Physical Model Approach in the Study of the Transport of Alkyl Amines Across a Silicone Rubber Membrane in a Two-Chamber Diffusion Cell

Abstract

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 (a) the simultaneous diffusion and rapid equilibrium of all the aqueous species, (b) the possible diffusion of both the amine and its protonated form through the membrane, and (c) the effect of a stagnant aqueous diffusion layer on each side of the membrane. The following conclusions were reached: (a) The thickness of the aqueous diffusion layer is ~ 100 μm, which is about the same as that previously measured for benzoic acid in this system. Transport of octylamine at pH ~ 10 is ~90% aqueous diffusion layer controlled, whereas hexylamine is ~50% aqueous diffusion layer controlled at high pH. (b) The membrane permeability of octylamine is ~15–20 times that of hexylamine. This gives an incremental π constant for the partition coefficient of ~0.61, as compared with the previously reported value of 0.56. (c) At low pH (~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 30,000 × with the scanning electron microscope, and no evidence of holes was found.