One must fully understand the history to appreciate today’s textbook thermodynamic theory of osmosis. Abby Nollett a scientist and a priest discovered osmosis in 1768. He was baffled. Pure water flowed uphill through a membrane into the wine. A considerable pressure applied to the wine was required to stop the flow.
Many others repeated the experiment over the next 118 years… They developed improved instruments, improved membranes, and showed that wine could be replaced with a sugar-water solution. Other solutes also worked. In 1886, van’t Hoff showed that osmotic pressure (pressure required to stop water flow through the membrane into the solution) was directly related to the concentration of the solute (for low concentrations of solute). This was the same as the newly discovered law for a perfect gas. This was the first thermodynamic theory for osmotic pressure. He received a Nobel prize for his work. (December 13, 1901) His Nobel Lecture covered the thermodynamic relationship for osmotic pressure and the importance of osmotic pressure in plant and animal life and in chemistry. Van’t Hoff noted that osmotic pressure was responsible for pushing sap up to the tallest branches of an Oak tree.
This osmotic pressure model did not work for an Oak tree (DrOsmosis.com, May 2019) or for osmosis. The osmotic pressure of pure water is greater than the osmotic pressure of a solution. A thermodynamic model for osmosis was developed with Gibbs Free energy. (Glasstone, Textbook of Physical Chemistry, 1951, 2nd edition, 7th printing). The chemical potential of pure water is greater than the chemical potential of a solution. Osmotic flow of water through a membrane is from high energy pure water to a lower energy solution. There were three separate models to describe the details of osmotic pressure, none were accepted. It was more important to discuss the application of osmotic pressure to chemical and solution properties than to know the details.
This thermodynamic model for osmosis along with van’t Hoff’s model for osmotic pressure is presented in Atkins textbook of Physical Chemistry. (1994, 5th edition, 2nd printing). Frank Borg discusses various models for osmosis and notes that the thermodynamic model uses the concept of chemical potential but does not give any clues to how the process really works. (What is Osmosis? Explanation and Understanding of a Physical Phenomenon, Frank Borg, Jyvaskyla University, 2003)
Today’s automobile provides an engineering analogy for understanding osmosis. The thermodynamic model for a heat engine is the Carnot cycle. The maximum useful work is the difference between the high temperature heat added and the low temperature heat rejected. We must develop the details to make the thermodynamic process produce work that will propel a car. For Osmosis, the details have been provided by nature. If we understand the details, we may be able to make improvements.
We have proposed a detail model.
As pointed out above, throughout history the actual mechanism of osmosis has never been explained. We have proposed a model based on fundamental concepts. (Osmosis: The Molecular Theory. Howlett, 2013)
- We proposed that vapor pressure is the driving force for pushing pure water through a membrane.
- Adding solute reduces water vapor pressure. External pressure increases water vapor pressure.
- Vapor pressure is caused by impact of molecules on a surface. (kinetic theory of gas)
- We proposed that free high-speed molecules exist with in liquids and solids.
- We proposed that these Free high-speed molecules have sufficient energy to escape the bonding forces in liquids and solids.
- We proposed a model for identifying the cut off speed using Maxwell-Boltzmann speed distribution.
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