Observation of Nanoconfinement Effect on the Kinetics of Hydrated Electron in the Nanoscale Water Pools of Water-AOT-Cyclohexane Microemulsions by Picosecond Pulse Radiolysis

The decay kinetics of the hydrated electron (eaq-) in aerosol OT (AOT)-based ternary microemulsions with pool sizes ranging from 0.34 to 4.85 nm were studied using picosecond pulse radiolysis coupled with transient absorption UV-vis spectroscopy. Electron transfer from oil to water and the subsequent solvation occurred within a time resolution of 7 ps. The decay kinetics of eaq- were accurately modeled using a double-exponential decay model, revealing the occurrence of two types of reactions, i.e., the recombination reaction at the water-oil interface and the radical-radical reactions in the water pools. The apparent lifetimes of both types of decays decreased significantly as the size of water pools decreased, indicating the influence of nanoconfinement effects. Moreover, the importance of the water-oil interface increased with decreasing water content, regardless of the presence or absence of NO3- as an electron scavenger in the water pools. Our findings provide a comprehensive understanding on the kinetics of the radiation reaction in AOT-based microemulsions.

Radiolytic syntheses of nanoparticles in supramolecular assemblies

Ionizing radiation is a powerful method in the syntheses of nanoparticles (NPs). The application of ionizing radiation in supramolecular assemblies can afford us more unique conditions to control the composition and morphology of the NPs. So far, most work focused on water-in-oil (W/O) microemulsions or reversed micelles. In this supramolecular organization, it has been proved that the effects of many conditions on the yield of e(aq)(-) play a key role, remarkably different from the mechanism in routine chemical method. Besides, some supramolecular assemblies of cyclodextrins and ionic liquids have been used in the syntheses of NPs by ionizing radiation, and many novel and interesting phenomena appeared. This review is intended to underline the three significant aspects of the radiolytic syntheses of NPs in supramolecular assemblies.

Formation of solid and hollow cuprous oxide nanocubes in water-in-oil microemulsions controlled by the yield of hydrated electrons

A local ordered structure constructed from solid Cu(2)O nanocubes was obtained by the radiolytic reduction of Cu(NO(3))(2) in a water-in-oil (W/O) microemulsion composed of Triton X-100, n-hexanol, cyclohexane, and water in the presence of ethylene glycol (EG). However, when Triton X-100 was replaced with Brij 56 in the microemulsion, hollow Cu(2)O nanocubes were synthesized. The addition of toluene into the Brij 56 system could decrease the ratio of hollow nanocubes. It was suggested that the balance between the reduction rate of Cu(2+) depending on the yield of hydrated electrons (e(aq)(-)) and the escape rate of the mixed solvent determined their final morphologies. The presence of EG influenced the rigidity of the interface of the microemulsion and thus the above balance, which resulted in the different morphologies of Cu(2)O nanoparticles in the Brij 56-based microemulsion.

Formation of nanoparticles in water-in-oil microemulsions controlled by the yield of hydrated electron: The controlled reduction of Cu2+

In the water-in-oil (W/O) microemulsions based on nonionic surfactants, i.e., Brij 30, Brij 56, or Triton X-100, the omega value (molar ratio of water to surfactant), anion, and surfactant could remarkably affect the radiolytic reduction of Cu2+ and the morphologies of the reduction products simultaneously. The addition of toluene or naphthalene could transform the reduction products from copper to cuprous oxide in the Brij 56-based microemulsion, and the efficiency of naphthalene was obviously higher than that of toluene. After the effects of pH value and cosurfactant were excluded, it could be concluded that the effects of the omega value, the anion, and the structure of the surfactant on the yield of hydrated electrons (eaq-) play a key role in the radiolytic reduction of Cu2+. It was also suggested that the morphology of the reduction product may be controlled by the yield of eaq-.

Photodetachment of Ferrocyanide in Reverse Micelles

Solvated electrons have been generated in reverse micelles (RMs) through photodetachment of ferrocyanide (Fe(CN)(6)(4-)) in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) RMs. We have measured both bleach recovery of the parent ferrocyanide CN stretch in the infrared and the decay of the solvated electron absorption at 800 nm. The bleach recovery has been fit to a diffusion model for the geminate recombination process. The fit parameters suggest a narrowing of the spatial distribution of ejected electrons due to confinement in the RMs when compared to bulk water. The diffusion coefficient of the solvated electron does not appear to be significantly affected by RM confinement. The decay of the solvated electron absorption exhibits an additional decay component that is not observed in bulk water and is smaller for larger RMs. No corresponding additional component is seen in the parent ferrocyanide IR bleach recovery, which supports our interpretation that the confinement-induced new decay process in RMs is due to electrons reacting with AOT headgroups.

Intermicellar material exchange in reverse micelles formed by ionic AOT and nonionic Igepal surfactants studied by means of pulse radiolysis. Influence of the temperature

The recombination of thiocyanate anion radicals, (SCN)2−, formed pulse radiolytically within the water pools of reverse micelles stabilized with anionic AOT and nonionic Igepal surfactants, was proved as an indicator reaction to study intermicellar exchange. It was found that the exchange process is slower inIgepal than in AOT reverse micelles with the same water to surfactant ratio. The apparent activation enthalpy and entropy of the exchange process were determined in different alkanes. For the AOT and Igepal reverse micelles the activation parameters increase with the droplet size, but for the AOT systems they do not significantly change with the increase of droplet concentration. For non-percolated systems the activation parameters for Igepal reverse micelles approach those for AOT reverse micelles. This result supports existing suggestions that the mechanism of intermicellar exchange does not differ in principle between reverse micelles stabilized with ionic and nonionic surfactants.

Hydrated Electrons in a Quaternary Microemulsion System: A Pulse Radiolysis Study

Using the technique of pulse radiolysis, hydrated electrons have been produced and studied in quaternary microemulsion (sodium lauryl sulfate (NaLS)/water/cyclohexane/1-pentanol) system. Remarkably high lifetimes ( approximately 20 &mgr;s) for hydrated electrons have been obtained. In general, these are two orders of magnitude higher than those reported earlier in reverse micelles. The water droplet sizes and location of the probes have been determined from the decay kinetics of hydrated electrons. The yields and half-lives (t 1/2 ) of the hydrated electrons vary smoothly as the water droplet sizes are changed. We believe that these studies will be useful not only in interpreting electron transfer reactions occurring in small water pockets in biological systems but also in the production of metal nanoclusters by radiation chemical method.