Surface heterogeneity nonpolar

Many simple systems that could be expected to form ideal Hquid mixtures are reasonably predicted by extending pure-species adsorption equiUbrium data to a multicomponent equation. The potential theory has been extended to binary mixtures of several hydrocarbons on activated carbon by assuming an ideal mixture (99) and to hydrocarbons on activated carbon and carbon molecular sieves, and to O2 and N2 on 5A and lOX zeoHtes (100). Mixture isotherms predicted by lAST agree with experimental data for methane + ethane and for ethylene + CO2 on activated carbon, and for CO + O2 and for propane + propylene on siUca gel (36). A statistical thermodynamic model has been successfully appHed to equiUbrium isotherms of several nonpolar species on 5A zeoHte, to predict multicomponent sorption equiUbria from the Henry constants for the pure components (26). A set of equations that incorporate surface heterogeneity into the lAST model provides a means for predicting multicomponent equiUbria, but the agreement is only good up to 50% surface saturation (9). 

As we discussed in Sec. VI,1 physical adsorption on charcoal and on metal surfaces is caused by the polarization of the adsorbed molecules in the electronic field over the surface of the conducting adsorbent (Sec. V,7), together with the nonpolar van der Waals forces between the adsorbent and the adsorbed molecules (Sec. V,2). As mentioned in Sec. V,12, the magnitude of the polarization of the adsorbed molecules by the electronic field is not seriously influenced by so-called active spots or by surface heterogeneity. The contribution by the nonpolar van der Waals forces, however, is more influenced by a heterogeneous character of the surface of the adsorbent. As those forces cooperate and as the surface of a metallic... 

The actual value of the double-layer capacitance depends on many variables including electrode type, electrochemical potential, oxide layers, electrode surface heterogeneity, impurity adsorption, media type, temperature, etc. [1, pp. 45-48]. Capacitance of the double layer also largely depends on the intermolecular structure of the analyzed media, such as the dielectric constant (or high-frequency permittivity), concentration and types of conducting species, electron-pair donicity, dipole moment, molecular size, and shape of solvent molecules. Systematic correlation with dielectric constant is lacking and complex, due to ionic interactions in the solution. In ionic aqueous solutions with supporting electrolyte ("supported system") the values of -10-60 pF/ cm are typically experimentally observed for thin double layers and solution permittivity e - 80. The double-layer capacitance values for nonpolar dielec-... 

Sorption of nonionic, nonpolar hydrophobic compounds occurs by weak attractive interactions such as van der Waals forces. Net attraction is the result of dispersion forces the strength of these weak forces is about 4 to 8 kj/mol ( 1 2 kcal/mol). Electrostatic interactions can also be important, especially when a molecule is polar in nature. Attraction potential can develop between polar molecules and the heterogeneous sod surface that has ionic and polar sites, resulting in stronger sorption. 

In Sec. V,1 we discussed the influence of crevices, cavities, the inside of cracks, recessed parts of the surface, and especially the inside of capillaries. In all these active spots for nonpolar van der Waals forces the adsorbed molecules can find far more direct neighbors than on a plane surface, and consequently the heat of adsorption is far higher in these spots than on plane surfaces. Owing to their structure many dielectric adsorbents, adsorbing molecules with nonpolar van der Waals forces show a rather heterogeneous distribution of adsorption sites of various strengths. If this were not the case, no smooth adsorption isotherms would be found, but isotherms showing sudden jumps, separated by hori-... 

The forces between ions and metal surfaces, discussed in Sec. V,3 are far less influenced by active spots. Those spots that are active for nonpolar van der Waals forces are not active here. According to the simplified picture described in Sec. V,3, all crystallographic faces should give the same attraction if the equilibrium distance r0 were the same. This distance, however, will not be the same and for this reason as well as because of other minor differences, we may expect the actual surfaces also to be heterogeneous with respect to this contribution of adsorption forces though quantitatively far less outspoken than for the nonpolar van der Waals forces. 

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