Adsorption isotherms specific types

Ag was used to detennine the adsorption of Ag ions on rutile or anatase T1O2. Atomic absorption and, in some cases, polarography were used to study the adsorption of Ag ions by precipitated silica or pyrogenic silica. The results are found in Surface Technology.VH 9%2) 65y including AG (for specific idsorption) and adsorption isotherms (Langmuir type). 

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. 

Fig. 4.11 (A) Adsorption isotherms of lysozyme meters of the CMK materials (a) specific surface on CMK-type mesoporous carbon materials at pH area (b) specific pore volume (c) pore diameter. 11 (a) CMK-1 (b) CMK-3 (c) CMK-3-130 (d) Reprinted with permission from [152], A. Vinu CMK-3-150. (B) Dependence of the adsorption etal.,J. Phys. Chem. B 2005, 109, 6436. 2005, capacities of lysozyme on the structural para- American Chemical Society.

The way in which a material adsorbs a gas is referred to as an adsorption isotherm. All adsorption isotherms can be described by five representative curves, given in Fig. 1. The isotherm shapes reflect specific conditions for adsorption, such as pore size and heats of adsorption [6]. The most common type of isotherm and the most useful for BET measurements is the Type II isotherm. The inflection point of this isotherm usually indicates monolayer coverage of the adsorbate [9]. 

The linear equilibrium isotherm adsorption relationship (Eq. 11) requires a constant rate of adsorption, and is most often not physically valid because the ability of clay solid particles to absorb pollutants decreases as the adsorbed amount of pollutant increases, contrary to expectations from the liner model. If the rate of adsorption decreases rapidly as the concentration in the pore fluid increases, the simple Freundlich type model (Eqs. 8 and 9) must be extended to properly portray the adsorption relationship. Few models can faithfully portray the adsorption relationship for multicomponent COM-pollutant systems where some of the components are adsorbed and others are desorbed. It is therefore necessary to perform initial tests with the natural system to choose the adsorption model specific to the problem at hand. From leaching-column experimental data, using field materials (soil solids and COMs solutions), and model calibration, the following general function can be successfully applied [155] ... 

The most reliable information about the mesoporous structure of solids comes from low-temperature nitrogen adsorption isotherms which enable the calculation of the specific surface area, pore volume, and pore size distribution Figure I shows the N2 adsorption isotherms of the purely siliceous MCM-41, niobium-containing MCM-41, and A1MCM-41 They are typical of reversible adsorption type IV and at relative low pressures (p/po < 0.3) are accounted for by monolayer adsorption of nitrogen on the walls of the mesopores. As the relative pressure increases (p/p0 > 0,3), the isotherm exhibits a sharp inflection, characteristic of the capillary condensation within uniform mesopores, where the p/po position of the inflection point is... 

The degree of adsorption depends on electrolyte concentration. The degree of coverage of a surface by specific adsorption of ions can be described by monolayer adsorption isotherms (Fig. 3.11). Three types of isotherm are generally considered ... 

In general, adsorption isotherms obtained with this and other zeolitic substrates are of Brunauer s Type I, the simple hyperbolic form also known as the Langmuir isotherm. Consequently, the asymptotic limit of adsorption is used instead of the value of Vm normally derived from the BET evaluation of specific surface area. It is, of course, not possible to define exact monolayer or multilayer adsorption in these three-dimensional interconnected pore systems. 

The early work of Kiselev (1957) revealed that the adsorption isotherms of n-pentane and n-hexane on non-porous quartz were intermediate in character between Types II and m. Values of C(BET) <10 were obtained and the differential enthalpies of adsorption decreased steeply at low surface coverage. More recently, the isotherms of isobutane (at 261 K) and neopentane (at 273 K) on TK800 have been found to be of a similar shape (Carrott et al., 1988 Carrott and Sing, 1989). Unlike those of benzene, these alkane isotherms do not undergo a pronounced change of shape as a result of surface dehydroxylation. This is consistent with the non-specific nature of their molecular interactions (see Chapter 1). 

The adsorption isotherms in Figure 11.3 are of interest for several reasons. First, it may seem surprising that an assemblage of kaolinite platelets should give a reversible isotherm. The adsorbent had a specific surface area of 17 m2g-1, which would appear to correspond to a platelet thickness of c. 50 nm. The particle rigidity and the house-of-cards packing have probably resulted in the formation of a macro-porous aggregate, which accounts for the appearance of the reversible Type II isotherm. 

In formulating adsorption Isotherm equations, assumptions have to be made about the kinds of ions that bind, and on the planes where they adsorb. Stem himself considered the specific adsorption of cations and anions, both at the outer Helmholtz plane (approximation (li) in the previous subsection)More likely are situations where only one ionic type adsorbs at the inner Helmholtz plane. For that case, the Langmuir equation is readily extended. We start with IA1.2al, which we write as 6J( -Q ) = K x. Here, = is the ratio... 

The last Issue to be dealt with Is the apparent irreversibility of the adsorption. One quite often encounters the opinion, especially In the older literature, that polymer adsorption would be an Irreversible phenomenon. These ideas are based on the hysteresis found In the adsorption isotherms desorption Isotherms (obtained by dilution with solvent) do not coincide with adsorption Isotherms (obtained by adding more polymer at given amount of solvent). Qualitatively, this was already discussed in sec. 5.3d. An experimental example Is given in fig. 5.31, for the adsorption of a polydisperse rubber from heptane on two types of carbon black (differing In specific surface area) ). The desorption Isotherms are found to He considerably above the adsorption Isotherms, the extent of desorption being very small. 

Adsorption of albumin, y-globulin, and fibrinogen from single solutions onto several hydrophobic polymers was studied using internal reflection IR spectroscopy. The adsorption isotherms have a Langmuir-type form. The calculated rate and amount of protein adsorbed was dependent on the polymer substrate and the flow rate of the solution. Competitive adsorption experiments were also investigated to determine the specific adsorption of each I-labelled protein from a mixture of proteins. Platelet adhesion to these proteinated surfaces is discussed in relation to a model previously proposed. 

More recentfy Mohlin and Gray (9J) determined adsorption isotherms on cellulose fibers for a variety of adsorbates (solutes). From the experimental type II isotherms specific surface areas of the fibers were computed, for eadi solute, with the results en in Table 11. The agreement observed between the different solutes is quite remarkable considering that the area of the solute molecule on the potymer surface must be known or estimated. Hie sirface area determined by nitrogen adsorption measurements at —196° was included for the purpose of comparison. The sli t di arity could possibly indicate that the area available to the smaller nitrogen molecule may be somewhat larger (1.9 compared to 1.6 m g" ). 

Vycor is a porous silica glass which is widely used as a model structure for the. study of the properties of confined fluids in me.soporous materials. The pores in vycor have an average radius of about 30-35 A (assuming a cylindrical geometry) and are spaced about 200 A apart [2-3]. A literature survey indicates that there are two kinds of (Corning) vycor glasses one type has a specific surface around 100 m /g while the other is characterized by a specific surface around 200 m /g (both values are obtained from N2 adsorption isotherms at 77 K). 

The adsorption-desorption isotherms of nitrogen at -196 °C obtained on all the catalysts under investigation were mainly of Type IV of Brunauer s classification [16], exhibiting hysteresis loops closed at P/Po ranging between 0.25 and 0.55. The adsorption data are summarized in Table 1, including BET-C constant, specific surface area(SBEj), total pore volume (Vp), estimated from the saturation values of the adsorption isotherms and average pore radius (r P), assuming cylindrical pore model for which superscript (cp) was used. 

---