Impermeability Towards Solvents

The membrane is critically important in osomometry. Selection of a membrane involves reconciliation of high permeability toward the solvent with virtual impermeability to the smallest polymer molecules present in the sample. Membranes of cellulose are most widely used. Commercially Regenerated cellulose film is a common source. The undried gel cellophane film is often preferred, but the dry film may be swollen in water (or in aqueous solutions of caustic or zinc chloride ) to satisfactory porosity. Useful cellulose membranes may also be prepared by denitration of nitrocellulose films/ and special advantages have been claimed for bacterial cellulose films. The water in the swollen membrane in any case may be replaced by a succession of miscible organic solvents ending with the one in which osmotic measurements are to be made. Membranes of varying porosity may be... 

Similar observations of the botanist Pfeffer, who had described the shrinking of plant cells placed in a concentrated medium, attracted the attention of the chemist Van t Hoff, who was working on the kinetic theory of gases. The results of both experiments were explained by proposing that water molecules move from the more concentrated solution toward the more dilute one. This movement, however, could be demonstrated only if the two solutions were separated by a membrane permeable to the solvent but impermeable to the solute. Membranes with such restricted selective permeabilities were called semipermeable. 

Here, we discuss the analytical solution of a reverse osmosis process in a channel flow situation. Let us assume that the velocity profile is fiilly developed at channel inlet and the channel wall membrane is totally impermeable to the solute R =1). The permeation velocity of the solvent brings the solute toward the wall resulting in an equilibrium concentration, c, at close to the wall (see Figure 4.19). 

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