Showing papers in "The Journal of Physical Chemistry in 1981"

A general small-curvature approximation for transition-state-theory transmission coefficients

Abstract: We derive an expression for the effective mass along the reaction coordinate in a chemically reacting system. This expression is used in two convenient algorithms for including quantum mechanical effects on the reaction coordinate into transition-state theory or generalized transition-state theory. One algorithm is based on a uniform semiclassical approximation; the other is based on a parabolic approximation to the effective potential. Examples are presented to show that the theory accurately accounts for reaction-path-curvature effects on the tunneling contributions to the reaction H + H/sub 2/, D + D/sub 2/, and D + H/sub 2/. Although the derivation, and hence expected reliability, of the new methods is most valid for systems with small reaction-path curvature, the methods are generally applicable and should provide useful accuracy even for medium-curvature and some large-curvature systems.

Quasi-elastic light scattering study of intermicellar interactions in aqueous sodium dodecyl sulfate solutions

Abstract: Quasi-elastic light scattering measurement have been performed on aqueous sodium dodecyl sulfate solutions in the 0.1 to 0.6 mole NaCl concentration range at 25 and 40 C. The aggregation number M, the hydrodynamic radius Rh, and the amphiphile concentration dependence of static and transport coefficients of micellar solutions are obtained from the experimental data. The micellar parameters M and Rh increase with salt concentration and slightly decrease with temperature. The concentration dependence of the apparent molecular weight and of the mass diffusion coefficient is interpreted on the basis of Derjaguin-Landau-Verwey-Overbeek theory of colloid stability. The fit to the experimental data, performed with the assumption that the Hamaker constant for micellar attraction A and the micellar electric charge Q do not depend on the salt concentration, is satisfactory and gives A = 4.5 x 10/sup -20/ J and Q = 37 electronic charges. The electric potential at the shear surface of the micelle goes from 70 MV at 0.1 mole NaCl to approximately 30 MV at 0.5 to 0.6 mole NaCl. 45 references.

Investigation of micelles, microemulsions, and phospholipid bilayers with the pyridinium N-phenolbetaine et(30), a polarity probe for aqueous interfaces

Abstract: The phenolbetaine et(30), a well-known internal probe of solvent polarity, is used to characterize the polarity of its environment in micelles, microemulsions, and phospholipid bilayers. The probe molecule, invariably solubilized in the aqueous interface, senses changes in polarity brought about by salt addition and variation of surfactant chain length and concentration, counterion, and temperature. These changes in polarity can be correlated with variations in the micellar aggregation number, signifying that the immediate probe surroundings become less polar when the micelle increases in size. With sodium dodecyl sulfate micelles a discontinuity in the variation of the et(30) value with the aggregation number is observed at 0.45 mole added NaCl, where the shape of the micelles has been reported to undergo a sphere-to-rod transition. The microemulsions behave as swollen micelles. The lowest polarity, comparable to that of 1-butanol, is observed for the phospholipid bilayers. 49 references.

Study of nickel-molybdenum-.gamma.-aluminum oxide catalysts by x-ray photoelectron and Raman spectroscopy. Comparison with cobalt-molybdenum-.gamma.-aluminum oxide catalysts

Abstract: Several series of nickel (or Co) molybdenum oxide catalysts supported on ..gamma..-Al/sub 2/O/sub 3/ have been studied by X-ray photoelectron and Raman spectroscopy. The combination of these two techniques is a complementary approach to the alumina coverage, the nature of the deposited species, and the interaction between these species. In particular, the characteristics of the monolayer coverage by the nickel species appear somewhat different from cobalt in systems without molybdenum. When molybdenum is present, formation of a phase similar to (a) NiMoO/sub 4/ is observed. No bulk NiO or Ni(OH)/sub 2/ phases are detected. Comparison with Co-Mo or Ni-Mo-..gamma..-Al/sub 2/O/sub 3/ industrial catalysts is also presented.

Track effects in radiation chemistry. Concentration dependence for the scavenging of hydroxyl by ferrocyanide in nitrous oxide-saturated aqueous solutions

Abstract: The radiation chemical yield for oxidation of ferrocyanide by OH produced in the fast electron radiolysis of N/sub 2/O-saturated aqueous solutions is found to increase from a low concentration limiting yield of 5.2 to values in excess of 6.7 for ferrocyanide concentrations above 0.1 M. This observed dependence reflects competition between the scavenging reaction and track recombination processes involving OH radicals which occurs on a time scale greater than 10/sup -9/s. By extrapolation an additional yield of -- 1.5 is estimated for OH radicals which recombine at still shorter times. These scavenging studies show that significant amounts of recombination occur within the track even at times as long as 10/sup -6/s. Secondary radicals produced within the track are also subject to recombination processes as is illustrated by the low concentration limit which shows that most OH radicals produced from N/sub 2/O within the track do not escape from the track. In general, for concentrations of reactive solutes greater than -- 10/sup -4/ M, i.e., for k(S) > 10/sup 6/ s/sup -1/, scavenging of OH from the track will contribute significantly to the observed yields and this effect must be taken into account in comparing the results from different radiolytic systems.

Reaction kinetics of nitrogen dioxide with liquid water at low partial pressure

Abstract: The reaction 2NO/sub 2/(g) + H/sub 2/O(l) ..-->.. 2H/sup +/ + NO/sub 3//sup -/ + NO/sub 2//sup -/ (l) has been studied for 1 x 10/sup -7/ less than or equal to P/sub NO2/ less than or equal to 8 x 10/sup -4/ atm. Since this reaction involves the transfer of a reactant from the gas into the aqueous phase, the rate depends upon the following: (a) physical mass transfer of the reactant, (b) the equilibrium solubility of NO/sub 2/, and (c) homogeneous aqueous-phase kinetics. In order for the observed rate to yield information most sensitive to (c), the rate of (a) must be comparable to that of (c). In the case of a second-order reaction, this can be achieved by working at low partial pressure of NO/sub 2/ as well as a high rate of physical mixing of the two phases. To facilitate the latter, the gas was brought into contact with the liquid as finely dispersed bubbles produced by flowing through a disk-frit; the mass transfer time constant, determined by uptake of CO/sub 2/, was tau/sub m/ = 1.7 to 5.3 s. The rate of reaction 1, monitored by observing the electrical conductivity of the aqueous solution, exhibitedmore » dependence on p/sub NO2/ and tau/sub m/ consistent with second-order kinetics and a steady-state aqueous-phase NO/sub 2/ concentration. Values of H/sub NO2/, the Henry's law coefficient, and k/sub 1/, the second-order aqueous-phase rate constant, were determined to be (7.0 +- 0.5) x 10/sup -3/M atm/sup -1/ and (1.0 +- 0.1) x 10/sup 8/M/sup -1/s/sup -1/, respectively, at 22/sup 0/C. Self-consistency of the gas-liquid reaction model has been demonstrated in terms of the identity of the diffusing species and the extent of mass-transport limitation.« less

Parabolic tunneling calculations

Abstract: A new analytic approximation is presented for thermally averaged transmission coefficients for tunneling through and reflection by truncated parabolic potential barriers. The approximation has a wide range of validity and avoids the spurious singularities of the well-known result for untruncated parabolic barriers. The analytic result is also applied to effective parabolic fits to other barrier shapes. The results are quite encouraging and suggest that the approximation may be useful for a wide variety of chemical applications.

Kinetics of the reactions of hydroxyl radical with benzene and toluene

Abstract: Absolute rate constants for the reactions of the hydroxyl radical with benzene and toluene were measured within the temperature and pressure ranges 213 less than or equal to T less than or equal to 1150 K and 20 less than or equal to P less than or equal to 200 torr by using He, Ar, and SF/sub 6/ as diluent gases. To help elucidate the variations in reaction mechanism with temperature, we also studied OH reactions with deuterated benzene (C/sub 6/D/sub 6/) and with selectively deuterated toluenes (C/sub 6/H/sub 5/CD/sub 3/, C/sub 6/D/sub 5/CD/sub 3/, and C/sub 6/D/sub 5/CH/sub 3/). Three major reaction channels were characterized kinetically. At T less than or equal to 298 K, electrophilic addition of the OH radical to the aromatic ring is the dominant reactive pathway in all systems studied. At temperatures above 500 K, rapid decomposition of the thermalized adduct back to reactants diminishes the importance of the addition channel and leads to bimolecular reaction rate-constant values significantly lower than those measured near room temperature. At elevated temperatures, the ring hydrogen abstraction (for benzene) and side-chain hydrogen abstraction (for toluene) pathways are shown to be predominant. The measured bimolecular rate constants increase monotonically withmore » increases in temperature above 500 K, and kinetic separation of the two hydrogen abstraction modes for toluene is achieved.« less

Growth and size distributions of cetylpyridinium bromide micelles in high ionic strength aqueous solutions

Abstract: The growth of cetylpyridinium micelles in high NaBr concentration aqueous solutions is observed by the use of quasi-elastic light scattering. The variations of the mean hydrodynamic radius, are measured over wide ranges of temperature (27 < T < 75 C), detergent concentration (1.5 x 10/sup -3/-40 x 10/sup -3/ M), and added NaBr concentration (0.2, 0.4, 0.6, and 0.8 M). These results are interpreted in the frame of a multiple equilibrium description of the micellar elongation. This classical description is rewritten in order to incorporate the counterion influence through the use of the crude ion binding approximation. A quantitative geometrical model is developed for the elongated micelles which are known to have the shape of long flexible spherocylinders. The obtained relation between the mean hydrodynamic radius and the mean aggregation number allows qualitative comparison of the experimental results to the predictions of the theory. 26 references.

Heterogeneous photocatalytic decomposition of benzoic acid and adipic acid on platinized titanium dioxide powder. The photo-Kolbe decarboxylative route to the breakdown of the benzene ring and to the production of butane

Abstract: : The heterogeneous photocatalytic decomposition of adipic acid in deaerated aqueous solutions containing a suspension of platinized TiO2 (anatase) powder leads to the formation of CO2 and butane with lesser amounts of valeric acid and negligibly small quantities of oligomeric materials. A mechanism for this photo-Kolbe reaction, which yields different products than the electrochemical Kolbe reaction, is proposed. The photodecomposition of benzoic acid in oxygen-containing solution at platinized TiO2 was also investigated. The reaction product is predominantly CO2, with intermediate production of salicylic acid and phenol. A mechanism for the reaction based on photogeneration of hydroxyl radicals and adsorption of intermediates on the powder surface is proposed. (Author)

Adsorption of fluorocarbon and hydrocarbon surfactants to air-water, hexane-water and perfluorohexane-water interfaces. Relative affinities and fluorocarbon-hydrocarbon nonideality effects

Abstract: Surface and interfacial tensions of dilute aqueous solutions of sodium perfluorobutyrate, sodium perfluorooctanoate, sodium perfluorodecanoate, sodium octyl sulfate, and sodium decyl sulfate at the air-water (a/w), hexane-water (h/w), and perfluorohexane-water (p-f-h/w) interfaces have been measured with the objective of determining the relative affinities for the interfaces at low degrees of adsorption. Up to approximately 2 dyn/cm the surface pressures were found to increase linearly with surfactant concentration (activity), in agreement with a thermodynamic model of adsorption. The free energies of adsorption, have been estimated by using the thermodynamic model. From these data the incremental changes in free energy of adsorption, ..delta.. free energy of adsorption values, on adding a-CH/sub 2/-group to the hydrocarbon chain and a-CF/sub 2/-group to the fluorocarbon chain have been calculated. The affinity of a hydrocarbon surfactant for the h/w interface is markedly higher than that for the a/w interface. A fluorocarbon surfactant at the p-f-h/w interface shows a much smaller increase in affinity as compared to the a/w interface. 31 references.

Kinetics of hydroxyl radical reactions with the atmospheric sulfur compounds hydrogen sulfide, methanethiol, ethanethiol, and dimethyl disulfide

Abstract: The flash photolysis-resonance fluorescence technique has been employed to study the kinetics of hydroxyl radical reactions with H/sub 2/S (k/sub 1/), CH/sub 3/SH (k/sub 2/), CH/sub 3/SCH/sub 3/ (k/sub 3/), and CH/sub 3/SSCH/sub 3/ (k/sub 4/) over the temperature range 244 to 367/sup 0/K. The following Arrhenius expressions adequately describe the measured rate constants as a function of temperature (units are cm/sup 3/ molecule/sup -1/ s/sup -1/): k/sub 1/ = (6.4 +- 1.3) x 10/sup -12/ exp((55 +- 58)/T); k/sub 2/ = (1.15 +- 0.39) x 10/sup -11/ exp((+338 +- 100)/T); k/sub 3/ = (6.8 +- 1.1) x 10/sup -12/ exp((-138 +- 46)/T); k/sub 4/ = (5.9 +- 3.3) x 10/sup -11/ exp((+380 +- 160)/T). The results are compared with previous investigations, and their implications regarding reaction mechanisms and atmospheric sulfur chemistry are discussed.