Effect of the Cationic Surfactant Moiety on the Structure of Water Entrapped in Two Catanionic Reverse Micelles Created from Ionic Liquid-Like Surfactants

The behavior of water entrapped in reverse micelles (RMs) formed by two catanionic ionic liquid-like surfactants, benzyl-n-hexadecyldimethylammonium 1,4-bis-2-ethylhexylsulfosuccinate (AOT-BHD) and cetyltrimethylammonium 1,4-bis-2-ethylhexylsulfosuccinate (AOT-CTA), was investigated by using dynamic (DLS) and static (SLS) light scattering, FTIR, and H-1 NMR spectroscopy techniques. To the best of our knowledge, this is the first report in which AOT-CTA has been used to create RMs and encapsulate water. DLS and SLS results revealed the formation of RMs in benzene and the interaction of water with the RM interface. From FTIR and H-1 NMR spectroscopy data, a difference in the magnitude of the water-catanionic surfactant interaction at the interface is observed. For the AOT-BHD RMs, a strong water-surfactant interaction can be invoked whereas for AOT-CTA this interaction seems to be weaker. Consequently, more water molecules interact with the interface in AOT-BHD RMs with a completely disrupted hydrogen-bond network, than in AOT-CTA RMs in which the water structure is partially preserved. We suggest that the benzyl group present in the BHD+ moiety in AOT-BHD is responsible for the behavior of the catanionic interface in comparison with the interface created in AOT-CTA. These results show that a simple change in the cationic component in the catanionic surfactant promotes remarkable changes in the RMs interface with interesting consequences, in particular when using the systems as nanoreactors.