PPh₄Cl in aqueous solution - the aggregation behavior of an antagonistic salt

The initial definition of hydrotropy by Neuberg in 1916 describes a hydrotrope as a molecule which enhances the solubilization of hydrophobic substances in water. Sodium dodecyl sulfate (SDS) and sodium xylene sulfonate (SXS) are typical representatives fulfilling this old definition. They are either surfactants with a critical micellar concentration (CMC) or hydrotropes in the current sense of the term, showing a minimum hydrotrope concentration (MHC), respectively. In the present contribution, we consider the antagonistic salt PPh₄Cl as a hydrotrope. Surface tension measurements and solubilization experiments on a hydrophobic dye confirm the solubilization behavior of PPh₄Cl, which is in-between the one of SDS and SXS. With the help of scattering techniques (DLS, SLS, SAXS), NMR and conductivity measurements, we show that in contrast to SDS as a hydrotrope with an inherent CMC, PPh₄Cl does not exhibit mesoscale aggregation. Therefore, PPh₄Cl can be classified rather as a hydrotrope in the modern sense, with an inherent MHC just as SXS.