Differential permeation, with permeate example

As can be expected, the derivations and calculations become increasingly complicated for multistage and differential permeation. The subject is detailed in the later pages of Hoffman (2003), with examples provided, including systematic spreadsheet calculations. 

The role of the skin as a portal of entry for chiral molecules is becoming an increasingly active and exciting field of research. To date, however, only a few investigations have been focused on the effect of enantiomeric differences of chiral drugs with respect to binding or metabolism in the skin on their transdermal delivery. There may be several sources for any observed enantioselectivity in terms of skin permeation of chiral drugs. For example, the components of the stratum corneum such as keratin and ceramides can serve as sources for chiral discrimination that could result in differential diffusion rates, dependent upon the stereochemistry of the solute. 

One can demonstrate such separation capabilities, even in the closed vessel considered so far, if we focus, for example, on the initial time period in the vessel when t — 0. We can calculate the moles of species A and B in the permeate chamber when f —> 0. For species A, differentiate the expression (4.3.30) for m,f,p with time t to obtain, by Taylor series expansion around f = 0,... 

Gas composition effects can be significant. The concenuation of carbon dioxide in natural gas, for example, can affect its rate of permeation (per unit of partial pressure differential). Data provided by Hogsett and Mazur (1983) indicate that increasing the CO2 concentration from IS to 50% in 200 psia natural gas increases the CO2 permeation rate from about 4.0 to 5.0 (scf)/(ft )(hrK100 psi) with GASEP membranes. 

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