Tannic Acid as a Promising Hydrotrope For Separation and Purification of High-Value Products

Most hydrotropes are made by dissolving organic salts at a concentration of at least 1M in water. Covalently bonded materials do exist that form hydrotropes. The best known is urea. Another inexpensive, non-ionic organic material that is highly soluble in water and that can be expected to promote the dissolution of other organic substances is tannic acid.     
                 
Molecular Formula - C₇₆H₅₂O₄₆
Molecular Weight - 1700
Melting point - 218°C
Water solubility - 1g/ 0.35 ml

Speaking roughly to produce a hydrotrope a chemical must dissolve in water to give a 1M solution. A 1M solution of tannic acid would contain 1700g of organic solid per liter of water. That would be 1.7 gm per milliliter. The solubility of tannic acid in water is 4.88 gm per milliliter. One could achieve a solubility of 2.87M if required in a saturated solution. Tannic acid is a material available in industrial quantities at a practical price. Sigma-Aldrich sells 500 grams for less than $100.00. Considering that only 60 g of urea are needed to produce a 1M aqueous solution that would give an effective hydrotrope and supposing that we provide three times as much tannic acid by weight, that would just be 180 g per liter that would not cost more than $75.00!

The molecule shown in the figure is only one representative (perhaps the major one) of the constituents of the organic mixture called ‘tannic acid’ but if we recognize that it is typical then each molecule can be approximated to contain 25 phenolic groups and 10 ester linkages. The phenolic groups alone would comprise over 15 hydrogen bond acceptors and 25 hydrogen bond donors.

 Opportunity for Undergraduate Chemistry Major Project

Could a hydrotrope of tannic acid selectively extract organic compounds dissolved in organic solvents typically used in industrial process chemistry? Could it simply solve otherwise difficult separation problems in the fine chemicals or pharmaceutical products industries? These are important questions that could be answered by the research for an undergraduate organic chemistry major's project.