Electronic Excited-State Behavior of Rhodamine 3B in AOT Reverse Micelles Sensing Contact Ion Pair to Solvent Separated Ion Pair Interconversion

Abstract

The amphiphile Aerosol OT (1,4-bis(2-ethylhexyl)sodium sulfosuccinate, AOT) forms, in reverse micellar nanoaggregates of water, RM, in isooctane, ion pairs (IPs) with the cationic fluorescent probe dye, rhodamine 3B, (3,6-bis(ethylamino)-9-[2-(ethoxycarbonyl)phenyl]-9H-xanthen-9-ylium, R3B), as either contact ion pairs, CIPs, or solvent (water) separated ion pairs, SSIPs. The ground-state AOT R3B ion pairs' equilibria as well as the dynamics of R3B electronic excited states show the progressive hydration of AOT(-) R3B(+) toward solvent separated ion pairs, SSIPs as the characteristic reverse micelle parameter w(0) = [H(2)O]/[AOT] increases. The apparent limiting hydration constant of R3B ion pairs, K(hyd) = 2.8 +/- 0.2, corresponds to full hydration of AOT, consistent with 1-3 water molecules per AOT polar head. Transient relaxations at w(0) = 0.2, with a 375 +/- 15 ps decay at 550 nm decrease to 115 +/- 15 ps at w(0) = 7.2 turning into corresponding rises at 588 nm. At higher w(0), water induced dynamics becomes faster. The lifetime is longer in RM with smaller w(0), due to the presence of CIPs that inhibit intrinsic nonradiative decay processes, which in contrast shorten the decay times at higher w(0), due to the presence of SSIPs. The pairs' electronic excited-state properties are sensitive to viscosity and local polarity of the surrounding environment of the interfacial regions of AOT reverse micellar nanoaggregates.