Showing papers by "Michael R. Hoffmann published in 1992"

Q-sized cadmium sulfide: synthesis, characterization, and efficiency of photoinitiation of polymerization of several vinylic monomers

Abstract: Q-sized CdS semiconductors have been synthesized in several nonaqueous solvents without the use of added stabilizers. The effects of solvent viscosity and dielectric strength, concentration of excess cadmium ions, temperature, and stirring upon the colloids' absorption spectra have been examined. In addition, efficiency of photoinitiation of polymerization of several vinylic monomers has been examined using both bulk and quantum-sized CdS, ZnO, and TiO_2. The Q-sized semiconductors demonstrated significantly higher quantum yields for photopolymerization than their bulk-sized counterparts. A correlation between the reactivity of a monomer toward polymerization and its Alfrey and Price Q and e values was observed. Hole scavenging by the solvent was necessary for efficient polymerization to occur. A correlation between the semiconductor photoinitation efficiency, and the reduction potential of its conduction band electrons was also found. On the basis of these observations, a mechanism of an anionic initiation step followed by free radical chain propagation steps has been proposed.

Photoinitiated polymerization of methyl methacrylate using Q-sized zinc oxide colloids

Abstract: The polymerization of methyl methacrylate has been shown to occur readily using Q-sized ZnO semiconductors as photoinitiators. We have examined the effects of solvent, monomer concentration, initiator concentration, light intensity, and semiconductor particle size upon the reaction rates. The reaction pathway appears to be via anionic initiation, followed by free-radical propagation steps. The holes formed upon illumination are scavenged by the solvent. Increasing the concentration of the photoinitiator increased polymer yield until a saturation value was achieved. The rate of polymerization rapidly increased with increasing monomer concentration, due to the Trommsdorf effect. The rate of polymerization was found to depend upon the square root of the incident light intensity, as predicted from simple kinetic theory. Quantum yields of polymerization decreased as particle size decreased, due to either increased surface defects or enhanced rates of competing electron-hole recombination. Under the same experimental conditions, no polymerization occurred with bulk-size ZnO particles as photoinitiators; thus Q-sized ZnO particles were more efficient photoinitiators of polymerization.

Decomposition of parathion in aqueous solution by ultrasonic irradiation

Abstract: Parathion (O,O-diethyl O-p-nitrophenyl thiophosphate) is a major pesticide that is used in large quantities worldwide. Organophosphate esters such as parathion have been used as alternatives to DDT and other chlorinated hydrocarbon pesticides. However, the organophosphate esters are not rapidly degraded in natural waters. At 20 oC and pH 7.4, parathion has a hydrolytic half-life of 108 days and its toxic metabolite, paraoxon, has a similar half-life of 144 days.

Simultaneous spectrophotometric measurement of iron(II) and iron(III) in atmospheric water

Abstract: A new analytical method employing di-pyridyl ketone benzoylhydrazone (DPKBH) as a colorimetric chelating agent for the simultaneous determination of iron(II) and iron (III) in cloudwater has been developed. A spectrophotometric detection limit of 4 nM for both Fe(III) and Fe(II) with a linear response from 4 nM to 0.1 µM was established for samples extracted with CHCl_3-H_2O. DPKBH chelation without CHCl_3 extraction showed a linear response from 0.1 to 30 µM. The molar extinction coefficients of the Fe(II)-bis(DPKBH) (є_2) and Fe(III)-bis(DPKBH) (є_1) complexes are є_l and є_2 = 3.6 X 10^4 L mol^(-1) cm^(-1) at 370 nm and є_2 = 1.1 X 10^4 L mol^(-1) cm^(-1) at 660 nm. Analytical interference studies on the possible changes in the oxidation state of iron with S(IV), oxalate, and other potential electron donors have also been carried out. This analytical method has been used to determine iron(II) and iron(III) simultaneously in cloudwater samples collected within the Los Angeles basin airshed. The concentration of Fe(II) varied from 0.3 to 5 µM, and the concentration of Fe(III) varied from 0.6 to 1.4 µM during several stratus cloud events.

Oxidation of hydrogen sulfide in aqueous solution by ultrasonic irradiation

Abstract: In 1933, Flosdorf and Chambers (1) reported that metal sulfides were oxidized in the presence of audible sound (1-15 kHz) while investigating the bactericidal action of audible sound. However, Schmitt et al. (2) were the first researchers to observe the rapid oxidation of dissolved H_2S gas to colloidal sulfur during sonication at 750 kHz with a 250-W power source. They reported that an increase in the total pressure of the system (PO_2) led to higher oxidation rates up to a limiting pressure.

The Chemical Effects of Collapsing Cavitation Bubbles

Abstract: A comprehensive mechanism is developed for aqueous-phase oxidation of S(-II), where [S(-II)] = [H_2S] + [HS^-] + [S^(2-)], by ·OH radical in the presence of oxygen. The oxidation of S(-II) is initiated by reaction with ·OH, but it is further propagated by a free-radical chain sequence involving O_2. This mechanism can adequately model the observed oxidation of S(-II) in air-saturated aqueous solutions sonicated at 20 kHz and 75 W/cm^2 at pH ≥ 10, assuming a continuous and uniform ·OH input into solution from the imploding cavitation bubbles. At this pH range, practically all S(-II) is present in the form of HS^- and cannot undergo thermal decomposition. Our work suggests that the use of simplified approaches for modeling the liquid-phase sonochemistry of a well-mixed solution may be justified when ·OH radical reactions predominate. For the immersion probe at 20 kHz and 75 W/cm^2, the effective ·OH uniform release into the bulk solution was estimated to be 3.5 μM/min with a corresponding steady-state ·OH concentration of ≤0.1 μM.

Photolnltlated Polymerlzation of Methyl Methacrylate Using @Sized ZnO Colloids

Abstract: The polymerization of methyl methacrylate has been shown to occur readily using Q-sized ZnO semiconductors as photoinitiators. We have examined the effects of solvent, monomer concentration, initiator concentration, light intensity, and semiconductor particle size upon the reaction rates. The reaction pathway appears to be via anionic initiation, followed by free-radical propagation steps. The holes formed upon illumination are scavenged by the solvent. Increasing the concentration of the photoinitiator increased polymer yield until a saturation value was achieved. The rate of polymerization rapidly increased with increasing monomer concentration, due to the Trommsdorf effect. The rate of polymerization was found to depend upon the square root of the incident light intensity, as predicted from simple kinetic theory. Quantum yields of polymerization decreased as particle size decreased, due to either increased surface defects or enhanced rates of competing electron-hole recombination. Under the same experimental conditions, no polymerization occurred with bulk-size ZnO particles as photoinitiators; thus Q-sized ZnO particles were more efficient photoinitiators of polymerization.