Pair-sites, competitive adsorption

In addition to the observation that the total uptake was only about 20% of the BET monolayer, in agreement with the previously discussed gravimetric studies, the data showed that when the CO2 pressure was increased at constant CO pressure, the amount of adsorbed CO decreased. Similarly, increasing the pressure of CO decreased the amount of adsorbed CO2. These results are consistent with adsorption on anion-cation pair-sites, where CO adsorbs on a cation and interacts with a neighboring anion, and where CO2 adsorbs as a bi dentate carbonate species. For competitive adsorption on a fixed number of surface sites, the coverages are given by the following expressions ... 

The idea of competitive adsorption on pair-sites has also been used to describe the interaction of Ho and H2O with magnetite ( ). When isotherms were collected using M2/H2O mixtures following the same approach as discussed above, it was found that the data fit a model where H2 adsorbed dissociatively and H2O adsorbed associative-ly, with both species competing for pair-sites. These studies were conducted at water-gas shift reaction temperatures (e.g., 650 K) and as for the adsorption of CO and CO2, only a fraction of the magnetite surface was capable of adsorbing H2 and H2O. 

Polar solvents may interact strongly with a mineral oxide surface. In principle, the adsorption of die solvent must be considered. Claesson [13] studied the adsorption of fatty acids by sihca from solvents of various polarities. The results show that polar solvents compete with the solute for available sites on the surface, while nonpolar solvents show little competition. The polarity of the solvent is often determined from the measured dielectric properties. Krishnakumar and Somasundaran [13] studied surfactant adsorption on to silica and alumina from solvents with various dielectric properties. The aim of the study was to look at the effect of adsorbent and smfactant acidities and solvent polarity on the adsorption properties of the surfactant molecules. They used anionic and cationic surfactants as adsorption probes. The results show that polar interactions control the adsorption from solvents of low dielectric properties while hydrocarbon chain interactions with the surface play an important role in determining adsorption from solvents of higher dielectric properties. It was also found that an acidic surfactant interacts strongly with a basic adsorbent, and vice versa. One should be aware that the polarity of a molecule as measmed from the dielechic properties is not always eorrelated with the ability of the molecules to form ion pairs. For example, dimethylformamide and nihomethane have almost equal dielechic constants. However, the extent of ion pairing in nihomethane is much greater than that in dimethylformamide. Thus, the solvent acidity and basicity are the physical properties which can best characterize the ability of the solvent to compete with the solute for available sites on the mineral surface. 

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