J.K. Thomas

Cadmium sulfide of small dimensions produced in inverted micelles

Abstract Inverted micelles are shown to be useful versatile systems for forming stable small (5 A radius) particles of CdS. The agglomeration number grows discontinuously, possibly obeying “magic agglomeration numbers”. The spectroscopic properties of these particles are studied as a parameter of radius and discussed in terms of aqueous CdS colloids and current colloid theory.

Room temperature phosphorescence of aromatic hydrocarbons in aqueous micellar solutions

Abstract The phosphoresence spectra of several simple arenas, pyrene, naphthalene, biphenyl and their brominated derivatives have been observed in aqueous micellar solutions at room temperature. The success of these experiments is due to the protective screening of the arene triplet state by solubilization in the micelles. Impurities in the aqueous phase are thus excluded from the excited arene. Small traces of oxygen efficiently remove the phosphorescence as oxygen readily penetrates into the micelle to quench the arene triplet. Hydrophilic quenchers such as Cu 2+ and Tl + also reduce the lifetime of the phosphorescence. In anionic micelles these quenchers are located at the surface of the micelle and efficient quenching take place at the lipid water interface. Cupric ions reduce the lifetime and yield of phosphorescence, while Tl + ions reduce the lifetime but also enhance the rate of emission of the phosphorescence via heavy atom spin orbit coupling. The technique and data are relevant to the measurement of kinetic parameters in micelles and membranes.

Reactions in micellar systems

Kinetic effects due to the solubilization of pyrene and biphenyl in aqueous surfactant solutions such as sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, and polyoxyethylene (23) dodecan...

On the nature of solubilized water clusters in aerosol OT/alkane solutions. A study of the formation of hydrated electrons and 1,8-anilinonaphthalene sulphonate fluorescence

Abstract The pulse radiolysis of dioctyl sulphosuccinate (aerosol-OT) H 2 O/heptane solutions leads to formation of hydrated electrons in the aqueous core of the inverted aerosol-OT micelles. The hydrated electrons are produced via direct interaction of the radiation with the aqueous regions and scavenging of electrons formed initially in the hydrocarbon phase by the water bubbles. The scavenging efficiency decreases with decreasing radius of the water cluster. Hydrated electrons are not formed below a critical size of the solubilized water particles. The quantum yield and wavelength of the maximum of the aniline-naphthalene sulphonate (ANS) fluorescence are strongly dependent on the water content of aerosol-OT inverted micelles. The fluorescence behavior indicates an increase of polarity with increasing cluster radius. The polarity of very large water clusters ( r ≈ 73 A) is still lower than that of bulk water. Water which is bound to Na + counterions cannot effectively participate in the solvation of the dipolar ANS excited states.

Photochemical processes on aluminium oxide. A pulsed laser study of photoinduced reactions

Abstract The photochemistry of pyrene and naphthalene adsorbed on Al 2 O 3 has been studied by pulsed laser techniques. Oxygen quenches the fluorescence of both molecules very efficiently while the triplet state of pyrene is quenched much more slowly (× 1/100). Nitrobenzene and dimethylaniline also quench pyrene fluorescence. Photolysis at high laser power photoionises pyrene, and the back reaction observed through the pyrene cation follows tunnelling kinetics. Co-adsorption of long chain alcohols on Al 2 O 3 leads to a decrease in the fluorescence quenching rates with both O 2 and nitrobenzene.

Laser photoionization of aromatic hydrocarbons in micellar solution

Abstract The 347.1 nm ruby laser photolysis of aromatic hydrocarbons such as pyrene, triphenylene and perylene solubilized in aqueous micellar solutions yields relatively high yields of ions (hydrated electrons and solute cations). The ionization process is biphotonic. In the case of triphenylene the yield of hydrated electrons decreases sharply with increasing chain length of the micellized surfactant molecules. This effect is less pronounced or absent in the photoionization of pyrene or perylene. The results are interpreted in terms of energy loss of photoelectrons in the hydrocarbon phase of micelles.

Transport of ions between water pools in alkanes

Abstract Laser photolysis studies in reversed micellar systems show that ion transport only occurs on collision of the water pools, with an efficiency of only 1% Various additives affect the efficiency of mixing.

Ion-pair complexes of methyl viologen and anionic solutes

Methyl viologen forms ion-pair complexes with anionic solutes such as EDTA, hexametaphosphate, acetate, citrate and sulfide ions. The luminescence properties of all complexes are similar, exhibiting an emission maximum centered about 520 nm and an excitation peak at 405 nm. The lifetime of the luminescence is on the order of 5 ns. For all complexes a new absorption band appears as a shoulder extending out to 500 nm. For the EDTA complex there is a strong pH dependence of both the intensity of the absorption and the stoichiometry of the complex suggesting that at pH values substantially greater than pK a 3 of EDTA (10.2) a 2:1 complex (2MV 2+ :⋯ EDTA 4− ) is formed. At pH values significantly less than pK a 3 of EDTA a 1 : 1 complex is formed (MV 2+ ⋯HEDTA 3− ). Photoexcitation of the complexes leads to the formation of reduced methyl viologen. The data draw attention to complications in previous model systems for the storage of solar energy.

Two photon laser photo-ionisation of molecules in liquids

Abstract Electrons are produced in several organic liquids by the two-photon laser photolysis of dissolved anthracene and pyrene. The rate constants for reaction of the electrons with biphenyl and O 2 are measured, and are in agreement with the known mobilities of the electrons in these liquids.

Photochemical reduction of carbonate to formaldehyde on TiO2 powder

Abstract Photochemical reactions at semiconductor particle surfaces have been used to drive energy uphill reactions to store light energy. Carbonate radical anion was found to be intermediate when carbonate was used to intercept the photogenerated hole on TiO 2 by exciting with light energy of more than 3.2 eV. The carbonate anion radical undergoes secondary reactions on the surface to give formaldehyde. The chemical energy stored in this reaction is 580 kj/mole. The quantum yield for the formation of HCHO is 4 × 10 −3 , but extended irradiation leads to loss of formaldehyde.