Parameter identifiability in application of soft particle electrokinetic theory to determine polymer and polyelectrolyte coating thicknesses on colloids

Soft particle electrokinetic models have been used to determine adsorbed nonionic polymer and polyelectrolyte layer properties on nanoparticles or colloids by fitting electrophoretic mobility data. Ohshima first established the formalism for these models and provided analytical approximations ( Ohshima, H. Adv. Colloid Interface Sci.1995, 62, 189 ). More recently, exact numerical solutions have been developed, which account for polarization and relaxation effects and require fewer assumptions on the particle and soft layer properties. This paper characterizes statistical uncertainty in the polyelectrolyte layer charge density, layer thickness, and permeability (Brinkman screening length) obtained from fitting data to either the analytical or numerical electrokinetic models. Various combinations of particle core and polymer layer properties are investigated to determine the range of systems for which this analysis can provide a solution with reasonably small uncertainty bounds, particularly for layer thickness. Identifiability of layer thickness in the analytical model ranges from poor confidence for cases with thick, highly charged coatings, to good confidence for cases with thin, low-charged coatings. Identifiability is similar for the numerical model, except that sensitivity is improved at very high charge and permeability, where polarization and relaxation effects are significant. For some poorly identifiable cases, parameter reduction can reduce collinearity to improve identifiability. Analysis of experimental data yielded results consistent with expectations from the simulated theoretical cases. Identifiability of layer charge density and permeability is also evaluated. Guidelines are suggested for evaluation of statistical confidence in polymer and polyelectrolyte layer parameters determined by application of the soft particle electrokinetic theory.     

Perceiving molecular themes in the structures and bonding of intermetallic phases: the role of Hückel theory in an ab initio era

Qualitative molecular orbital theory is central to our understanding of the bonding and reactivity of molecules and materials across chemistry. Advances in computational technology and methodology, however, have made ab initio or density functional theory calculations a simpler alternative, offering reliable results on increasingly large systems in a reasonable time-scale without the need for concerns about the approximations and parameterization of semi-empirical one-electron based methods. In this perspective, we illustrate how the availability of higher-level computational results can augment, rather than supplant, the insights provided by approaches such as the simple and extended Hückel methods. We begin by describing a way to parameterize Hückel-type Hamiltonians against DFT results for intermetallic systems. The potential for chemical understanding embodied by such orbital-based models is then demonstrated with two schemes of bonding analysis that originated in them (but can be extended to DFT results): the μ(3)-acid/base model and the μ(2)-Hückel chemical pressure analysis, which translate the molecular concepts of acidity and electronic/steric competition, respectively, into the context of intermetallic chemistry.     

Hannah's Prior Knowledge About Chemicals: A Case Study of One Fourth‐Grade Child

Chemical principles are taught in elementary education across much of the United States because the National Science Education Standards include concepts about the nature of matter, states of matter, and changes in matter among other science concepts within the first to fifth grade levels. “Chemicals” is a word related to the nature of matter that is used not only in formal instruction, but also in everyday conversations. Children's prior knowledge about chemicals gained from everyday experiences will influence how they learn about chemical principles. The research described herein reveals insights into one child's conceptual structure related to the word “chemical,” which includes how she uses the word both inside and outside of school. Hannah was purposefully chosen for this case study because she exemplified “children's science.” Her understanding of chemicals as cleaners, in foods, and used for a purpose were primarily gained from everyday experiences. The implications of these findings are discussed with respect to both future research and elementary science education.     

Background-free detection of trapped ions

We demonstrate a Doppler cooling and detection scheme for ions with low-lying D levels which almost entirely suppresses scattered laser light background, while retaining a high fluorescence signal and efficient cooling. We cool a single ion with a laser on the $^{2}\mathrm{S}_{\mbox{\tiny$1/2$}}\leftrightarrow {^{2}\mathrm{P}}_{\mbox{\tiny$1/2$}}$ transition as usual, but repump via the $^{2}\mathrm{P}_{\mbox{\tiny$3/2$}}$ level. By filtering out light on the cooling transition and detecting only the fluorescence from the $^{2}\mathrm{P}_{\mbox{\tiny$3/2$}}\rightarrow {^{2}\mathrm{S}}_{\mbox{\tiny$1/2$}}$ decays, we suppress the scattered laser light background count rate to 1 s^?1 while maintaining a signal of 29000 s^?1 with moderate saturation of the cooling transition. This scheme will be particularly useful for experiments where ions are trapped in close proximity to surfaces, such as the trap electrodes in microfabricated ion traps, which leads to high background scatter from the cooling beam.     

Front Cover: Can Cyanuric Acid and 2,4,6-Triaminopyrimidine Containing Ribonucleosides be Components of Prebiotic RNA? Insights from QM Calculations and MD Simulations (ChemPhysChem 11/2019)

As a step toward assessing their fitness as pre-RNA nucleobases, we employ DFT and MD simulations to analyze the noncovalent interactions of cyanuric acid (CA) and 2,4,6-triaminopyrimidine (TAP), and the structural properties of the associated ribonucleosides (rNs) and oligonucleotides. Our calculations reveal that the TAP : CA pair has a comparable hydrogen-bond strength to the canonical A : U pair. This strengthens the candidature of CA and TAP as prebiotic nucleobases. Further, the stacking between two canonical nucleobases is stronger than those between TAP or CA and a canonical base, as well as those between two TAP and/or CA, which indicates that enhanced stacking may have served as a driving force for the evolution from prebiotic to canonical nucleobases. Similarities in the DFT-derived anti/syn rotational barriers and MD-derived (anti) glycosidic conformation of the CA and TAP rNs and canonical rNs further substantiate their candidature as pre-RNA components. Greater deglycosylation barriers (as obtained by DFT calculations) for TAP rNs compared to canonical rNs suggest TAP rNs indicate higher resistance to environmental factors, while lower barriers indicate that CA rNs were likely more suitable for less-challenging locations. Finally, the tight packing in narrow CA:TAP-containing helices suggests that the prebiotic polymers were shielded from water, which would aid their evolution into self-replicating systems. Our calculations thus support proposals that CA and TAP can act as nucleobases of pre-RNA.     

Synthesis of triblock and multiblock methacrylate polymers and self‐assembly of stimuli responsive triblock polymers

Multisegmented poly(methacrylate)s were synthesized using one pot reversible addition fragmentation chain transfer polymerization. Initially, a series of triblock copolymers were synthesized with different ratios of trimethylsilyl methacrylate, di(ethylene oxide) methacrylate, and oligo(ethylene oxide) methacrylate, and different total polymer molecular weights. Additionally, a polymer containing seven distinct blocks of methacrylic monomers was synthesized in one pot. For the triblock copolymers, the trimethylsilyl group was subsequently hydrolyzed, and the self‐assembly of the triblock copolymer was studied in water, under different pH and thermal conditions. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 2548–2555     

Electrochemical behavior of small biological molecules at organic donor—acceptor electrodes in aqueous media

The electrochemical behavior of NADH, xanthine, uric acid, 6-mercaptopurine, 6-thioxanthine, dopamine and ascorbic acid was investigated at the organic metal-like polymer paste electrodes, tetrathiafulvalene- and N-methylphenazene-tetracyanoquinodimethane. The reactivity of small biological molecules at polymer paste electrodes is generally lower than at most active graphite electrodes. Typical detection limits of about 10^?5 M and the typical upper limit of linearity of 10^?3 M are observed for the compounds studied.     

A search for spectator isobars in deuteron collisions

A search has been made for spectator delta resonances from the breakup of deuterons by a 4 GeV/c incident π⁺ beam. It is shown that the narrow pπ enhancement produced backwards in the laboratory system is mostly not Δ, and an upper limit of 0.8% is estimated for the Δ Δ component of the deuteron.     

Practical Vehicle Routeing Using Computer Programs

This article gives an account of a trial by a large company of VANPLAN, Scicon Consultancy's Vehicle routeing computer package.It covers the various logical stages in the trial of project design, data collection and validation, staff training and acceptance criteria.The article also highlights the advantages of using computers interactively to plan routes using the skills of existing staff. This is compared with the ‘traditional’ operational research approach using automatic routeing algorithms. Suggestions are also made regarding how to properly and fully test new routeing algorithms or techniques.