Accreditation of RM producers: what does it mean?

A comparative study of different papers published in this special edition of ACQUAL "Accreditaton of RM producers" (see Editorial) is presented. The main question is: Is accreditation the key to guaranteeing the quality of RMs or is it certification? The arguments and different options of the authors are compared and evaluated against existing rules and present practice. In the conclusion options are given which depend on the type of RM producer (namely, laboratory or product certification body), and which could be acceptable to the community of analysts and metrologists, as well as to the market.     

Weighted least-squares in calibration: what difference does it make?

In univariate calibration, an unknown concentration or amount x(0) is estimated from its measured response y(0) by comparison with a calibration data set obtained in the same way for known x values. The calibration function y = f(x) contains parameters obtained from a least-squares (LS) fit of the calibration data. Since minimum-variance estimation requires that the data be weighted inversely as their true variances, any other weighting leads to predictable losses of precision in the calibration parameters and in the estimation of x(0). Incorrect weighting also invalidates the apparent standard errors returned by the LS calibration fit. Both effects are studied using Monte Carlo calculations. For the strongest commonly encountered heteroscedasticity, proportional error (sigma(i) proportional, varianty(i)), neglect of weights yields as much as an order of magnitude precision loss for x(0) in the small x region, but only nominal loss in the calibration mid-range. Use of replicates gives great improvement at small x but can underperform unweighted regression in the mid-to-large x region. Variance function estimation approximates minimum-variance, even though the true variance functions are not well reproduced. A relative error test applied to the calibration data themselves is predisposed to favor 1/y(2) (or 1/x(2)) weighting, even if the data are homoscedastic. This predisposition weakens when replicate measurements are taken and disappears when the test is applied to an independent set of data. The distinction between a priori and a posteriori parameter standard errors is emphasized. Where feasible, the a priori approach permits reliable assignment of weights, application of a chi(2) test, and use of the normal distribution for confidence limits.     

Surface tension of an ideal solid: What does it mean?

The surface tension (ST) of an ideal (rigid, smooth, and inert) solid surface is usually calculated from a set of two equations: the Young equation and an additional equation that expresses the correlation between the individual STs of two phases and their interfacial tension. The present discussion suggests that this calculated ST may not be an appropriate characteristic of wettability of a solid surface. The reasons include the nonmeasurability of this ST and theoretical aspects related to the rigidity and inertness of an ideal solid. Instead, it is suggested to measure the contact angles of a set of a few standard liquids, from which a ‘wettability index’ can be calculated by properly averaging the wettability indices calculated for each of standard liquid. The optimal identity of these liquids and the optimal weights of the averages should be found by experimentation.     

New insights on the photophysical behavior of PRODAN in anionic and cationic reverse micelles: from which state or states does it emit?

6-propionyl-2-(N,N-dimethyl)aminonaphtahalene, PRODAN, is widely used as a fluorescent molecular probe because of its significant Stokes shift in polar solvents. It is an aromatic compound with intramolecular charge-transfer states (ICT) that can be particularly useful as a sensor. The nature of the emissive states has not yet been established despite the detailed experimental and theoretical investigations done on this fluorophore. In this work, we performed absorption, steady-state, time-resolved fluorescence (TRES) and time-resolved area normalized emission (TRANES) spectroscopies on the molecular probe PRODAN in the anionic water/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/n-heptane and the cationic water/benzyl-n-hexadecyl dimethylammonium chloride (BHDC)/benzene reverse micelles (RMs). The experiments were done by varying the surfactant concentrations at a fixed molar ratio (W = [H2O]/[Surfactant]) and changing the water content at a constant surfactant concentration. The results obtained varying the surfactant concentration at W = 0 show a bathochromic shift and an increase in the intensity of the PRODAN emission band due to the PRODAN partition process between the external solvent and the RMs interface. The partition constants, Kp, are quantified from the changes in the PRODAN emission spectra and the steady-state anisotropy () with the surfactant concentration in both RMs. The Kp value is larger in the BHDC than the AOT RMs, probably due to the interaction between the cationic polar head of the surfactant and the aromatic ring of PRODAN. The partition process is confirmed with the TRES experiments, where the data fit to a continuous model, and with the time-resolved area normalized emission spectroscopy (TRANES) spectra, where only one isoemissive point is detected. On the other hand, the emission spectra at W = 10 and 20 show a dual fluorescence with a new band that emerges in the low-energy region of the spectra, a band that was previously assigned to the PRODAN emission from the water pool of RMs. Our studies demonstrate that this band is due to the emission from an ICT state of the molecular probe PRODAN located at the interface of the RMs. These results are also confirmed by the lifetime measurements, the TRES experiments where the results fit to a two-state model, and the time-resolved area normalized emission spectroscopy (TRANES) spectra where three or two isoemissive points are detected in the AOT and BHDC RMs, respectively. In the AOT RMs, Kp values obtained at W = 10 and 20 are almost independent of the water content; the values are higher for the BHDC RMs due to the higher micropolarity of this interface.     

Hydrogen bonding in the mixed HF/HCl dimer: Is it better to give or receive?

The ClH⋯FH and FH⋯ClH configurations of the mixed HF/HCl dimer (where the donor⋯acceptor notation indicates the directionality of the hydrogen bond) as well as the transition state connecting the two configurations have been optimized using MP2 and CCSD(T) with correlation consistent basis sets as large as aug‐cc‐pV(5 + d)Z. Harmonic vibrational frequencies confirmed that both configurations correspond to minima and that the transition state has exactly one imaginary frequency. In addition, anharmonic vibrational frequencies computed with second‐order vibrational perturbation theory (VPT2) are within 6 cm⁻¹ of the available experimental values and deviate by no more than 4 cm⁻¹ for the complexation induced HF frequency shifts. The CCSD(T) electronic energies obtained with the largest basis set indicate that the barrier height is 0.40 kcal mol⁻¹ and the FH⋯ClH configuration lies 0.19 kcal mol⁻¹ below the ClH⋯FH configuration. While only modestly attenuating the barrier height, the inclusion of either the harmonic or anharmonic zero‐point vibrational energy effectively makes both minima isoenergetic, with the ClH⋯FH configuration being lower by only 0.03 kcal mol⁻¹. © 2018 Wiley Periodicals, Inc.     

A combined photoacoustic DSC for simultaneous temperature modulated measurements: does it really work?

Combination of two techniques, photoacoustic (PA) and differential scanning calorimetry (DSC), is a combination of quasi stationary thermodynamic DSC method and nonstationary dynamic PA method. Especially favorable and easy to realize is the combination with power compensated type of DSC. It has several advantages over the use of two techniques separately and allow to perform measurements simultaneously. The most obvious is simultaneous determination of thermodynamic properties such as specific heat, heat of transition and dynamic properties such as effusivity, thermal conductivity at the different phase transitions and complex specific heat at the glass transition. Unlike other temperature modulated techniques PA–DSC is especially suitable for studying polymer materials since their low thermal conductivity is an additional advantage. Conditions for simultaneous measurements are examined. It is proved that the combination of two techniques and necessary changes in construction do not essentially change adequate work of the instrument. A little disbalance of DSC operation due to the construction change can be corrected simply by recalibration. The procedures for testing and calibration for the proper operation of the combined PA–DSC are given together with some details of experimental methodology. Several measurements could serve as examples of widespread applicability of PA–DSC to study different types of phase transitions as well as time dependent processes such as glass transition.     

How does bovine serum albumin prevent the formation of kidney stone? A kinetics study

To attain a better understanding of the crystallization of calcium oxalate crystals under the influence of the protein bovine serum albumin, we examined not only the nucleation kinetics but also the structural synergy between the biomineral and the biosubstrate. It follows that during the crystallization process of calcium oxalate crystals bovine serum albumin inhibits the nucleation of calcium oxalate by increasing the kink kinetics barrier. The results of scanning electron microscopy and X-ray diffraction show, however, that bovine serum albumin promotes the formation of calcium oxalate dihydrate. Apart from this, bovine serum albumin facilitates the ordered calcium oxalate crystal assembly by suppressing the supersaturation-driven interfacial structure mismatch. The physics questions behind the mentioned effects have been addressed from the kinetics point of view. This may explain why bovine serum albumin plays an important role in suppressing urine stone formation.     

Predictive electrochemistry: will it be possible?

Journal of Solid State Electrochemistry -     

Can tandem interactions give rise to motion?

We outline the simplest principles which, taking advantage of tandem interaction, can give rise to long range motion with the use only short range forces.     

Fluorine in the Earth and the solar system,where does it come from and can it be found?

This review (1) presents a summary of the distribution of fluorine in different fluid (surficial, subterranean, metamorphic, and magmatic–hydrothermal–geothermal) and solid (oceanic and continental crust, mantle, and core) domains of the Earth, and various extraterrestrial materials and bodies (meteorites, planets and moons, and the Sun); (2) it provides an estimate of the total fluorine abundance for the Earth and in its dominant reservoirs contributing to the Earth's fluorine endowment; and (3) it discusses key observations that could further improve our understanding of fluorine abundances and geochemical systematics.     

Polymer modeling: where has it been and where is it going?

The development of the area of polymer modeling often referred to as molecular modeling has been reviewed from its early beginnings to the present day. Key forces influencing the development include computational power, algorithmic advances and access to computational resources. The desire to apply modeling techniques to predict the properties of increasingly complex polymer-containing systems, taken in conjunction with a number of current limitations discussed in this brief review, is expected to define in part some essential future developments.     

Competition between alkenes in intramolecular ketene-alkene [2 + 2] cycloaddition: what does it take to win?

In the course of developing a new synthetic methodology using ketenes in sequential cycloaddition steps, we were faced with a competition problem with molecules containing a ketene tethered to more than one reacting partner. To pinpoint the electronic and tethering requirements for a chemoselective reaction, we undertook a series of ketene-alkene [2 + 2] cycloaddition competition experiments. Those experiments were conducted on molecules containing either two identical alkenes having different tether lengths or two alkenes having the same tether length but being electronically different. We demonstrated that the reaction is much faster for forming five-membered rings than six-membered rings and calculated the Hammett constant rho for intramolecular ketene-alkene [2 + 2] cycloadditions to be -1.39.     

How long does it take to melt a gold nanorod?: A femtosecond pump–probe absorption spectroscopic study

Using pump–probe femtosecond transient absorption spectroscopy, we determined the rate of the bleach of absorption around 700–800 nm due to the longitudinal surface plasmon band of gold nanorods. Using TEM of the spotted, completely irradiated solutions suggest that the dominant products of the photothermal conformation of the rods are spheres of comparable volume. This lead to the conclusion that the melting of the rods is at least 30–35 ps, independent of the power used (5–20 μJ) or the nanorod aspect ratio (1.9–3.7).     

The ester group: how hydrofluoroalkane-philic is it?

Pressurized metered-dose inhalers (pMDIs) have been recognized as potential devices for the delivery of systemically acting drugs, including biomolecules, to and through the lungs. Therefore, the development of novel excipients capable of imparting stability to suspension formulations in hydrofluoroalkane (HFA) propellants is of great relevance because many of the drugs of interest are poorly soluble in HFAs. In this work, we use ab initio calculations and chemical force microscopy (CFM) to determine the HFA-philicity of the biodegradable and biocompatible ester moiety quantitatively. The complementary information obtained from the binding energy calculations and adhesion force measurements are used to gain microscopic insight into the relationship between the chemistry of the moiety of interest and its solvation in HFA. A lactide (LA)-based copolymer surfactant was synthesized and characterized, and its ability to stabilize a dispersion of micronized budesonide in HFA227 was demonstrated. These results corroborate the ab initio calculations and CFM and show that the LA-based moiety is a suitable candidate for enhancing the stability of dispersions in HFA-based pMDIs.