Mobile adsorption

THE.9.1. Prigogine et R. Defay, Les phases superficielles ideales a adsorption mobile (The ideal superficial phases with moving adsorption). Bull. Soc. Chim. Belg. 53, 115-140 (1944). 

Sharom, M.S. and Edgington, L.V. The adsorption, mobility, and persistence of metalaxyl in soil and aqueous systems. Can. J. Plant Pathol, 4 334-340, 1982. 

The difference between IGC and conventional analytical gas-solid chromatography is the adsorption of a known adsorptive mobile phase (vapour) on an unknown adsorbent stationary phase (solid state sample). Depending on experiment setup, IGC can be used at finite or infinite dilution concentrations of the adsorptive mobile phase. The latter method is excellent for the determination of surface energetics and heat of sorption of particulate materials [3]. With IGC at finite dilution, it is possible to measure sorption isotherms for the determination of surface area and porosity [4], The benefits of using dynamic techniques are faster equilibrium times at ambient temperatures. 

Majka, J.T. and T.L. Lavy (1977). Adsorption, mobility, and degradation of cyanazine and diuron in soils. Weed Sci., 25 401 -06. Mallawatantri, A.P. and D.J. Mulla (1992). Herbicide adsorption and organic carbon contents on adjacent low-input versus conventional farms. J. Environ. Qual., 21 546-551. 

Peter, C.J. and J.B. Weber (1985). Adsorption, mobility, and efficacy of metribuzin as influenced by soil properties. Weed Sci., 33 868-873. 

Sprankle, P., Meggitt, W.F., Penner, D. (1975) Adsorption, mobility and microbial degradation of glyphosate in the soil. Weed Sci. 23, 229-234. 

The influence of anion on K+ rate of adsorption, mobility, and retention were investigated by Sadusky and Sparks (1991) on two Atlantic coastal plain soils. They reported that the type of anion had little effect (if any) on the rate of K+ adsorption, but had an effect on the amount of K+ adsorbed. They found that K+ adsorbed in the presence of a particular accompanying anion was of the order Si03 > P04 > S04 > Cl... 

In contrast to localized adsorption, mobile adsorption models assume that molecules can diffuse freely on the surface. One of the most popular equations used to describe mobile adsorption is that proposed by Hill and de Boer [105] as an analogue of the FG isotherm. This equation can be obtained by combining the two-dimensional form of van der Waals equation with the Gibbs adsorption isotherm. Note that the pre-exponential factors for localized and mobile adsorption are different. In the case of localized adsorption, the pre-exponential factor Kq takes into account the vibrations of adsorbing molecules in X, y and z direction, whereas the factor for the mobile adsorption contains only the partition functions for vibration in the z-direction and the transnational partition function describing mobility of adsorbing molecules in the (x,y)-plane. 

Soil sorption (adsorption/mobility) this parameter provides information on the potential mobility of a chemical in soil. 

When the model involves moleeular interactions in the adsorbed film or adsorption mobility, the topography of surface should be strictly defined. 

Some authors have extended the above adsorption model by including lateral, attractive interactions between admolecules, the multilayer nature of the surface film, and adsorption mobility (see Refs. 5,9,167,168 and references therein). 

Fixed-bed adsorption mobile feed liquid in axial flow... 

Steber, J. Wierich, P. "Properties of aminotris(methylenephosphonate) affecting its environmental fate degradability, sludge adsorption, mobility in soils, and bioconcentration", Chemosphere 1987,16, 1323-1337. 

The succeeding material is broadly organized according to the types of experimental quantities measured because much of the literature is so grouped. In the next chapter spread monolayers are discussed, and in later chapters the topics of adsorption from solution and of gas adsorption are considered. Irrespective of the experimental compartmentation, the conclusions as to the nature of mobile adsorbed films, that is, their structure and equations of state, will tend to be of a general validity. Thus, only a limited discussion of Gibbs monolayers has been given here, and none of such related aspects as the contact potentials of solutions or of adsorption at liquid-liquid interfaces, as it is more efficient to treat these topics later. 

These effects can be illustrated more quantitatively. The drop in the magnitude of the potential of mica with increasing salt is illustrated in Fig. V-7 here yp is reduced in the immobile layer by ion adsorption and specific ion effects are evident. In Fig. V-8, the pH is potential determining and alters the electrophoretic mobility. Carbon blacks are industrially important materials having various acid-base surface impurities depending on their source and heat treatment. 

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

The state of an adsorbate is often described as mobile or localized, usually in connection with adsorption models and analyses of adsorption entropies (see Section XVII-3C). A more direct criterion is, in analogy to that of the fluidity of a bulk phase, the degree of mobility as reflected by the surface diffusion coefficient. This may be estimated from the dielectric relaxation time Resing [115] gives values of the diffusion coefficient for adsorbed water ranging from near bulk liquids values (lO cm /sec) to as low as 10 cm /sec. 

The case of a vapor adsorbing on its own liquid surface should certainly correspond to mobile adsorption. Here, 6 is unity and P = the vapor pressure. The energy of adsorption is now that of condensation Qu, and it will be convenient to define the Langmuir constant for this case as thus, from Eq. xvn-39. 

Thus the entropy of localized adsorption can range widely, depending on whether the site is viewed as equivalent to a strong adsorption bond of negligible entropy or as a potential box plus a weak bond (see Ref. 12). In addition, estimates of AS ds should include possible surface vibrational contributions in the case of mobile adsorption, and all calculations are faced with possible contributions from a loss in rotational entropy on adsorption as well as from change in the adsorbent structure following adsorption (see Section XVI-4B). These uncertainties make it virtually impossible to affirm what the state of an adsorbed film is from entropy measurements alone for this, additional independent information about surface mobility and vibrational surface states is needed. (However, see Ref. 15 for a somewhat more optimistic conclusion.)... 

The standard entropy of adsorption AS2 of benzene on a certain surface was found to be -25.2 EU at 323.1 K the standard states being the vapor at 1 atm and the film at an area of 22.5 x T per molecule. Discuss, with appropriate calculations, what the state of the adsorbed film might be, particularly as to whether it is mobile or localized. Take the molecular area of benzene to be 22 A. ... 

The preceding treatments are based on the concept of localized rather than mobile adsorption. The distinction may be difficult experimentally note Ref. 109 and the discussion in connection with Fig. XVII-25. There are also conceptual subtleties see Section XVIII-5. 

The matter of surface mobility has come up at several points in the preceding material. The subject has been a source of confusion—see Ref. 112. Actually, two kinds of concepts seem to have been invoked. The first is that invoked in the discussion of physical adsorption, which has to do with whether the adsorbate can move on the surface so freely that its state is essentially that of a two-dimensional nonideal gas. For an adsorbate to be mobile in this sense, surface barriers must be small compared to kT. This type of mobile adsorbed layer seems unlikely to be involved in chemisorption. 

In general, it seems more reasonable to suppose that in chemisorption specific sites are involved and that therefore definite potential barriers to lateral motion should be present. The adsorption should therefore obey the statistical thermodynamics of a localized state. On the other hand, the kinetics of adsorption and of catalytic processes will depend greatly on the frequency and nature of such surface jumps as do occur. A film can be fairly mobile in this kinetic sense and yet not be expected to show any significant deviation from the configurational entropy of a localized state. 

Neither the thermodynamic nor the rheological description of surface mobility has been very useful in the case of chemisorbed films. From the experimental point of view, the first is complicated by the many factors that can affect adsorption entropies and the latter by the lack of any methodology. 

The sequence of events in a surface-catalyzed reaction comprises (1) diffusion of reactants to the surface (usually considered to be fast) (2) adsorption of the reactants on the surface (slow if activated) (3) surface diffusion of reactants to active sites (if the adsorption is mobile) (4) reaction of the adsorbed species (often rate-determining) (5) desorption of the reaction products (often slow) and (6) diffusion of the products away from the surface. Processes 1 and 6 may be rate-determining where one is dealing with a porous catalyst [197]. The situation is illustrated in Fig. XVIII-22 (see also Ref. 198 notice in the figure the variety of processes that may be present). 

Figure Bl.19.13. (a) Tliree STM images of a Pt(l 11) surface covered witli hydrocarbon species generated by exposure to propene. Images taken in constant-height mode. (A) after adsorption at room temperature. The propylidyne (=C-CH2-CH2) species that fomied was too mobile on the surface to be visible. The surface looks similar to that of the clean surface. Terraces ( 10 mn wide) and monatomic steps are the only visible features. (B) After heating the adsorbed propylidyne to 550 K, clusters fonn by polymerization of the C H...

A study of Table 1.1 reveals interesting features as to the mobility of the adsorbed atoms. Thus, for an argon atom on the (100) face, the easiest path from one preferred site S to the next is over the saddle point P, so that the energy barrier which must be surmounted is (1251 — 855) or 396 X 10 J/molecule. Since the mean thermal energy kT at 78 K is only 108 J/molecule, the argon molecule will have severely limited mobility at this temperature and will spend nearly all of its time in the close vicinity of site S its adsorption will be localized. On the other hand, for helium on the... 

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