Molecular mechanisms of osmosis.

Osmosis across a semipermeable membrane is usually treated in terms of thermodynamics, but the equations for osmosis can also be derived from kinetic considerations. Since fewer solvent molecules bombard the semipermeable membrane from the solution side, a kinetic pressure difference (osmotic potential) is generated into pore openings. Intermolecular forces cancel each other and do not affect the osmotic potential. On the other hand, osmotic flow is dependent on intermolecular cohesive forces permitting the generation of large negative pressures in the membrane pores. Osmosis is therefore a unique property of liquids, whereas intermolecular cohesive forces do not affect diffusion. Osmotic pressure up to 180 atm can be correctly determined from the reduction in saturated vapor pressure above the solution because osmotic pressure and reduction in vapor pressure to some extent are analogous phenomena. Osmotic pressures up to 180 atm may also be correctly determined from kinetic considerations by accounting for binding between solvent and solute molecules (4-5 water molecules per solute molecule for sucrose solutions).