Showing papers in "Transactions of The Faraday Society in 1969"

Aluminum + water reaction

Abstract: During the aluminum + water reaction, aluminum ions and electrons are removed in separate steps at different sites on the surface. Ions are removed nearly uniform over the entire surface, which is covered at all times with a thin amorphous oxide film. The outer surface of this oxide is first hydrolyzed and then dissolves to yield soluble species which either remain in solution or, at intermediate values of pH, precipitate as a porous hydroxide of extremely small particle size. The hydroxide appears to be identical to pseudoboehmite. The overall rate of the corrosion reaction is controlled by this dissolution of the film and by the disposition of the soluble products. The corrosion rate is nearly independent of specimen potential, solution pH below 10, and of the presence in solution of many salts at concentrations as high as 1 mole/1. The rate is strongly dependent on temperature, on the presence of specific inhibitive salts, and increases rapidly at high pH. In environments which preclude oxide dissolution, e.g., water + dioxane mixtures or water vapour, the corrosion rate is drastically reduced. The corrosion rate is constant when no solid hydroxide is formed, and is otherwise a strong function of the amount of reaction. At high temperatures the rate decreases with time as the precipitated hydroxide hinders transport. At lower temperatures the rate may first increase with time as nucleation of hydroxide provides sinks for soluble species close to the interface, and then at long times the rate decreases as the hydroxide layer thickens. Electrons are removed more readily at special sites, and in our specimens these sites were primarily at the grain boundaries. Electron removal results in increased hydroxyl ion concentration, and this in turn results in more rapid attack on the protective oxide. At such cathodes the oxide film is maintained at a small constant thickness at which there is a balance between the rates of its dissolution by the basic solution and growth because of the great affinity of aluminum for oxygen. At cathodic sites there is thus anodic activity as well, but electron removal predominates. The metal is corroded more rapidly at cathodes, producing pronounced grain boundary attack in our specimens. Application of anodic potentials eliminates grain boundary attack.

Pulse radiolysis studies on the oxidation of organic radicals in aqueous solution

Abstract: Pulse radiolysis has been used to measure directly the absolute rates of oxidation by ferricyanide ion of various radicals produced by OH attack on organic solutes. These include mono, di- and polyhydroxylic compounds, hydroxy acids, polyethylene oxides of molecular weight 200, 6000 and 20 000 and the amino acid serine. Radicals produced by hydrogen abstraction from α carbon atoms in alcohols are oxidized at, or near, diffusion-controlled rates, whereas the reactions are much slower for radicals formed by OH-attack elsewhere. The technique has been used to measure the percentage OH-attack at the α position for a series of straight and branched-chain alcohols. Oxygen competes with ferricyanide for radical oxidation. The data for oxygen-containing solutions fit a simple radical-competition scheme which has been used to measure rates of peroxy-radical formation. These approach diffusion-controlled limits.

Interaction of spherical colloidal particles with planar surfaces

Abstract: The rate of deposition of spherical polystyrene latex particles on smooth plastic films has been determined with the rotating disc technique. When the particles were negatively charged and the film positively charged, the deposition rate was closely in agreement with the Levich theory of diffusion-controlled transport to a rotating disc. With negative particles and negative films, high electrolyte concentrations were required to produce measurable deposition and the kinetics could not be accounted for by the appropriate modification of the Derjaguin-Landau-Verwey-Overbeek theory of colloid stability for sphere-plate interaction (including a treatment for diffusion of particles in a linear potential field). Furthermore, the anomalous deposition in this system was not proportional to time or to concentration of sol, although the sol itself was substantially unaffected by electrolyte under the conditions of the experiment. The evidence strongly suggests that—even with this seemingly ideal model for studying sphere-plate interaction—anomalous deposition occurred preferentially on to areas of locally favourable potential or geometry (or both). This phenomenon may prove to be important in practical deposition systems.

Gas-liquid chromatographic determination of cross-term second virial coefficients using glycerol. Benzene + nitrogen and benzene + carbon dioxide at 50°C

Abstract: This paper extends the theory of gas-chromatographic elution of a highly dilute solute to the second order in gas-phase imperfection terms, yielding a precise expression for the ideal retention volume in terms of the thermodynamic parameters of the system. The effect of carrier gas dissolution in the solvent is included explicitly; the cross-term second virial coefficient B12 for the system solute + carrier gas cannot be evaluated unambiguously for an appreciably soluble carrier gas. The use in such cases of polar solvents in which non-polar carrier gases are effectively insoluble is discussed with reference to surface adsorption and chromatographic non-ideality, which are important in polar solvents. Extrapolation of observed retention volume to zero flowrate for each of a series of columns covering a range of solvent loadings should give, corresponding to the limit of infinite loading, unambiguous values of bulk-phase solute-in-solvent activity coefficient and of B12. The theory has been applied to measurements on the systems benzene + nitrogen + glycerol, and benzene + carbon dioxide + glycerol, using four columns loaded at 15.7, 25.3, 33.6 and 23.3 % by weight glycerol on Celite, and the system benzene + hydrogen + glycerol using a column loaded at 44.0 %. The flowrate-dependence of the net retention volume is approximately linear on all columns, the gradient correlating well with empirical plate height and with solvent loading, in accordance with theory and with the known facts about the distribution of a polar solvent on Celite. The zero-flowrate B12 values are effectively the same with all columns. These values of B12 at 50°C are, for benzene + nitrogen, –98 ± 9 cm3 mole–1, and for benzene + carbon dioxide, –250 ± 15 cm3 mole–1, both being in fair agreement with theoretical predictions for systems of non-spherical molecules of different sizes. The activity coefficient for benzene at infinite dilution in glycerol at 50°C is logγ∞13= 2.084 ± 0.005.

Reaction of nitrous oxide with copper surfaces. Application to the determination of free-copper surface areas

Abstract: A study is made on the reaction of nitrous oxide with copper surfaces. As with germanium, the reaction has an activation energy that increases with coverage. The transition from surface to bulk oxidation sometimes involves an incubation time. The reaction at 90°C and 20 cmHg gives a coverage of about unity, and is used for determining free-copper surface areas of catalysts. The results are compared with those of X-ray line broadening and electron microscopy. Emmett and Skau's carbon monoxide method for determining free-copper surface areas is briefly discussed.

Absorption spectra of dimer cations. Part 2.—Benzene derivatives

Abstract: Radical cations of benzene, fluorobenzene, toluene, m-xylene, mesitylene, hexamethyl-benzene and p-tert-butyl-toluene have been prepared by γ-radiolysis of glassy solutions in a mixture of isopentane and n-butyl chloride at 77 K. The effects of concentration and of slight warming are used to assign the absorption bands to monomeric and dimeric species (M+ and M+2); the latter presumably have “sandwich” configurations like excimers. The assignments are supported by the resemblance of the dimer bands to those of para-cyclophane : some α,ω-diphenyl alkanes were also studied. Attempts at preparing “dimer anions” were unsuccessful, except with para-cyclophane. The spectra of the cations of the methyl-benzenes showed extra peaks, probably due to carbonium ions formed by hydrogen-atom transfer.

Excess energies in mass spectra of some oxygen-containing organic compounds

Abstract: Excess energies of the electron-impact induced dissociation of several oxygen-containing compounds have been estimated by measuring the translational energies of the fragment ions. The heats of formation of CH3O, C2H5O, CH3CO, HCO, COOH, and their positive ions have been determined. The formyl radical has a heat of formation of 8 ± 3 kcal/mole. Ions of composition CH3O+ and C2H5O+ from these compounds always have the hydroxy carbinyl structure at threshold energies. The method is shown to be competitive with the best methods of chemical kinetics for determining bond dissociation energies.

Conductance of electrolyte solutions. A modified expression for its concentration dependence

Abstract: Fuoss and Hsia have recalculated the relaxation field of the Fuoss and Onsager (F–O) theory for electrolytic conductance since the F–O equation failed to fit experimental data with sufficient accuracy. The resulting expression, which is complicated, was used in the present work to calculate the coefficients J1 and J2 of the equation, Λ=Λ0–Sc½+Ec ln c+J1c–J2c. The new expression is superior to the previous Fuoss and Onsager expression and the discrepancies between the latter theory and that of Pitts are much reduced when the new J1 and J2 coefficients are used.

Phase equilibria of dodecylhexaoxyethylene glycol monoether in water

Abstract: Low-angle X-ray diffraction has been used to characterize the two lyotropic mesomorphic phases (neat phase and middle phase) which are formed in the dodecylhexaoxyethylene glycol monoether + water system. The occurrence of these two anisotropic mesomorphic phases does not affect the behaviour of the thermodynamic excess functions which have been interpreted in terms of solute-water interactions. Careful measurements of water vapour pressure have been necessary to establish the limited extent of the two phase co-existence regions which separate homogeneous mesomorphic phase regions from isotropic solution in this system. Volume and heat capacity measurements as a function of temperature across mesomorphic phase boundaries have confirmed that the neat phase/isotropic solution and the middle phase/isotropic solution transformations are first-order phase transitions with very small volume and enthalpy changes.

Kerr effect in methane and its four fluorinated derivatives

Abstract: Electric birefringence has been measured at a wavelength of 632.8 nm over a range of temperature and pressure in the gases CH4, CH3F, CH2F2, CHF3 and CF4. The results yield values of the second hyperpolarizability γ for CH4 and CF4 and, for CH3F, CH2F2 and CHF3, the first hyperpolarizability β and the difference (α33–α) between the optical polarizability component in the direction of the molecular dipole and its mean. For CH2F2(α33–α) is unusually small; the birefringence changes sign as the pressure is increased, the change occurring at 1.7 bar at 244°K. Limitations of a bond additivity model for β are exposed, and the importance of hyperpolarizability contributions to the Kerr constant demonstrated. The density dependence of the birefringence in the five gases is compared with the predictions of simple theoretical models, but agreement is in most cases poor.

Distribution of n-alkanols between water and n-alkanes

Abstract: The distribution ratios for 20°C of four n-alkanols between water and each of three n-alkanes have been determined. Combination of the results with calorimetrically-determined heats of solution of the alkanols in water and in alkanes at high dilution have yielded standard free energies, heats and entropies of transfer of the alkanols from water to the alkanes. The standard free energies and entropies of solution of alkanols in the alkanes have also been derived. In addition, integral heats of solution of 1-octanol in n-decane at 20, 25 and 30°C have been determined in a concentration region where association of the alcohol is occurring. These heats can be adequately explained using an association model proposed by Heller and Fletcher, based on infra-red measurements.

Infra-red spectra and spectral shifts of CO adsorbed on evaporated alkali-halides. Part 1.—Films of sodium salts

Abstract: Infra-red spectra of CO adsorbed on NaCl, NaBr and NaI films were recorded at –196°C and for different surface coverages. The high surface area films were prepared by a method described previously.1 The absorption bands were shifted, with respect to the gas phase, towards higher energies. For NaCl as adsorbent, the heat of adsorption was calculated for various adsorption sites and orientations of the adsorbate, and was found in agreement with experiment for molecules adsorbed perpendicularly on the surface Na+ ions. A high contribution from the quadrupole field-gradient interaction was found. Spectral shifts were calculated, using the perturbation method. The perturbing potential was taken as a sum of four attractive and repulsion potentials. Good agreement with the experimental value of Δν was found.

Vaporization, thermodynamics and structures of species in the tellurium + oxygen system

Abstract: High-temperature mass-spectrometric investigations of the equilibrium vapours over crystalline tellurium dioxide have shown the existence of the molecular species (TeO2)n and (TeO)n, where n= 1, 2, 3, 4 as well as Te2 and O2. Under these conditions the most abundant species are TeO2(g) and TeO(g). Second- and third-law heats of reaction and entropies for the various equilibria among these species have been obtained. Fundamental frequencies obtained from infra-red absorption studies of the species isolated in rare-gas matrices were used for calculations of thermodynamic functions at various temperatures. Heats of atomization for (TeO2)2 and (TeO)2 were obtained. Using the average of the second and third-law heats, ΔH°298= 2.5 ± 2 kcal mol–1, for the reaction TeO(g)=½Te2(g)+½O2(g) one obtains the dissociation energy for TeO(g), D°0= 92.5 ± 2 kcal mol–1. A bent geometry (110°) is indicated for TeO2 and evidence for cyclic structures of (TeO2)2 and (TeO)2 is given.

Evaluation of microporosity, with special reference to a carbon black

Abstract: If an adsorbent yields a type II isotherm (BDDT classification), then the value of the monolayer capacity, and therefore of the specific surface, calculated from the isotherm will be incorrect unless the micropore contribution is allowed for. A method for the evaluation of the micropore contribution of a carbon black to the adsorption isotherm of nitrogen at –195°C is described. The micropores are filled with nonane at –195°C by adsorption from the vapour phase, and the excess pumped off at room temperature. The adsorption isotherm of nitrogen at –195°C is determined on the sample, both before and after thus filling the micropores. The volume of the micropores is then given by the distance between the (parallel) portions of the two isotherms corresponding to the multilayer region.

Critical state in chain-molecule mixtures

Abstract: The Prigogine corresponding states theory and a cell model are used to predict the main features of the critical line in P,T, composition space for a mixture of chain-molecules. A simple van der Waals dependence of the configurational energy on volume (as used by Flory) is assumed in most of the calculations. For a mixture of homologues of small chain-length difference, a continuous critical line is predicted; it ends at the critical points of the pure components rising to a high maximum critical pressure at an intermediate concentration. Comparison is made between the predicted and observed lines for normal alkanes. With a large chain-length difference between components, the line is broken at low concentration of the longer molecules by a three-phase line running between a lower critical solution point (L.C.S.P.) and an upper critical end-point (U.C.E.P.). For normal alkane mixtures, methane + C7, ethane + C20, propane + C32, are predicted to be the system of minimum chain-length difference which show the L.C.S.P., in agreement with experiment. Satisfactory predictions without adjustment of any parameters are made for portions of the critical line found by Ehrlich and Kurpen for polyethylene in the n-alkanes from ethane to pentane. The pressure dependence of the lower critical solution temperature at zero pressure is successfully predicted for polymer + solvent systems. The free energy of mixing is obtained as a function of pressure and temperature for chain-molecules of different species. The pressure dependences of the upper and lower critical solution temperatures are compared.

Absorption spectra of dimer cations. Part 3.—Naphthalene and anthracene derivatives and pyrene

Abstract: The absorption spectra of γ-irradiated solutions of naphthalene in glasses at 77 K provide further evidence for the existence of dimer cations (C10H8)+2. Results are also presented for several methyl-naphthalenes, pyrene and 1,2-di(1-naphthyl)-ethane in which a weak intra-molecular interaction occurs. Attempts at producing a dimer anion were unsuccessful. Dimer cations of 9-methyl- and 9-bromo-anthrancene were produced by radiolysis of dilute solutions of the di-anthracenes; there appear to be two distinct dimers in this case.

Absorption spectra of dimer cations. Part 1.—Olefins

Abstract: The absorption spectra of γ-irradiated glasses at 77 K containing cyclohexene, butadiene and penta-1,3-diene have been studied. The effects of warming and recooling and of concentration show that the bands can be assigned to monomeric and dimeric radical cations (M+ and M+2). Assignments of the corresponding spectra of a number of olefins obtained by other workers are discussed.

Migration and trapping of extrinsic charge carriers in polymer films

Abstract: The behaviour of thin polymer films charged by a negative corona discharge in air can be described quantitatively in terms of diffusional migration and trapping of extrinsic charge carriers injected into the material Up to a field strength of about 105 V cm–1 charge migration is characterized by a field-independent mobility At higher fields the mobility becomes an approximately linear function of field strength In the polymers investigated mobilities range from 10–9 to 10–13 cm2 V–1 s–1, corresponding to diffusion coefficients of 10–10 to 10–14 cm2 s–1 Mobilities increase with the solvent content of the polymer and with temperature The temperature dependence of mobility is adequately described by an Arrhenius equation with activation energies between 10 and 40 kcal mol–1 Charge trapping is indicated by a persistent residual potential which is a function of the injected charge density, the film thickness and the mobility of the charges The inherent trapping capability of the material is characterized by a trapping frequency In a few cases, cross sections of the charge traps could be estimated and were of molecular magnitude (5–13 A)

Electrical conductivity and thermoelectric power of the molten Tl + Se and Tl + Te systems

Abstract: The electrical conductivity and thermoelectric power of the liquid T1 + Se and T1 + Te systems have been measured as a function of concentration and temperature. Both of the systems show a deep minimum in the electrical conductivity at the composition corresponding to T12Se and T12Te. The absolute thermoelectric power changes its sign around this composition, being negative for the T1-rich solutions and positive for the Se- and Te-rich solutions. The impurity effect was examined in the T12Se melt. The electrical conductivity of the saturation range, where the carrier density can be considered to be constant, is discussed in the light of the scattering model.

Infra-red cryogenic studies. Part 1.—Hydrogen halide multimers

Abstract: Infra-red spectra are reported of HCl, HBr and HI in argon and other matrices at 20 K. Studies have been made over a wide concentration range from matrix/absorber ratio 2000 to 50 which, together with diffusion (“warm-up”) experiments, enable an interpretation to be offered of the complex pattern of frequencies. The first two intense association bands which arise are assigned to cyclic dimers and trimers respectively. Bands are also assigned to various multimeric species and the frequencies compare favourably with values calculated on the basis of a model based on that proposed by Keyser and Robinson.

Mechanism of S2 chemiluminescence in the reaction of hydrogen atoms with hydrogen sulphide

Abstract: Ground-state sulphur atoms were generated in a discharge flow system by the rapid reaction between H and H2S: H + H2S = H2+ HS (1), H + HS = H2+ S (2) The resulting chemiluminescence by the B3Σ–u state of S2 is shown to arise (a) by two-body combination of S atoms into level v′= 10; (b) by three-body recombination into lower levels: S(3P)+ S(3P)+ M = S*2(B3Σ–uv′⩽9)+ M (3), S*2= S2(X3Σ–g)+hv(4), S*2+ M = S2+ M (5) with k3=(1.0 ± 0.2)× 1015 cm6 mole–2 sec–1 and k4/k5= 2.0 × 10–7 mole cm–3 for M = Ar at 298°K; also D(S—S)= 101.7 ± 0.4 kcal/mole. The mechanism of the sulphur-catalyzed heterogeneous recombination of hydrogen atoms is studied and discussed.

Catalytic oxidation of methane over noble metals

Abstract: The complete oxidation of methane over palladium, platinum, rhodium and iridium has been studied using a microcalorimetric technique. The results indicated that methane may be adsorbed on two types of reaction site, one of which also adsorbs oxygen. The activation energies are proportional to the Pauling bond energies of the oxygen-metal bond in the region where the order in oxygen is low.

Statistical thermodynamics of two-fluid models of mixtures

Abstract: An approximation of a type devised by van der Waals has been shown to be superior to the random mixing approximation, and to theories derived from it, when applied to mixtures of molecules of different sizes. In this paper, van der Waals's approximation is extended to a two-fluid model, and so takes into account the departures from a random distribution induced by differences of intermolecular energy. The treatment is further extended to mixtures of molecules of different shapes. The two-fluid van der Waals approximation is compared with the experimental excess thermodynamic functions for ten simple binary mixtures. These yield unambiguous evidence that the intermolecular energy between unlike molecules is less than the geometric mean of that between the like molecules by up to 2 %. Independent and quantitative confirmation of such differences is provided by recent measurements of the second virial coefficients and of the critical temperatures of the same mixtures.

Formation and decomposition of surface oxide in carbon combustion

Abstract: The formation of surface oxide on graphitized carbon black by reaction with oxygen at 500°C and the subsequent desorption of the oxide as CO at 600–750°C have been studied gravimetrically. The surface oxide is formed exclusively during the initial transient period of fast combustion observed by previous workers, and for each O atom chemisorbed one labile C atom is removed from the surface. The kinetics of desorption follow the relation: –dθ/dt=Dθ exp {–[E1+β(1 –θ)]/RT}, where log10Dθ= log10D1+α(1 –θ), and θ is the fraction of O on the surface at time t related to that present during steady combustion. D1, E1, α and β are constants. Values of 70 and 57 kcal mole–1 were found for E1 with two specimens at lower and higher burn-off respectively; the corresponding values of β were 43 and 47 kcal mole–1. Apart from the effect of catalytic impurities, two distinct processes are required to account for the kinetics of combustion. Only one of these produces stable oxide. A hypothesis is proposed which identifies this with reaction at “arm-chair”{112l} edges of the carbon lattice.

Infra-red cryogenic studies. Part 3.—Hydrogen halides in doped argon matrices

Abstract: Infra-red spectra are reported of binary mixtures of HCl, HBr or HI (HX) with O2, N2, CO2, CO or C2H4(D) in argon matrices at 20 K. With the exception of O2, three induced absorption bands are observed which are interpreted as arising from HX—D, HX—D—HX and (HX)2—D interactions.

Excess volume of mixing

Abstract: Excess volumes VE of mixing for eight binary mixtures as a function of temperature and composition have been determined and the results have been used in examining the theory of Flory for binary mixtures differing in shape and size. Prigogine's refined version of the cell theory using average potential model has also been examined.

Rotational structure of some hydrocarbon flame bands

Abstract: A number of hydrocarbon flame bands have been photographed in high orders of diffraction of a 3.4-m Ebert spectrograph, with an achieved resolving power of about 100,000. The spectrum contains both sharp and diffuse bands, and there are two distinct types of band envelope. Strong evidence is presented that the lower state of the hydrocarbon flame bands is the [graphic omitted], 2A′ ground state of the HCO radical, and that the A and B bands form two separate band systems. The origins are assigned as : [graphic omitted], 2A′ state, T00= 38691 cm–1, [graphic omitted] state, T00= 41270 cm–1. Partial rotational analyses of three bands in the A1 progression show that these are type a bands, and lead to the following structure for HCO in the [graphic omitted], 2A′ state : rCH= 1.16 A(assumed), rCO= 1.36 ± 0.02 A, HCO = 111 ± 4°.

Electrical conductivities of aqueous solutions of KCl, KOH and HCl, and the ionization of water at high shock pressures

Abstract: Measurements have been made of the electrical conductivities of aqueous solutions of KCl, KOH and HCl at shock wave pressures in the range 70–133 kbar. The molar conductivity of KCl was not measurably different from its normal value, and this finding is interpreted as meaning that although the temperature jump in the shock front tends to increase the mobilities of K+ and Cl– ions, the simultaneous jump in density has an opposite and cancelling effect. On the other hand, the conductivities of HCl and KOH increased several fold, indicating that the excess mobilities of hydrogen ions and hydroxyl ions increase by a factor of 3-6 under the shock conditions. By combining the measured ionic conductivities with earlier measurements of the conductivity of pure water it has been estimated that the ionization product Kw of water increases to about 0.09 mol2 kg–2 at a shock pressure af 133 kbar. It is suggested that the ionization of water into H3O+ and OH– ions becomes nearly complete at 200 kbar and that this transformation may be responsible for the difference in the pressure-volume behaviour of shocked water below and above the 150–200 kbar region.

Solution of hydrogen in niobium

Abstract: Ultra-high vacuum techniques have been used to investigate the phase diagram of the hydrogen + niobium system by pressure and electrical resistance measurements over the ranges –78 to 300°C, H/Nb = 0–0.65, and 10–7–10 torr [1 torr = 1.33 × 103 N m–2]. Rapid sorption of gaseous hydrogen was observed at all temperatures down to –78°C. The boundary of the two-phase region is symmetrical about H/Nb = 0.256. The critical point is at 211°C, H/Nb = 0.256, 1 × 10–1 torr. The plateaux pressures p in the two-phase region are given by log10p(torr)=(7.87 ± 0.14)—(82.5 ± 1.1) kJ (mol 2.303 RT)–1. Isotherms of electrical resistivity plotted against hydrogen concentration show changes of slope at H/Nb ratios corresponding to the onset of hydride formation and reach maxima at H/Nb = 0.5. Matthiessen's rule is obeyed in the solid-solutions (α) phase while in the mixed-phase region a marked resistivity-temperature hysteresis occurs near 86°C. The hysteresis is thought to be associated with a change in lattice structure of the hydride.

Unified theory of explosions, cool flames and two stage ignitions. Part 2

Abstract: A unified (chain-thermal) theory is applied to cool flame and critical phenomena observed in the static oxidations of many organic compounds in the low-temperature region. The cool-flame oscillations, negative temperature coefficient, lobes on the critical curve and numerous physical discontinuities often observed are reproduced in the theory and causally linked thereby. General requirements of the type of kinetic scheme necessary are found, but the theory can throw no light on the species involved.