Relation Between Pressure and Concentration Difference Across Membranes Permeable to Solute and Solvent ∗

Abstract

Summary

1. Experiments are presented which demonstrate, as expected from theoretical considerations, that the classical van't Hoff equation cannot be employed to ascertain the relation between pressure difference and concentration difference across a membrane in the dynamic situation existing in osmotic systems involving membranes permeable to solute as well as to solvent. It is shown that the pressure difference necessary to prevent a net osmotic movement of fluid across a leaky membrane may be smaller by several orders of magnitude than the pressure difference necessary to accomplish the same effect with a semipermeable membrane; i.e., a membrane to which the van't Hoff equation applies. It is also shown that, contrary to what would be expected on the basis of reasoning from the van't Hoff equation, the osmotic transport rate across a leaky membrane is a function not only of the solute concentration difference but also of the nature of the solute molecules. Further, it is demonstrated that net transport of liquid may occur across leaky membranes separating iso-osmotic solutions of different non-electrolytes. 2. In all these cases, the use of equations derived for the equilibrium state across a semipermeable membrane (e.g. the van't Hoff equation) would lead to decidedly erroneous results. 3. These observations demonstrate the use of van't Hoff's equation in a recent analysis of capillary permeability to be incorrect.