Impact of counterion on water motion in aerosol OT reverse Micelles

We have measured the dynamics of water solvation in two different reverse micellar environments with low water content, w(0) = 1.7. In the water/Aerosol OT (AOT)/isooctane reverse micelles, the characteristic ultrafast bulk water motion is completely absent. In contrast, when the sodium cation normally complexed to the AOT surfactant head group is exchanged for an ammonium cation, solvation dynamics measurements reveal that the water is significantly more mobile. However, the water present in these modified reverse micelles is still less mobile than bulk water. These studies reveal that the Na+ ion in normal AOT reverse micelles is responsible for reducing a large fraction but not all of the water motion in these environments.