Additive molecules, selective adsorption

Selective surface diffusion is governed by a selective adsorption of the larger (nonideal) components on the pore surface. The critical temperature, 7), of a gas will thus indicate which component in a mixmre is more easily condensable. The gas with the highest T will most likely be the fastest permeating component where a selective surface flow can take place. Eor a mixed gas an additional increase in selectivity may be achieved if the adsorbed layer now covering the internal pore walls restricts the free pore entrance so that the smaller nonadsorbed molecules cannot pass through. 

Molecular probe analyses of the Type lib materials showed that they adsorbed significant quantities of CO2 and n-butane, but nearly zero amounts of the two larger probe molecules (see Table 3, Figures 7a and b). Because of this selective adsorption, these materials behaved similarly to the 5A size molecular sieves, but without the sharp cutoffs displayed by zeolites. However, it is remarkable that these Type lib materials adsorbed more of n-butane than the Type Ila materials. It may be that the addition of the fine oxidic particles led to interstices, which did not form in the preparation starting with the metal alkoxides. These interstices may provide adsorption sites for the larger molecules. Further investigations will be conducted to determine the structural factors that give rise to these different adsorptive properties. 

Flotation Additives. Molecules adsorb selectively onto particles of crushed ore, thereby rendering them hydrophobic. They can then be separated by adsorption in a foam. An example is the xanthates, used with lead and copper ores ... 

It is shown in a subsequent section that in a water solution of oligomer salt containing a distribution of molecular sizes, addition of latex results in selective adsorption of smaller molecules at the particle-water interface. Use is made of this in refining crude fractions, as mentioned in the preceding section ... 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

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