Computer simulation studies of water states in perfluoro polyether reverse micelles: Effects of changing the counterion

We performed molecular dynamics computer simulations to study the effect of counterions on the structural and dynamical properties of water pool enclosed by a reverse micelle containing ionic surfactants. Our simulations were performed on a system containing PFPE surfactants, water, and supercritical carbon dioxide. We chose NH4+, Na+, and Ca++ to be the counterions. The results of the simulations showed that structural and dynamical properties of water in the pool created by the micellar system depend on the type of the counterion. We observed that the orientational dynamics of water inside the PFPE reverse micelles was slower as compared to the dynamics in bulk water or in an ionic aqueous solution irrespective of the type of the counterion present in the solution. The calculations showed that water dynamics got slower as we progressed from the NH4+ ion to the Na+ ion and to the Ca++ ion. Finally, we observed the existence of water bridges connecting ions and charged headgroups in our simulations. These bridges contribute significantly into the slow orientational dynamics of water.