Sorption surface analysis

Schwenke H, Beaven PA, Knoth J (1999) Applications of total reflection X-ray fluorescence spectrometry in trace element and surface analysis. Fresenius J Anal Chem 365 19-27 Towle SN, Brown GE, Parks GA (1999a) Sorption of Co(II) on metal oxide surfaces I. Identification of specific binding sites of Co(II) on (110) and (001) surfaces of Ti02 (rutile) by grazing-incidence XAFS spectroscopy. J Coll Interface Sci 217 299-311... 

Surface analysis via physisorption is the appropriate technique to analyze the specific surface of a catalyst The catalyst sample is contacted at different pressures (normally from vaccum to atmosphere) with a gas that physisorbs under appropriate temperatme conditions (normally N2 at 77 K) in a defined manner on its surface. Owing to this physisorption process the pressure in the sample chamber changes. From this pressure drop, the physisorbed amount of gas per sample mass is determined. The adsorbed amount versus the applied pressure is called the sorption isotherm and with different models, such as, for example, the most common BET (Brunauer, Emmett, and Teller) model, the available surface can be calculated from the isotherm. Other models like the BJH (Barrett, Joyner, and... 

Halenda) or modem DFT (density fimctional theory) methods also allow to evaluate the pore size distribution from the same data. Sample preparation, highly defined experimental conditions, and very precise pressure measurements are the key factors for accurate surface analysis. While sorption experiments probe pores in the size range from approximately 0.3 to 100 nm, mercury porosim-etry is the method of choice to determine the total pore volume and the pore size distribution from 5 nm up to 500 p,m. The method pushes liquid mercury under high pressure into the porous material and the Hg volume accommodated in the solid is monitored as a function of pressure. Following the Kelvin equation, a higher pressure is necessary to push the mercury into smaller pores. Therefore, from the amount of mercury infiltrated into the solid as a function of pressure the pore size distribution can be obtained. 

The conducted researches of complexing processes of noble metals on a sulfur-containing CMSG surface formed the basis for development of sorption-photometric, sorption-luminescent, soi ption-atomic-absoi ption, sorption-atomic-emission and sorption-nuclear-physic techniques of the analysis of noble metals in rocks, technological objects and environmental objects. Techniques of separation and detenuination of noble metals in various oxidation levels have been proposed in some cases. 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

Here we also consider sorption kinetics as the mass-transfer barrier to surfactant migration to and from the interface, and we follow the Levich framework. However, our analysis does not confine all surface-tension gradients to the constant thickness film. Rather, we treat the bubble shape and the surfactant distribution along the interface in a consistent fashion. 

The difference between this technique and GC or HPLC is that the separation process occurs on a flat essentially two-dimensional surface. The separated components are not usually eluted from the surface but are examined in situ. Alternatively, they can be removed mechanically for further analysis. In thin-layer chromatography (TLC), the stationary phase is usually a polar solid such as silica gel or alumina which is coated onto a sheet of glass, plastic, or aluminium. Although some moisture is retained by the stationary phase, the separation process is predominantly one of surface adsorption. Thin layers are sometimes made from ion-exchange or gelpermeation materials. In these cases the sorption process would be ion-exchange or exclusion. 

Pump-down of the antechamber, following solution experiments, involved sorption pumping and cryo-pumping resulting in the pressure decreasing from ambient to 10 Torr in 5 minutes. The resulting sample surfaces were subsequently examined by both LEFT) and AES after transfer back to the analysis-chamber. 

When used in direct soil analysis, XRF suffers from the fact that it is largely a surface phenomenon. For this reason, only surface elements will be determined. However, because it is a surface phenomenon, it has been extensively used to study sorption on the surfaces of soil components. An extensive list of investigations using XRF to investigate various sorption mechanisms is given in the text by Sparks [33],... 

Gibbs free energies of water sorption, AG "(/l), can be extracted from isopiestic vapor sorption isotherms this analysis shows that AG (T) < AG", where AG" = -44.7 kj moH is the Gibbs free energy for vapor sorption at a free water surface at ambient condihons. Water absorbed by the membrane is therefore more strongly bound than water at a free bulk water surface this affirms the hydrophilic nature of water sorption in PEMs. 

Still, this theory is over-simplified, and holds only for a limited part of the sorption isotherm, which is usually the case for relative pressures between 0.05-0.30, and the presence of point B (Fig. 1.14). Thus, isotherms of Types II (macroporous polymer supports) and IV (mesoporous polymer supports), but not Type I and III, are those amenable to BET analysis [21, 80]. Attention should also be paid to the constant C, which is exponentially related to the enthalpy of adsorption of the first layer. A negative or high value of C exceeding 200-300, is likely to indicate the presence of micropores and the calculated surface area should be questioned since the BFT theory would not be applicable [79, 80]. 

The substrates used for DNA immobihsation can be, at the level of the AFM analysis, either rigid (e.g., glass) or flexible (e.g., polymers). An atomically flat surface is preferable, but sometimes this is difficult to achieve. The DNA immobilisation mechanisms can be (i) physio-sorption, (ii) charge complementarity and (iii) covalent binding. Depending on the actual objective of the AFM study, each combination of the surface-immobilisation mechanism has both benefits and drawbacks. 

Feltz, A. Martin, A. (1987) Solid-state reactivity and mechanisms in oxide systems. 11 Inhibition of zinc ferrite formation in zinc oxide - a-iron(lll) oxide mixtures with a large excess of a-iron(lll) oxide. In Schwab, G.M. (ed.) Reactivity of solids. Elsevier, 2 307—313 Fendorf, S. Fendorf, M. (1996) Sorption mechanisms of lanthanum on oxide minerals. Clays Clay Miner. 44 220-227 Fendorf, S.E. Sparks, D.L. (1996) X-ray absorption fine structure spectroscopy. In Methods of Soil Analysis. Part 3 Chemical Methods. Soil Sd. Soc. Am., 377-416 Fendorf, S.E. Eick, M.J. Grossl, P. Sparks, D.L. (1997) Arsenate and chromate retention mechanisms on goethite. 1. Surface structure. Environ. Sci. Techn. 31 315-320 Fendorf, S.E. Li,V. Gunter, M.E. (1996) Micromorphologies and stabilities of chromiu-m(III) surface precipitates elucidated by scanning force microscopy. Soil Sci. Soc. Am. J. 60 99-106... 

The concept of transport resistances localized in the outermost regions of NS crystals was introduced in order to explain the differences between intracrystalline self-diffusion coefficients obtained by n.m.r methods and diffusion coefficients derived from non-equilibrium experiments based on the assumption that Intracrystalline transport is rate-limiting. This concept has been discussed during the past decade, cf. the pioneering work [79-81] and the reviews [2,7,8,23,32,82]. Nowadays, one can state that surface barriers do not occur necessarily in sorption uptake by NS crystals, but they may occur if the cross-sections of the sorbing molecular species and the micropore openings become comparable. For indication of their significance, careful analysis of... 

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