Chemical Compounds as Hydrotropes

Features

Hydrotropes are small molecules and are typically salts with a negatively charged or anionic region. The negative charge ensures this part of the molecule is very hydrophilic. The remainder of the molecule, by contrast, is usually composed of hydrocarbon rings, which are hydrophobic. The rings are often aromatic, which means they enjoy added stability thanks to the way electrons are shared among the atoms of the ring. High concentrations of hydrotropes are typically required to make other organic compounds dissolve in water.

Surfactants are similar in some respects; like hydrotropes, they reduce the surface tension of water and can help dissolve organic compounds. When compared with surfactants, however, hydrotropes exhibit some interesting similarities and differences. The hydrophilic portion of the molecule generally constitutes a greater portion of the total molecular weight in a hydrotrope. Like surfactants, they reduce the surface tension of water, albeit to a lesser extent. Additionally, surfactants usually feature a hydrocarbon tail rather than the fused or aromatic rings found in hydrotropes.

The mechanism by which hydrotropes increase the solubility of organic compounds is incompletely understood. According to a 2011 article in the journal Soft Matter, it seems likely they form aggregates around the solute, although it's also possible they change the structure of the solvent in a way that makes it easier for the solute to dissolve. Generally a minimum hydrotrope concentration is required for these effects, and this concentration is generally higher than for surfactants.

Industry employs some hydrotropes as adjuncts to surfactants, adding the hydrotrope to fine tune the surfactant solution and lend it desirable physical properties. Sometimes they are used on their own instead. Additives like other salts can sometimes reduce the concentration of hydrotropes required for the solute to dissolve. Alkyl hydrotropes are also an interesting group of compounds midway between surfactants and hydrotropes in terms of their structure and properties.