Relation between the particle size distribution and the kinetic performance of packed columns. Application to a commercial sub-2 microm particle material

To study the influence of the particle size distribution (PSD), we measured the chromatographic performance of a series of sub-2 microm particle high performance liquid chromatography (HPLC) columns packed with four different particle mixtures having a purposely imposed different size distribution. Using the reduced kinetic plot representation by plotting the separation impedance (E(0)) versus the plate number ratio (N(opt)/N), the different columns could be classified according to their chromatographic performance without the need to specify a mean particle diameter or a molecular diffusion coefficient, as is needed in the classical reduced plate height and flow resistance analysis. The present analysis shows that it is not so much the width or span of the particle size distribution, but rather the presence of fines that greatly determines the chromatographic performance of particulate columns.     

Intersystem crossing processes in nonplanar aromatic heterocyclic molecules

A comprehensive study of the photophysical properties of a series of monoaza[5]helicenes is presented on the basis of joint optical spectroscopy and quantum chemistry investigations. The molecules have been characterized by absorption and CW/time-resolved luminescence measurements. All quantities related to spin-orbit-coupling processes, such as intersystem crossing rates and radiative phosphorescence lifetimes, were found to depend strongly on the nitrogen position within the carbon backbone. Density functional theory and semiempirical quantum-chemical methods were used to evaluate the molecular geometries, the characteristics of the excited singlet and triplet states, and the spin-orbit coupling matrix elements. We demonstrate that the magnitude of spin-orbit coupling is directly correlated with the degree of deviation from planarity. The trends from the calculated photophysical quantities, namely, radiative fluorescence and phosphorescence decay rates and intersystem crossing rates, of the mono-aza-helicenes are fully consistent with experiment.     

Rapid determination of p<Emphasis Type="Italic">K</Emphasis><Subscript>a</Subscript> values of 20 amino acids by CZE with UV and capacitively coupled contactless conductivity detections

A rapid and universal capillary zone electrophoresis (CZE) method was developed to determine the dissociation constants (pK _a) of the 20 standard proteogenic amino acids. Since some amino acids are poorly detected by UV, capacitively coupled contactless conductivity detection (C⁴D) was used as an additional detection mode. The C⁴D coupling proved to be very successful on a conventional CE-UV instrument, neither inducing supplementary analyses nor instrument modification. In order to reduce the analysis time for pK _a determination, two strategies were applied: (i) a short-end injection to reduce the effective length, and (ii) a dynamic coating procedure to generate a large electroosmotic flow (EOF), even at pH values as low as 1.5. As a result, the analysis time per amino acid was less than 2 h, using 22 optimized buffers covering a pH range from 1.5 to 12.0 at a constant ionic strength of 50 mM. pK _a values were calculated using an appropriate mathematical model describing the relationship between effective mobility and pH. The obtained pK _a values were in accordance with the literature. Figure a UV (1) and C⁴D (2) detectors placed on-line on the CE capillary. b Curve of effective mobility as a function of pH for histidine     

Photooxidative degradation of polyether-based polymers

The mechanisms of photooxidation of several polyether based polymers were compared in order to give general rules of orientation of the oxidative reactions. These polymers were a poly(ether-ester) and two poly(ether-urethane)s. The behavior of the polyether component as homopolymer was also studied. This study was extended to copolymers of fluorinated olefins and allyl or vinyl ethers, and the non fluorinated homologues. The elucidation of the photooxidation mechanisms was based on the identification of the photoproducts by analysis of both the solid polymeric matrix and the gaz phase formed on irradiation. It was shown that the photochemical sensitivity of the polyether component was responsible for the rapid oxidation of the copolymers. However, in the case of the fluorinated compounds, different behaviors were observed. It was shown that the presence of the fluorine atoms strongly influenced the orientation of the reaction and modified the oxidation kinetics. The reactivity of the methylene groups in α-position of the oxygen atom of the ether groups were not found to be equivalent regarding oxidation and the secondary was more oxidizable than the tertiary one.     

On-line solid-phase extraction and multisyringe flow injection analysis of Al(III) and Fe(III) in drinking water

A new analytical method was developed for on-line monitoring of residual coagulants (aluminium and iron salts) in potable water. The determination was based on a sequential procedure coupling an extraction/enrichment step of the analytes onto a modified resin and a spectrophotometric measurement of a surfactant-sensitized binary complex formed between eluted analytes and Chrome Azurol S. The optimization of the solid phase extraction was performed using factorial design and a Doehlert matrix considering six variables: sample percolation rate, sample metal concentration, flow-through sample volume (all three directly linked to the extraction step), elution flow rate, concentration and volume of eluent (all three directly linked to the elution step). A specific reagent was elaborated for sensitive and specific spectrophotometric determination of Al(III) and Fe(III), by optimizing surfactant and ligand concentrations and buffer composition. The whole procedure was automated by a multisyringe flow injection analysis (MSFIA) system. Detection limits of 4.9 and 5.6 μg L⁻¹ were obtained for Al(III) and Fe(III) determination , respectively, and the linear calibration graph up to 300 μg L⁻¹ (both for Al(III) and Fe(III)) was well adapted to the monitoring of drinking water quality. The system was successfully applied to the on-site determination of Al(III) and Fe(III) at the outlet of two water treatment units during two periods of the year (winter and summer conditions).     

Blends of a new thermoplastic in a thermoset epoxy matrix

Properties of thermoplastic modified epoxy network have been studied. The particularity of this work is the use of new thermoplastic epoxies whose structure is close to the final matrix. Blends of thermoset epoxy (Diglycidyl Ether of Bisphenol A/4-4′ methylenebis [3-chloro 2,6-diethylaniline]) with a thermoplastic content from 5 to 40%w have been realised. Initial miscibility in the thermoset precursors shows an UCST behaviour with a maximal value near 130°C for a thermoplastic content of 10%. Due to the presence of tertiary amine and pendant hydroxyl groups on the thermoplastic backbone, epoxy amine reactions are faster than for the neat system but the thermoplastic seems not to have reacted with the thermoset network. The final blends are transparent but toughening increase is rather low.     

Starch Nanocrystal Fillers in an Acrylic Polymer Matrix

Aqueous suspensions of nanocrystals can be obtained by acid hydrolysis of native waxy maize starch granules. The disruption of waxy maize starch granules by 2.2N HCl hydrolysis has been followed by scanning and transmission electron microscopy and laser granulometry. The mechanical properties of composite materials made of poly(styrene-co-butyl acrylate) filled with starch nanocrystals were characterized by dynamic mechanical analysis. These nanocrystals appeared to be an interesting reinforcing phase in a thermoplastic matrix.     

Theoretical investigation of the molecular structure of aluminium triisopropoxide and its complexes in ring-opening polymerization

We present a theoretical study on aluminium triisopropoxide (Al(OⁱPr)₃) using both empirical (Molecular Mechanics, MM, with Dreiding II force field) and quantum-chemical (Austin Model 1, AM1, semiempirical Hartree-Fock) techniques. We determine the most stable geometries for both the tetramer and trimer of aluminium triisopropoxide as well as the thermodynamic characteristics of the equilibrium existing between these two aggregated structures. The theoretical results are compared to experimental data from X-ray diffraction and ²⁷A1 NMR measurements. For the tetramer, it appears that the optimal equilibrium geometries are in good agreement with the experimental X-ray diffraction geometry; another geometry is also obtained with both theoretical approaches, which is slightly less stable but of higher symmetry. On the basis of the most stable configurations for the tetramer and trimer aggregates, the variation of free enthalpy (ΔG) between the two aggregated structures has been estimated. The evolution of the theoretical ΔG values indicates a displacement of equilibrium towards the trimer species with temperature, in good agreement with experimental ¹H and ²⁷A1 NMR data. Moreover, the AM1 heats of formation show a gain of 33.9 kcal/mol due to the aggregation of four A1(OⁱPr)₃ instead of three, and thus a better stability of the tetramer. The molecular geometries being well described by the theoretical methods used in this study, we also present a model for the ring-opening polymerization complexes of ε-caprolactone and lactides.     

Ion implantation in poly(para-phenylene): Stability of the resulting electrical conductivity and influence of the implantation parameters

: In an attempt to obtain a semiconducting poly(paraphenylene) PPP we have looked at the effect of ion implantion on this conjugated polymer (Ref. 1). The properties of the material may be varied according to the implantation parameters (Ref. 2) and the electrical conductivity after ion implantation of PPP remains almost constant for a long period of time.