Adsorption potential energy effects

Two computer programs for determination of i, vn, and ( ) are listed in Appendices II and III of reference [9]. The former accounts for adsorption potential energy effects. The latter neglects these contributions. 

The parameters /min and /max are the lower and upper limits, respectively, for the inicropore size range. The lower limit / min is for a pore that has the same adsorption potential energy as that of a single lattice layer or for a pore that has zero potential energy. The volume of pores having sizes between these two values of / min is very small, and hence either value of / mm will suffice. The upper limit /-max is cut off at 3.5a for convenience, since the adsorption potential energy in pores with halfwidths larger than this value is effectively zero,... 

The trapping efficiency of polymeric, microporous adsorbents [e.g., polystyrene, polyurethane foam (PUF), Tenax] for compound vapors will be affected by compound vapor density (i. e., equilibrium vapor pressure). The free energy change required in the transition from the vapor state to the condensed state (e.g., on an adsorbent) is known as the adsorption potential (calories per mole), and this potential is proportional to the ratio of saturation to equilibrium vapor pressure. This means that changes in vapor density (equilibrium vapor pressure) for very volatile compounds, or for compounds that are gases under ambient conditions, can have a dramatic effect on the trapping efficiency for polymeric microporous adsorbents. 

Effect of Phase-Boundary Potential on Adsorption Free Energy... 

The interaction between the adsorbed molecules and a chemical species present in the opposite side of the interface is clearly seen in the effect of the counterion species on the HTMA adsorption. Electrocapillary curves in Fig. 6 show that the interfacial tension at a given potential in the presence of the HTMA ion adsorption depends on the anionic species in the aqueous side of the interface and decreases in the order, F, CP, and Br [40]. By changing the counterions from F to CP or Br, the adsorption free energy of HTMA increase by 1.2 or 4.6 kJmoP. This greater effect of Br ions is in harmony with the results obtained at the air-water interface [43]. We note that this effect of the counterion species from the opposite side of the interface does not necessarily mean the interfacial ion-pair formation, which seems to suppose the presence of salt formation at the boundary layer [44-46]. A thermodynamic criterion of the interfacial ion-pair formation has been discussed in detail [40]. 

The adsorption of gas molecules on the interior surfaces of zeolite voids is an ionic interaction with a characteristic potential energy called the heat of adsorption. The molecular adsorption process results in an exothermic attachment of the gas molecules to the surface of the voids, and is characterized by a high order of specificity. Zeolites exhibit a high affinity for certain gases or vapors. Because of their "effective" anionic frameworks and mobile cations, the physical bonds for adsorbed molecules having permanent electric moments (N2, NH-j, H20) are much enhanced compared with nonpolar molecules such as argon or methane. 

Figure 4 Illustration of the steering effect on a potential energy surface with a coexistence of purely attractive and repulsive paths towards dissociative adsorption. Three typical trajectories corresponding to the low, medium and high kinetic energy regime are included.

The enhancement of the potential energy in micropores can be directly assessed by microcalorimetric measurements of Aads li. Moreover, a comparison between N2 and Ar adsorption allows one to distinguish between the enhancement due to the confinement in micropores and that due to specific adsorbent-adsorbate interactions. Both effects are manifested in the low-pressure range of the nitrogen isotherm, but the specific interactions are virtually absent with argon. 

A relatively constant Tafel slope for reactions not involving adsorption, and those involving adsorption with complete charge transfer across the double layer, distorted by second order effects, may also be explained in terms of a non-Franck-Condon process. Since adsorbed intermediates in charge transfer processes also show adsorption energies depending on potential in the same way as the potential energy barrier maxima, these should also follow the same phenomena. 

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