The “Multiple Partition Coefficient” Hypothesis in Relation to Permeability

Abstract

Irwin's “multiple partition coefficient” theory 1 has been examined with reference to the adequacy of its theoretical and experimental bases. This theory proposes that the rate of diffusion of a substance from (A)/to (C) in a system: aqueous phase (A)/ non-aqueous phase (B)/ aqueous phase (C) depends upon the partition coefficients between adjacent phases. Suppose the diffusing substance to be a dye originally dissolved in (A) : with time its fugacity in (B) will approach but not exceed that in (A) (which we may assume to be kept constant). Its stoichiometric concentration in (B) will assume a value which characterizes the partition coefficient, but its fugacity from (B) will be independent of the partition coefficient. The rate of diffusion out of (B) into (C) will depend upon both its fugacity and its stoichiometric concentration in (B), and also of course in (C). The rate of diffusion across either or both phase boundaries may also be affected by local conditions at the interface peculiar to the nature of the 2 phases and the diffusing substance. For these reasons the hypothesis, insofar as it is based on partition coefficients alone, is physically unjustifiable, and we would expect it to have a very limited applicability.

The supposed experimental proof 1 of the hypothesis rests upon parallelism between the relative rates of uptake of a relatively small number of dyes into the sap of living cells of Valonia or Nitella, and their uptake by the “sap” of an artificial cell. This artificial cell consists of a horizontal glass tube bearing 3 upright arms; the horizontal tube is filled with CHCl₃, which separates sea water in one end arm from natural or artificial sap in the other.