Hydrotropes as Solvents for Extraction and Separation

Hydrotropes are water solutions, most frequently but not always of ionized substances, with concentrations of at least 1 mole/liter that increase the solubility of organic solutes which would have very poor solubility in water alone. For practical reasons, useful hydrotrope forming materials need to be inexpensive. Typical hydrotrope forming substances are:

• aromatic sulfonate salts
• aromatic sulfonic acids
• salts of benzoic acid and substituted benzoic acid
• glycols
• urea
• 4-isopropyl benzenesulfonic acid calcium salt
• 2,4-dimethyl benzenesulfonic acid sodium salt 40%
• p-toluenesulfonate sodium
• ethylene glycol monobutyl ether O-sulfonate potassium
• potassium salicylate

One can have a partitioning of substrate compounds between the hydrotrope and an apolar solvent such as heptane, cyclohexane, and methylcyclohexane. Use of these very non-polar solvents increases the proportion of the substrate extracted into the aqueous phase because the substrate has limited solubility in the alternative hydrocarbon phase.

An example from recent literature is provided:

 Hydrotropic separation of mixtures of o/p-hydroxy acetophenone

KOPARKAR Y. P. ; GAIKAR V. G. ;

Separation Science and Technology  (Sep. Sci. Technol.) 2004, vol. 39, n^o16, pp. 3879-3895 

A new extractive separation technique has been developed for the separation of o-/p-hydroxy acetophenone (HAPs) using hydrotropy. Hydrotropes are freely water-soluble organic salts, which enhance the solubility of otherwise water-insoluble or sparingly soluble organic compounds in aqueous solutions. The ability of hydrotropes to differentiate even isomeric organic compounds is explored in this extractive separation. o/p-HAPs were extracted from their solutions in organic solvents of different polarities using aqueous solutions of hydrotropes. The solvent nature has a significant effect on the selective extraction of both phenols. The combination of heptane and aq. Na-p-toluenesulfonate (at least 194 g/1000 g. water) solution gave almost pure p-HAP in the aqueous phase, whereas with chloroform as the solvent, it was possible to retain with complete selectivity p-HAP in the chloroform layer.

Any organic solvent can be used as the 2^nd phase in a separation using a hydrotrope so long as that solvent is not strongly soluble in the hydrotrope. Such a combination can be used particularly when one only wants to remove a particular element of a mixture ie taking a small amount of impurity into the hydrotrope phase. This could be particularly used to purify a material with less than a percent of a particular impurity.  When the impurity seems to be partially soluble in water but not sufficiently soluble to remove it with a practical amount of washing, resorting to a hydrotrope could solve the problem. 

The use of hydrotropes for extraction is an example of ‘salting in’ when it uses a concentrated solution of an inexpensive hydrophobic salt.

Another possibility is that the solid crude substance could be partially dissolved in the hydrotrope solution.

The solubilization capacity of the hydrotrope is a strong function, usually exponential, of the hydrotrope's concentration, and mere dilution with water is enough to recover the dissolved materials.

Hydrotropes can be expected to be useful to replace water when it is used in an aqueous trituration of an impure solid mixture as in ‘swish’ chromatography. This can be expected to be advantageous when very little of either the predominant component or the minor impurities dissolves in water alone.