Water and ion flow across cellulosic membranes

Abstract

The primary objective of this investigation has been to explain why cellulose acetate behaves as a semipermeable membrane in saline water. To explain this phenomenon, two different mechanisms for the transfer to water and ions through cellulose acetate membranes were formulated. Those ions and molecules that cannot enter into hydrogen bonding with the membrane are transferred by hole‐type diffusion. The rate of diffusion appears to be governed by a water‐cellulose acetate structure. The reaction between water and the cellulose polymers to form bound water regions is favored by compressing the membrane. As pressure is applied on the membrane, more bound water is produced, which causes the rate of hole‐type diffusion to decrease. On the other hand, those ions and molecules that can associate with the membrane and are transported through it by alignment‐type diffusion. The formation of the water‐cellulose acetate structure does not appreciably diminish the diffusion rate of water through the membrane. Cellulose acetate begins to behave as an effective semipermeable membrane in saline water when it is compressed sufficiently to retard greatly the diffusion of NaCl. Several types of experiments were conducted to support these hypotheses. The most important evidence was obtained from resistance experiments. The electrical resistances of specific ions was measured across cellulose acetate at various pressures by using permselective membranes to prevent migration of the ion of opposite charge. It was observed that the rate of diffusion of those ions that cannot combine with the membrane actually does decrease as the membrane is compressed. The rate of diffusion of H 3 O+, which can enter into hydrogen bonding with cellulose acetate, is much higher and is not appreciably reduced as the membrane is compressed. These resistance‐pressure relationships are correlated with the semipermeability of the cellulose acetate.