Solvent dynamics in a reverse micellar water-pool: a spectroscopic investigation of DDAB-cyclohexane-water systems

We have measured the hydrogen bonded structure and sub-ns relaxation dynamics of water molecules encapsulated in the DDAB-cyclohexane (Cy)-water reverse micellar (RM) water-pool dependent on water concentration (w(0) = [water]/[DDAB]) and temperatures. The interfacial film of DDAB-Cy undergoes significant alteration upon addition of water as the microscopic phase changes from cylindrical aggregates to discrete droplets which is in contrast to the conventional RM systems. FTIR spectroscopy in mid-infrared (MIR) and far-infrared (FIR) regions suggests the encapsulated water molecules to undergo a transition with increasing w(0) towards a bulk-like behavior. Time resolved fluorescence spectroscopy using Coumarin-500 as the fluorophore reveals a decrease in solvation time constant with increasing w(0) as well as with increasing temperature, a behavior consistent with conventional RM systems. The temperature dependent relaxation dynamics is found to follow an Arrhenius type behavior with a value for E(act) in the range of 2.5-3 kcal mol(-1) for all the studied systems. Our results show that phase modification has a marginal effect on the relaxation dynamics.