The substances that are most commonly associated with the term
hydrotrope are the sodium salts of short alkylbenzene sulfonates, such
as xylenesulfonate and cumenesulfonate. In such compounds, the
anion is amphiphilic, and bulky, and the amphiphilicity is not as pronounced
as for regular surfactants, although they may foam considerably.
Another class comprises water soluble monoethers of ethylene
glycols or higher polyols. Depending on the alkyl chain length, they
can be “simple” co-solvents, hydrotropes, or real surfactants.
Even urea can be considered as a hydrotrope. Urea is of particular
interest because of its biological function and since, like water, it can
act as both a donor and an acceptor of hydrogen bonds. Using spectroscopic
techniques [21], as well as molecular dynamics simulations
[22], it has been demonstrated that urea self-associates into relatively
small clusters, on average 3–4molecules. However, as soon as an organic
compound is added, urea clusters around it, thus fulfilling the abovementioned
definition of a hydrotrope [4••].