Development and optimization of an HPLC–DAD method for quantification of six petroleum hydrocarbon compounds in aqueous samples

Benzene, toluene, and xylene isomers (BTXs) and pyrene, benzo(b)fluoranthene, and benzo(a)pyrene (polycyclic aromatic hydrocarbons) are common pollutants found in many industrial effluents and in aquifers due to fossil fuels spill from underground storage reservoirs. For these reasons, the determination of these compounds has gained importance in the last decades. In this work, a simultaneous, fast, and accurate quantification of six petroleum hydrocarbon compounds (such as BTXs, pyrene, benzo(b)fluoranthene, and benzo(a)pyrene) using a high-performance liquid chromatography–diode array detector method has been demonstrated. The proposed method is suitable for the direct aqueous sample evaluation and also brings advantages, including the use of small volumes of organic solvents, with high resolution, reducing the analysis cost. The method was also checked using synthetic and real samples, including those containing surfactants, commercial gasoline, and river water samples spiked with petroleum hydrocarbon compounds.     

Aggregation of the hairy rod conjugated polyelectrolyte poly{1,4-phenylene-[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} in aqueous solution: an experimental and molecular modelling study

The aggregation of the fluorescent hairy rod, anionic conjugated polyelectrolyte poly{1,4-phenylene-[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) has been studied in aqueous solutions by molecular dynamics simulations, fluorescence and light scattering. Formation of clusters leads to considerable increases in light scattering, decreases in the fluorescence quantum yields and red shifts in emission maxima. Molecular dynamics simulations considering two isolated tetramers in aqueous solution show that they rapidly form aggregates, and support experimental evidence for the association of polymer chains involving both electrostatic and hydrophobic interactions. They also provide indications for proximity of aromatic rings, which is likely to be the main factor responsible for the observed fluorescence behaviour. However, there are no indications of extensive pi-stacking. The organic co-solvents methanol, acetonitrile and dioxane break up these aggregates. From studies of the dependence of the aggregation behaviour on dielectric constant or the empirical solvent parameters E(N)(T) and B(KT) for binary mixtures with water, it can be seen that this is not simply an effect of changing solvent polarity, but is due to preferential solvation of the polymer chains. This is supported by molecular dynamic simulations on two tetramers in water-dioxane mixtures (70:30%). It is suggested that similar factors are involved in both the association behaviour and aggregate disruption with other hairy rod conjugated polyelectrolytes in water.     

The effect of cationic gemini surfactants upon lipid membranes. An experimental and molecular dynamics simulation study

Gemini surfactants possess interesting interfacial and aggregation properties that have prompted comprehensive studies and successful applications in a wide variety of fields. However, a systematic study on the effect of gemini tail and spacer length upon the organization of lipid membranes has not been presented so far. In this study, we analyze the action of dicationic alkylammonium bromide gemini surfactants on DPPC liposomes, the latter employed as a model of lipid membranes. Differential scanning calorimetry results indicate that the surfactants presenting shorter tails (12 carbons) induce a decrease in the overall order of the bilayer, while those with longer tails (16 and 18 carbons) lead to the formation of more ordered structures. The respective influence on the degree of lipid order transverse to the bilayer was additionally studied resorting to a detailed fluorescence anisotropy study. In this case, it is observed that among the shorter tail surfactants, those with longer spacers (6 and 10 carbons) are responsible for a more pronounced disrupting effect upon the membrane, especially close to the lipid polar heads. Molecular dynamics simulation supports the most important findings and provides insight into the mechanism that governs this interaction. Accordingly, the interplay between tail and spacer length accounts for the differential vertical positioning of the gemini molecules and atom-density in the core of the bilayer, that provide a rationale for the experimental observations.     

Initial-value problems for linear partial difference equations

The method presented in [4] for the solution of linear difference equations in a single variable is extended to some equations in two variables. Every linear combination of a given functionf and of its partial differences can be obtained by the discrete convolution product off by a suitable functionl (which depends on the considered linear combination), and we want to solve in a convolutional form difference equations in the whole plane. However, the convolution of two functions may not be possible if their supports contain half straight lines with opposite directions. To avoid this, we take four sets of functions corresponding to the quadrants such thatl belong to every set, every set endowed with the convolution and with the usual addition is a ring, and there is an inverse ofl in each of the four rings. This is attained by taking, for each ring, a set of functions whose supports belong to suitable cones. After choosing such rings, a very natural initial-value first-order Cauchy Problem (in partial differences) is reduced to a convolutional form. This is done either by a direct method or by introducing the forward difference functions _i f(i=1,2) in a general way depending on the shape of the support off so that Laplace-like formulas with initial and final values) hold. Applications to difference equations in the whole plane and to partial differential problems are made.     

Enantiomers separation by simulated moving bed chromatography. Non-instantaneous equilibrium at the solid-fluid interface

The simulated moving bed (SMB) technology, first conceived for large bulk-scale separations in the petrochemical industry, has found increasingly new applications in the pharmaceutical industry. Among these, the separation of fine chemicals has been the subject of considerable study and research. This work presents the modeling, simulation and design of the operation of a SMB plant in order to separate a binary chiral mixture. The usual assumption of instantaneous equilibrium at the solid-fluid interface is questioned and a first-order kinetics of adsorption is taken into account. The cases of linear, Langmuir and modified Langmuir equilibria are studied. The equivalent true moving bed (TMB) model was used assuming axial dispersion for the fluid flow and plug flow for the solid-phase flow. Intraparticle diffusion was described by a linear driving force (LDF) approximation. Simulation results indicate that, under certain conditions, equilibrium is not actually reached at the adsorbent surface. This leads to different unit performances, in terms of product purities and recoveries, as compared to those predicted assuming instantaneous equilibrium. Moreover, SMB units may be improperly designed by the usual methods (flow-rate ratio separation regions) if non-equilibrium effects are overlooked.