Rheology of wormlike micelles from non-equilibrium molecular dynamics

In this work, the rheology of complex fluids, i.e., surfactants of varying concentration in a Lennard–Jones fluid, is analyzed with non-equilibrium molecular dynamics simulations. The molecular model considers that the surfactant molecule is composed of a hydrophilic head, affine to solvent, and a hydrophobic tail made of four monomers. The solvent is modeled by a Lennard–Jones fluid, which shows mostly a Newtonian behavior, but at relatively high shear rates, a slight shear-thinning followed by a slight shear thickening are exhibited. The intermolecular potential produces an equilibrium configuration, in which the surfactant molecules self-assemble in a wormlike micelle. With the aim to analyze the system behavior with various stress fields, two flows are simulated under non-equilibrium conditions: (1) simple shear and (2) Poiseuille's flow. In simple shear, by keeping the velocity of the upper plate of the flow cell constant, a monotonic flow curve is predicted within a range of shear rates. At low shear rates, a concentration-dependent Newtonian region of viscosity η₀ corresponds to an isotropic condition in which the wormlike micelle preserves its equilibrium conformation. At intermediate shear rates, the solution exhibits a slight shear thinning, generating bands placed normal to the gradient direction (gradient banding). At high shear rates the solution exhibits shear-thickening, with bands now generated normal to the vorticity direction. These predictions by molecular models explain, to our knowledge for the first time, experiments in shear-thickening wormlike micellar solutions, where shear-thickening appears simultaneously with bands generated perpendicular to the vorticity axis. In Poiseuille's flow, we also find agreement between predictions of the model with theoretical developments and experiments performed by other authors.     

On the evolution of the rate of polymerization,number and size distribution of particles in styrene emulsion polymerization above CMC

This work is an extension of a communication reported by two of the authors [Carro and Herrera‐Ordoñez, Macromol Rapid Commun 2006, 27, 274], where bimodal particle size distributions (PSD), obtained by asymmetric flow‐field flow fractionation (AFFF, AF⁴), were taken as evidence of certain degree of stability of primary particles. Now, emulsion polymerizations of styrene were performed under conditions employed before by other researchers, intending to examine if the behavior observed is general. The number of particles (N) and PSD were studied by means of dynamic light scattering and AF⁴. By the later, bimodal PSDs were detected in all cases, where the population corresponding to primary particles (diameter <20 nm) depends on reaction conditions. Regarding N, AF⁴ results show that it is constant during interval II, in contrast to DLS results. Primary particle coagulation was evidenced as minimums in N evolution and the rate of polymerization curves, monitored by calorimetry and gravimetry, which are enhanced when higher particle number is generated and/or the ionic strength is increased. These results suggest that particle coagulation is not as extensive as it would be expected according to the coagulative theory. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3152–3160, 2010     

Random lasing in an organic light‐emitting crystal and its interplay with vertical cavity feedback

The simultaneous vertical‐cavity and random lasing emission properties of a blue‐emitting molecular crystal are investigated. The 1,1,4,4‐tetraphenyl‐1,3‐butadiene samples, grown by physical vapour transport, feature room‐temperature stimulated emission peaked at about 430 nm. Fabry‐Pérot and random resonances are primed by the interfaces of the crystal with external media and by defect scatterers, respectively. The analysis of the resulting lasing spectra evidences the existence of narrow peaks due to both the built‐in vertical Fabry‐Pérot cavity and random lasing in a novel, surface‐emitting configuration and threshold around 500 μJ cm⁻². The anti‐correlation between different modes is also highlighted, due to competition for gain. Molecular crystals with optical gain candidate as promising photonic media inherently supporting multiple lasing mechanisms.     

Solid-phase micro-extraction procedure for the determination of 1,3-dichloro-2-propanol in water by on-fibre derivatisation with bis(trimethylsilyl)trifluoroacetamide

The headspace solid-phase micro-extraction technique with on-fibre derivatisation followed by gas chromatography-tandem mass spectrometry has been evaluated for the analysis of 1,3-dichloro-2-propanol in water. An asymmetric factorial design has been performed to study the influence of five experimental factors: extraction time and temperature, derivatisation time and temperature and pH. The best extraction performance is achieved in the headspace mode, with 5 mL stirred water samples (pH 4) containing 1.3 g of NaCl, equilibrated for 30 min at 25 °C, using divinylbenzene-carboxen-polydimethylsiloxane as the fibre coating. On-fibre derivatisation has been used for the first time with 50 μL of bis(trimethylsilyl)trifluoroacetamide at 25 °C during 15 min, leading to effective yields. The proposed method provides high sensitivity, good linearity and repeatability (relative standard deviation of 5.1% for 10 ng mL⁻¹ and n = 5). The limits of detection and quantification were 0.4 and 1.4 ng mL⁻¹, respectively. Analytical recoveries obtained for different water samples were approx. 100%.     

Development of a fast liquid chromatography–tandem mass spectrometry method for the determination of endocrine-disrupting compounds in waters

A fast liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS-MS) method was developed to study five endocrine-disrupting compounds (4-n-nonylphenol, bisphenol A, estrone, 17β-estradiol and 17α-ethinylestradiol) in water. Different columns were tested; the chromatographic separation of the analytes was optimized on a Pinnacle DB biphenylic column with a water–acetonitrile gradient elution, which allowed the separation of the selected endocrine-disrupting compounds (EDCs) in less than 6 min. Quantitative analysis was performed in selected reaction monitoring (SRM) mode; two transitions were chosen for each compound, using the most abundant for quantitation. Calibration curves using bisphenol A-d ₁₆ as internal standard were drawn, showing good correlation coefficients (0.9993–0.9998). All figures of merit of the method were satisfactory; limits of detection were in the low pg range for all analytes. The method was then applied to the determination of the analytes in real water samples: to this aim, polar organic chemical integrative samplers (POCIS) were deployed in the influent and in the effluent of a drinking water treatment plant in Liguria (Italy). The EDC level was rather low in the influent and negligible in the outlet, reflecting the expected function of the treatment plant.     

Passive sampling and stir bar sorptive extraction for the determination of endocrine-disrupting compounds in water by GC-MS

A new method using the extraction and preconcentration capabilities of stir bar sorptive extraction, combined with high-resolution gas chromatography and mass spectrometry, was developed for the determination of five selected endocrine-disrupting compounds (4-n-nonylphenol, bisphenol A, estrone, 17β-estradiol, and 17α-ethinylestradiol) in water. In situ derivatization to transform the phenolic compounds into lipophilic and volatile analytes was carried out with acetic anhydride. Two different methods of headspace derivatization to further improve the chromatographic properties of 17β-estradiol and 17α-ethinylestradiol were developed and compared. The optimized method provided good sensitivity (limits of quantitation 1.2–2.6 ng), repeatability (relative standard deviation 2–9%), and reproducibility (relative standard deviation 10–17%). Passive sampling by means of polar organic chemical integrative samplers was applied to monitor river waters used as supply sources for drinking water treatment plants in the Liguria region of Italy. The analytes showed a different distribution at the three sites considered; bisphenol A proved to be the most abundant, ranging from 185 to 459 ng per sampler.     

Interpolation of Limited and Weakly Compact Operators on Families of Banach Spaces

In this paper, we study how the limited and weakly compact properties of operators are preserved by interpolation of the real method for infinite families of Banach spaces introduced by Carro in Studia Math. 109 (1994). We apply these results to the case of Sparr, Fernández and Cobos–Peetre methods of interpolation for finite families.     

Combined solid-phase extraction and gas chromatography-mass spectrometry used for determination of chloropropanols in water

A sensitive and rapid derivatization method for the simultaneous determination of 1,3-dichloro-2-propanol (1,3-DCP) and 3-chloropropane-1,2-diol (3-MCPD) in water samples has been developed. The aim was to research the optimal conditions of the derivatization process for two selected reagents. A central composite design was used to determine the influence of derivatization time, derivatization temperature and reagent volume. A global desirability function was applied for multi-response optimization. The analysis was performed by gas chromatography-mass spectrometry. During the optimization of the extraction procedure, four different types of solid-phase extraction (SPE) columns were tested. It was demonstrated that the Oasis HLB cartridge produced the best recoveries of the target analytes. The pH value and the salinity were investigated using a Doehlert design. The best results for the SPE of both analytes were obtained with 1.5 g of NaCl and pH 6. The proposed method provides high sensitivity, good linearity (R(2)≥0.999) and repeatability (relative standard deviations % between 2.9 and 3.4%). Limits of detection and quantification were in the range of 1.4-11.2 ng/mL and 4.8-34.5 ng/mL, respectively. Recoveries obtained for water samples were ca. 100% for 1,3-DCP and 3-MCPD. The method has been successfully applied to the analysis of different samples including commercially bottled water, an influent and effluent sewage.     

A rapid analytical method based on microwave-assisted extraction for the determination of drugs of abuse in vitreous humor

Robust and simple validated analytical methods are required in postmortem toxicology to confirm immunoassay screening analysis of drugs of abuse. In this work, microwave-assisted extraction (MAE) was evaluated as an alternative method for extraction of target compounds such as cocaine, benzoylecgonine, cocaethylene, morphine, codeine, 6-monoacetylmorphine, methadone, and 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine from vitreous humor. The MAE procedure parameters, namely, extraction temperature, time, and solvent volume, were optimized using a central composite design and applying desirability functions. The optimal conditions for extraction were 80 °C, 8 min, and 15 mL of dichloromethane solvent. The MAE–high-performance liquid chromatography–diode-array detection method was validated, showing its capability for the detection of concentrations in the range from 33 to 76 ng mL⁻¹ and recoveries in the range from 87 to 99.3% for all drugs. The MAE-based method was tested for 15 vitreous humor samples from forensic cases and its performance was compared with that of a solid-phase extraction method previously developed by our group. In general, better recovery and precision were achieved with the use of the MAE-based procedure.