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D Vollhardt

Publications and source records attributed to D Vollhardt.

At least 37 records · Page 2Linked to original sources
PubMed2005

Quantum chemical semiempirical approach to the structural and thermodynamic characteristics of fluoroalkanols at the air/water interface.

In the framework of quantum chemical PM3 approximation, the geometrical structure and thermodynamic functions characteristics of the formation of monomers (n = 1-14, 34), dimers (n = 1-14, 34), and trimers and tetramers (n = 1-8) of fluoroalkanols with the composition C(n)F(2)(n+1)CH(2)CH(2)OH are calculated. It is shown that, in contrast to the fatty alcohols, which have a flat zigzag structure, the fluoroalkanol monomers are helical with an average backbone torsion angle equal to 162 degrees. For the minimum-energy structure of dimers, the self-organization of the molecules in a dimer was observed; that leads to an opposite alternation of the torsion angles corresponding to the matching atoms in the two molecules that form the dimer. This results in the fact that the most stable conformation of the dimer is the double helix. The lead (39.5 A) and diameter (7.3 A) of the double helix are determined from the calculations of C(34)F(69)CH(2)CH(2)OH dimers. Enthalpy, entropy, and Gibbs energy of the clusterization are shown to be linearly dependent on the length of the fluorinated chain. From the analysis of these thermodynamic quantities, it is concluded that dimerization of fluoroalkanols at the air/water interface takes place if the hydrocarbon link number exceeds 6, whereas for ordinary alcohols this characteristic number is 11. These calculated values agree with experimental data. The additive scheme for the evaluation of the clusterization free energies for arbitrary clusters is developed and applied to obtain the estimate of the Gibbs clusterization energy for infinitely large clusters.

Air↗
PubMed2004

Mutual experimental and theoretical validation of bulk photoemission spectra of Sr1-xCaxVO3.

We report high-resolution high-energy photoemission spectra together with parameter-free LDA + DMFT (local density approximation + dynamical mean-field theory) results for Sr1-xCaxVO3, a prototype 3d(1) system. In contrast to earlier investigations the bulk spectra are found to be insensitive to x. The good agreement between experiment and theory confirms the bulk sensitivity of the high-energy photoemission spectra.

Journal Article↗
PubMed2004

Effect of hydroxyl group position and system parameters on the features of hydroxystearic Acid monolayers.

The characteristic features of hydroxystearic acid monolayers OH-substituted in the mid position of the alkyl chain deviate considerably from those of the usual nonsubstituted stearic acid. The phase behavior, domain morphology, and two-dimensional lattice structure of 9-, 11-, and 12-hydroxystearic acids are studied, using pi-A isotherms, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD), to obtain detailed information on the effect of the exact position of the OH-substitution. The pi-A isotherms of all three hydroxyoctadecanoic acids have an extended flat plateau region, the extension of which only slightly decreases with the increase of temperature. At the same temperature, the extension of the plateau region increases and the plateau pressure decreases from 9-hydroxyoctadecanoic acid to 12-hydroxyoctadecanoic acid. The absolute -DeltaH and -DeltaS values for the phase transition increase slightly from 9-hydroxyoctadecanoic acid to 12- hydroxyoctadecanoic acid and indicate differences in the ordering of the condensed phase under consideration of the special reorientation mechanism of these bipolar amphiphiles at the fluid/condensed phase transition. The morphology of the condensed phase domains formed in the fluid/condensed coexistence region is specific for the position of the OH-substitution of the alkyl chain, just as the lattice structures of the condensed monolayer phase. 11-hydroxyoctadecanoic acid monolayers form centered rectangular lattices with the chain tilt toward the NNN (next nearest neighbor) direction, and 12-hydroxyoctadecanoic acid monolayers have an oblique lattice over the entire pressure range. A special feature of 9-hydroxystearic acid monolayers is the phase transition between two condensed phases observed in the pi-A isotherm of 5 degrees C at approximately 18 mN/m, where the centered rectangular lattice shows a NNN/NN transition. The morphology of the condensed phase domains formed in the fluid/condensed coexistence region, just as the lattice structures of the condensed monolayer phase, reveal the high specifity of the monolayer feature of the bipolar hydroxystearic acids OH-substituted in the mid position.

Journal Article↗
PubMed2004

Effect of substance properties on the appearance and characteristics of repeated surface tension auto-oscillation driven by Marangoni force.

The effect of substance properties (solution viscosity and density, surfactant bulk and surface diffusion coefficient, activity, and solubility) on the appearance and characteristics of surface tension auto-oscillation that occurs by dissolution of a surfactant drop under the water-air interface is considered in the framework of a simple mathematical model, taking into account the convection driven by the Marangoni effect and convective diffusion together with adsorption/desorption processes at the air-water interface. Numerical simulations show that apart from the Marangoni and Schmidt number, the system behavior is governed also by the exchange number, which determines the surfactant exchange with the interface. The criterion for the instability onset in a system with both normal and tangential (with respect to the interface) concentration gradient, the correlation between the global and local Marangoni numbers, as well as a comparison with experiment are discussed.

Journal Article↗
PubMed2003

Experimental studies on the geometrical characteristics determining the system behavior of surface tension autooscillations.

Autooscillation of the surface tension is a phenomenon related to Marangoni instability periodically arising and fading by dissolution of a surfactant droplet under a water-air interface. A detailed experimental investigation was performed to clear up the influence of the system geometry on development and characteristics of autooscillations. It was found that the aspect ratio is an additional dimensionless parameter that determines the system behavior equally to the Marangoni number. The influence of the cell diameter, capillary immersion depth, and droplet radius on the autooscillation period and amplitude was studied as well.

Journal Article↗
PubMed2003

Prominent quasiparticle peak in the photoemission spectrum of the metallic phase of V2O3.

We present the first observation of a prominent quasiparticle peak in the photoemission spectrum of the metallic phase of V2O3 and report new spectral calculations that combine the local-density approximation with the dynamical mean-field theory (using quantum Monte Carlo simulations) to show the development of such a distinct peak with decreasing temperature. The experimental peak width and weight are significantly larger than in the theory.

Journal Article↗
PubMed2002

Theoretical description of repeated surface-tension auto-oscillations.

A mathematical model is proposed to follow the behavior of a system where a droplet of a surfactant with limited solubility on the tip of a capillary under the free liquid surface dissolves in a container with a unity aspect ratio. Numerical simulations show that instability repeatedly arises and fades in the system, resulting in auto-oscillations of the surface tension in agreement with the experimental data. The system evolution during the oscillation is discussed in detail. It is established that the presence of a boundary in the radial direction is a necessary precondition for the appearance of the second and following oscillations.

Journal Article↗
PubMed2001

Mott-hubbard metal-insulator transition in paramagnetic V2O3: an LDA+DMFT(QMC) study.

The electronic properties of paramagnetic V2O3 are investigated by the computational scheme LDA+DMFT(QMC). This approach merges the local density approximation (LDA) with dynamical mean-field theory (DMFT) and uses quantum Monte Carlo simulations (QMC) to solve the effective Anderson impurity model of DMFT. Starting with the crystal structure of metallic V2O3 and insulating (V0.962Cr0.038)2O3 we find a Mott-Hubbard transition at a Coulomb interaction U approximately 5 eV. The calculated spectrum is in very good agreement with experiment. Furthermore, the orbital occupation and the spin state S = 1 determined by us agree with recent polarization dependent x-ray-absorption experiments.

Journal Article↗
PubMed2001

Numerical study of the Marangoni instability resulting in surface tension auto-oscillations: general regularities of the system evolution.

Theoretical studies are performed to explain the mechanism of surface tension auto-oscillations recently found. The Marangoni instability in a system containing a surfactant droplet under the air-water interface is investigated numerically. The simulations, based on the equations of fluid mechanics, take into account convective diffusion and adsorption of the surfactant. The behavior of the system is determined by nonstationary concentration gradients that are nonuniform on the surface as well as in the normal to the surface direction. Initially a slow diffusion dissolution of the drop material takes place. The convective transfer of the surfactant is negligible, the surface tension remains nearly constant and the system parameters change rather slowly during the induction period. With the increase of the concentration gradients the system becomes unstable, resulting in a jump in the convection velocity, surface tension, and adsorption on the surface. The concentration and velocity distributions in the bulk and on the surface are obtained from the numerical solution of the problem. The contributions of different mechanisms of the mass transfer are compared in different stages of the process.

Journal Article↗
PubMed2000

Electronic correlations in manganites

The influence of local electronic correlations on the properties of colossal magnetoresistive manganites is investigated. To this end, a ferromagnetic two-band Kondo lattice model is supplemented with the local Coulomb repulsion missing in this model and is analyzed within dynamical mean-field theory. Results for the spectral function, optical conductivity, and the paramagnetic-to-ferromagnetic phase transition show that electronic correlations have drastic effects and may explain some experimental observations.

Journal Article↗
PubMed2000

Penetration of dissolved amphiphiles into two-dimensional aggregating lipid monolayers.

The review demonstrates the recent theoretical and experimental progress in the understanding of penetration systems at the air-water interface in which a dissolved amphiphile (surfactant, protein) penetrates into a Langmuir monolayer. The critical review of the existing theoretical models which describe the thermodynamics of the penetration are critically reviewed. Although a rigorous thermodynamic analysis of penetration systems is unavailable due to their complexity, some model assumptions, e.g. the invariability of the activity coefficient of the insoluble component of the monolayer during the penetration of the soluble component results in reasonable solutions. New theoretical models describing the equilibrium behaviour of the insoluble monolayers which undergo the 2D aggregation in the monolayer, and the equations of state and adsorption isotherms which assume the existence of multiple states (conformations) of a protein molecule within the monolayer and the non-ideality of the adsorbed monolayers are now available. The theories which describe the penetration of a soluble surfactant into the main phases of Langmuir monolayers were presented first for the case of the mixture of the molecules possessing equal partial molar surfaces (the mixture of homologues), with further extension of the models to include the interesting process of the protein penetration into the monolayer of 2D aggregating phospholipid. This extension was based on a concept which subdivides the protein molecules into independent fragments with areas equal to those of the phospholipid molecule. Various mechanisms for the effect of the soluble surfactant on the aggregation of the insoluble component were considered in the theoretical models: (i) no effect on the aggregate formation process; (ii) formation of mixed aggregates; and (iii) the influence on the aggregating process via the change of aggregation constant, but without any formation of mixed aggregates. Accordingly depending on the mechanism, different forms of the equations of state of the monolayer and of the adsorption isotherms of soluble surfactant are predicted. Based on the shape of the experimental pi-A isotherms, interesting conclusions can be drawn on the real mechanism. First experimental evidence has been provided that the penetration of different proteins and surfactants into a DPPC monolayer in a fluid-like state induces a first order main phase transition of pure DPPC. The phase transition is indicated by a break point in the pi(t) penetration kinetics curves and the domain formation by BAM. Mixed aggregates of protein with phospholipid are not formed. These results agree satisfactorily with the predictions of the theoretical models. New information on phase transition and phase properties of Langmuir monolayers penetrated by soluble amphiphiles are obtained by coupling of the pi(t) penetration kinetics curves with BAM and GIXD measurements. The GIXD results on the penetration of beta-lactoglobulin into DPPC monolayers have shown that protein penetration occurs without any specific interactions with the DPPC molecules and the condensed phase consists only of DPPC.

Adsorption↗
PubMed2000

Anderson impurity in a correlated conduction band

We investigate the physics of a magnetic impurity with spin 1/2 in a correlated metallic host. Describing the band by a Hubbard Hamiltonian, the problem is analyzed using dynamical mean-field theory in combination with Wilson's nonperturbative numerical renormalization group. We present results for the single-particle density of states and the dynamical spin susceptibility at zero temperature. New spectral features (side peaks) are found which should be observable experimentally. In addition, we find a general enhancement of the Kondo scale due to correlations. Nevertheless, in the metallic phase, the Kondo scale always vanishes exponentially in the limit of small hybridization.

Journal Article↗
PubMed2000

N-n-alkyl N,N-dimethylammonioacetic acid bromides: the first complete series of crystal and molecular structure determinations of an amphiphilic compound with alkyl chain lengths n = 1,..., 16

The molecular and crystal structures of 16 N-n-alkyl N,N-dimethylammonioacetic acid bromides with chain lengths between n = 1 and n = 16 have been determined. All compounds from n = 5 to n = 16 form bilayers with interdigitated chains. The even-numbered chains display the chain packing type M2(II). The chain packing of the odd-numbered chain compounds is less regular. The head groups of all compounds are connected via electrostatic N+...Br- interactions, and by OH...Br- hydrogen bonds. The compounds with short chains are packed in different ways. Their molecular conformation depends on the crystal packing.

Journal Article↗
PubMed1999

Auto-oscillation of surface tension.

Long-time auto-oscillation of the surface tension can evolve when in an aqueous system a diethyl phthalate droplet is placed under the free water surface. The experimental conditions for development of surface tension auto-oscillations are described. Based on a theoretical analysis the mechanism of these auto-oscillations is proposed. The mechanism of the auto-oscillations results from a switching between diffusion and convection transfer of diethyl phthalate in the solution. A periodic Marangoni flow on the water surface resulting from a surface layer instability is discussed. The solubility of the amphiphile in the water and its surface activity are the main characteristics that determine the system behavior.

Journal Article↗
PubMed1996

Long-range tilt orientational order in phospholipid monolayers: a comparative study.

Monolayers of dipalmitoyl-phosphatidyl-N-monomethylethanolamine (DPP(Me)E) and dipalmitoyl-phosphatidyl-N,N-dimethylethanolamine (DPP(Me2)E) are studied and compared with dipalmitoyl-phosphatidylethanolamine (DPPE), dimyristoyl-phosphatidylethanolamine (DMPE), and dipalmitoyl-phosphatidylcholine (DPPC) to characterize the influence of the headgroup size. The properties of the condensed phases of DPP(Me2)E and DPP(Me)E are between those of DPPC and DMPE or DPPE. DPPC domains are elongated and the orientation changes continuously, whereas DMPE domains are compact and the orientation jumps at curved lines. The domains of DPP(Me2)E and DPP(Me)E are compact, and not elongated. The orientation changes continuously by 360 degrees around a point in the centered domains, and jumps of the orientation occur only in the case of twinning. Furthermore, the size of the headgroup influences the erection of the aliphatic chains. For DPPC and (DPP(Me2)E), no complete erection of chains occurs, whereas for DPP(Me)E the surface pressure required for the complete erection of chains is much higher than for DPPE. The same tendency is found for the collapse. DPPC monolayers do not collapse. DPP(Me2)E monolayers collapse at a much higher surface pressure than those of DPP(Me)E and DPPE.

1,2-Dipalmitoylphosphatidylcholine↗