Entropy Effects in Phase Distribution Porous Media

The second factor governing the solvent power of an SF is the state of compression of the gas, specifically the gas density. The critical role of density (as opposed, for example, to pressure) in SF solubility was first discussed at some length by this author and his colleagues [25], who showed that pressures of M000 atmospheres are needed to gain liquid-like solvent power. Density effects have recently been discussed by a number of authors [12,19-21]. 

The distribution coefficient K for species i partitioning between a porous matrix and bulk solution is 

The exclusion process described above occurs because the effective volume of occupancy of the cylinder is reduced below its true volume. (A reduction in entropy naturally accompanies the shrinkage in effective volume.) The distribution coefficient (in the absence of disturbing forces) is simply the volume ratio 

If dc is replaced by Ms, where s is the wall area of the capillary per unit volume of pore space, we get 

This expression is more widely applicable than Eq. 2.43 because few pores are true cylinders and dc, but not 5, loses meaning for noncylindrical pore geometries. Equation 2.44 can consequently be used as an approximation for other pore shapes and even for more complex pore space. For example, Eq. 2.44 proves to be exactly applicable to long pores of square cross section [27] Eq. 2.43 cannot be applied without arbitrarily defining an apparent pore diameter to replace dc. For any given pore geometry, s l is proportional to mean pore size. 

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ENTROPY EFFECTS IN PHASE DISTRIBUTION POROUS MEDIA 31... 

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