Adsorption excess

The study of metal ion/metal(s) interfaces has been limited because of the excessive adsorption of the reactants and impurities at the electrode surface and due to the inseparability of the faradaic and nonfaradaic impedances. For obtaining reproducible results with solid electrodes, the important factors to be considered are the fabrication, the smoothness of the surface (by polishing), and the pretreatment of the electrodes, the treatment of the solution with activated charcoal, the use of an inert atmosphere, and the constancy of the equilibrium potential for the duration of the experiment. It is appropriate to deal with some of these details from a practical point of view. 

In nitrate and sulfate media, Cr(III) is not very stable and the reaction Cr(II) -> Cr tends to be irreversible. The rate constants obtained in these media are tentative because of excessive adsorption of chromous ions in these media. The rate constants have been... 

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... 

Y.V. Kazakevich, R. LoBrutto, F. Chan and T. Patel, Interpretation of the excess adsorption isotherms of organic components on the surface of reversed-phase adsorbents. Effect on the analyte retention. J. Chromatogr.A 913 (2001) 75-87. 

Figure 6. Comparison of the contributions which excess adsorption and compression in the void space have on the total storage capacity at 298 K (left) and at 77K. (right).

For application purposes not only the excess adsorption, but also the compressed H2 in the void space of the material needs to be taken into account for calculating the total storage capacity. As an example, in Figure 6 the contribution of excess adsorption and compression on the total storage amount is visualised in a qualitative way for hydrogen stored at 298 K and 77 K. This example shows that for high temperatures and pressures the compression contribution gains importance, while at 77 K. the contribution of excess adsorption is more important. 

A second form of storage iron is haemosiderin (Weir et al., 1984). This is deposited in humans as a response to the condition of iron overload. Haemosiderin forms as insoluble granules with electron dense cores surrounded by a protein shell. It exists in two forms primary haemosiderin is the result of iron overload due to excessive adsorption of iron in the gut, whereas the secondary form is caused by the numerous blood transfusions which are used to treat thallassaemia (a form of anaemia). Electron diffraction indicated that the iron core in primary haemosiderin is a 3-line ferrihydrite with magnetic hyperfine splitting only below 4 K and, in the secondary form, consists of poorly ordered goethite. As goethite is less soluble in ammonium oxalate buffer solution (pH 3) it has a lower intrinsic toxicity (Mann et al., 1988). 

FIGURE 9.11 Classification of excess adsorption isotherms for binary solutions with o° solubility. After Kipling [39]. Reproduced by permission from Quantitative Review by Ripling Royal Society of Chemistry, Cambridge, UK, 1951. 

FIGURE 9.12 Excess adsorption isotherms for ideal bindary solution with infinite solubility for different values of K the adsorption distribution constant given by equation 9.44. 

The absolute adsorption is predicted from theoretical calculations and is the amount of hydrogen which is adsorbed in the porous material, not considering the gas phase. The relation between the excess adsorption (N and the absolute adsorption (N ds) can be esily derived considering a typical adsorption experiment [21] The amount of gas adsorbed on a sample (Nads) is expressed as the total amount of gas introduced in the sample cell (N ) minus the free molecules in the gas phase. (2.4) ... 

Therefore the relation between experimentally measured excess adsorption and the absolute adsorption can be expressed as in (2.6) ... 

Especially under conditions for which the density ofthe gas phase is comparable to or higher than the density of the adsorbed phase (Qg/Qa 1), that is, low temperatures and high pressures, the excess adsorption differs from the absolute adsorption. 

Figure 2.2 Typical shapes of the absolute adsorption isotherm and excess absorption isotherm. The inflection of the excess adsorption isotherm can occur at high pressures.

Adsorbent nonpermeability is an important condition, since it essentially states that all processes occurs in the liquid phase. Since adsorption is related to the adsorbent surface, it is possible to consider the analyte distribution between the whole liquid phase and the surface. Using surface concentrations and the Gibbs concept of excess adsorption [20], it is possible to describe the adsorption from binary mixtures without the definition of adsorbed phase volume. 

In a liquid binary solution, this accumulation is accompanied by the corresponding displacement of another component (solvent) from the surface region into the bulk solution. At equilibrium a certain amount of the solute will be accumulated on the surface in excess of its equilibrium concentration in the bulk solution, as shown in Figure 2-6. Excess adsorption E of a component in binary mixture is defined from a comparison of two static systems with the same liquid volume Vo and adsorbent surface area S. In the first system the adsorbent surface considered to be inert (does not exert any surface forces in the solution) and the total amount of analyte (component 2) will be no = VoCo. In the second system the adsorbent surface is active and component 2 is preferentially adsorbed thus its amount in the bulk solution is decreased. The analyte equilibrium concentration Ce can only be measured in the bulk solution, so the amount VoCe is thereby smaller than the original quantity no due to its accumulation on the surface, but it also includes the portion of the analyte in the close proximity of the surface (the portion U Ce, as shown in Figure 2-6 note that we did not define V yet and we do not need to define... 

This expression describes the analyte retention in binary system using only the total volume of the liquid phase in the column, Vq, and total adsorbent surface area S as parameters and the derivative of the excess adsorption by the analyte equihbrium concentration. It is important to note that the position of Gibbs dividing plane in the system has not been defined yet. 

In case of injection of a very small amount of analyte, its concentration is in the linear region of adsorption isotherm (Henry region of linear variation of adsorption with the equilibrium concentration of the analyte) and the derivative could be substituted with the slope of excess adsorption isotherm, also known as Henry constant, Kh, to get... 

It is essential that while setting the conditions for the differential mass-balance equation we did not define the function of the excess adsorption isotherm. We can now use the expression (2-46) for measurement of the model independent excess adsorption values. It is convenient to use it for the study of the adsorption behavior of binary eluents [22]. 

Integration of function (2-46) allows the calculation of the excess adsorption values [22] ... 

Integration of this dependence through the whole concentration range actually allows the calculation of the column void volume, or the total volume of the liquid phase in the column. Since excess adsorption of pure component is equal to 0 (F(0) = r(lOO) = 0), then... 

Since the measurement of the excess adsorption isotherm of a component in the binary system does not require a priori introduction of any model, it is possible to consider the excess adsorption isotherm as being model-independent (within a framework of adsorption process) and it is possible to derive the properties of the adsorbed layer on the basis of consideration of... 

Figure 2-8. Typical excess adsorption isotherm of acetonitrile from water on the surface of reversed-phase silica.

In another words, the negative slope of the excess adsorption isotherm in the linear region is equal to the volume of adsorbed layer, which was derived from the consideration of the adsorption process and not from a prior introduction of the model. A similar expression was derived by Everett [27]. 

The analysis of experimental excess adsorption isotherms using equation (2-50) had shown unusual results [22]. The adsorbed layer thickness of acetonitrile adsorbed from water on different types of reversed-phase adsorbents calculated as the ratio of adsorbed layer volume and adsorbent surface area appears to be on average equal to 14 A, which is equivalent to approximately five monolayers of acetonitrile molecules adsorbed on the hydrophobic surface. At the same time, the adsorbed layer thickness of methanol adsorbed from water on the same adsorbents is equal to only 2.5 A, which is equivalent to the monolayer-type adsorption. 

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