Determination of a Mass-Transfer Coefficient Using the Limiting-Current Technique

In this work, mass-transfer coefficients of a cross-corrugated plate have been determined by the limiting-current technique. A simple Microsoft Excel spreadsheet has been created, which allows students to correlate data. Students will be able to calculate the mass-transfer coefficient of different processes, for example, cross-flow membrane filtration processes or membrane reactors. The possibility of using the spreadsheet with different correlations and to discriminate between them is also validated by comparing the model results with published experimental data available in the recent literature. It is possible as well to study the influence of different turbulence promoters on the flow.     

Direct measurement of the internal electron quasi-Fermi level in dye sensitized solar cells using a titanium secondary electrode

The spatial dependence of the electron quasi-Fermi level (QFL) in the interior of dye sensitized nanocrystalline solar cells (DSC) under short circuit conditions can be inferred from calculations based on a diffusive electron transport model. The calculations predict that the difference in the QFL between the electrolyte and contact sides of the TiO(2) layer under short circuit conditions at 1 sun could be as much as 0.5-0.7 eV. The predicted QFL profiles depend on assumptions made about energy positions, electron mobility, and the conduction band density of states. In this work, the position of the QFL at the electrolyte side of the dye sensitized TiO(2) film in a DSC has been measured using a thin passivated titanium contact deposited on top of the nanocrystalline TiO(2) by evaporation. The method allows changes in the electron QFL at all points on the IV characteristic of the cell to be monitored under dark and photostationary conditions. In addition, cells incorporating the titanium electrode can give information about the behavior of the QFL under dynamic conditions.     

Larvicide Activity on Aedes aegypti of Essential Oil Nanoemulsion from the Protium heptaphyllum Resin

The aim of this work was to prepare a nanoemulsion containing the essential oil of Protium heptaphyllum resin and to evaluate the larvicidal activity and the residual larvicidal effect against Aedes aegypti. The essential oil was identified by gas chromatography coupled to a mass spectrometer, and the nanoemulsions were prepared using a low-energy method and characterized by photon correlation spectroscopy. The results indicated the major constituents as p-cimene (27.70%) and α-Pinene (22.31%). Nanoemulsions had kinetic stability and a monomodal distribution in a hydrophilic-lipophilic balance of 14 with particle diameters of 115.56 ± 1.68 nn and zeta potential of −29.63 ± 3.46 mV. The nanoemulsion showed larvicidal action with LC₅₀ = 2.91 µg∙mL⁻¹ and residual larvicidal effect for 72 h after application to A. aegypti larvae. Consequently, the nanobiotechnological product derived from the essential oil of P. heptaphyllum resin could be used against infectious disease vectors.     

Laser desorption/ionization determination of molecular weight distributions of polyaromatic carbonaceous compounds and their aggregates

Molecular weight distributions (MWDs) of model polyaromatic hydrocarbons (PAHs) and complex asphaltene samples have been investigated in laser desorption/ionization mass spectrometry (LDI-MS) experiments. Special efforts are devoted to the characterization of aggregation effects during the desorption process. It is found that non-covalent clusters of the PAHs and asphaltenes form readily in the desorbing plume. Aggregation is favoured in the experiments performed on dense samples at high laser energy and under continuous ion extraction conditions. In the absence of polar groups in the analyte molecules, the aggregation propensity correlates well with the size of the polycondensed system and with its degree of pericondensation, in qualitative agreement with previous theoretical predictions. For the polydispersed asphaltenes from two different crude oils, MWDs peaking at masses smaller than 500 amu with a high-mass tail extending up to about 3000 amu have been observed, yielding average weights around 900 amu. Such MWDs are in good agreement with previous mass spectrometric measurement, as well as with diffusion studies in solution. In addition, stable asphaltene aggregates have been detected giving rise to two broad bands in the mass spectrum corresponding to average molecular weights of 2200-3100 amu and 15,000-19,000 amu, respectively. It is concluded that the strong aggregation propensity of asphaltenes is likely to be responsible for the apparent inconsistency between the MWD for these compounds determined by different groups in independent LDI-MS experiments. The reliability of different sample preparation procedures, including solvent-free methods, is discussed, and strategies are outlined that serve to apply the potentiality of LDI mass spectrometry to the characterization of covalent and non-covalent compounds in complex carbonaceous systems.     

Reliability-Based Optimization Using Differential Evolution and Inverse Reliability Analysis for Engineering System Design

In this contribution, a new methodology based on a double-loop iteration process is proposed for the treatment of uncertainties in engineering system design. The inner optimization loop is used to find the solution associated with the highest probability value (inverse reliability analysis), and the outer loop is the regular optimization loop used to solve the considered reliability problem through differential evolution and multi-objective optimization differential evolution algorithms. The proposed methodology is applied to mathematical functions and to the design of classical engineering systems according to both mono- and multi-objective contexts. The obtained results are compared with those obtained by classical approaches and demonstrate that the proposed strategy represents an interesting alternative to reliability design of engineering systems.     

Oxazoline-based antimicrobial oligomers: synthesis by CROP using supercritical CO2

A method using supercritical CO(2) to obtain biocompatible 2-oxazoline-based oligomers quaternized with different amines is described. The synthesized oligo(2-oxazoline)s display partial carbamic-acid insertion at one end. The syntheses of quaternary oligo(2-bisoxazoline)s and linear oligoethylenimine hydrochlorides are reported. Oligo(2-methyl-2-oxazoline) and oligo(2-bisoxazoline) quaternized with N,N-dimethyldodecylamine are the most efficient biocidal agents showing fast killing rates against Staphylococcus aureus and Escherichia coli. Linear oligoethylenimine hydrochloride shows the lowest MIC values but higher killing times against both bacteria. Based on the antimicrobial activity studies, a cooperative action of carbamic acid with the ammonium end group is proposed.     

The role of ethanol-water solvent mixtures in N719 sensitization of electrodeposited ZnO nanorods

Water plays an important role in N719 sensitization of ZnO films for application as photoanodes in DSC devices. The role of water content in ethanolic N719 sensitization solutions was examined resorting to N719-solvent interaction studies based on Kamlet-Taft solvatochromic parameters. Results show that as the water content increases, solvent’s HBA ability decreases, hindering dye aggregation in solution and increasing the fraction of dye carboxylic groups available for anchorage onto the charged ZnO surface. The impact of dye-dye-solvent equilibria in solution on ZnO nanorod films sensitization and device behavior was evaluated. Devices assembled with films sensitized in N719 solutions containing equal parts of ethanol and water showed a twofold increase in short-circuit current densities when compared to those sensitized in ethanol only, despite exhibiting significantly less stained films. Data indicate that the presence of water in the sensitization solution reduces the amount of dye aggregates in solution and on the ZnO surface.     

On the determination of molecular weight distributions of asphaltenes and their aggregates in laser desorption ionization experiments

Molecular weight distributions (MWD) of asphaltenes and their aggregates have been investigated in laser desorption ionization (LDI) mass spectrometric experiments. A systematic investigation of the dependence of the measured MWD on the asphaltene sample density and on the laser pulse energy allows the assignment of most probable molecular weights within 300-500 amu and average molecular weights of 800-1000 amu for the monomeric asphaltenes, as well as for the estimation of the contribution from asphaltene clusters in typical LDI measurements. The results serve to reconcile the existing controversy between earlier mass spectrometric characterizations of asphaltenes based on laser desorption techniques by different groups. Furthermore, the MWD measurements performed on particularly dense samples yield an additional differentiated broad band peaking around 9000-10,000 amu and extending over 20,000 amu, not observed previously in LDI experiments, thereby revealing a strong propensity of the asphaltenes to form clusters with specific aggregation numbers, which is in qualitative agreement with previous theoretical predictions and with the interpretation of measurements performed with other techniques.