Use of circular variance to quantify the deviation of a macromolecule from the spherical shape

It is shown that the extent of deviation of a molecular shape from spherical can be characterized by comparing the distribution of the circular variances, a measure originally proposed to quantify angular spread, of the vectors from each atom to the rest of the molecule to the circular variance of a collection of atoms filling the unit sphere. Different measures for quantifying the difference between distribution are proposed and compared.     

Precipitation and association in a mixture of dodecylammonium chloride and sodium dodecyl sulfate in aqueous medium

The precipitation boundary in aqueous mixtures of dodecylammonium chloride (DDACl) and sodium dodecyl sulfate (NaDS) was determined in the absence and in the presence of 1 mol dm^–3 NaCl. The structure and the composition of the solid phase was characterized by x-ray and chemical analysis, respectively. Activities of Cl^– and Na⁺ ions in the aqueous phase were measured by ion-selective electrodes. As determined by calorimetry, precipitation is an exothermic process.The DDA·DS precipitate was formed in the equimolar region of the precipitation components. Its crystallographic structure is described. In an excess of sodium dodecyl sulfate, the precipitate incorporated substantial amounts of NaDS, as detected by analyses of both solid and liquid phases. X-ray analysis of the dry sample showed peaks of crystalline NaDS. According to the polarization microscopy of wet samples, one may conclude that liquid crystals, containing an excess of NaDS, are incorporated in the solid DDA·DS phase. The same was found in the case of an excess of DDACl; mixed liquid crystals with an excess of DDACl were incorporated in the solid DDA·DS.Interpretation of the solubility boundary points to the presence of DDA⁺·DS^– ion pairs. Formation of these species at low ionic strengths is characterized by the equilibrium constantK _a ⁰ 10⁶. However, in the 1 mol dm^–3 NaCl, the association of DDA⁺ and DS^– ions into pairs is inhibited (K _a ⁰ 0). This finding can be explained in terms of ionic clouds around the charged surfactant heads: if these heads are not in close contact, but separated due to structural effects of the chains, the dense distribution of counterions around them at high ionic strengths may compensate for electrostatic attraction and, thus, inhibit ion-pairing.     

Surface potential at the hematite-water interface

Construction of a metal oxide electrode enabling measurement of surface potential is described. The electrode was made using a hematite monocrystal, which avoids the problems arising from the possible porosity of the oxide layer. The potential of this electrode was measured as a function of pH. The hematite electrode provides reproducible results, especially in the acidic region. Surface potentials were calculated from electrode potentials using the electrokinetic isoelectric point. The slope of surface potential with respect to pH was found to be lower than the Nernstian, especially in the basic region. The effect was more pronounced at higher ionic strengths. It was shown how the measurement of surface potential can help to interpret the equilibrium data and evaluate the choice of a theoretical model describing the interfacial equilibrium at the metal oxide-water interface.     

Interlaboratory Evaluation of Ultraviolet Radiation Emissions from Compact Fluorescent Lamps

There have been many recent reports regarding the potential risks of UV emissions from compact fluorescent lamps (CFLs). In some of these reports, the robustness of the measurements was difficult to discern. We conducted round‐robin measurements, involving three lamp manufacturers and two government research laboratories to gather reliable data on the UV emissions from commercially available CFLs. The initial sample of lamps consisted of 71 spiral‐shaped CFLs purchased from local retailers. From the initial sample, 14 “high UV emitting” CFLs were chosen for further evaluation. We compared the UV emissions at a distance of 20 cm with the UV exposure limits (ELs) published by the International Commission on Non‐ionizing Radiation Protection (ICNIRP). We found that the allowable exposure time for measured lamps ranged from 21 to 415 h. This indicates that the emissions would not exceed the short‐term ELs that have been established by the ICNIRP for healthy individuals. We also evaluated the potential long‐term risk and found it to be insignificant. There was a large variation in the UV emissions found, even for lamps from a single package, indicating that it is impossible to predict the UV output of a CFL based on its physical appearance and model designation.     

Code interoperability and standard data formats in quantum chemistry and quantum dynamics: The Q5/D5Cost data model

Code interoperability and the search for domain‐specific standard data formats represent critical issues in many areas of computational science. The advent of novel computing infrastructures such as computational grids and clouds make these issues even more urgent. The design and implementation of a common data format for quantum chemistry (QC) and quantum dynamics (QD) computer programs is discussed with reference to the research performed in the course of two Collaboration in Science and Technology Actions. The specific data models adopted, Q5Cost and D5Cost, are shown to work for a number of interoperating codes, regardless of the type and amount of information (small or large datasets) to be exchanged. The codes are either interfaced directly, or transfer data by means of wrappers; both types of data exchange are supported by the Q5/D5Cost library. Further, the exchange of data between QC and QD codes is addressed. As a proof of concept, the H + H₂ reaction is discussed. The proposed scheme is shown to provide an excellent basis for cooperative code development, even across domain boundaries. Moreover, the scheme presented is found to be useful also as a production tool in the grid distributed computing environment. © 2013 Wiley Periodicals, Inc.     

Long-range contribution to inverse-distance power and Morse energy and pressure terms of a molecular liquid,tested on benzene

Analytical expressions were derived to estimate the contribution to the intermolecular energy and pressure from pairs of rigid molecules farther than a cutoff distance; inverse power terms (like in the Lennard-Jones potential) and exponential terms (like in the Morse potential) were considered. The Lennard-Jones case was tested on liquid benzene at room temperature in the Gibbs ensemble using the cavity-biased technique. The density produced by simulations using the derived cutoff correction was found to be independent of the cutoff used.     

Reconstruction of a plane convex body from the curvature of its boundary

Let denote the class of plane convex bodies having a width functionw, wherew′ is absolutely continuous. It is proved that a body in is determined (up to translation) by the radius of curvature function of its boundary. This result is then used for a characterization of the extreme (indecomposable) bodies in and for a density theorem for Reuleaux polygons in . The content of this paper is a revised version of a part of the Master of Science thesis written by the author under the supervision of Professor Micha A. Perles at the Hebrew University of Jerusalem and submitted in October, 1971.