Osmotic Pressure: Conceptions \& Misconceptions from Pfeffer, to Van't Hoff, to Einstein, to Fermi, to the Present

A phenomenological approach to understanding osmotic pressure in terms of competing effects of solute-solvent, solute-solute, and solvent-solvent interactions helps to avoid the misconceptions about osmotic pressure that were held by Van't Hoff, Einstein, Fermi, and many others. It also provides a better understanding of the concept of ``negative'' osmotic pressure and its role in explaining water of crystallization, gel syneresis, and in biological tissue aging and pathological states. Negative osmotic pressure may be conceptualized as due to a transient metastable state wherein the chemical potential of solvent is temporally increased above that of pure solvent in the standard state. Measureable changes in differential osmotic pressure-volume relationships of osmotic systems are readily related to differential changes in solvent chemical potential and number of mols of solvent in a system.