Adsorption determination

Measurements of the adsorption of inhibitors on corroding metals are best carried out using the direct methods of radio-tracer detection and solution depletion measurements . These methods provide unambiguous information on uptake, whereas the corrosion reactions may interfere with the indirect methods of adsorption determination, such as double layer capacity measurements", coulometry", ellipsometry and reflectivity Nevertheless, double layer capacity measurements have been widely used for the determination of inhibitor adsorption on corroding metals, with apparently consistent results, though the interpretation may not be straightforward in some cases. 

While all rates of these unimolecular reactions can be fit quantitatively by LH expressions. Equation 11, the heats of adsorption determined from the temperature dependence of the adsorption equillb-rium constant. Equation 14, do not agree with the measured reaction activation energy except for NH3 where = 16 2 kcal/mole. NO... 

Most workers try to explain the final stereochemistry of the OH group on the basis of adsorption. That is, the mode of adsorption determines the resulting stereochemistry, assuming that the addition of hydrogen occurs cis from the surface up to the bottom of the adsorbed carbonyl group. Most investigators seem to assume that the carbonyl is protonated in acidic media and the mode of adsorption of this species is different from the unprotonated species (in neutral and basic media). 

Hohl, H., L. Sigg, and W. Stumm (1980), "Characterization of Surface Chemical Properties of Oxides in Natural Waters The Role of Specific Adsorption Determining the Specific Charge," in M. C. Kavanaugh and J. O. Leckie, Eds., Particulates in Water, Advances in Chemistry Series, ACS 189, 1-31. 

As few as six different kinds of adsorption have been proposed as being responsible for a great variety of hydrocarbon transformations over metal catalysts (14). We fully accept this approach—that the character of primary adsorption determines the structure of the product. One of the main points that will be stressed is that very different reactions may often be concealed behind the expression cyclization. ... 

He concludes that the first (associative) mechanism gives values nearest the observed heat of adsorption determined by Beeck (30), and is therefore accepted as nearest the truth (34) (Qo (calculated) = 42 kcal./ mole Qo (observed) = 58 kcal./mole). Experiments on tungsten and nickel films (Beeck (35), Trapnell (36), and more recent work in Rideal s laboratory) have shown that when ethylene is added to a clean metal surface ethane appears in the gas phase. A self hydrogenation mechanism must be operative and at least in these cases dissociation of ethylene must occur on the catalyst. It is suggested that the calculations might be complicated by the energy of bond strain in the adsorption of an ethylene molecule to the fixed lattice distances of the metal. 

The proposed mechanism of the effect of water can be supported by two other findings (i) the calculations of Maatman et al. [410] revealed that the active sites could be identified with surface silanol groups [Sect. 4.1.2.(a)] and(ii) independent studies of other authors [424—426] showed that silica gel could actually adsorb two layers of water the first layer is strongly chemisorbed whereas the second is less strongly adsorbed and retains much of the character of free water. The standard enthalpy and entropy changes on adsorption determined from kinetic adsorption coefficients, Kr and Kr, for the first and second layer, respectively [411], are consistent with this observation. 

Weak adsorption is understood to be chemisorption simply determined by charge transfer, in Mulliken s sense (137) for instance, adsorption of xenon to transition metals (138) as compared to physical adsorption determined by dispersion forces. In terms of this secondary ensemble effect the influence of alloying is smaller for strong adsorbates than for weak adsorbates, if adsorption on the same sites is considered. 

Figure 1. The isotherm of Platidiam adsorption (determined as Pt(II) content) on polyacrylamide-coated magnetite nanoparticles.

A second set of measurements consists of the so-called heats of adsorption, determined calorimetrically under a variety of conditions considered later. The quantities are usually reported as a function of the amount of adsorbate held on the surface. For both sets of measurements, the fundamental variable representing the quantity of adsorbate is the surface concentration T ns/As. 

J. R. Katzer (University of Delaware, Newark, Del. 19711) I was interested in the comment in your paper that you observed a variation in the rate of sorption with a change in the cation present in the type X zeolite. How much variation did you find, and how were your rates of adsorption determined ... 

Fia. 9. Effect of drying temperature on water content and adsorptivity of aJumina. Drying time, 6 hours. Adsorptivity determined by immersion in Aids vapor at 430°F. and atmospheric pressure. 

Later studies by Garner and his co-workers showed that the fraction of carbon monoxide or hydrogen reversibly chemisorbed at room temperature varied from oxide to oxide. Zinc oxide was shown to be a case where the adsorption of carbon monoxide at room temperature was completely reversible. The heat of adsorption, determined both calorimetrically (5) and isosterically 6), was in the range 12-20 kcal./mole. For several other oxides, however, notably chromia, Mn20s and Mn20s Cr20s, the heat of adsorption of carbon monoxide was higher and the chemisorption was... 

S the potential sites for adsorption, determined by the specific surface area (particle size) and crystallinity of the mineral. 

Two macroscopic methods to design adsorption columns are the scale-up and kinetic approaches. Both methods rely on breakthrough data obtained from pilot columns. The scale-up method is very simple, but the kinetic method takes into account the rate of adsorption (determined by the kinetics of surface diffusion to the inside of the adsorbent pore). The scale-up approach is useful for determining the breakthrough time and volume (time elapsed and volume treated before the maximum allowable effluent concentration is achieved) of an existing column, while the kinetic approach will determine the size requirements of a column based on a known breakthrough volume. 

The model also correctly predicts that the maximum in the NOx conversion is coincident with 100% C3H6 conversion being reached. This can be understood as follows. Oxygen is deposited onto the Pt surface from dissociative O2 adsorption and from NO dissociation. This oxygen is rapidly removed from the Pt surface by reaction with the C3H6-derived species present. The competition between NO dissociation and O2 adsorption determines the NOx... 

A core-flood for adsorption determination consists of injecting a measured volume of surfactant solution containing a nonadsorbing tracer into a brine-saturated core and collecting effluent fractions at the core outlet. Chemical analysis of the effluent samples allows the calculation of an adsorption level based on material balance considerations and also results in a set of effluent profiles for the surfactant and the tracer. In addition to the material balance, adsorption is evaluated by matching experimental effluent concentrations from the core-floods with a convection—dispersion—adsorption numerical model. The model parameters then allow calculation of a complete adsorption isotherm. 

The above analysis, while very tentative, indicates that vapor adsorption determinations are a desirable part of the study of contact angle systems. In fact, through such data, it may be possible to develop a sufficiently valid analysis to allow prediction from them of contact angle values. In any event, the above approach suggests that the fundamental quantity in determining a contact angle is the difference, y sv° ysL > such, and not either quantity separately, in the sense of being viewed as independent. Complications that may arise with a two-component liquid are discussed further below. 

The adsorption of three argon/nitrogen binary mixtures at 310 K and up to 0.6 bar are presented. A continuous, quasi-equilibrium flow technique of adsorptive introduction was used to allow high-resolution isotherms to be obtained. These are compared to differential enthalpies of adsorption determined using adsorption microcalorimetry. 

The surface excess obtained by the second-harmonic generation in the concentration range below the CMC, however, changes with concentration in contradiction to the usual interpretation of surface tension data. Moreover, the absolute values of the adsorption determined by two experimental methods differ by one order of magnitude. These discrepancies were explained by means of the concept of a depth-dependent distribution of surfactant molecules [66]. Different distributions can lead to identical adsorption values. The surface excess determined by the second-harmonic generation can be attributed only to the very top layer, whereas the values obtained from surface tension techniques are apparently more sensitive to the near-surface layer. 

---