Indirect determination of chloride and sulfate ions in alcohol fuel by capillary electrophoresis

A capillary zone electrophoresis method using indirect UV detection for the analysis of chloride and sulfate in alcohol fuel samples was developed. The anions were analyzed in less than 3 min using an electrolyte containing 10 mmol l^–1 chromate and 0.75 mmol l^–1 hexamethonium bromide (HMB) as electroosmotic flow modifier. Coefficients of variation were better than 0.6% for migration time (n=10) and between 2.05 and 2.82% for peak area repeatabilities. Analytical curves of peak area versus concentration in the range of 0.065–0.65 mg kg^–1 for chloride and 0.25–4.0 mg kg^–1 for sulfate were linear with coefficients of correlation higher than 0.9996. The limits of detection for sulfate and chloride were 0.033 and 0.041 mg kg^–1, respectively. Recovery values ranged from 85 to 103%. The method was successfully applied for the quantification of sulfate and chloride in five alcohol fuel samples. The concentration of sulfate varied from 0.45 to 3.12 mg kg^–1. Chloride concentrations were below the methods LOD.     

Specific ion effects in solutions of globular proteins: comparison between analytical models and simulation

Monte Carlo simulations have been performed for ion distributions outside a single globular macroion and for a pair of macroions, in different salt solutions. The model that we use includes both electrostatic and van der Waals interactions between ions and between ions and macroions. Simulation results are compared with the predictions of the Ornstein-Zernike equation with the hypernetted chain closure approximation and the nonlinear Poisson-Boltzmann equation, both augmented by pertinent van der Waals terms. Ion distributions from analytical approximations are generally very close to the simulation results. This demonstrates that properties that are related to ion distributions in the double layer outside a single interface can to a good approximation be obtained from the Poisson-Boltzmann equation. We also present simulation and integral equation results for the mean force between two globular macroions (with properties corresponding to those of hen-egg-white lysozyme protein at pH 4.3) in different salt solutions. The mean force and potential of mean force between the macroions become more attractive upon increasing the polarizability of the counterions (anions), in qualitative agreement with experiments. We finally show that the deduced second virial coefficients agree quite well with experimental results.     

Polymeric nanoparticles assembled with microfluidics for drug delivery across the blood-brain barrier

The blood-brain barrier (BBB) is a challenge in the treatment of some diseases, since it prevents many drugs from reaching therapeutic concentrations in the brain. In this context, there is a growing interest in nanoparticles for drug delivery, since they are able to cross this barrier and target the brain. The use of polymeric materials in the development of these nanoparticles has been extensively studied. It has already been demonstrated that these nanosystems have the ability to cross the BBB, which allows effective drug release into the brain. Biodegradable polymers provide a great advantage in the development of nanosystems, but modifications of the nanoparticles’ surface is essential. The traditional batch methods lack precise control over the processes of nucleation and growth, resulting in poor control over final properties of the nanoparticles. Therefore, microfluidics could be used to achieve a better production environment for the fabrication of nano- structured drug delivery systems. This study provides a brief review of: the BBB, the polymeric nanoparticles with the ability to overcome the barrier, the properties of the most used polymeric matrices, and the nanostructured drug delivery systems assembled with microfluidics.     

Cure kinetic parameter estimation of thermosetting resins with isothermal data by using particle swarm optimization

The heuristic algorithms have shown to be a powerful tool in parameter estimation. Among these algorithms, particle swarm optimization (PSO) has become a method whose application has been increasing quickly. In the present work a new way for parameter estimation from cure kinetic model of polymeric resin using a differential-algebraic approach is shown. The PSO was applied to minimize the least squares function and to find the parameters from an autocatalytic model for describing cure kinetics of thermosetting resins. The isothermal data were obtained at four temperatures: 318, 333, 348 and 363 K. Three parameter estimation procedures were compared for finding a parameter set for all temperatures simultaneously. In the first one, called classical method, a curing rate was calculated with experimental values of the degree of cure and the temperature. In the second and third methods, the curing rate was obtained from the integration of a differential-algebraic system and the main difference between them is the objective function and the way to determine the ultimate reaction heat. All methods showed good results; however, the third method was more accurate than the others. The confidence regions of all parameters were found and they were used to give us indication whether the parameters estimated here by different methods are statistically different.     

The numerical integration of relative equilibrium solutions. The nonlinear Schrodinger equation

We analyse different error propagation mechanisms for conservativeand nonconservative time-integrators of nonlinear Schrödingerequations. We use a geometric approach based on interpretingwaves as relative equilibria.     

A remark about the density of the orbits of the Collatz permutation

In this work, we study the density of the orbits of the Collatz Permutation and discuss the long-standing problem of determining whether this permutation has only a finite number of distinct orbits.     

On a theorem of Kisin

This note provides a short proof of a theorem of Kisin on crystalline representations.     

Single-shot measurement of solids and liquids T1 values by a small-angle flip-flop pulse sequence

We propose the small-angle flip-flop (SAFF) pulse sequence as an alternative procedure for the rapid measurement of the ¹H spin–lattice relaxation time in the laboratory frame (T₁) of solid and liquid substances, in a time-domain NMR experiment. Based on the original flip-flop pulse sequence, this technique allows the fast estimation of T₁ values of samples that require minutes to hours of acquisition time if traditional pulse sequences are employed. We have applied SAFF to different substances, with T₁ ranging from microseconds up to seconds, including natural clays, polymers, and organic and inorganic solvents. We also demonstrate the potential of the pulse sequence in the real-time monitoring of dynamic processes, such as the conformational changes of polymeric materials during heating. The results we obtained with SAFF are comparable with those acquired with the inversion-recovery pulse sequence, with the addition of several benefits. This pulse sequence obeys steady-state and magnetization-conserving principles, making it possible to dismiss the need for relaxation delay times of the order of 5T₁. SAFF has shown high sensitivity in the resolution of individual components of T₁ in multiexponential systems and can be easily integrated to well-established pulse sequences, such as Magic Sandwich Echo and Carr–Purcell–Meiboom–Gill, for the single-shot determination of T₁ and T₂ or T_2*.     

Progress in the electrochemical reduction of CO2 on hierarchical dendritic metal electrodes

Electrochemical reduction of CO₂ (ERC) is a promising way to deal with CO₂ emissions in the atmosphere, recycle CO₂, and achieve a carbon-neutral economy. Electrodes made of hierarchical dendritic materials are investigated for the ERC owing to their structural advantages, including large surface areas, presence of facets with low-coordinated atoms, and needle-like tips. Selected examples are presented to illustrate the state-of-the-art investigation of these dendritic electrodes. The mechanisms and the function of the dendritic structure are discussed, combined with a comparison of performance results of ERC.