Adsoiption isotherms

Freundlich adsoiption isotherm constants for several compounds on several adsor-bents/water systems at ambient temperature can be found in Table 1.24, Appendix I. 

The adsorption of sodium oleate on a fluorite (CaF2) IRE has been quantitatively studied more recently (116). Adsoiption proceeded rapidly during the first two hours, but slowed at longer times, taking as much as 30 hours to reach equilibrium. The formation of interfacial calcium dioleate was clearly indicated by the pair of carboxylate bands at 1571 and 1534 cm 1 in an in - situ spectrum of an adsorbed layer formed from sodium oleate at 9 x 10 5 M. Calculation of the adsoiption isotherm from the FT-IR data, at sodium oleate concentrations between 5 x 10 and 5 x 104 M, yielded results in good agreement with values obtained by other methods. 

In a further study of autophobicity by Blake and Wade (1971), adsoiption isotherms were determined for water vapour and the first five n-aliphatic alcohols on the surface of oxidized aluminium foil. The results were very similar to those obtained for a-Al203. A Type II water isotherm was obtained, the initial steep slope being indicative of a high affinity of adsorption. The residual uptake of water was... 

The adsoiption isotherm is obtained by gradually increasing the pressure. The smallest pores are filled first the gas then condenses in successively larger pores until a saturated vapour pressure level is reached at which the entire porous volume is saturated with liquid. By measuring, from Pq, the quantities of gas that remain adsorbed for decreasing relative pressure levels, the desorption isotherm is obtained. Equation (1.4) is used to ascertain, for each equilibrium pressure value, the dimension of the pores in which the liquid is in equilibrium with the vapour phase. 

Figure 6.2. Fit of the Freundlich adsoiption isotherm equation to copper adsorption by two soils. (From Kurdi and Doner, 1983.)...

Because of the substantial scatter of the degree of elimination it is advisable in critical cases to determine P-values experimentally in simulated systems or from field measurements. In an ETAD-project [11] P-values have been determined by Zahn-Wellens measurements but such values have to be considered as worst-case situations because test conditions require relatively high soluble dye concentrations in relation to the amount of suspended sludge [11], Furthermore, P-values are pH-dependent and can be determined from adsoiption isotherms. In the case of wool and polyamide dyeing, P-values can be estimated from the data on fixation (F), due to the similarity of the surface functional groups of sludge particles to those of the textile substrate. 

This is a general type of adsoiption isotherm. In the case of ion exchange, it is easy to conelate data in a way to have the same type of relationship for the ion that is removed from the liquid phase. When equihbrium is reached. 

For maity of activated microporous carbons, the isotherms exhibit prominent adsorption at low relative pressures and then level off, i.e., the isotherms exhibit Type I behavior. Type I isotherms may be also observed in the mesoporous materials with pore sizes close to the micropore range. In particular, in the case of gas adsorption on highly uniform cylindrical pores, the adsoiption isotherms exhibit discernible steps at relative pressures down to 0.1 or perhaps even lower. Such Type I behavior can be indicative of some degree of broadening of the mesopore size distribution. 

Biunauer S, Emmett PH (1937) The use of low temperature van der Waals adsoiption isotherms in detramining the surface areas of various adsorbents. J Am Chem Soc 59 2682-2689... 

Figure 5.2. represents several adsoiption isotherms for n-decane on poly(methyl methacrylate) [126]. They correspond to type III isotherms... 

The nitrogen adsoiption isotherms were measured volumetrically at 77 K using an automatic adsorption apparatus (Quantachrmne, Autosoib-l-MP). Each sample was outgassed at 423 K and 1 mPa for 12 h before the adsorption measurmnents. 

Figure 5. Adsoiption isotherms for SF in slit-shaped pores of various widths.

Adsorption volumetry of N2 or Ar is carried out at 77, 87 or 90 K, either with a conventional point-by-point procedure or with the quasi-equilibrium procedure allowing a continuous recording of the adsoiption isotherm (ref. 14). When needed, the latter procedure is associated with adsoiption microcalorimetry (ref. IS). 

If one plots the adsoiption isotherms of N2 and Ar adsorbed on the same sample at a given temperature (with P/Pq on the abscissa), as is done in Fig. 1, one may or may not get a crossing of the isotherms. The presence of such crossing points in different silicas was also reported previously in the literature by different research groups (refs. 16,17). The reason for the crossing may be derived from previous works (refs. 8,9) where a systematic Comparison was carried out... 

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