Determination of the thickness of the unstirred layers in the moving-drop method for measuring aqueous-nonaqueous interfacial transport rates.

A technique for measuring the thickness of the unstirred layer in the aqueous phase surrounding a rising octanol drop is described. Measurements of the rate of movement of both acetic and valeric acids from the drop into buffered solutions were made using an apparatus previously described (Miller, D.M. (1986) Biochim. Biophys. Acta 856, 27-35). The rate of movement of valeric acid was shown to be dependent on the pH of the buffer while that of acetic acid was not. The thickness of the unstirred layer in the buffered solution (w.u.l.) was found to be 7.0 microns, and the thickness of the unstirred layer in the drop (o.u.l.) was estimated to have an upper limit of 4.6 microns. The rate constant for the movement of valeric acid from the octanol phase to the buffer, kow, has a value restricted to the range 21 to 46 microns s-1 while that for movement in the opposite direction, kwo, ranges from 600 to 1300 microns s-1. The permeability of the w.u.l. is 120 microns s-1, while that of the o.u.l. has a lower limit of 37 microns s-1. Thus diffusion through the w.u.l. was not rate-limiting during movement into the drop, but was during movement outward. The rate of uptake of acetic acid was found to be 39 microns s-1 and was independent of pH, from which it was concluded that the resistance of the w.u.l. was negligible for this compound.