophilic ones. The adsorption plateau-values at silica and PS- (EO)8, approximately... 

The enzymatic activities of O -chymotrypsin in solution and adsorbed at the different surfaces are presented in Fig. 11, where the specific enzymatic activity (defined as activity per unit mass of protein) is plotted as a function of temperature. The enzyme loses activity due to adsorption. On the hydrophobic Teflon and PS surfaces, the activity is completely gone, whereas on the hydrophilic silica surface, or-chymotrypsin has retained most of its biological function. These differences are in agreement with the adsorption isotherms and the circular dichroism spectra. The influence of the hydrophobicity of the sorbent surface on the affinity of the protein for the sorbent surface, as judged from the rising parts of the adsorption isotherms (Fig. 8), suggests that the proteins are more perturbed and, hence, less biologically active when adsorbed at hydrophobic surfaces. Also, the CD spectra indicate that adsorption-induced structural perturbations are more severe at hydrophobic surfaces. 

Determination of the segmental adsorption energy. The determina-tion of x ° is also possible. Since x d can be found from Equation 5 if Xsc an the solvency terms are known, we can add xf° and find x ° by Equation 1. The determination of xf° calls for a separate experiment, e.g., an adsorption isotherm of the displacer from solvent, in the absence of polymer. Following such a scheme we used the values of cr obtained from the displacement isotherms of Figure 3 and 4 to determine segmental adsorption energy parameters Xg° for PVP on silica. The required additional information on xdo was obtained from the initial slopes of dis-... 

The adsorption isotherms of PEG 6000 and AO,000 in fig. 1 show the typical high affinity character for precipitated and for pyrogenic silica with the following trends ... 

Figure 1.6 Top Low-temperature nitrogen adsorption ( ) and desorption (x) isotherms measured on a calcined SBA-15 mesoporous silica solid prepared using an EO20PO70EO20 block copolymer [54]. Bottom Pore size distribution derived from the adsorption isotherm reported at the top [54]. A high surface area (850 m2/g), a uniform distribution of cylindrical nanopores (diameter —90 A), and a large pore volume (1.17 cm3/g) were all estimated from these data. These properties make this material suitable for use as support in the preparation of high-surface-area solid catalysts. (Reproduced with permission from The American Chemical Society.)...

The retention of analyses in RP-HPLC markedly depends on the adsorption of the organic constituent of the mobile phase on the surface of the stationary phase. The excess adsorption isotherms of ACN, THF and methanol were measured on silica support modified with C, C6, C8, C10, C12 and C18 monomeric phase and a model was developed for the description of the retention of solutes from the binary mobile phase. The dependence of the retention factor on the partition coefficient can be described by... 

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