J. K. Thomas

Small CdS particles in inverted micelles

Abstract Colloidal semiconductor particles of CdS, CuS, and PbS of extremely small dimensions have been obtained in reversed micelles, water-in-oil CTAB, and SDS mieroemulsions and lecithin vesicles. The effect of the relation between reactant and water-pool concentration and the effect of the water pool size have been studied. The spectroscopic characteristics of the particles have been examined and necessary comparisons have been made.

Benzylhexadecyldimethylammonium chloride in microemulsions and micelles

Abstract Several physical techniques have been used to study the nature and properties of benzylhexadecyldimethylammonium chloride micelles (BHDC), both as inverted micelles in benzene with aqueous cores, and as conventional “Hartley” micelles in water with hydrocarbon-like cores. For inverted micelles the nature of the water core or pool was investigated using IR spectroscopy, the absorption spectra of Co 2+ , and NMR spectroscopy. These data showed that the properties of bulk water were approached only when solvation of the surfactant ions was complete. The sizes of the micelles, both inverted and regular, were determined by a fluorescence technique. The data obtained for inverted micelles were in agreement with earlier measurements. The regular micelles in water were similar in size to micelles of cetyltrimethyl ammonium bromide (CTAB). Several laser-induced photoreactions also showed that the surface of a BHDC micelle was quite similar to that of CTAB. NMR data showed that the benzyl group of BHDC is directed toward the NCH 3 groups, and does not affect the physical processes studied. However, in inverted micelles the benzyl group is located toward the surfactant chain, and toward the bulk solvent benzene.

On the nature of surfactant vesicle and micelle systems

Abstract Several physical techniques, including fluorescence spectroscopy, laser photolysis, NMR, and electrical conduction were used to investigate the structural and kinetic properties of synthetic vesicles formed from didodecyldimethylammonium bromide. These structures are like their natural counterparts but unlike micelles in exhibiting broad lines in the proton NMR spectrum and in occluding glucose in their interiors, though diffusion through the vesicle walls is much more rapid than in the lecithins. The surface charge is low, with tighter binding of counterions such as I− and Br− than in micelles, and the head groups are in close proximity, both features typical of vesicles. The surface region appears to have a polarity much like that of cationic micelles, though the gradation to a nonpolar environment is much steeper in the vesicles. The vesicles modify photochemical reactions of species solubilized in these structures, in a fashion reminiscent of analogous micellar structures.

Photochemistry of iron oxide colloids

Abstract The photochemistry of two aqueous colloids of iron oxides (hematite and amorphous), have been investigated by laser flash photolysis and steady-state techniques. Excited ruthenium tris-bipyridine, ∗Ru(bpy)32+, is quenched by the amorphous colloid provided polyacrylic acid (PAA) used to stabilize the colloid and promote adsorption of Ru(bpy)32+ onto the oxide surface. No net chemical change takes place. The quenching effect is most marked at pH 7. Generation of reduced methyl viologen, MV+ ·, by irradiation of the PAA-MV2+ complex causes a dissolution of the pure oxide by e− transfer from MV+ · to the colloid. The measured rate of this reaction is a factor of 10 less than that calculated for a diffusion-controlled process.

Effect of interlayer spacing on intercalation of CdS in clays

The effect of laponite clay interlayer space on intercalation of CdS has been investigated. Results indicate that the degree of “openness” of the interlayer space of the laponite film determines whether the intercalation of CdS in laponite film can occur. In these systems, solvents play the dominant role in opening the interlayer space. As methanol molecules cannot open the interlayer space sufficiently to allow S2− anions to penetrate into the clay, CdS is formed only on the external surface of the laponite-methanol systems. However, ethanol and n-propanol molecules expand the layers sufficiently for CdS to form inside the interlayers. The minimum basal d-spacing of an intercalated clay can be estimated from summation of the thickness of the tetrahedra-octahedra-tetrahedra sheet of clays and the diameters of guest molecules, if a monolayer of guest molecules is intercalated into the clay.

Photophysical properties of pyrene in zeolites 3. A direct time-resolved diffuse reflectance study in alkali cation-exchanged zeolites X and Y

Abstract Time-resolved diffuse reflectance spectroscopy is used to investigate the photolysis of pyrene (Py) impregnated alkali cation-exchanged zeolites X and Y. These studies suggest that pyrene molecules are distributed in at least two different sites (active and non-active) on the zeolites X and Y. Excitation of pyrene adsorbed in non-active sites produces singlet excited states of pyrene with characteristic spectrum and photoionized species by absorbing a second photon, and triplet excited states through inter-system crossing; however, pyrene adsorbed in active sites is photoionized through a single-photon process. The products of the photoionization are the cation radical of pyrene and the ionized electron that is captured by the zeolite framework cation (e.g. Na 3+ 4 for sodium-exchanged zeolite Y). The absorption spectrum of the captured electron in the zeolite framework cation depends on the alkali cation exchanged. Coadsorbed water blocks the oxygen quenching of 3 Py* and trapped electron whereas the absorption spectrum of the trapped electron shifts to about 620 nm regardless of the alkali cation exchanged in the zeolite.

Photophysical properties of polyaromatic molecules adsorbed on TiO2 powder: the effect of coadsorbant

Abstract Pyrene adsorbed on TiO2 particles in aqueous suspension exhibits a single fluorescence peak at 395 nm. The fluorescence exhibits biexponential decay, and only the slower decay is quenched by cations. Coadsorption of OH−, dimethylaniline, platinum metal, or metal oxides, tends to produce a single exponential decay with a long lifetime, >200 nsec. Coincidental with coadsorption is the appearance of a 373-nm emission peak, thus leading to a pyrene fluorescence spectrum with two maxima at 373 and 395 nm, reminiscent of pyrene in a polar environment. Similar effects are observed for pyrene car☐aldehyde adsorbed on TiO2 in aqueous suspension. These photophysical effects are discussed in terms of discrete adsorption sites for pyrene on TiO2.

Photophysical and photochemical properties of pyrene-doped TiO2 particle suspensions in water

Both crystalline and amorphous TiO2 impregnated with pyrene have been prepared and the photophysical behavior of the arene in the porous particles is reported. The pyrene can be leached out of the TiO2 particles with hexane. The exact behavior of the pyrene in TiO2 depends on the physical state of the TiO2. In crystalline TiO2 pyrene occupies two polar sites in the particle, one of which is accessible to water and ionic solutes, while the other is isolated from such molecules. The two sites exhibit different photophysical behavior. Pyrene in amorphous TiO2 apparently occupies only one site, which gives ready access to water and ionic quenchers. Photo products normally associated with excited pyrene are not observed in TiO2 particles, a fact that is probably due to the close proximity of the products which are formed under conditions of limited diffusion, thus enhancing back reaction.

Photophysical studies in liquid crystal solutions and liquid crystal foams

Abstract Photophysical studies were used to probe features of micelle-alkanol systems in particularl as they form swollen micelles and liquid crystals in both the solution and the foam phase. Two systems were studied, sodium dodecyl sulfate (SDS)-octanol-water system I an cetyltrimethylammonium bromide-hexanol-water system II. The fluorescence spectroscopy and quenching studies show that progressive addition to octanol to SDS in system I leads to a more hydrophobic environment for the fluorescent probe molecule. Quenching studies show that reactions are slower in liquid crystal structures compared to micelles, both in liquid and foam vehicles; this applies to both system I and system II. Mixed micelle-liquid crystal systems gave complex kinetics which could be analyzed by a multiple experimental decay mechanism. Poisson kinetics were used to measure the aggregation number of the swollen SDS-octanol micelle, the entity intermediate between the SDS micelles and the liquid crystal structures. The aggregation number found was 214 indicating that this structure cannot be spherical but could be disk-like or rod-like in shape. Swollen micelles and liquid crystals in the liquid and foam vehicles exhibited similar photophysical properties.

Photophysical studies of calcium carbonate colloids

Abstract Pyrene carboxylic acids, PC n A, where n = 1, 4, 10 or 16 have been used to study colloidal CaCO 3 and Ca(OH) 2 solutions in benzene. The quenching of the probe excited state depends on the quencher and its location in the system. Excimer formation shows a dependence on chain length. For pyrene butyric acid, PBA, excimer formation is very fast while pyrene decanoic acid, PDA, and pyrene hexadecanoic acid, PHA, exhibit slower formation. All excimers are formed dynamically. The data are interpreted in terms of kinetics previously established for other heterogeneous systems. The studies comment on the differing nature of the interaction of fatty acids with CaCO 3 and Ca(OH) 2 colloidal surfaces.