"All Pe and SD(Pe) are

in units of 1G-

6 cm/s

; (nd) = compound


detected in the acceptor



is missing in the cyclodextrin system. Some molecules (e.g., metoprolol, carba-mazepine) may have the suitable shape to take advantage of strong cyclodextrin binding, and thus indicate substantially increased permeabilities.

7.7.12 Effects of Buffer

Gutknecht and Tosteson [535] considered the effect of buffer on the transport of salicylic acid across a single bilayer (BLM). Buffers affect the magnitude of the pH gradient formed in the unstirred water layer as the result of the diffusion of ionizable permeants. (This is in addition to bulk solution pH gradient conditions formed by the added buffers.) In turn, the pH at the membrane-water interface affects the concentration of the uncharged (membrane-permeant) species, and thus contributes to the magnitude of the permeant concentration gradient in the membrane phase. The gradient pH permeation cell considered in the abovemen-tioned study [535] (unbuffered in Fig. 7.52a or buffered in Fig. 7.52b) consisted of a pH 3.9 donor solution, a membrane, and a phosphate buffered acceptor solution. The flux (10~8 molcm~2 units) was measured to be 0.09 in the unbuffered solution and 3.9 in the buffered solution. The buffer attenuates the pH gradient in the donor-side unstirred water layer and causes the pH at the donor-side surface of the membrane to be 4.81, (Fig. 7.52a) compared to pH 7.44 (Fig. 7.52b) in the unbuffered donor solution. With the lower pH, the fraction of uncharged salicylic acid at the membrane-water interface is higher, and so transport is increased (43 times), over the condition of the unbuffered solution.



Unstirred water layer

G.15 M NaCl

K in

Unstirred water layer

40 mM salicylic acid Pu 6X10-4 cm s-1, Po 0.7 cm s-1

0.15 M phosphate buffer Net flux 0.09 x 10-8mol cm-2s-1


0 0

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