Stability of non-aqueous dispersions. Paper 7. Electrostatic stabilization of concentrated colloidal dispersions

The stability of colloidal dispersions in apolar media is reviewed beginning with dilute dispersions. A criterion for the definition between dilute and concentrated dispersions is presented followed by a new interpretation of Levine's theory for concentrated colloidal dispersions in apolar media [J. Colloid Interface Sci., 54 (1976) 34]. We conclude that concentrated dispersions containing submicron particles are generally unstable leading to coagulation. Successive coagulations lead to the formation of larger secondary particles, “coagulates”, which are ultimately stable to further coagulation and exist in energetic minima.     

Ultrafast far-infrared dynamics probed by terahertz pulses: a frequency domain approach. I. Model systems

Time-resolved terahertz spectroscopy has become a widely used experimental tool for the investigation of ultrafast dynamics of polar systems in the far infrared. We have recently proposed an analytical method for the extraction of a transient two-dimensional susceptibility from the experimental data [N?mec, Kadlec, and Kuzel, J. Chem. Phys. 117, 8454 (2002)]. In the present paper the methodology of optical pump-terahertz probe experiments is further developed for direct application in realistic experimental situations. The expected two-dimensional transient response function is calculated for a number of model cases (including Drude dynamics of free carriers, harmonic and anharmonic oscillator modes); these results serve as a basis for the interpretation of experimental results. We discuss also the cases where only partial (one-dimensional) information about the system dynamics can be experimentally obtained.     

Kinetics of partly diffusion-controlled reactions. XIV. Effects of reversible energy transfer

The general effect of back energy transfer is to reduce the apparent quenching constant which is an important parameter in the interpretation of energy transfer data. However, this interpretation may be erroneous when possible diffusion effects and the existence of nonuniform configurational distributions are not taken into account.     

A formalism for scattering of complex composite structures. I. Applications to branched structures of asymmetric sub-units

We present a formalism for the scattering of an arbitrary linear or acyclic branched structure build by joining mutually non-interacting arbitrary functional sub-units. The formalism consists of three equations expressing the structural scattering in terms of three equations expressing the sub-unit scattering. The structural scattering expressions allow composite structures to be used as sub-units within the formalism itself. This allows the scattering expressions for complex hierarchical structures to be derived with great ease. The formalism is generic in the sense that the scattering due to structural connectivity is completely decoupled from internal structure of the sub-units. This allows sub-units to be replaced by more complex structures. We illustrate the physical interpretation of the formalism diagrammatically. By applying a self-consistency requirement, we derive the pair distributions of an ideal flexible polymer sub-unit. We illustrate the formalism by deriving generic scattering expressions for branched structures such as stars, pom-poms, bottle-brushes, and dendrimers build out of asymmetric two-functional sub-units.     

Conformational effect on luminescence and polarization of some aniline derivatives. Intersystem crossing from higher singlet states

We present an analysis of emission spectra, obtained in polar and non-polar solvents and at different temperatures, for a series of aniline derivatives excited at different wavelengths. Important changes in the intensity and polarization of the emission are shown to be due to conformational effects of the amino group which strongly changes the molecular desactivation dynamics. This interpretation is shown to be consistent also with the absorption spectra.     

Determination of complex reaction mechanisms. Analysis of chemical, biological and genetic networks

We present several methods of determining, not guessing, complex chemical reaction mechanisms and their functions. One method is based on the theory of correlation functions of measured time series of concentrations of chemical species; another is on measurements of temporal responses of concentrations to various perturbations of arbitrary magnitude; a third deals with the analysis of oscillatory systems; a fourth is on the use of genetic algorithms to determine functions of chemical reaction networks. All methods are applicable to chemical, biochemical, and biological reaction systems and to genetic networks and systems biology. The methods depend on the design of appropriate experiments on the whole system and corresponding theories for interpretation that lead to information on the causal chemical connectivity of species, on reaction pathways, on reaction mechanisms, on control centers in the system, and on functions of the system. The first three methods require no assumption of a model or hypothesis, nor extensive calculations, unlike the interpretation of measurements made on a gene network at only one time.     

Molecular dynamics simulation of thermomechanical properties of montmorillonite crystal. 1. Isolated clay nanoplate

The structure and mechanical properties of clay nanoparticles is a subject of growing interest because of their numerous applications in engineering. We present the results of molecular dynamics simulation for a single nanoplate of pyrophyllite - a 2:1 clay mineral consisting of two tetrahedral sheets of SiO4 and an intervening octahedral AlO6 sheet. Simulations were performed in the temperature interval from 5 to 750 K using the ionic-type potentials of Cygan et al. On this basis the temperature dependences of structural parameters, characterizing both tetrahedral and octahedral sheets as well as single lamella, have been studied. Two slightly different structures were observed in this wide temperature interval. The mechanical properties of the nanoplate were calculated from stress-strain diagrams, which have been obtained at relatively slow rates of deformation (for molecular simulations). Using different types of loading, we calculated the full elasticity tensor and estimated the influence of temperature on its components. We estimated also the bending and torsion stiffnesses of the nanoplate as specific characteristics of this type of particle. Because the nanoplate is atomically thin, a reasonable determination of the thickness is a nontrivial problem, both in the modeling of mechanical properties and in physical interpretation of the obtained data. We propose a procedure for its calculation.     

Protein structural motif recognition via NMR residual dipolar couplings.

NMR residual dipolar couplings have great potential to provide rapid structural information for proteins in the solution state. This information even at low resolution may be used to advantage in proteomics projects that seek to annotate large numbers of gene products for entire genomes. In this paper, we describe a novel approach to the structural interpretation of dipolar couplings which is based on structural motif pattern recognition, where a predefined gapless structural template for a motif is used to search a set of residual dipolar couplings for good matches. We demonstrate the applicability of the method using synthetic and experimental data. We also provide an analysis of the statistical power of the method and the effects of order tensor frame orientation, motif size, and structural complexity on motif detection. Finally, we discuss remaining problems that must be overcome before the method can be used routinely to identify protein homologies.     

Retention in analytical ion exchange chromatography-1. Strong electrolytes on strong ion exchangers

A retention equation of general validity is adapted to ion exchange chromatography providing a self-consistent interpretation of retention of solutes and system peaks. Discussion is limited to eluent mixtures containing ions of invariant charge and to stationary phases with pH-independent surface charge. An interpretation is proposed for the retention volume of system peaks in binary eluents (mixtures of two eluent counter-ions). Retention volumes of labelled counter-ions of the eluent appear to provide useful information about the chromatographic system with clear interpretation in any multicomponent eluent.     

Superimposing two-dimensional gels to study genetic variation in malaria parasites.

Two-dimensional polyacrylamide gel electrophoresis is a valuable tool for studying genetic variation in the human malaria parasite, Plasmodium falciparum. It involves examining the position of protein spots in gel produced from different isolates. Some spots have been seen to vary, while others have had a constant position in all isolates so far examined. These invariant spots provide a reference frame to compare variations in other spots. This paper discusses the usefulness of digital image handling, warping and superimposition in a personal computer environment. Rather than produce a fully automatic interpretation system, we show how the computer may be used as a tool for manipulating gel images, although interpretation of the gels' features remains with the human expert. Autoradiographs are scanned on a desktop scanner, and the images in digital form can be displayed on a monitor attached to a personal computer. The coordinates of the invariant spots on each of several gels are identified by the user. Each of the gels is then warped so that the invariant spots of all the gels coincide as closely as possible. The variable spots are then examined. We have used both affine warping transformations, which match the invariant spots as closely as possible, and thin plate spline transformations, which match them exactly. Colour superimposition proved a useful way of examining the gels.     

Photodissociation dynamics of the NO dimer. I. Theoretical overview of the ultraviolet singlet excited states

Molecular orbital theory and calculations are used to describe the ultraviolet singlet excited states of NO dimer. Qualitatively, we derive and catalog the dimer states by correlating them with monomer states, and provide illustrative complete active space self-consistent field calculations. Quantitatively, we provide computational estimates of vertical transition energies and absorption intensities with multireference configuration interaction and equations-of-motion coupled-cluster methods, and examine an important avoided crossing between a Rydberg and a valence state along the intermonomer and intramonomer stretching coordinates. The calculations are challenging, due to the high density of electronic states of various types (valence and Rydberg, excimer and charge transfer) in the 6-8 eV region, and the multiconfigurational nature of the ground state. We have identified a bright charge-transfer (charge-resonance) state as responsible for the broadband seen in UV absorption experiments. We also use our results to facilitate the interpretation of UV photodissociation experiments, including the time-resolved 6 eV photodissociation experiments to be presented in the next two papers of this series.     

On the nonlinear interpretation of a vibrating wire viscometer operated at large amplitude

A non-linear interpretation is proposed for the vibrating wire (VW) viscometer operated with the transient or ringdown method. This new interpretation is motivated by the necessity of having an acceptable signal to noise ratio (SNR) such that a measurement can be performed in 1 s. A large SNR is achieved by a large oscillation amplitude such that the requisite conditions for the linear interpretation are no longer met. We demonstrate the applicability of a new non-linear interpretation with wires of length 40 mm and 50 mm over a viscosity range from 0.3 to 159 mPa s where the maximum amplitude of motion is approximately two thirds of the radius.     

The influence of medium conductivity on electropermeabilization and survival of cells in vitro.

Electropermeabilization and cell death caused by the exposure to high voltage electric pulses depends on the parameters of pulses, as well as the composition of the extracellular medium. We studied the influence of extracellular conductivity on electropermeabilization and survival of cells in vitro. For this purpose, we used a physiological medium with a conductivity of 1.6 S/m and three artificial media with conductivities of 0.14, 0.005, and 0.001 S/m. Measurements of pH, osmolarity, and cell diameter were made to estimate possible side effects of the media on the cells. Our study shows that the percentage of surviving cells increases with the decreasing medium conductivity, while the percentage of electropermeabilized cells remains unaffected. Our results show that cell survival in experiments involving electropermeabilization can be improved by decreasing the medium conductivity. To provide an interpretation of experimental results, we have theoretically estimated the resting transmembrane voltage, the induced transmembrane voltage, the time constant of the voltage inducement, and heating of the cell suspension for each of the media used. These calculations imply that for accurate interpretation of experimental results, both the induced and the resting transmembrane voltage must be considered, taking into account the conductivity and the ionic composition of the extracellular medium.     

Photooxydation du polyethylene. I. Cinetique de photooxydation du PEbd

The kinetics of photooxidation of ldPE films are characterized by the superposition of two phenomena: the first, corresponding to an exponential increase of CO concentration with time, is observed when oxygen diffusion controls the kinetics: the second, corresponding to a linear increase of [CO] with time and higher activation energy, becomes predominant in degraded samples in which [CO] ? 100 cm^?1. We suppose that the modifications of morphology following chain breaking in the amorphous phase favours oxygen access to the internal zones of the sample. Therefore, O₂ diffusion is no longer the rate determining process and we can observe the change of kinetic parameters. The disappearance of the vinylidene groups can be due to an attack by the alkoxyl macroradicals giving cross-linking. The rate of appearance of vinyl groups is temperature independent. At present, no interpretation of this result can be made.     

Nucleophilic ring opening and the formation of saturated heterocycles. 11. Quantum-chemical investigation of the reactivities of aziridine and 2-methylaziridine in aminolysis reactions

The aminolysis of aziridine and 2-methylaziridine in the gas phase was investigated by the MNDO method. Using the Germer solvatone model, modified for the MNDO computational scheme, we studied the effect of a polar medium on the ammonolysis of aziridine and 2-methylaziridine. We showed that the most probable mechanism for the investigated reactions is an A_c2 mechanism; however, in polar media the realization of the A_a2 mechanism is possible in the case of sterically hindered amines. We found that the determining factor for the nucleophilic opening of aziridine rings is the three-dimensional structure of the nucleophile rather than its basicity.See [1] for communication 10.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7. pp. 905–912, July, 1991.We thank Yu. V. Puzanov for supplying us with sets of programs for the quantum-chemical calculations. We especially thank S. G. Koldobskii for his assistance in discussing the results of our calculations and providing us with an interpretation of the solvatone method for MNDO.     

A practical solution for eliminating artificial image contrast in aberration-corrected TEM.

We propose a simple and practical solution to remove artificial contrast inhibiting direct interpretation of atomic arrangements in aberration-corrected TEM. The method is based on a combination of "image subtraction" for elimination of nonlinear components in images and newly improved "image deconvolution" for proper compensation of nonflat phase contrast transfer function. The efficiency of the method is shown by experimental and simulation data of typical materials such as gold, silicon, and magnesium oxide. The hypothetical results from further improvements of TEM instruments are also simulated. It is concluded that we can approach actual atomic structures by using the present method, that is, a proper combination of a Cs corrector, image subtraction, and image deconvolution processes.     

Analysis of reduced density matrices in the coordinate representation. I. Definitions and basic formulas

The probability interpretation of density matrics is reviewed and certain quantities suggested for study by means of diagrams. The case of a “spin-symmetric ensemble” is further discussed.     

Molecular quantum similarity measures and N-dimensional representation of quantum objects. I. Theoretical foundations

A conceptual general definition of quantum similarity and similarity indices are given. The interpretation of molecular similarity from a quantum theoretical point of view is analyzed. Various approximations are also discussed.     

Analyse des spectres de vibration infrarouge et Raman de benzothiazolines et de 2H-chromènes, éléments constitutifs de spiropyrannes benzothiazoliniques. Aspect synthétique

Study of infrared and Raman spectra of benzothiazolines and 2H-chromenes, constitutive parts of benzothiazolinic spiropyrans. Synthetic aspect. We have synthesized some benzothiazolines and some substituted 2 H-chromenes as models of the two halves of benzothiazolinic spiropyrans with the aim of interpretation of infrared and Raman spectra of spiropyranic structure. While doing that, we have been able to improve the synthesis method of chromenes obtained from coumarins and we have prepared new nitro-chromenes. By studying IR. and Raman spectra of the two halves of spiropyranic molecule, we could propose a rather good attribution of fondamental modes of vibration. This attribution will used as base for the interpretation of IR. and Raman spectra of a series a benzothiazolinic spiropyrans.