Preferential adsorption, multicomponent mixture

Accounting for size differences can also be realized in terms of distribution functions, assuming certain interaction energies. Simply because of size differences between molecules preferential adsorption will take place, i.e. fractionation occurs near a phase boundary. In theories where molecular geometries are not constrained by a lattice, this distribution function is virtually determined by the repulsive part of the interaction. An example of this kind has been provided by Chan et al. who considered binary mixtures of adhesive hard spheres in the Percus-Yevick approximation. The theory incorporates a definition of the Gibbs dividing plane in terms of distribution functions. A more formal thermodynamic description for multicomponent mixtures has been given by Schlby and Ruckenstein ). 

In summary, one can see that separation selectivity for gas and vapor molecules depends on the category of pores (mesopores, supermicropores, and ultramicropores) and on the related transport mechanisms. Either size effect or preferential adsorption effect (irrespective of molecular dimension) is involved in selective separation of multicomponent mixtures. The membrane separation selectivity for two gases is usually expressed either as the ratio between the two pure gas permeation fluxes (ideal selectivity) or between each gas permeation flux measured from the mixture of the two gases (real selectivity). More detailed information on gas and vapor transport in porous ceramic membranes can be found in Ref. [24]. 

Most real protein-containing fluids of interest, such as blood, are multicomponent systems so that the influence of one protein on another may become important. With respect to adsorption, the main consideration is whether adsorbed amounts are in proportion to solution concentration or whether surfaces "select one protein in preference to another. Presumably if proteins act independently of each other one should be able to predict adsorbed amounts in mixtures from relative affinities derived from single protein studies. Although there have been no systematic attempts to make such predictions it seems likely that they would fail. In general it has been found that preferential or selective adsorption occurs so that certain proteins may be enriched in the surface relative to the solution and vice versa. There have as yet been no attempts to determine the properties of protein-surface systems that govern the relative surface affinity of different proteins. More will be said on this topic when adsorption from plasma is discussed. 

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