Adsorption, types

The present discussion is restricted to an introductory demonstration of how, in principle, adsorption data may be employed to determine changes in the solid-gas interfacial free energy. A typical adsorption isotherm (of the physical adsorption type) is shown in Fig. X-1. In this figure, the amount adsorbed per gram of powdered quartz is plotted against P/F, where P is the pressure of the adsorbate vapor and P is the vapor pressure of the pure liquid adsorbate. 

Type I isotherms are encountered when adsorption is limited to, at most, only a few molecular layers. This condition is encountered in chemisorption where the asymptotic approach to a limiting quantity indicates that all of the surface sites are occupied. In the case of physical adsorption, type I isotherms are encountered with microporous powders whose pore size does not exceed a few adsorbate molecular diameters. A gas molecule, when inside pores of these small dimensions, encounters the overlapping potential from the pore walls which enhances the quantity adsorbed at low relative pressures. At higher pressures the pores are filled by adsorbed or condensed adsorbate leading to the plateau, indicating little or no additional adsorption after the micropores have been filled. Physical adsorption that produces the type I isotherm indicates that the pores are microporous and that the exposed surface resides almost exclusively within the micropores, which once filled with adsorbate, leave little or no external surface for additional adsorption. 

Most corrosion inhibitors are of the adsorption type. In general, these are compounds which adsorb on the metal surface and act to suppress anodic and/or cathodic corrosion processes 147 . 

The activation energies for the two types of reactions may be qualitatively considered. Type A adsorption will have a certain activation energy, x, not defined by our model, which will limit the rate of Type A adsorption. Type B adsorption will also have this activation energy Ea, which may limit its rate. However, as seen from Fig. 8, it will in addition have an activation energy of the electron transfer type, Eb = Ei (in Fig. 1) =0.8 e.v. = 18 Kcal./mole. Thus if E. > Eb, the rates of the two types of adsorption will be equal if Eb > Ea, Type B adsorption will be slower, controlled by an activation energy of about 0.8 e.v. 

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... 

It appears from Table 1 that 2, in which molecular oxygen is oriented along the [110] direction, is the most stable form. Though, 1 and 3 are only about 7-8 kcal/mol more unstable than 2. The Fermi levels for the 1 - 3 are all found at about -11 eV. The Ag-O overlap populations show that the Ag-O bond is weakest in 2 and strongest in 1, whereas the 0-0 overlap populations are nearly identical for the three different adsorption types. 

After maximum coverage of platinum films upon hydrogen adsorption, three desorption peaks have been observed. The same peaks have been found for the alloys, but the relative populations of the various adsorption types were different. Here again, the peak corresponding to the larger heat of adsorption is most influenced, leading to the conclusion that it corresponds to hydrogen atoms bonded to several metal atoms. 

One important aspect of method development in SFE is that the extractability of analytes from sample matrices is strongly dependent upon the nature or type of the sample matrix. The efficiency of the extraction of a discrete set of analytes needs to be optimized for that set of analytes from a standard matrix and also optimized for the extractability of those specific analytes from a particular sample matrix. However, the optimum set of parameters for both of these cases may be distinctly different. This is due to the analytes, depending on the nature of the sample matrix, having an affinity for the physical outside surface of the matrix or actually being present within the sample matrix particles. This affinity results in either the predominance of a solubility mechanism, a diffusion type mechanism or a physical adsorption type mechanism. Therefore, the optimum extraction conditions will vary if the sample of interest is a polymer, soil or vegetable matrix. 

Figure 4.15 The six types of adsorption isotherms (V = volume adsorbed) Type I shows a monolayer (Langmuir isotherm) types II and III show multilayer adsorption type IV shows first a monolayer, followed by filling of mesopores. The knee in isotherms I, II, and IV, indicated bya blackdot, indicates the point of monolayer formation (point B ).

It is in many cases difficult to decide whether a certain adsorption phenomenon belongs to the physical adsorption type or whether it is a case of chemisorption. If we define physical adsorption as the phenomenon which occurs when the molecules are bound to the surface of the adsorbent by van der Waals cohesion forces in the widest sense of the word, hence... 

All of the above features are connected with the fact that the interaction of these carbons with lithium does not involve intercalation but adsorption-type... 

The rationale of using hybrid simulation here is that a classic diffusion-adsorption type of model, Eq. (2), can efficiently handle large distances between steps by a finite difference coarse discretization in space. As often happens in hybrid simulations, an explicit, forward discretization in time was employed. On the other hand, KMC can properly handle thermal fluctuations at the steps, i.e., provide suitable boundary conditions to the continuum model. Initial simulations were done in (1 + 1) dimensions [a pseudo-2D KMC and a ID version of Eq. (2)] and subsequently extended to (2 + 1) dimensions [a pseudo-3D KMC and a 2D version of Eq. (2)] (Schulze, 2004 Schulze et al., 2003). Again, the term pseudo is used as above to imply the SOS approximation. Speedup up to a factor of 5 was reported in comparison with KMC (Schulze, 2004), which while important, is not as dramatic, at least for the conditions studied. As pointed out by Schulze, one would expect improved speedup, as the separation between steps increases while the KMC region remains relatively fixed in size. At the same time, implementation is definitely complex because it involves swapping a microscopic KMC cell with continuum model cells as the steps move on the surface of a growing film. 

Pore diameter, specific pore volume. A controlled, macroporous silica can be obtained by hydrolytic polycondensation of polyethoxysiloxane, while silica gel structure modification results from thermal or rather hydrothermal treatment. When hydrothermal treatment is carried out with silica of a pore size of 100 A at 250 C and 50 atm for a period of 15-20 h, formation of silica gel with a homogeneous pore size of 900 A is possible. Of coarse, the increase of pore diameter reduced the surface area thereby the Rf. values are also generally increased in the case of adsorption type chromatography. El Rassi et al. 44] studied the effect of water and that of hydrothermal treatment on the activity of silica gel. 

Calibration is necessary for in-situ spectrometry in TLC. Either the peak height or the peak area data are measured, and used for calculation. Although the nonlinear calibration curve with an external standard method is used, however, it shows only a small deviation from linearity at small concentrations [94.95 and fulfils the requirement of routine pharmaceutical analysis 96,97J. One problem may be the saturation function of the calibration curve. Several linearisation equations have been constructed, which serve to calculate the point of determination on the basis of the calibration line and these linearisation equations are used in the software of some scanners. A more general problem is the saturation function of the calibration curve. It is a characteristic of a wide variety of adsorption-type phenomena, such as the Langmuir and the Michaelis-Menten law for enzyme kinetics as detailed in the literature [98. Saturation is also evident for the hyperbolic shape of the Kubelka-Munk equation that has to be taken into consideration when a large load is applied and has to be determined. 

We obtained adsorption kinetic data and isotherm curves by determining the amount of protein adsorbed onto polymers by internal reflection IR spectroscopy which gave good reproducibility (19). The adsorption character was consistent with the Langmuir adsorption type and adsorption vs. time curves showed the expected plateau usually found in macro-molecular adsorption. A competitive adsorption study is being carried... 

Interfacial carrier molecules are presented by their hydrophUic fragments in adjacent aqueous solutions and by their hydrophobic parts in an organic phase of a hquid membrane. Thus, the reaction can be treated as a homogeneous type occurring in an adjacent reaction zone instead of a heterogeneous adsorption-type reaction process. 

Mamyrin (1979), King (1980b), Reimer (1980), Shapiro et al. (1981, 1982), Mooney (1982) and Pirkle and Jones (1983). Particularly intriguing examples have been published by Antropov (1981) of atmospheric methane flux related to petroleum deposits (Fig.5-14) and seismic shock (Figs. 5-15 and 5-16). These measurements were made with adsorption-type gas lasers one type makes point measurements of the sample in an adsorption tube (lskatel-2) the other (Luch) measures the specific gas adsorption along a path length (1-100 m). 

In other cases, stationary phases have been tailored to achieve specific separations. In one case, a new stationary phase was designed to achieve the separation of a particular mixture of volatile priority water pollutants whose separation has posed a real problem (6, 7). A serendipitous finding was that this new stationary phase, DB-1301, also promises to be very useful for the separation of some chlorinated pesticides. Nor have we reached the end of this road an Immobilized form of 2330, with utility for those Interested In dioxins, In positional Isomers of the fatty acids, and In other challenging separations, will soon be available. Fused silica columns with bonded particulate materials, reminiscent of the old PLOT-type columns, are also available. The primary utility of the PLOT-type columns currently available Is for fixed gas analysis, but newer types on the horizon will permit a choice of adsorptive-type separations, partition-type separations, or a combination of both. 

With metal ions, some organic reagents form coloured compounds that are sparingly soluble both in water and in organic solvents. They are either polynuclear complexes, such as formed by phenylfluoron with Sn(IV) or with Ge(IV), or adsorption-type compounds such as those formed by titanium yellow with Mg. In such cases, the absorbance is measured for suspensions of coloured pseudo-solutions stabilized with protective colloids [e.g., gum arabic, gelatine, poly(vinyl alcohol)]. 

The adsorption isotherm of nonionic surfactants are in many cases Langmuirian, much like those of most other highly surface active solutes adsorbing from dilute solutions, and the adsorption is generally reversible. However, several other adsorption types are produced [29], and these are illustrated in Figure 5.7. The steps... 

Rate of Reversible Adsorption Type of Isotherm Q = fads ( ori) - fdes (ri)... 

Kbuta e> butenes, and KH2 are, respectively, the adsorption equilibrium constants for butane, butenes, and hydrogen in a provisional adsorption type expression which accounts for multicomponent adsorption. For the terminal dehydrogenation conditions in which pure n-butane feed was converted to butenes and hydrogen to a very small degree, both butenes and hydrogen terms may be preliminarily neglected. This leads to... 

For the case of the frequency response of a real catalyst in which the number and magnitude of different types of adsorption is not known beforehand, the response may be interpreted by curve fitting to give a distribution of adsorption types versus rate constant. The interpretation of an experimentally determined frequency response curve would not be too dissimilar, in principle, from the interpretation of the output of an infrared spectrum where two or more unknown compounds are to be identified and quantitatively estimated from a single IR scan. 

Each of the observed spectra could be separated into curves caused by two types of adsorption type [Pg.134]

It was assumed that tlie rate constants of elementary steps do not depend on the parameter q (see eq.2.30). The rate constants of the steps in the forward direction (e.g. adsorption type) which correspond to eq. (7.129) contain the term e while the desorption type constants have a term e1". The values of the transfer coefficients a and p are considered to be the same and distribution functions are similar for all surface species. 

---