Effect of temperature and surfactant structure on the microviscosity at the micelle-water interface: isomeric alkylbenzenesulfonates used as their own probe

We investigated the effect of temperature and surfactant structure on the microviscosity in aqueous micellar solutions formed by isomeric hexadecylbenzenesulfonates (xphiC16, where x=4-6 and indicates the position of the benzene ring [phi] along the alkyl chain) by fluorescence polarization and excimer emission spectroscopy. For a given isomer, the degree of polarization (p) was found to decrease with increasing temperature, with no evidence for changes in micellar structure. etaint/tau ratios, where etaint is the microviscosity of the benzene environment in micelles and tau its natural lifetime, were derived from fluorescence polarization measurements and showed a similar temperature behavior to that observed with the degree of polarization, suggesting that a thermal effect is the determinant factor in the variation of etaint. Interestingly, the microviscosity around the benzene ring was found to depend on the isomer structure in the entire range of temperatures investigated (8-60 degrees C) and is mainly determined by the orientation of the surfactant at the micelle-water interface in which the short alkyl chain is preferentially located at the interface and the long alkyl chain in the micellar core. This micelle conformation was found to prevail in the entire range of temperatures. In contrast to the dependence of p with temperature, excimer to monomer maximum emission ratios (IE/IM) were found to increase with increasing temperature, showing that when IE/IM is high (strong excimer emission), the degree of polarization is low (low microviscosity) and vice versa. Thus, the two independent measurements (IE/IM and p) yield the same information, namely, that the benzene moiety in all xphiC16 aqueous micelles resists both translational and rotational diffusion in a similar manner in the entire range of temperatures investigated (approximately 8-60 degrees C).     

Subnanosecond multiphonon relaxation of divalent copper impurities in LiTaO3

The temperature dependence of the lifetime of the excited ²T₂ state of the Cu²⁺ ion in a LiTaO₃ crystalline host has been measured with picosecond optical pulses. Its value ranges from 450 psec at 22°K to 10 psec at 423°K. Consistent with this result is the observation of weak fluorescence at 1.75 μm having a low temperature quantum efficiency of ～ 10^?6. The experimental results are in reasonably good agreement with recent theoretical models based on strong coupling of the Cu²⁺ impurities to the host lattice.     

THE LIFETIME OF THE EXCITED SINGLET STATE OF β-CAROTENE: CONSEQUENCES TO PHOTOSYNTHETIC LIGHT HARVESTING

Abstract The mechanism of singlet-singlet energy transfer to chlorophyll from carotenoid auxiliary pigments in photosynthetic apparatus is considered. Transmittance studies and resonance-enhanced Raman spectroscopy on a picosecond time scale lead to the conclusion that the de–excitation lifetime of the β-carotene singlet state is not greater than one picosecond. This would require close contiguity on the part of the transferring partners.     

Mechanochemical reactions of elastomers with metals

Metal powders were incorporated into various elastomers and the mixtures were subjected to intense shearing in air or a nitrogen atmosphere. The molecular weight after shearing was still relatively high, however; greater than 100 kg/mol. The elastomer was then dissolved and centrifuged to remove the metal particles and the solution was analyzed for reaction products. A strongly ultraviolet (UV)-absorbing species was formed by shearing a styrene–butadiene copolymer (SBR) with iron powder. The concentration increased with the concentration of iron powder in the mixture and the extent of shearing, and it was greater after shearing in a nitrogen atmosphere. These results are attributed to the direct reaction of macromolecular radicals, formed by shearing, with iron powder to yield an iron–polymer compound. Natural rubber (cis-polyisoprene), which absorbed 100–200 μg of iron per gram of polymer, was about as reactive as SBR. Polybutadiene was less reactive, and an ethylene–propylene copolymer did not react with or solubilize a significant amount of iron. Zinc was, in general, solubilized to a somewhat lesser degree than iron, whereas aluminum powder appeared to undergo virtually no reaction with sheared elastomers. The maximum extent of reaction (solubilization) was of the order of one metal atom per final polymer molecule, which was consistent with the proposed mechanism. Moreover, an analogous reaction took place between simple organic radicals and iron particles in suspension.     

Experimental and numerical studies of sheet electron beampropagation through a planar wiggler magnet

Detailed experimental studies on sheet relativistic electron beam propagation through a long planar wiggler are reported and compared with numerical simulations. The planar wiggler has 56 periods with a period of 9.6 mm. Typically, the wiggler field peak amplitude is 5 kG. The experimental efforts are focused on controlling the deviation of the beam toward the side edge of the planar wiggler along the wide transverse direction. It is found that a suitably tapered magnetic field configuration at the wiggler entrance can considerably reduce the rate of deviation. The effects of the following techniques on beam transport efficiency are discussed: side focusing, beam transverse velocity tuning at the wiggler entrance, and beam spread limiting. High beam transport efficiency (almost 100%) of a 15-A beam is obtained in some cases     

Fluorescence from pyrene solubilized in aqueous micelles. A model for quenching by inorganic ions

When pyrene is solubilized within cetyl trimethyl ammonium bromide micelles the lifetime of its fluorescent state is very dependent upon the concentration of iodide ions within the surrounding aqueous phase. Quantitative examination of the kinetic data throws doubt on the existing model for quenching which requires pyrene to diffuse from within the hydrocarbon core to the peripheral region where encounter and quenching can occur. An alternative model is proposed which argues that the water-lipid interface penetrates deep into the micellar body close to the included pyrene such that quencher ions can encounter pyrene without the latter having to move appreciably. The observed effects of ionic strength add support to the new model.     

Photo-properties of a silicon naphthalocyanine: a potential photosensitizer for photodynamic therapy

Abstract— The photoproperties of derivatized silicon naphthalocyanine have been investigated in order to assess its potential as a photosensitizer for photo-dynamic therapy. Absorption, fluorescence, phosphorescence and triplet absorption spectra have been measured. Oxygen quenching of the triplet state formed singlet oxygen in significant yields.     

Role of the cocatalyst in the copolymerization of CO2 and cyclohexene oxide utilizing chromium salen complexes

The mechanism of the copolymerization of cyclohexene oxide and carbon dioxide to afford poly(cyclohexylene)carbonate catalyzed by (salen)CrN3 (H2salen = N,N,'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylene-diimine) in the presence of a broad range of cocatalysts has been studied. We have previously established the rate of copolymer formation to be very sensitive to both the electron-donating ability of the salen ligand and the [cocatalyst], where N-heterocyclic amines, phosphines, and ionic salts were effective cocatalysts. Significant increases in the rate of copolymerization have been achieved with turnover frequencies of approximately 1200 h(-1), thereby making these catalyst systems some of the most active and robust thus far uncovered. Herein we offer a detailed explanation of the role of the cocatalyst in the copolymerization of CO2 and cyclohexene oxide catalyzed by chromium salen derivatives. A salient feature of the N-heterocyclic amine- or phosphine-cocatalyzed processes is the presence of an initiation period prior to reaching the maximum rate of copolymerization. Importantly, this is not observed for comparable processes involving ionic salts as cocatalysts, e.g., PPN+ X-. In these latter cases the copolymerization reaction exhibits ideal kinetic behavior and is proposed to proceed via a reaction pathway involving anionic six-coordinate (salen)Cr(N3)X- derivatives. By way of infrared and 31P NMR spectroscopic studies, coupled with in situ kinetic monitoring of the reactions, a mechanism of copolymerization is proposed where the neutral cocatalysts react with CO2 and/or epoxide to produce inner salts or zwitterions which behave in a manner similar to that of ionic salts.