Osmosis

After our papers on seed storage were rejected several times, we found that our design approach for seed storage warehouses was different from published works on the theory of osmosis.   We had several warehouses installed around the country.   All installations  are still working.   They are maintaining temperature and relative humidity at design conditions.    Either the textbooks on osmosis were in error or  we were in deep trouble.    So, we studied osmosis.     

We wrote the theory of osmosis and reverse osmosis about 20 years ago.   After carefully reviewing osmosis publications and applying fundamental theories to the osmosis experiment,   we showed that water flow across a membrane was related to the water vapor pressure difference.     Pore size in the membrane prevented the sugars in the wine from flowing into the pure water but allowed a net flow of water into wine.    Early osmosis experiments were successful with a sugar water solution because sugar molecules are much larger than water molecules and the membrane pore size was not tightly controlled.   

Two Step Process

Osmosis Experiment

The osmosis experiment is really a two-step process.  Initially, there is zero net flow when pure water is on both sides of the membrane.. Next, step 1, sugar added to one side lowers the effective water vapor pressure in the sugar solution.   Net flow of water then occurs from the pure water side across the membrane into the sugar water solution.  From higher water vapor pressure to lower vapor pressure.   Notice that this is not a bulk flow process.    Applied pressure is the same on both sides of the membrane.     Finally, step 2,  pressure is applied to the sugar water solution.    This also increases the effective water vapor pressure.  Pressure is increased until water flow stops.    The effective water vapor pressure in the sugar water solution then,  equals the water vapor pressure of pure water. 

Further pressure increase on the sugar solution causes a net flow of water from the sugar water to the pure water side of the membrane.   This is called reverse osmosis;  it is used to generate pure water from polluted water.    

Thus, we showed that osmosis water flow is simply caused by the water vapor pressure difference across a membrane.    Pore size in the membrane allows water molecules to pass but stops larger molecules.  

 Water vapor pressure can be increased by increasing applied pressure or by heating the water.        Our efforts have  reduced a complicated physical experiment to its fundamental principles.   We note that our efforts today were much easier than  in 1748 when Nollet observed the first osmosis experiment or in the 1890’s when vant Hoff applied thermodynamics to the experimental relation between osmotic pressure and sugar solution concentration.   

 We have explained the osmosis experiment in terms of fundamental thermodynamics.   It is important that we try to explain all physical processes in terms of fundamental principles.  

©   Larry Howlett     HTMD Engineering     2020

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