Showing papers in "Physical Chemistry Chemical Physics in 2004"

Miniature biofuel cells

Abstract: The history of electrochemical power sources shows that batteries or fuel cells were introduced only when the development of new electrical or electronic system demanded these. At this time, the already established feasibility of miniaturization of implantable sensor-transmitter systems to volumes smaller than 1 mm3, and the demand for spatially and temporally resolved information on local temperature, flow, pressure and chemical concentrations, are likely to create a demand for a miniature, low cost glucose-O2 biofuel cell that would power the autonomous sensor–transmitters for a few weeks. Prototypes of these cells are in hand. Their most unique feature which is their structural simplicity, is made possible by the selectivity of their “wired” enzyme catalysts: the cells consist merely of two 7 μm diameter carbon fibers, each coated with a different “wired” enzyme bioelectrocatalyst. On one, catalyzing the two-electron electrooxidation of glucose at a reducing potential, glucose oxidase is co-immobilized in and electrically connected (“wired”) by an electron conducting hydrogel of a reducing redox potential. On the other, catalyzing the four electron electroreduction of O2 to water, bilirubin oxidase is co-immobilized in and electrically “wired” by an electron conducting hydrogel of an oxidizing potential. The cells are the smallest ever built. When the volume of the fibers is 0.0026 mm3, the current of the cell operating at 0.52 V in a physiological buffer solution at 37°C is 8.3 μA. The 4.3 μW power output of the cell is expected to suffice for the operation of implanted sensors and for the intermittent transmission of the data collected to an external receiver.

Induced magnetic fields in aromatic [n]-annulenes—interpretation of NICS tensor components

Abstract: The components of nucleus-independent chemical shift (NICS) tensors for Dnhn-annulenes are discussed as indexes of the aromatic character of electronic π systems. The component corresponding to the principal axis perpendicular to the ring plane, NICSzz, is found to be a good measure for the characterisation of the π system of the ring. Isotropic NICS values at ring centres contain large influences from the σ system and from all three principal components of the NICS tensor. At large distances away from the ring center, NICSzz, which is dominated by contributions from the π system, characterizes NICS well.

Assessment of aromaticity via molecular response properties

Abstract: The possibility of founding a quantitative theory of aromaticity on the basis of measurable response properties is discussed. The reasons suggesting that nonmeasurable parameters are unsuitable for quantitative evaluation of aromaticity are analyzed.

First local minimum of the formic acid dimer exhibits simultaneously red-shifted O?H?O and improper blue-shifted C?H?O hydrogen bonds

Abstract: The first local minimum of the formic acid dimer exhibits simultaneously red-shifted O–H⋯O and blue-shifted C–H⋯O hydrogen bonds. The improper, blue-shifted hydrogen bond was investigated by means of the NBO analysis, rehybridization model and optimization in the electric field. It was shown that the electrostatic model cannot describe the nature of the blue-shifted H-bond. From the NBO analysis it becomes evident that the formation of the O–H⋯O hydrogen bond and C–H⋯O improper hydrogen bond can be explained on the basis of an increase of electron density in the σ* antibonding O–H orbital and a decrease of electron density in the σ* antibonding C–H orbital. While the former effect is easily explained on the basis of hyperconjugation, the latter requires the existence of a new mesomeric structure characterized by delocalization of electron density from the C–H σ* antibonding orbital to the remaining part of the complex. The rehybridization model explains properly the formation of both hydrogen bonds but fails to interpret the changes of the other bonds.

Relativistic quantum chemistry: the multiconfigurational approach

Abstract: A multiconfigurational approach to the quantum chemistry of heavy element compounds is described. Relativistic effects are treated in two steps, both based on the Douglas–Kroll Hamiltonian. Scalar terms are included in the basis set generation and are used to determine wave functions and energies, which include static (through the use of the CASSCF method) and dynamic correlation effects (using multiconfigurational perturbation theory, CASPT2). Spin–orbit coupling is treated in a configuration interaction model, which uses CASSCF wave functions as the basis states. The method is shown to work for all atoms of the periodic system, with the possible exception of the heavier fifth row main group atoms. Illustrative results are presented for the main group atoms (spin–orbit splittings), the electronic spectrum of the iridium atom, the ground state of Tl2 and Pb2, and for the electronic spectrum of PbO. Some applications in actinide chemistry are also discussed.

Microemulsions with ionic liquid polar domains

Abstract: In this work 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4], an ionic liquid)/Triton X-100 (TX-100)/cyclohexane microemulsions have been prepared and characterized by phase behavior, conductivity measurement, dynamic light scattering, freeze-fracturing electron microscopy, and UV-vis techniques, and our attention is concentrated on the microemulsions with the ionic liquid as the nano-sized polar domains.

Measurement of single molecule conductivity using the spontaneous formation of molecular wires

Abstract: A technique to measure the electrical conductivity of single molecules has been demonstrated. The method is based on trapping molecules between an STM tip and a substrate. The spontaneous attachment and detachment of α,ω-alkanedithiol molecular wires was easily monitored in the time domain. Electrical contact between the target molecule and the gold probes was achieved by the use of thiol groups present at each end of the molecule. Characteristic jumps in the tunnelling current were observed when the tip was positioned at a constant height and the STM feedback loop was disabled. Histograms of the measured current jump values were used to calculate the molecular conductivity as a function of bias and chain length. In addition, it is demonstrated that these measurements can be carried out in a variety of environments, including aqueous electrolytes. The changes in conductivity with chain length obtained are in agreement with previous results obtained using a conducting AFM and the origin of some discrepancies in the literature is analysed.

Electronic relaxation dynamics in DNA and RNA bases studied by time-resolved photoelectron spectroscopy

Abstract: We present femtosecond time-resolved photoelectron spectra (TRPES) of the DNA and RNA bases adenine, cytosine, thymine, and uracil in a molecular beam. We discuss in detail the analysis of our adenine TRPES spectra. A global two-dimensional fit of the time and energy-resolved spectra allows for reliable separation of photoelectron spectra from several channels, even for overlapping bands. Ab initio calculations of Koopmans’ ionization correlations and He(I) photoelectron spectra aid the assignment of electronically excited states involved in the relaxation dynamics. Based upon our results, we propose the following mechanism for electronic relaxation dynamics in adenine: Pump wavelengths of 250, 267 and 277 nm lead to initial excitation of the bright S2(pp*) state. Close to the band origin (277 nm), the lifetime is several picoseconds. At higher vibronic levels, i.e. 250 and 267 nm excitation, rapid internal conversion (t < 50 fs) populates the lower lying S1(np*) state which has a lifetime of 750 fs. At 267 nm, we found evidence for an additional channel which is consistent with the dissociative S3(ps*) state, previously proposed as an ultrafast relaxation pathway from S2(pp*). We present preliminary results from TRPES measurements of the other DNA bases at 250 nm excitation.

Ab initio studies on the photophysics of the guanine?cytosine base pair

Abstract: The low-lying excited singlet states of the Watson–Crick form of the guanine–cytosine base pair have been investigated with multi-reference ab initio methods (complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory based on the CASSCF reference (CASPT2)). The reaction paths and energy profiles for single proton transfer from guanine to cytosine in the 1ππ* guanine-to-cytosine charge-transfer state and for twisting of the CC double bond of the cytosine ring in the locally excited 1ππ* state of cytosine have been explored by excited-state geometry optimization using the configuration-interaction-with-singles (CIS) method and single-point energy calculations at the CASPT2 level. Avoided crossings of the 1ππ* potential-energy functions with the electronic ground-state potential-energy function have been identified along both reaction paths. The results suggest the existence of low-lying conical intersections of the 1ππ* potential-energy surface with the S0 surface which become accessible by possibly barrierless single proton transfer as well as out-of-plane deformation of cytosine and may trigger an ultrafast radiationless decay to the ground state. The relevance of these results for the rationalization of the photostability of the genetic code is briefly discussed.

Crystal Structure of Cyanobacterial Photosystem II at 3.2 A Resolution: A Closer Look at the Mn- Cluster

Abstract: In the crystal structure of photosystem II (PSII) from the cyanobacterium Thermosynechococcus elongatus at 3.2 A resolution, several loop regions of the principal protein subunits are now defined that were not interpretable previously at 3.8 A resolution. The head groups and side chains of the organic cofactors of the electron transfer chain and of antenna chlorophyll a (Chl a) have been modeled, coordinating and hydrogen bonding amino acids identified and the nature of the binding pockets derived. The orientations of these cofactors resemble those of the reaction center from anoxygenic purple bacteria, but differences in hydrogen bonding and protein environment modulate their properties and provide the unique high redox potential (1.17 V) of the primary donor. Coordinating amino acids of manganese cluster, redox-active TyrZ and non-haem Fe2+ have been determined, and an all-trans β-carotene connects cytochrome b-559, ChlZ and primary electron donor (coordinates are available under PDB-code 1W5C).

Zeta potential and photocatalytic activity of nitrogen doped TiO2 thin films

Abstract: Nitrogen doped TiO2 films were fabricated by annealing anatase TiO2 films in gaseous NH3. Nitrogen atoms in these films were substitutionaly introduced into oxygen sites of TiO2 lattice. We have evaluated the zeta potential of these films and have found that it became negative with the amount of nitrogen doping. The photocatalytic decomposition of gaseous 2-propanol was examined under light-limited conditions, and the quantum yield (QY) was estimated while illuminating with ultraviolet (UV) and visible light (VIS). The QY values for the nitrogen doped TiO2 film were 15.4% (UV) and 0.41% (VIS), whereas those for the pure TiO2 film were 18.4% (UV) and 0% (VIS). In addition, the photocatalytic decomposition of ethylamine ions (CH3CH2NH3+) was evaluated in an aqueous solution under UV illumination in a light-rich condition. Consequently, the decomposition rate of the nitrogen doped TiO2 for aqueous ethylamine ions was higher than that of the pure TiO2 because the surface of the nitrogen doped TiO2 film was negatively charged and cationic ethylamine ions were efficiently adsorbed on its surface. The surface structure and the band structure of the nitrogen doped TiO2 are discussed in this paper.

Efficient evaluation of three-center two-electron integrals over Gaussian functions

Abstract: The RI (resolution of the identity) technique achieves significant increases in efficiency for various molecular electronic structure methods This results from the approximation of four-center two-electron integrals by corresponding three-center integrals It is shown that the three-center integrals required can be evaluated with a much simpler algorithm than for the general case This further increases the advantage of RI procedures

Tests of second-generation and third-generation density functionals for thermochemical kinetics

Abstract: We report tests of second- and third-generation density functionals, for pure density functional theory (DFT) and hybrid DFT, against the BH6 representative barrier height database and the AE6 representative atomization energy database, with augmented, polarized double and triple zeta basis sets. The pure DFT methods tested are G96LYP, BB95, PBE, mPWPW91, VSXC, HCTH, OLYP, and OPW91 and the hybrid DFT methods tested are B1B95, PBE0, mPW1PW91, B97-1, B98, MPW1K, B97-2, and O3LYP. The performance of these methods is tested against each other as well as against first-generation methods (BP86, BLYP, PW91, B3PW91, and B3LYP). We conclude that the overall performance of the second-generation DFT methods is considerably better than the first-generation methods. The MPW1K method is very good for barrier height calculations, and none of the pure DFT methods outperforms any of the hybrid DFT methods for kinetics. The B1B95, VSXC, B98, OLYP and O3LYP methods perform best for atomization energies. Using a mean mean unsigned error criterion (MMUE) that involves two sizes of basis sets (both with polarization and diffuse functions) and averages mean unsigned errors in barrier heights and in atomization energy per bond, we find that VSXC has the best performance among pure functionals, and B97-2, MPW1K, and B1B95 have the best performance of all hybrid functionals tested.

The nature of water on surfaces of laboratory systems and implications for heterogeneous chemistry in the troposphere

Abstract: A number of heterogeneous reactions of atmospheric importance occur in thin water films on surfaces in the earth's boundary layer. It is therefore important to understand the interaction of water with various materials, both those used to study heterogeneous chemistry in laboratory systems, as well as those found in the atmosphere. We report here studies at 22 °C to characterize the interaction of water with such materials as a function of relative humidity from 0–100%. The surfaces studied include borosilicate glass, both untreated and after cleaning by three different methods (water, hydrogen peroxide and an argon plasma discharge), quartz, FEP Teflon film, a self assembled monolayer of n-octyltrichlorosilane (C8 SAM) on glass, halocarbon wax coatings prepared by two different methods, and several different types of Teflon coatings on solid substrates. Four types of measurements covering the range from the macroscopic level to the molecular scale were made: (1) contact angle measurements of water droplets on these surfaces to obtain macroscopic scale data on the water-surface interaction, (2) atomic force microscopy measurements to provide micron to sub-micron level data on the surface topography, (3) transmission FTIR of the surfaces in the presence of increasing water vapor concentrations to probe the interaction with the surface at a molecular level, and (4) X-ray photoelectron spectroscopy measurements of the elemental surface composition of the glass and quartz samples. Both borosilicate glass and the halocarbon wax coatings adsorbed significantly more water than the FEP Teflon film, which can be explained by a combination of the chemical nature of the surfaces and their physical topography. The C8 SAM, which is both hydrophobic and has a low surface roughness, takes up little water. The implications for the formation of thin water films on various surfaces in contact with the atmosphere, including building materials, soil, and vegetation, are discussed.

Ice perfection and onset of anomalous preservation of gas hydrates

Abstract: Anomalous preservation is the well-established but little-understood phenomenon of a long-term stability of gas hydrates outside their thermodynamic field of stability. It occurs after some initial decomposition into ice in the temperature range between 240 and 273 K. In situ neutron diffraction experiments reveal that the low-temperature on-set of this effect coincides with the annealing of stacking faults of the ice formed initially. The defective, stacking-faulty ice below 240 K apparently does not present an appreciable diffusion barrier for gas molecules while the annealed ordinary ice Ih above this temperature clearly hinders gas diffusion. This is supported by further experiments showing that the so-called ice Ic formed from various high-pressure phases of ice, gas hydrates or amorphous ices does transform fully to ordinary ice Ih only at temperatures near 240 K, i.e. at distinctly higher temperatures than generally assumed. In this light, some quite disparate observations on the transformation process from ice Ic to ice Ih can now be better understood. The transformation upon heating is a multistep-process and its details depend on the starting material and the sample history. This ‘memory’ is finally lost at approximately 240 K for laboratory time-scale experiments.

Effect of a conjugated carbonyl group on the photophysical properties of carotenoids

Abstract: Effects of introducing a carbonyl group into the conjugation system of carotenoids were studied for four naturally occurring carotenoids: peridinin, fucoxanthin, siphonaxanthin and spheroidenone. The conjugated carbonyl group affects energetics and dynamics of all these carotenoids in a similar way, although the magnitude of the changes depends strongly on the carotenoid structure. Firstly, presence of a carbonyl group considerably narrows the S1/ICT–S2 gap, and this effect does not depend on polarity. The S1/ICT energies of carotenoids were measured by recording S1/ICT–S2 spectral profiles in the near-infrared region and the resulting energies were 16100 cm−1 for peridinin, 16520 cm−1 for fucoxanthin and 16610 cm−1 for siphonaxanthin. Narrowing of the S1/ICT–S2 gap has important consequences for functionality of these carotenoids in light-harvesting systems of oceanic organisms, since while the S2 state is red-shifted to capture green light, the S1/ICT state is still high enough to transfer energy to chlorophyll. The S1/ICT energy of spheroidenone was determined to be 13000 cm−1. Secondly the carbonyl group introduces some polarity-dependent effects: (1) polarity-induced change of the S1/ICT lifetime. When changing from nonpolar to polar solvent, the S1/ICT lifetime is changed from 160 to 8.5 ps for peridinin, from 60 to 30 ps for fucoxanthin, from 60 to 20 ps for fucoxanthin, while for the longer carotenoid spheroidenone the S1/ICT lifetime remains 6 ps regardless of solvent polarity. This effect is explained in terms of stabilization of charge-transfer character of both ground and excited states. (2) stabilization of the charge-transfer character in polar solvents is also demonstrated by appearance of new polarity-induced bands in the transient absorption spectra. (3) polarity-induced changes of the ground state are manifested by asymmetric broadening of the absorption spectrum accompanied by a loss of vibrational structure.

X-ray absorption spectroscopy under reaction conditions: suitability of different reaction cells for combined catalyst characterization and time-resolved studies

Abstract: Structure–activity correlations of solid catalysts and time-resolved studies generally require that the structure within a solid catalyst is probed simultaneously and at the same location where the catalysis and the structural changes occur These requirements lead to a compromise between the spectroscopic arrangement and the optimum design for an in situ reactor cell Opportunities and limitations of in situ and time-resolved X-ray absorption spectroscopy (XAS) combined with gas analysis are critically analysed with the help of two different cell designs, an in situ EXAFS cell designed for solids in the form of pressed wafers and a capillary cell where the catalyst is packed similarly to a plug flow reactor On the basis of three examples, the reduction of CuO/ZnO, the reduction of PdO/ZrO2 and methane oxidation over PdOx/ZrO2, criteria are developed which allow to judge the appropriate cell design in solid–fluid reactions The prerequisites for the design of an in situ cell including the catalyst shape strongly depend on the time resolution required Important issues embrace the type of reaction to be investigated (slow vs fast), the reaction medium, and the porosity of the catalyst material Criteria for assessing the role of pore and film diffusion in in situ studies are of paramount importance for a proper experimental design

Measurements of the material properties of metal nanoparticles by time-resolved spectroscopy

Abstract: An important aim of nanoparticle research is to understand how the properties of materials depend on their size and shape. In this Invited Article I describe how time-resolved spectroscopy can be used to measure physical properties of nanometre sized objects such as the characteristic time scales for electron–phonon coupling and heat dissipation, and their elastic moduli. The electron–phonon coupling and heat dissipation measurements are important for applications of particles that involve conduction of heat or electricity. On the other hand, the elastic moduli studies provide fundamental information about the properties of nanomaterials. The results of these experiments show that nanometre sized particles can have very different properties compared to the corresponding bulk material. For example, we have recently shown that gold nanorods produced by wet chemical methods have much smaller elastic moduli (an 18% decrease in Young’s modulus) compared to bulk gold.

Structure-based mechanism of photosynthetic water oxidation

Abstract: The recently-published 3.5 A resolution X-ray crystal structure of a cyanobacterial photosystem II (PDB entry 1S5L) provides a detailed architecture of the oxygen-evolving complex (OEC) and the surrounding amino-acids [K. N. Ferreira, T. M. Iverson, K. Maghlaoui, J. Barber and S. Iwata, Science, 2004, 203, 1831–1838]. The revealed geometry of the OEC lends weight to certain hypothesized mechanisms for water-splitting, including the one propounded by this group, in which a calcium-bound water acts as a nucleophile to attack the oxygen of a MnVO group in the crucial O–O bond-forming step [J. S. Vrettos, J. Limburg and G. W. Brudvig, Biochim. Biophys. Acta, 2001, 1503, 229–245]. Here we re-examine this mechanism in the light of the new crystallographic information and make detailed suggestions concerning the mechanistic functions (especially the redox and proton-transfer roles) of calcium, chloride and certain amino-acid residues in and around the OEC. In particular, we propose an important role for an arginine residue, CP43–Arg357, in abstracting protons from a substrate water molecule during the water-splitting reaction.

A novel flow battery: A lead acid battery based on an electrolyte with soluble lead(II). Part II. Flow cell studies

Abstract: This series of papers will describe the chemistry, electrochemistry and performance of a flow battery with no separator and a single electrolyte, lead(II) in methanesulfonic acid. Voltammetry at rotating disc electrodes is used to define the conditions for the high rate deposition and dissolution of lead and lead dioxide in aqueous methanesulfonic acid. The determination of lead methanesulfonate solubility and solution conductivity as a function of lead methanesulfonate and methanesulfonic acid concentrations was carried out in order to aid the selection of the electrolyte for the battery.

High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of pyrrole

Abstract: The fragmentation dynamics of pyrrole molecules following excitation at many wavelengths in the range 193.3 < λphot < 254.0 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Excitation at the longer wavelengths within this range results in (vibronically induced) population of the 11A2(πσ*) excited state, but once λphot ≤ 225 nm the electric dipole allowed 11B2 ← X1A1(π* ← π) transition becomes the dominant absorption. All of the total kinetic energy release (TKER) spectra so derived show a ‘fast’ peak, centred at TKER ∼7000 cm−1. Analysis of the structure evident in this peak, particularly in spectra recorded at the longer excitation wavelengths, reveals selective population of specific vibrational levels of the pyrrolyl co-fragment. These have been assigned by comparison with calculated normal mode vibrational frequencies, leading to a precise determination of the N–H bond strength in pyrrole: D0 = 32850 ± 40 cm−1, and the enthalpy of formation of the pyrrolyl radical: ΔfH0°(C4H4N) = 301.9 ± 0.5 kJ mol−1. The recoil anisotropy of the fast H atom photofragments formed following excitation to, and dissociation on, the 11A2(πσ*) potential energy surface (PES) is seen to depend upon the vibrational level of the pyrrolyl co-fragment. This observation, and the finding that the mean TKER associated with these fast H + pyrrolyl fragments is essentially independent of λphot, can be explained by assuming that, upon N–H bond fission, the skeletal vibrational motions in pyrrole(11A2) molecules evolve adiabatically into the corresponding modes of the ground state pyrrolyl fragment. A second, ‘slow’ peak is increasingly evident in TKER spectra recorded at shorter photolysis wavelengths, and becomes the dominant feature once λphot ≤ 218 nm. This component exhibits no recoil anisotropy; its TKER profile is reminiscent of that observed in many other dissociations that yield H atoms by ‘statistical’ decay of highly vibrationally excited ground state molecules. The form of the TKER spectra observed at these shorter excitation wavelengths is rationalised by assuming two possible decay routes for pyrrole molecules excited to the 1B2(ππ*) state. One involves fast 11B2 11A2 radiationless transfer and subsequent fragmentation on the 11A2 PES, yielding ‘fast’ H atoms (and pyrrolyl co-fragments)–reminiscent of behaviour seen at longer excitation wavelengths where the 11A2 PES is accessed directly. The second is assumed to involve radiationless transfer to the ground state, either by successive 11B2 11A2 X1A1 couplings mediated by conical intersections between the relevant PESs or, possibly, by an as yet unrecognised direct 11B2 X1A1 coupling, and subsequent unimolecular decay of the resulting highly vibrationally excited ground state molecules yielding ‘slow’ H atoms (together with, most probably, cyanoallyl co-fragments).

Evaluation of different mechanistic proposals for water oxidation in photosynthesis on the basis of Mn4OxCa structures for the catalytic site and spectroscopic data

Abstract: Recent progress in EPR and EXAFS spectroscopy, quantum mechanical calculations and X-ray crystallography led to a tremendous improvement in our picture of the catalytic site of water oxidation in photosystem II. It is now likely that the four Mn ions are grouped in a 3 + 1 fashion with three short Mn–Mn distances of about 2.7 A and one long Mn–Mn distance of 3.3 A. In addition, Ca has been firmly localized close to the Mn centers, with an average distance of 3.4 A and an average angle of the Mn–Ca vectors close to the membrane normal (≤23°). The recent crystal structure of Ferreira et al. (Science, 2004, 303, 1831–1838) suggests that the Mn3 unit and Ca form a distorted cubane like structure. The fourth Mn is ‘dangling’ from this unit either via a μ4-oxo bridge or a mono μ-oxo bridge. However, the precise Mn–Mn distances and the bridging situation still need to be worked out. On this structural basis and the available spectroscopic data two possible mechanisms for photosynthetic water oxidation are discussed in order to highlight questions that still need to be solved for a full understanding of this fascinating reaction.

Control and characterisation of a single aerosol droplet in a single-beam gradient-force optical trap

Abstract: Optical tweezers are used to control aerosol droplets, 4–14 μm in diameter, over time frames of hours at trapping powers of less than 10 mW When coupled with cavity enhanced Raman scattering (CERS), the evolution of the size of a single droplet can be examined with nanometre accuracy Trapping efficiencies for water and decane droplets are reported and the possible impact of droplet heating is discussed We demonstrate that the unique combination of optical tweezing and CERS can enable the fundamental factors governing the coagulation of two liquid droplets to be studied

Biomimetic synthesis of silica nanospheres depends on the aggregation and phase separation of polyamines in aqueous solution

Abstract: Long-chain polyamines extracted from the highly siliceous cell walls of diatoms are known to precipitate silica nanospheres from aqueous, silicic-acid containing solutions at near-neutral pH in vitro The same is true for synthetic polyamines such as polyallylamine In the present contribution we show that the microscopic phase separation of polyallylamine in aqueous solution is strictly correlated with the silica precipitation activity of polyallylamine/silicic acid solutions Multivalent anions such as phosphate or sulfate efficiently induce this microscopic phase separation At higher anion concentrations, macroscopic phase separation occurs In contrast to the multivalent phosphate and sulfate ions, the monovalent chloride ions are much less efficient in polyallylamine aggregate formation

Heterogeneous reaction of ozone with liquid unsaturated fatty acids: detailed kinetics and gas-phase product studies

Abstract: Detailed kinetics and product yield studies have been performed for the heterogeneous reaction between gas-phase ozone and three liquid fatty acids using a coated-wall flow tube and chemical ionization mass spectrometry. Gas-surface reaction probabilities for ozone loss of (8.0 ± 1.0) × 10−4, (1.3 ± 0.1) × 10−3, and (1.8 ± 0.2) × 10−3 have been measured at room temperature (298 K) for oleic acid, linoleic acid and linolenic acid, respectively. The temperature dependence of the uptake coefficients was found to be small and positive. Comparison of these results to the kinetics of the equivalent gas-phase reactions implies that there is a definite enhancement in the rate for the heterogeneous process due to entropic factors, i.e. due to collisional trapping of ozone in the surface layers of the liquid, and a possible effect on the activation energy of the reaction. For linoleic acid, the reaction probability was found to be independent of relative humidity (up to 55%), to ±10%, at 263 K. Volatile reaction products were observed using proton-transfer-reaction mass spectrometry. Nonanal was observed with a 0.50 (±0.10) yield for the reaction with oleic acid, whereas hexanal and nonenal were observed for linoleic acid with 0.25 (±0.05) and 0.29 (±0.05) yields, respectively. These results indicate that the primary ozonide formed initially in the reaction can decompose via two equal probability pathways and that a secondary ozonide is not formed in high yield in the aldehydic channel. These reactions represent a source of oxygenates to the atmosphere and will modify the hygroscopic properties of aerosols.

On the role of interface polymers for the mechanics of natural polymeric composites

Abstract: Research on the deformation mechanisms of tendons and wood has shown that these tissues deform mostly by shearing of a soft matrix between stiff fibres. For this type of composite to be both strong and tough, tight binding between matrix and fibres is required. Recent results suggest that Nature may have evolved special interface polymers, capable both of binding to the fibres and of forming a matrix. Proteoglycans could play this role in tendons by binding to collagen fibrils with their protein-like ends and by forming an aqueous matrix with their sugar-like ends. Hemicelluloses could play a similar role in the plant cell wall, as they are binding to cellulose fibrils and forming aqueous networks between them. This observation suggests that new biomimetic composites might be developed on the basis of amphiphilic polymers capable of binding to stiff fibres and of forming a gel-like matrix around them.

Cavity Enhanced Droplet Spectroscopy: Principles, Perspectives and Prospects

Abstract: Micron-sized liquid droplets exhibit unique optical properties, including lower threshold energies for non-linear optical processes than are observed in the bulk liquid phase. Whispering gallery modes (WGMs) and cavity quantum electrodynamics lead to cavity enhancement of fluorescence and Raman scattering at wavelengths commensurate with WGMs. The principles of cavity enhanced droplet spectroscopy will be reviewed, along with the major developments in the field over two decades. The prospect of applying cavity enhanced techniques in the simultaneous determination of droplet size and composition is discussed. A systematic experimental examination of the cavity enhanced Raman spectroscopy of water droplets is presented. In particular, the influences of laser power and illumination geometry, ionic strength and the presence of a second Raman active scatterer, nitrate, on the shape of the water Raman band are explored. Prospects for applying cavity enhanced techniques to studies of aerosol dynamics are also discussed.

Hydrogen storage by physisorption on nanostructured graphite platelets

Abstract: The physisorption energy of molecular hydrogen (H2) on flat carbon nanoparticles (graphitic platelets) and polycyclic aromatic hydrocarbons (PAHs) is determined to be attractive between 3.5 and 7.2 kJ mol−1, depending on the orientation of H2 and on the particle size. Entropy, estimated from experimental data, reduces the interaction energy by 3.4 kJ mol−1 at room temperature. Therefore, nanostructured graphitic platelets might be suitable for hydrogen storage. Computations have been carried out for PAHs from benzene to coronene using second order Moller–Plesset (MP2) theory at the basis set limit, and the results are extrapolated to graphene layers.

Absorption and fluorescence of toluene vapor at elevated temperatures

Abstract: Absorption and fluorescence of the S0 → S1 (π,π*) transition in toluene are studied in the temperature range 300 K to 1130 K and 300 K to 930 K, respectively. Experiments are conducted in a shock-tube and in a heated flow-cell. Fluorescence spectra are investigated after excitation at 248 nm and 266 nm using a nitrogen diluent at a total pressure of 1 bar. Over the temperature range studied the fluorescence quantum yield decreases exponentially by three orders of magnitude for 266 nm excitation and double exponentially by three orders of magnitude for 248 nm excitation. The fluorescence spectrum shifts to the red with increasing temperature. The vibrational structure of the absorption spectrum found at ambient conditions vanishes above 600 K. The absorption feature broadens and the maximum shifts to the red. Taking advantage of the distinctive temperature dependence of the fluorescence, we suggest potential techniques using toluene as a sensitive tracer molecule for temperature imaging in both homogeneously and inhomogeneously mixed flow-fields.

Singlet oxygen formation in photocatalytic TiO2 aqueous suspension

Abstract: The formation of reactive singlet oxygen molecule (1O2), the a1Δg state of excited O2, in the photocatalytic TiO2 aqueous suspension system was shown to be evident by detecting the near-infrared phosphorescence at 1270 nm by means of a gated photon counting method. The lifetimes of O2(1Δg) in three different media (water, ethanol, 1∶1 water–ethanol mixture) were elucidated and compared with those reported in corresponding homogeneous media. Based on the comparison of the lifetime with those of the other reactive species such as OH radicals and trapped holes, the contribution of 1O2 to the actual photocatalytic procedure was discussed.