Use of Hydrotropes Hydrotropic Solubilization

Hydrotropes have many features in common with micelles. The most important is the presence of a minimum hydrotrope concentration (CHC) analogous to the minimum micellar concentration (CMC) described earlier (Balasubramanian et al., 1989). The most important difference is that in hydrotropes, the dissolved solute is precipitated on dilution, whereas with surfactants dilution leads to emulsification with consequent problems of separation. Another difference is that surfactants show solubility enhancements at low concentrations, usually in the millimolar range, whereas hydrotropic solubilization occurs in the molar concentration range. Yet another difference is that, unlike micellar solubilization which is general and nonselective, hydrotropes do not solubilize all hydrotropic substances and are hence selective. This is obviously an advantage where reactant selectivity is important. 

Possible Mechanisms of Hydrotropic Action Similarity to Salting-In 

Any technique that enhances the solubility of a sparingly soluble solute in a liquid finds immediate application in organic synthesis. Thus research over the 

In the production of certain common dyes, for example, Metanil Yellow or Orange IV, the reactant (diphenylamine) has to be at least slightly soluble. Ordinarily, it is dissolved in strong sulfuric acid, and this objectionable acid is later neutralized with alkali. When diphenylamine is dissolved in a hydrotropic solution and diazotized metanilic acid and NaOH are added, brilliant yellow crystals of Metanil Yellow separate in excellent yield. 

When sulfanilic acid is used instead of metanilic acid, Orange IV is likewise obtained in equally good yield. 

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