Problem of boundary layers in the exchange diffusion of water across bimolecular lipid membranes

The correct value of the permeability of artificial lipid bilayers to water, as given by the diffusion of tritiated water, is in general difficult to obtain owing to the presence of relatively thick boundary layers. It is shown that forced convection usually ruptures the membrane before the boundary layers become insignificant. Three other approaches to the problem are described: the thickness of the equivalent stagnant layers has been estimated explicitly by the use of glass and cellophane membranes of known permeability, and the resulting data have been used to correct the apparent permeability of the lipid membrane, the possibility of determining the true permeability from the measurement of the apparent permeability and the time lag of the transient state has been examined and shown to be impracticable in the present systems, and the true lipid membrane permeability has been calculated from the diffusion of tritiated water in known linear velocity fields in the aqueous phases. The first and third methods give permeability coefficients in agreement with each other which are, within experimental error, equal to the osmotic permeability coefficient.