Molecular fractionation in isotactic polypropylene during isothermal crystallization

The nucleation frequency of isotactic polypropylene shows for certain molecular weight distributions during isothermal crystallization a “stepwise” development of the nuclei as a function of time. The resulting curve can be fitted by a superposition of exponential functions assuming a separate nucleation density for each nucleation step. The multimodal nucleation is discussed as a consequence of molecular fractionation effects during crystallization. Received: 23 October 1997 Accepted: 12 May 1998     

The physics of athermal nuclei in polymer crystallization

The lower temperature limit for sporadic “thermal” nucleation, as estimated in a previous paper (Janeschitz-Kriegl (1997) Colloid Polym Sci), can be verified by a variety of earlier experiments which are compiled in this paper. Below the said limit only “athermal” nuclei are of importance in samples containing no large numbers of hetero-nuclei. The number of athermal nuclei increases tremendously (by many decades) with decreasing temperature. Why these nuclei are dormant and “awake” only at “their” temperature is explained in terms of their growth conditions, which depend on the length of the incorporated (helical) sequences. Also the sluggishness of molecular processes, occurring in the temperature range of metastable conditions, where sporadic nucleation can occur is demonstrated by some of the recalled experiments. Except for very fast industrial processes, where flow plays a dominant role, this sluggishness makes a noticeable influence of the sporadic nucleation on structure formation very improbable. Received: 30 July 1998 Accepted in revised form: 5 September 1998     

Origins of two surfactant phases (D and Dp′) in a temperature-composition space of water/C12EO6 / propanol / heptane system: tricritical point and critical double end point

The origins of middle surfactant phases (D and Dp′) were investigated in the composition-temperature (49–73 °C) space of a water/hexaethyleneglycol dodecyl ether (C₁₂EO₆)/propanol/heptane system at atmospheric pressure. Two types of three-phase bodies exist in the dilute C₁₂EO₆ region of the four-component composition tetrahedron at 49.0 °C. A cone-like three-phase body consisting of aqueous (W), Dp′, and oil (O) phases becomes thinner with increasing temperature, and collapses into the superimposed critical tie lines (CTL) at 51.7 °C. One end of the superimposed CTL is a critical double end point where the closed-loop coexistence curve of the Dp′ and O phases disappears. The chiral three-phase body consisting of W, D, and O phases shrinks with increasing temperature, and finally vanishes at the tricritical point near 72.5 °C. The roles of C₁₂EO₆ and propanol are discussed and the conditions for the middle surfactant phases are discussed on the basis of the whole series of three-phase behavior. Received: 9 July 1998 Accepted in revised form: 25 August 1998     

Towards an order-N DFT method

One of the most important steps in a Kohn-Sham (KS) type density functional theory calculation is the construction of the matrix of the KS operator (the “Fock” matrix). It is desirable to develop an algorithm for this step that scales linearly with system size. We discuss attempts to achieve linear scaling for the calculation of the matrix elements of the exchange-correlation and Coulomb potentials within a particular implementation (the Amsterdam density functional, ADF, code) of the KS method. In the ADF scheme the matrix elements are completely determined by 3D numerical integration, the value of the potentials in each grid point being determined with the help of an auxiliary function representation of the electronic density. Nearly linear scaling for building the total Fock matrix is demonstrated for systems of intermediate size (in the order of 1000 atoms). For larger systems further development is desirable for the treatment of the Coulomb potential. Received: 30 March 1998 / Accepted: 6 July 1998 / Published online: 15 September 1998     

Selective membrane test for crude oil based model emulsions by use of video-enhanced microscopy

 A method for testing water/oil emulsion droplet membranes selectively has been demonstrated. The method uses electric fields to induce attraction, membrane thinning and coalescence between aqueous droplets deposited in an oil continuum. The coalescence process is monitored visually by the use of videomicroscopy. A set of model oils containing indigenous surfactants (asphaltenes) from a crude oil has been studied, and the effects of asphaltene concentration, oil phase aromaticity, aging of oils and interfacial exposure time have been investigated. The strength of the field at the point of coalescence is defined as the critical parameter describing membrane strength. In the current experiments a.c. fields were used and droplet sizes were of the order of 500–600 μm. Received: 8 October 1998 Accepted in revised form: 11 January 1999     

Electrosynthesis and characterization of poly(3-methylthiophene) on different substrates

 We report on the study of the electropolymerization of 3-methylthiophene onto Pt, p-Si and indium-tin oxide deposited on polyester using quartz crystal microgravimetry, electrochemical and spectroelectrochemical measurements. We observe that potential variation during the galvanostatic electrodeposition is correlated with the nucleation process. The mass variation of poly(3-methylthiophene) is linear with the amount of the electrochemical charge whereas the absorbance variation curve shows two distinct slopes. The latter result has been explained in terms of conjugation length variation during polymer growth. Scanning electron microscopy analysis allowed the detection of morphological changes from a dense and compact structure to an open morphology when the poly(3-methylthiophene) film thickness increases. The roughness of the films did not change considerably with the substrate, as observed by atomic force microscopy. Absorption and photocurrent spectra at different film thicknesses show a shift of the peaks to lower wavelengths when the thickness increased, thus confirming the decrease of mean conjugation length in thicker films. Received: 1 September 1998 / Accepted: 11 November 1998     

Nucleation and growth processes in the colloidal crystallization of silica spheres in the presence of sodium chloride as studied by reflection spectroscopy

Time-resolved reflection spectroscopic measurements are made for the kinetic analyses of the nucleation and growth processes of soft-type colloidal crystals of silica spheres (110 nm in diameter) in the presence of sodium chloride. Fast-scanning reflection spectra are taken using a continuous circulating-type stopped-flow cell system. The cell system is composed of a peristaltic pump and a quartz flow cell, which are connected with a PharMed tube in a closed circuit. The volume fraction of the spheres is 0.028. Induction periods range from 0.2 to 1.3 s and increase as salt concentration increases. Nucleation rates are 1 × 10⁴ to 7 × 10⁴ spheres/mm³s and decrease as salt concentration increases. The crystallization process has been observed from the sharpening and the increase in intensity of the reflection peaks. The crystal growth rate in the absence of salt is 23 μm/s, and decreases as salt concentration increases. The importance of electrostatic intersphere repulsion through the electrical double layers and the cooperative and synchronous fluctuation of colloidal spheres in the crystallization processes is supported. Received: 15 July 1998 Accepted in revised form: 18 September 1998     

Poisson-Boltzmann calculations versus molecular dynamics simulations for calculating the electrostatic potential of a solvated peptide

We performed a very long molecular dynamics simulation of a peptide in explicit water molecules and ions and averaged the electrostatic potential caused by peptide, water and ions at eight points in the vicinity of the peptide. These electrostatic potential values were directly compared to the potential calculated by solving the non-linear Poisson-Boltzmann equation for the system, which describes the solvent using continuum electrostatics. We analyze the contribution of dielectric constant, conformational flexibility and solvation effects on the electrostatic potential at these eight points. Received: 24 April 1998 / Accepted: 4 August 1998 / Published online: 23 November 1998     

Nucleation kinetics of a new nonlinear optical crystal

The induction period of various proportion of urea–thiourea zinc chloride crystal in water has been measured experimentally using the visual observation method. The induction period, which is inversely proportional to the nucleation rate, has been used to estimate the interfacial tension between the urea–thiourea zinc chloride and water; hence, the nucleation parameters like critical radius (r*), number of molecules in the radius (r*) and Gibbs free energy change for the formation of a critical nucleus (∆G*) have been calculated.     

The saddle point of the nucleophilic substitution reaction Cl− + CH3Cl: results of large-scale coupled cluster calculations

On the basis of large-scale coupled cluster calculations including connectedz triple substitutions in a perturbative way, the geometrical parameters of the D _{3 h} saddle point of the Walden inversion reaction Cl⁻ + CH₃Cl′→ ClCH₃ + Cl′⁻ are predicted to be R _s (C—Cl) = 2.301 ? and r _s (C—H) = 1.069 ?. The barrier height with respect to the reactants is recommended to be 11.5 ± 1.0 kJ mol⁻¹. Connected triple substitutions lower the barrier height by almost a factor of 2, but have very little influence on the geometric structure of the saddle point. Received: 26 June 1998 / Accepted: 15 July 1998 / Published online: 28 September 1998     

Phase field theory of interfaces and crystal nucleation in a eutectic system of fcc structure: II. Nucleation in the metastable liquid immiscibility region

In the second part of our paper, we address crystal nucleation in the metastable liquid miscibility region of eutectic systems that is always present, though experimentally often inaccessible. While this situation resembles the one seen in single component crystal nucleation in the presence of a metastable vapor-liquid critical point addressed in previous works, it is more complex because of the fact that here two crystal phases of significantly different compositions may nucleate. Accordingly, at a fixed temperature below the critical point, six different types of nuclei may form: two liquid-liquid nuclei: two solid-liquid nuclei; and two types of composite nuclei, in which the crystalline core has a liquid "skirt," whose composition falls in between the compositions of the solid and the initial liquid phases, in addition to nuclei with concentric alternating composition shells of prohibitively high free energy. We discuss crystalline phase selection via exploring/identifying the possible pathways for crystal nucleation.     

A new “hydrophobic template” method detects segments forming transmembrane α-helical bundles in ion channels

We present a “hydrophobic template” method enabling recognition of α-helix bundles in membrane channels from sequence analysis. Inspection of hydrophobic properties of pore-forming helices in proteins with known structure (A-B₅ toxins) permits delineation of a common polarity motif: two hydrophobic surface stretches separated by polar areas. The bundles are stabilized by nonpolar interhelical contacts. A number of transmembrane segments were checked for presence of this motif, and it was detected for pore-forming helices of several ion transporters (segments M2 of acetylcholine and GABA_A receptors, α5 peptide of δ-endotoxin), which reveal five α-helix bundle architecture. Applications of the method to modeling of membrane channels are discussed. Received: 24 April 1998 / Accepted: 3 September 1998 / Published online: 7 December 1998     

The mechanism of silver granular electrodeposits formation

Granules as a possible form of metal electrodeposit can be formed during deposition of metals, such deposition processes being characterized by large exchange current density values. Because of this, zero nucleation zones around growing grains are formed, permitting granular metal growth. In some cases of prolonged deposition, macro-crystalline deposits can be formed as well as granular ones, e.g. in the case of silver deposition at overpotentials lower than the critical value for dendrite growth initiation. The mechanism of granular deposit growth as a final form of metal electrocrystallization is proposed. Silver boulders were deposited on␣platinum and silver substrates. At low deposition potentials, various crystallographic forms, some of them ideal or derived from cube-octahedron-type morphology, were obtained as a result of independent grain growth inside zones of zero nucleation. In addition to cube-octahedra, twinned and multiply twinned silver particles were also observed. The nucleation density was found (1) to increase with increasing deposition overpotential, (2)␣to decrease with increasing silver concentration, and (3) to be greater on Ag than on Pt for the same deposition overpotential and dendrite precursors. Increasing overpotential leads to increase of density of twinned grains. The grain growth at greater overpotentials from more concentrated solution is less ideal, producing a granular deposit on prolonged deposition. Received: 21 April 1997 / Accepted: 18 September 1997     

The transition of three types of three-phase behavior in a temperature-composition space of water/C12EO6/propanol/heptane system

 The change of three-phase behavior of a water/hexaethylene-glycol dodecyl ether (C₁₂EO₆)/propanol/heptane system was studied with increasing temperature (30.0–48.9 °C). A cone-like three-phase body consisting of aqueous (W), surfactant (D_p), and oil (O) phases is formed in the composition tetrahedron at lower temperatures. The body is expanded with increasing temperature, and touches the water–C₁₂EO₆–heptane triangle (the base) at 44.8 °C. At about 45.6 °C the loci of the D_p and O phases meet and cut off at a critical double end point and the three-phase body separates into two bodies: one has a chiral shape and the other is unclosed. The unclosed body is transformed with the change of shape of the tie triangle on the base as temperature increases and leaves from the base at 48.9 °C. A closed-loop miscibility gap between middle surfactant (D^′ _p) and O phases or a cone-like three-phase body is formed inside the composition tetrahedron at higher temperatures. The roles of C₁₂EO₆ and propanol are also discussed. Received: 14 April 1998 Accepted: 15 July 1998     

The quality approach and fundamental research: Working towards a constructive alliance (Part I)

 The quality approach is being progressively used in scientific research and is likely to become a key point in the future. This article critically examines the applicability of the quality approach to fundamental research. A critical survey of the basic concepts of the quality approach is proposed (conformity to a reference, satisfaction of requirements and/or adaptation to use, planning and formalization). The need for quality in fundamental research is then discussed through a review of the scientific, economic and financial, human, social and environmental issues that research will face in the future. An attempt is made to demonstrate that the quality approach incorporates concerns and current practices of the scientific community which are not normally identified and labelled as pertaining to the field of Quality. Received: 8 March 1998 / Accepted: 8 December 1998     

Sequence classification of water channels and related proteins in view of functional predictions

We have worked with a classification method based upon a notion of probabilistic similarity or “likelihood of similarity” between aligned sequences. One important parameter, among others, affecting the sequence similarities and hence the classification results is the amino acid similarity matrix. We present a method for choosing the most adapted matrix to classify protein sequences. This method has been applied to the transmembrane channels of the major intrinsic protein (MIP) family. At present, two functional subgroups have been well characterized in this family: (1) specific water transport by the aquaporins and (2) small neutral solutes transport. The aim of the present study is to show the usefulness of the classification method in the prediction of sequence segments important for substrate selectivity. Moreover, we show that this method can also be used to predict the function of undetermined MIP proteins. The method could be applied to other protein families as well. Received: 24 April 1998 / Accepted: 4 August 1998 / Published online: 2 November 1998     

The phenomenon of water penetration into sodium octanoate micelles studied by molecular dynamics computer simulation

 In this publication we have studied the penetration process of water molecules into the hydrophobic core of a sodium octanoate micelle. The analysis of this phenomenon was based on a molecular dynamics computer simulation. We calculated the probability to find water molecules within a specific sphere which was adjusted in the center of the micelle. It turned out that the position of the micellar mass and geometry center was not too different, so that this reference point was well characterized. Water penetration was observed within the whole aggregate but if the radius is smaller than 300 pm, polar solvent molecules are very rarely observed. The results of our computer simulations suggest that significant water diffusion into the micelle occurs at larger distances from the micellar center with a lower threshold value of about 400 pm. In addition to these calculations, we used the Connolly algorithm in order to determine the solvent accessible surfaces of different micellar equilibrium structures. We observed large dynamical fluctuations with the formation of pores and channels. These structures are occasionally filled with water molecules. Received: 29 April 1998 Accepted: 27 May 1998     

Choosing GTO basis sets for periodic HF calculations

We investigate numerical linear dependencies of Gaussian-type orbital basis sets employed in the framework of the Hartree-Fock self-consistent field method for periodic structures, which so far have hampered the use of extended basis sets for non-ionic crystals. These linear dependencies occur when diffuse basis functions are included in a basis set in an uncontrolled manner. We use the condition number of the overlap matrix to lead us in the construction of extended basis sets for periodic structures which avoid numerical linear dependencies. Extended basis sets of high quality are optimized for a number of periodic structures (fcc He, α-Be, α-BN, and B1 NaF) with respect to the energy of the constituent atoms or ions. The results obtained with our basis sets, which do not require reoptimization in the crystal environment, compare favorably with those obtained with other extended basis sets reported in the literature. Received: 20 July 1998/Accepted: 21 August 1998 / Published online: 19 October 1998