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

• Published in: The journal of physical chemistry. B Vol. 114; no. 32; pp. 10417 - 10426
• Main Authors: Ferreira, José A. B, Costa, Sílvia M. B
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 19-08-2010
• Subjects: Anisotropy; B: Surfactants, Membranes; Dioctyl Sulfosuccinic Acid - chemistry; Fluorescent Dyes - chemistry; Humans; Ions - chemistry; Micelles; Rhodamines - chemistry; Solvents - chemistry; Spectrometry, Fluorescence; Surface-Active Agents - chemistry
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp100571t

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 = [H2O]/[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.