Sink Condition in Acceptor Wells

In Section 7.7.5.4, we discuss the effects of additives in the acceptor wells that create a sink condition, by strongly binding lipophilic molecules that permeate across the membrane. As a result of the binding in the acceptor compartment, the transported molecule has a reduced active (unbound) concentration in the acceptor compartment, cA(t), denoted by the lowercase letter c. The permeability equations in the preceding section, which describe the nonsink process, are inappropriate for this condition. In the present case, we assume that the reverse transport is effectively nil that is, CA(t) in Eq. (7.1) may be taken as cA(t) 0. As a result, the permeability equation is greatly simplified  

Note that we call this the apparent permeability, since there is a hidden assumption (unbound concentration is zero). 

When very insoluble samples are used, sometimes precipitate forms in the donor wells, and the solutions remain saturated during the entire permeation assay. Equations (7.20) and (7.21) would not appropriately represent the kinetics. One needs to consider the following modified flux equations [see, Eqs. (7.1) and (7.2)] 

The donor concentration becomes constant in time, represented by the solubility, S = C/)(0j = Reverse flux can still occur, but as soon as the sample 

Ordinarily it is not possible to determine the membrane retention of solute under the circumstances of a saturated solution, so no R terms appear in the special equation [Eq. (7.25)], nor is it important to do so, since the concentration gradient across the membrane is uniquely specified by S and CA (t). The permeability coefficient is effective in this case. 

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