bluuebunny/crossref_metadata_2025_split huggingface dataset:
Shear Induced Micellar Structures
Authors: Hofmann S. , Rauscher A. , Hoffmann H. 
Published: July 2012

AbstractMixed solutions of the anionic surfactant Sodiumdodecylsulfate (SDS) and the zwitterionic Tetradecyldimethylaminoxid (TDMAO) were studied under shear in a Couette‐type apparatus. For shear rates exceeding a critical shear gradient γc the solutions show birefringence which approaches a constant value ΔnMAX with increasing shear rate. At the same time properties like the shear viscosity and the normal stress difference increase dramatically. A structural modification must thus have taken place for γ > γc: a shear‐induced structure (SIS) has developed. – The structure of the micelles in the solution at rest was determined by SANS‐, electric birefringence‐ and viscosity experiments for different mixing ratios, concentrations and ionic strength. The solutions under shear were studied by SANS, flow‐birefringence and rheological measurements. – At rest small, rodlike micelles which consist of mixed surfactants are present. The length of the micelles are smaller than their mean distance and the viscosity is low in this case. The dimensions of the micelles were determined from the positions of the correlation peaks in the SANS‐patterns which are due to repulsive forces between the charged aggregates. At a mixing ratio of 9:1 (TDMAO:SDS; C = 100 mM) the solutions are viscoelastic and no SIS is detected. The SIS is thus formed only in solutions with surfactant concentrations which are below the overlap concentration. At γc – at which the SIS is beginning to form – the product of γc and τROT, the rotational relaxation time of the rods, is much smaller than one. – As soon as a critical shear gradient is reached these former isotropic micellar solutions show birefringence, an extinction angle of zero degrees, normal stress difference and an increased viscosity. Former isotropic SANS‐patterns deform and peaks show the appearence of a highly ordered structure in equilibrium with unordered micelles. All micelles taking part in the SIS are completely aligned in the direction of flow. Formation of the SIS is highly dependent on concentration, mixing ratio and ionic strength. Finally, two proposals for the nature of SIS are presented: A shear induced phase transition into domains of (pseudo)nematic phases seems most likely to us and is consistent with our experimental results. But also a pearl‐string like array in domains of oriented micelles is conceivable.
Hopping of Quantum Particles on Crystals with Energy Disorder: Influence on Spin Resonance
Authors: Reineker P. , Köhler J. 
Published: July 2012

AbstractFor spin 1/2 quantum particles hopping on energetically disordered lattices paramagnetic lineshapes and free induction decay (FID) signals are calculated as a function of the hopping rate γ between nearest neighbours. The energy disorder is modeled by local magnetic fields chosen from a Gaussian or dichotomic distribution. For motion on a linear chain it is derived – using a continued fraction method – that the FID signal shows a cross over from an exp(–t3/2) law to an asymptotic exp(–t) behaviour. Preliminary investigations for 2‐ and 3‐dimensional lattices have been performed using algebraic multigrid methods.
Thermodynamic Study of the Gaseous Heterocomplexes NaSnCl3 and NaSnBr3
Authors: Hilpert K. , Miller M. , Venugopal V. 
Published: July 2012

AbstractThe NaCl – SnCl2 and NaBr – SnBr2 systems were investigated under equilibrium conditions in the temperature ranges between 713 to 963 K and 653 to 893 K, respectively, by using Knudsen effusion mass spectrometry with a two‐compartment Knudsen cell. The gaseous species NaX, (NaX)2, SnX2, and NaSnX3 (X = Cl, Br) are present in the vapour of the two systems. The enthalpy and entropy changes of the dissociation reactionsNaSnCl3(g) ⇄ NaCl(g) + SnCl2(g) (1)andNaSnBr3(g) ⇄ NaBr(g) + SnBr2(g) (2)resulted asΔHo298(Eq. (1)) = 197 ± 6 kJ mol−1,ΔSo298(Eq. (1)) = 153 ± 7 J mol−1 K−1,ΔHo298,(Eq. (2)) = 190 ± 6 kJ mol−1, andΔHo298(Eq. (2)) = 160 ± 7 J mol−1 K−1.The enthalpy and entropy changes of the reactionsSnX2(g) + (NaX)2(g) ⇄ NaSnX3(g) NaX(g), X = Cl, Br, (3)are additionally evaluated as well as equilibrium constants for the reactions Eqs. (1) to (3). The results obtained are discussed.
The Dielectric Constant and Density of Benzene to 400 °C and 3000 bar
Authors: Deul R. , Rosenzweig S. , Franck E. U. 
Published: July 2012

AbstractA high pressure apparatus is described to measure simultaneously within one cell the dielectric constant and the density of non‐polar supercritical fluids to 400 °C and 3000 bar. Thus relations between the dielectric constant and the fluid density at any given temperature can be discussed with particular reliability. The apparatus consists of a cylindrical autoclave, which encloses a stainless steel belows containing the sample. A cylindrical capacitor (ca. 60 pF) is mounted inside the bellows. A back‐up fluid of heptane is used to produce the pressure on the bellows. Measurements are made between 10 kHz and 1 MHz. Values of the dielectric constant ϵ are reported for 10 temperatures between 27.5 and 400 °C and for pressures between 1 bar and 3000 bar. At 27.5°C and 1 bar ϵ is 2.273. At 100°C it varies from 2.124 to 2.425 between 1 and 2800 bar. At 400°C and 3000 bar ϵ = 2.198. The probable error is below 0.2%. Density measurements cover the same range. The temperature dependence of ϵ at constant density is very small. The Clausius‐Mossotti‐function is presented for the whole region. In accordance with existing theories it can be described as a three‐term series in powers of density and a T−1‐term.
Enthalpy of Demixing of a Binary Liquid Mixture of Critical Composition: Study of the System Iso‐Butoxyethanol/Water
Authors: Zimmer H.‐J. , Woermann D. 
Published: July 2012

AbstractA fast pressure jump technique is used to measure calorimetrically the temperature dependence of the enthalpy of demixing of an iso‐butoxyethanol(i‐C4E1)/water mixture of critical composition (xc(i‐C4E1) = 0.0664; Tc = 26.46 °C) in the vicinity of its lower critical point. It is found that the enthalpy of demixing has a positive sign as expected thermodynamically for a lower critical point. A linear relation is observed between the molar enthalpy of demixing ΔdemixHm(Xc) and the temperature difference (T – Tc) (Tc, critical temperature; T, temperature; (T > Tc) with a slope of (∂ ΔdemixHm(Xc)/∂T)p = (5.5 ± 0.3) J mol−1 K−1. The enthalpy of demixing ΔdemixHm(Xc) becomes zero at the critical temperature Tc. The value of (∂ ΔdemixHm(Xc)/∂T)p is of the same order of magnitude as the absolute value of (∂ ΔdemixHm(Xc)/∂T)p of the system isobutyric acid/water (upper critical point; (∂ ΔdemixHm(Xc)/∂T)p = −(5.2 ± 0.4) J mol−1 K−1) using the same technique.
Osmotic Properties of Crystalline Inorganic Precipitation Membranes: Study of Barium Sulfate Membranes
Authors: Milz M. , Forke R. , Woermann D. , Sunn U. 
Published: July 2012

AbstractBaSO4 precipitation zones with membrane properties (“conditioned state”: characteristic values of membrane potential, non‐ohmic electric current‐voltage characteristic) are generated by interdiffusion of iso‐osmotic aqueous solutions of BaCl2 and Na2SO4 within cellophane skins. In the “conditioned state” they are found to be impermeable to glucose, sucrose, and raffinose. Osmotic differences of these substances between the bulk phases as well as osmotic differences of BaCl2 and Na2SO4 cause an osmotic volume flow across the precipitation zone from which the hydrodynamic permeability of the precipitation membrane imbedded within cellophane skin can be calculated. The hydrodynamic permeability of a cellophane skin containing a BaSO4 precipitation zone in its “conditioned state” is smaller by a factor of more than 10 compared with that of an untreated cellophane skin. A precipitation zone looses its characteristic properties (“deconditioned state”) if the ions forming the precipitate (Ba++, SO4–) are removed from the bulk phases. This triggers changes of the properties of the surface of the BaSO4 crystallites and Ostwald ripening processes set in. Such a precipitation zone becomes permeable to glucose, sucrose, and raffinose. In the “deconditioned state” the hydrodynamic permeability of precipitations zones approaches nearly the value of an untreated cellophane skin. Adding again Ba+ + and SO4– ions to the bulk phase regenerates the electrochemical and osmotic properties of the precipitation zone. Low angle scattering patterns of synchroton radiation of freeze dried cellulose skins containing a BaSO4 precipitation zone in its “conditioned state” are analyzed in terms of a random structure model. It is found that the correlation length of local electron density fluctuations has a value of about 6 nm. This reflects the tight packing of BaSO4 crystallites forming a precipitation zone with membrane properties.
The Frequency Dependence of the Dynamical Conductivity of Molten Alkali Nitrates in the Microwave and Millimetre Wave Range
Authors: Happ H. , Fend Th. , Yen S. 
Published: July 2012

AbstractMeasurements are reported of the dynamical conductivity of the alkali nitrates in the molten state near their melting points Tm (1 < T/Tm < 1.2) in the microwave and millimetre‐wave range. The conductivity of KNO3, RbNO3 and CsNO3 shows no dispersion up to the far‐infrared whereas in LiNO3 and NaNO3 it passes through a shallow minimum in the millimetre‐wave range. The temperature dependence of the conductivity indicates that it is due to activated processes up to the millimetre‐wave region. The shallow minimum in LiNO3 and NaNO3 indicates a high frequency limit of the diffusive response at about 50 GHz.
Formation and Reduction of Semiconductor‐Like Aggregates of Silver‐Carboxy‐Alkane‐Thiolates in Aqueous Solution
Authors: Fojtik Anton , Mulvaney Paul , Linnert Thomas , Giersig Michael , Henglein Arnim 
Published: July 2012

AbstractIn slightly acidic solutions of Ag+ and carboxy‐methane‐thiol (CMT) or carboxy‐ethane‐thiol (CET), aggregates are formed whose absorption spectrum is significantly different from that of the monomeric silver‐carboxy‐alkane‐thiols. The build‐up of the aggregates occurs slowly. They develop a band at 335 nm (AgCMT) or 355 nm (AgCET), and simultaneously, acquire fluorescence. Silver is produced upon the photolysis of the aggregates. A very small amount of silver atoms deposited on the aggregates is sufficient to quench the fluorescence substantially. Both the absorption and fluorescence bands are blue‐shifted as the aggregates become smaller during photolysis or dissolution in alkaline solution. The aggregates are proposed to have semiconductor‐like ionic planes separated from each other by the organic groups. Electron microscopy shows that the aggregates are partly crystalline and have a size of about 25 Å when they are freshly prepared. The crystallinity increases and the structure changes upon aging for several days.—When Ag+ is reduced by NaBH4 in the presence of carboxy‐alkane‐thiols, stable silver aggregates of different sizes are formed. The absorption bands and chemical behaviour of some of the aggregates resemble those of oligomeric silver clusters investigated previously. The stability of the clusters is explained in terms of “pre‐complexation” of silver surface atoms by the carboxy‐thiols.
Solvent and Acidity Function Effects in the Kinetics of the Acid Hydrolysis of Ditert‐Butylsuccinate in Aqueous Acetone
Authors: Hanna M. T. , Khalil Fayez Y. , Asaad Adel N. 
Published: July 2012

AbstractThe rates for the consecutive acid hydrolysis of ditert‐butylsuccinate were measured in a series of an aqueous dipolar aprotic solvent, namely, acetone. The observed rates of the two steps of the reaction were analysed into four separate terms due to the unimolecular (AAL1) and bimolecular (AAC2) mechanisms. These rates were correlated with the measured acidity function H0 of the medium, where linear relationships exist at lower acetone contents for the unimolecular rate constants (k′I1 & k′II1). The bimolecular rate constants (k′I2 & k′II2), however, follow a non‐linear trend indicating the operation of the AAC2 mechanism. The activation parameters and the rate‐acidity function relationships were computed and discussed as evidences of mechanistic pathway and solvation effects involved in the reaction.
Photochemistry of Propynal in the Gas Phase Following S1(nπ*) ← S0 Excitation
Authors: Opitz Joachim 
Published: July 2012

AbstractFollowing excitation of the S1(nπ*) ← S0 transition the photochemical decomposition of propynal (HCCCHO) and propynal α‐D (HCCCDO) is analyzed. Quantum yields for the main products CO and acetylene are reported for excitation wavelengths 382 nm ≥ λ ≥ 308 nm. Depending on the wavelength two photochemical pathways seem to occur leading to product formation with ϕ < 1 for λ ≥ 343.5 nm and ϕ > 1 for λ < 343.5 nm respectively. For the pathway leading to ϕ < 1 a mechanism is proposed which is based on a dissociation into molecular products from the excited ground state S0 by a concerted reaction.
Kinetics of Diabatic and Adiabatic Electron Exchange in Organic Systems Comparison of Theory and Experiment
Authors: Grampp Günter , Jaenicke Walther 
Published: July 2012

AbstractThe rate constants ket of electron self exchange between neutral molecules and the corresponding radical ions, measured by ESR line broadening, are discussed for seven organic ϕ‐systems in 18 aprotic solvents of a wide range of relaxation times, τL (0.2 < τL/ps < 10), and polarities, γ(0.05 < γ 0.53) in terms of the theoretical approaches of Marcus and Sumi and of Jortner and Bixon, both valid for comparable magnitudes of inner and outer reorganization energies. (γ = < γ = 1/n2‐1/ε; n = refractive index; ε = dielectric constant.) The electron transfer is diabatic for four substituted p‐phenylenediamines, it is adiabatic relaxation overdamped for tetracyanoquinodimethane (TCNQ), tetrathiafulvalene (TTF) and tetracyano‐ethylene (TCNE). By changing the solvents no transition between both mechanisms is found. To explain the results, the reactants are represented by ellipsoids. Their centre to centre distance in the transition state is derived from slopes of lnket τL γ−1/2 vs. γ (for adiabatic reactions) or of Infketγ1/2 vs. γ (for diabatic reactions). The mutual orientation of the reactants follows the minimum of electrostatic energy. The resonance splitting V for this orientation, calculated from the overlap integrals agrees well for all systems with the observed diabatic or adiabatic behaviour of the reaction, except for TCNE. The resonance splitting and the equilibrium constant of the precursor complex are derived experimentally from the temperature dependence of the electron transfer rate constant ket. Negative association energies of the complex are due to ion‐dipole interactions of the reactants. An additional reorganization term is observed for the aliphatic TCNE system in ethereal solvents. To describe the experiments quantitatively, both theories used are nearly equivalent.
The Superconductor/Ionic Conductor Interface at Low Temperatures
Authors: Lorenz W. J. , Saemann‐Ischenko G. , Breiter M. W. 
Published: July 2012

AbstractThe charge transfer across the interface superconductor/ionic conductor represents a relatively unknown field at present. This paper deals with the first electrochemical studies in this area. Both n‐type classical and p‐ and n‐type high Tc superconductors (HTSC) are considered. Different superionic conductors (Ag β‐alumina, Ag + ion conducting glass, and RbAg4I5) are used as solid electrolytes in experiments covering a temperature range down to 10 K. A transient technique in the time domain and Electrochemical Impedance Spectroscopy (EIS) in the frequency domain are used to study the silver deposition as Faradaic charge transfer process at these interfaces. The results show an admittance peak in transient measurements or a corresponding negative peak of the polarization resistance in EIS‐measurements at T ≈ Tc. These findings are attributed to an increase of the exchange current density around Tc. The effect is much less on an A15 compound than on HTSCs. The effect can only be interpreted as a quantumelectrochemical phenomenon caused by the contribution of Cooper pairs as correlated charge carriers crossing the electrochemical double layer. A proximity‐like phenomenon can be excluded.
Tracer Particle Diffusion in Crystal‐ and Fluid‐like Ordered Colloidal Suspensions
Authors: Härtl W. , Versmold H. , Zhang‐Heider X. 
Published: July 2012

AbstractStatic and dynamic light scattering experiments with crystal‐ and fluid‐like ordered colloidal suspensions are reported. The systems were composed of two electrostatically interacting particle species of similar size and effective particle charge but different refractive index. The refractive index of the abundant species was matched to the index of the dispersing medium. The incoherent scattering from the mismatched second species was used to investigate the self‐diffusion in the ordered suspension. Leakage of coherent scattering due to a slight mismatch was used to monitor simultaneously the structure factor S(Q) of the suspensions. – The self‐dynamic structure factor Fs(Q,t) at several ionic strength of the suspension is analyzed in terms of mean‐square displacements 〈R(t)2〉 and velocity autocorrelation functions v(0) v(t). The long time self‐diffusion coefficient is determined as a function of the effective hard sphere volume fraction Φ.
Neutron Scattering Experiments in the Neighborhood of the Glass Transition in Polybutadiene – a Test of Mode Coupling
Authors: Richter D. , Zorn R. , Frick B. , Farago B. 
Published: July 2012

AbstractWe review recent quasielastic and inelastic neutron scattering experiments taken in the neighborhood of the glass transition in polybutadiene. The experimental results are compared with predictions of the mode coupling theory. We specially emphasize experimental evidence for the existence of a dynamical transition above Tg. We present several experiments which from characteristic anomalies consistently obtain critical temperatures between 213 K ≤ Tc ≤ 220 K. Furthermore, we discuss experimental results for the dynamic structure factor S(Q,t) in the regime of the structural relaxation, showing striking similarities with the predictions of the mode coupling approach.
Dynamical Lightscattering of Pretransitional Phenomena in Liquid Mixtures
Authors: Schröer W. , Wiegand S. , Staude W. , Peters Th. 
Published: July 2012

AbstractWe report measurements of the scattering intensity and of the autocorrelation function of the pseudo binary mixture guaiacol/glycerol‐water as a function of temperature and water content. The guaiacol glycerol ratio was kept constant 1:1, while the mass ratio of water/glycerol varied between 0.4% <x< 1.4%. The system shows a closed solubility loop ifx> 1.17%. A phenomenological model for the temperature dependence of the interaction parameter is used in order to describe the scattering intensity and the diffusion coefficient as function of temperature.
Ellipsometry on Anisotropic Langmuir‐Blodgett Films
Authors: Geiß G. , Hickel W. , Lupo D. , Praß W. , Scheunemann U. 
Published: July 2012

AbstractMultiple angle null ellipsometry has been used to obtain information about the optical anisotropy of multilayer Langmuir‐Blodgett films. The data are evaluated with a simplified uniaxial (or biaxial) model system. The optical constants of films made of 22‐tricosenoic acid have been determined to nx,y = 1.51 and nz = 1.54 with a monolayer thickness of 3.2 nm. The data are compared with results from waveguide mode spectroscopy. Phenylhydrazone films have been measured in the spectral region between 420 nm (λmax) and 700 nm. The resulting data imply a tilt of the chromophore dipole direction greater than 45° with respect to the surface normal.
Thin Layers of Phospholipids with Branched Chains: Thermodynamic Analysis and Comparison
Authors: Brezesinski G. , Förster G. , Rettig W. , Kuschel F. 
Published: July 2012

AbstractMonolayers and bulk phases of glycerophosphocholines with a branched fatty acid chain in 2‐position of the glycerol backbone were investigated. In the bulk the frustration caused by the drastic change of the molecule shape within the homologous series influences the structures of the gel and liquid‐crystalline phases in a different way. Gel phases with interdigitated chains and non‐bilayer liquid‐crystalline phases were found. All of the lipids investigated form stable monolayers at the air/water interface. The analysis of the temperature dependence of the compression isotherms leads to the characteristic temperature Tc (critical temperature of the monolayer). The comparison with the main‐transition temperature Tm of the bulk phases can be used as an indicator for structural differences between monolayers and bilayers.
Langmuir‐Blodgett Mono‐ and Multilayer Assemblies of Amphiphilic Liquid‐ Crystalline n‐Alkyl‐4′‐n‐alkoxybiphenyl‐4‐carboxylates. I
Authors: Decher G. , Sohling U. 
Published: July 2012

AbstractFive n‐Alkyl‐4′‐Alkoxy‐biphenyl‐4‐carboxylates were investigated as Langmuir monolayers at the air/ water interface and as Langmuir‐Blodgett (LB) films on solid supports. All five compounds form stable monolayers at the air/water interface and can be transferred onto solid supports yielding LB‐films, whose structures and kinetic behavior strongly depend on the headgroup. The structures of the LB‐films were investigated by means of optical textures and X‐ray diffraction and compared with the structures of the corresponding bulk phases which were investigated by means of Differential Scanning Calorimetry and X‐ray powder diffraction. The LB‐films of the acid and the two methyl esters exhibit bilayer lamellar structures which are very similar to the bulk crystalline phases at room temperature. The two ethyl esters show a mixture of two phases after transfer, a monolayer and a bilayer phase, the percentage of bilayer phase decreasing with time. For these two compounds the bilayer phase is a deposition enforced phase, which has not been observed in bulk material.
Influence of Thin Superstrate Films on Evanescent Waves in Surface Waveguides
Authors: Gauglitz G. , Ingenhoff J. 
Published: July 2012

AbstractSurface waveguide structures in glass substrates arranged as Mach‐Zehnder interferometers allow the measurement of slightest influences on a superstrate coated on one arm of the integrated optical device. The refractive indices of the substrate's, the waveguide's, and the superstrate's material determine the characteristics of the modal field in the interferometer arms. For this reason, any influence on the effective refractive index in the superstrate can be monitored with high sensitivity. By use of the wave theory of light, the amplitudes and the phase correlation of the interfering waves can be calculated for any given combination of the materials mentioned above. To prove these considerations, one arm of the Mach‐Zehnder interferometer is coated with polysiloxane as a thin superstrate film (0,5–2 μm). If organic hydrocarbons are passed over the superstrate, the molecules penetrate this interface and change its refractive index in dependence on the gas concentration. This experimental arrangement allows to compare measurements and calculations. Furthermore, the set‐up can be used to monitor physical and chemical effects taking place in the thin superstrate layer. Therefore, besides a verification of the theory, the Mach‐Zehnder chip can be used as a sensor. It can be shown that multi‐wavelength measurements increase the information obtained.