Physical model approach in the study of the transport of alkyl amines across a silicone rubber membrane in a two-chamber diffusion cell.

The transport of hexylamine and octylamine through a silicone rubber membrane was studied as a function of buffer (phosphate) concentration and pH. The results were interpreted using a physical model which assumed a steady-state rate of transport and which accounted for the simultaneous diffusion and rapid equilibrium of all the aqueous species, the possible diffusion of both the amine and its protonated form through the membrane, and the effect of a stagnant aqueous diffusion layer on each side of the membrane. The following conclusions were reached: The thickness of the aqueous diffusion layer is approximately 100 micron, which is about the same as that previously measured for benzoic acid in this system. Transport of octylamine at pH greater than or equal to 10 is approximately 90% aqueous diffusion layer controlled, whereas hexylamine is approximately 50% aqueous diffusion layer controlled at high pH. The membrane permeability of octylamine is approximately 15-20 times that of hexylamine. This gives an incremental pi constant for the partition coefficient of approximately 0.61, as compared with the previously reported value of 0.56. At low pH (less than or equal to 5), the transport of the protonated species becomes important. The membrane permeabilities of these ammonium ions are about four or five orders of magnitude less than the membrane permeabilities of the corresponding amines. The membranes were examined at 30,000X with the scanning electron microscope, and no evidence of holes was found.