Joan Llorens

Emergence of Supramolecular Chirality by Flows

Hydrodynamic forces in stirred solutions induce chirality in some supramolecular species of J-aggregates, as detected at the level of the electronic transition. However, the mechanism that explains the phenomenon remains to be elucidated, although the basic effect of hydrodynamic gradients of the shear rate is most probably the folding or bending of the nanoparticles in solution. Herein, we demonstrate a correlation between chiral flows in different regions of circular and square stirred cuvettes and the emergence of true circular dichroism (CD). The results show that chaotic flows lead to a racemic mixture of chiral shaped supramolecular species, and vortical flows to scalemic mixtures. In a magnetically stirred flask the descending and ascending flows are of different chiral sign and the CD reading depends on the weighting of these two flows of inverse chiral sign. The effect of the gradient of shear rates of the flows leading to chiral shape objects depends on the shape of the cuvette, which suggests that the flask shape and the controlled addition of reagents in defined regions of the stirred solutions may exert a control in self-assembly processes.

Reversible Mechanical Induction of Optical Activity in Solutions of Soft‐Matter Nanophases

Nanophases of J-aggregates of several achiral amphiphilic porphyrins, which have thin long acicular shapes (nanoribbons), show the immediate and reversible formation of a stationary mechano-chiral state in the solution by vortex stirring, as detected by their circular dichroic signals measured by 2-modulator generalized ellipsometry. The results suggest that when a macroscopic chiral force creates supramolecular chirality, it also creates an enantiomeric excess of screw distortions, which may be detected by their excitonic absorption. An explanation on the effect of the shear flow gradients is proposed on the basis of the orientation of the rotating particles in the vortex and the size, shape, and mechanical properties of the nanoparticles.

Discrimination of the effects of surfactants in gas absorption

By means of a suitable experimental scheme the hydrodynamic effect and the barrier effect due to the presence of surfactants in the liquid phase have been separately evaluated. The experimental system was a lab absorption column of alternate spheres and cylinders. CO2, in its pure form and diluted with N2, was absorbed into water and solutions of NaOH, without surfactants and with five different surfactants. Under the conditions studied the barrier effect appears to be of less significance. The hydrodynamic effect provides a good explanation for the observed reduction of the absorption. An equation is presented to estimate the reduction of the absorption rate due to the surfactants as a function of the interfacial pressure.

Polydispersity index from linear viscoelastic data: unimodal and bimodal linear polymer melts

Abstract This article describes a method for determining the polydispersity index I p 2 = M z / M w of the molecular weight distribution (MWD) of linear polymeric materials from linear viscoelastic data. The method uses the Mellin transform of the relaxation modulus of a simple molecular rheological model. One of the main features of this technique is that it enables interesting MWD information to be obtained directly from dynamic shear experiments. It is not necessary to achieve the relaxation spectrum, so the ill-posed problem is avoided. Furthermore, a determinate shape of the continuous MWD does not have to be assumed in order to obtain the polydispersity index. The technique has been developed to deal with entangled linear polymers, whatever the form of the MWD is. The rheological information required to obtain the polydispersity index is the storage G′(ω) and loss G″(ω) moduli, extending from the terminal zone to the plateau region. The method provides a good agreement between the proposed theoretical approach and the experimental polydispersity indices of several linear polymers for a wide range of average molecular weights and polydispersity indices. It is also applicable to binary blends.

Determination of equilibrium distribution constants of phenol between surfactant micelles and water using ultrafiltering centrifuge tubes

Abstract Equilibrium distribution constants, K s , of phenol between surfactant micelles and water have been determined by micellar enhanced ultrafiltration (MEUF) using commercial ultrafiltering centrifuge tubes. Three surfactants: sodium dodecyl sulphate (SDS), polyoxyethylene 20 cetyl ether (C16E20) and cetylpiridinium chloride (CPC) were tested with a 10 000 molecular weight cut off (MWCO) membrane. Additionally, membranes of 5000 and 30 000 MWCO were used for CPC. A phenomenological mathematical model has been proposed for the batch MEUF process and checked with the experimental permeate or retentate composition. The model is based on two assumptions: monomeric molecules are not rejected by the membrane and the rejection of micelles is independent of the retentate concentration. The measured micelles rejections for different surfactants and the equivalent molecular weight of the micelles are correlated and they are not significantly affected by the addition of phenol. The estimates of K s for SDS and CPC agree with previously reported values determined by other methods. K s values for CPC, calculated using 5000, 10 000 and 30 000 MWCO membranes, have not been significantly different. K s estimate has allowed to predict the phenol permeate concentration measured in continuous tangential MEUF experiments.

Two-phases model for calcium removal from aqueous solution by polymer enhanced ultrafiltration

Abstract Water softening has been carried out by polymer enhanced ultrafiltration (PEUF). Poly(acrylic acid–maleic anhydride) sodium salt (PAM-Na), a polyelectrolyte containing COO − groups, attracts Ca 2+ and releases Na + ions. When pH is lowered, H + replaces the bound Ca 2+ . Thus, the polyelectrolyte can be regenerated and reused. A model is proposed to explain the observed calcium and sodium rejections at different experimental conditions. The feed solution is considered to be divided into two phases. One phase is occupied by the macro ions and bound counter ions and the other is the remaining solution. The model only needs to determine two equilibrium constants ( K Na,H and K Ca,Na ) which indicate the relative tendencies of Na + versus H + , and Ca 2+ versus Na + to bind the polymer, respectively. K Na,H value was determined by titration and it is pH dependent. K Ca,Na value was calculated from ultrafiltration runs at neutral pH and its value goes well with other ultrafiltration runs when pH is varied.

Chirality generated by flows in pseudocyanine dye J-aggregates: revisiting 40 years old reports.

Spontaneous symmetry breaking in J-aggregates of cyanine dyes has a long history in chemical literature. In 1976, Honda and Hada claimed that they had achieved chiral induction (CD) by stirring J-aggregates of pseudocyanine. However, this report is controversial, as the combinations of linear dichroism and birefringence can lead to artifactual circular dichroic signals that are unrelated to molecular chirality. A Mueller matrix spectroscopy study, with an approach for the application of a gradient of the shear rate (solution layer between a rotating and a fixed disk) that differs from the simple vortex stirring used in the original report, shows that true CD can be induced in the sample. The phenomenon is discussed, taking into account the flow dynamics that allows the alignment of the aggregate particles and the gradient of shear rates that determines their folding/torsion, which leads to a chiral excitonic transition.

NOM characterization by LC-OCD in a SWRO desalination line

Abstract The quantification and characterization of natural organic matter (NOM) in seawater reverse osmosis desalination (SWRO) is of great importance to improve our understanding of NOM role and to evaluate the performance of the different units involved in desalination facilities. In this study, we compared the effectiveness of two pre-treatment methods, operated in a pilot facility fed with Mediterranean seawater, in terms of determining NOM composition and NOM removal using liquid chromatography coupled with organic carbon detection (LC-OCD). LC-OCD was also used to assess the SWRO operation. Conventional pre-treatment using a flotation unit followed by dual-media filtration achieved 12% of dissolved organic carbon (DOC) removal. The same level of DOC removal was achieved by coupling flotation and ultrafiltration. In both pre-treatment methods, low-molecular-weight (LMW) neutrals and biopolymers were reduced by 33–40% and 18–19%, respectively. Reverse osmosis (RO) membranes acted as almost a complete...

Improvement of the analysis of the biochemical oxygen demand (BOD) of Mediterranean seawater by seeding control.

Biochemical oxygen demand (BOD) is a useful parameter for assessing the biodegradability of dissolved organic matter in water. At the same time, this parameter is used to evaluate the efficiency with which certain processes remove biodegradable natural organic matter (NOM). However, the values of BOD in seawater are very low (around 2 mgO(2)L(-1)) and the methods used for its analysis are poorly developed. The increasing attention given to seawater desalination in the Mediterranean environment, and related phenomena such as reverse osmosis membrane biofouling, have stimulated interest in seawater BOD close to the Spanish coast. In this study the BOD analysis protocol was refined by introduction of a new step in which a critical quantity of autochthonous microorganisms, measured as adenosine triphosphate, is added. For the samples analyzed, this improvement allowed us to obtain reliable and replicable BOD measurements, standardized with solutions of glucose-glutamic acid and acetate. After 7 days of analysis duration, more than 80% of ultimate BOD is achieved, which in the case of easily biodegradable compounds represents nearly a 60% of the theoretical oxygen demand. BOD(7) obtained from the Mediterranean Sea found to be 2.0±0.3 mgO(2)L(-1) but this value decreased with seawater storage time due to the rapid consumption of labile compounds. No significant differences were found between two samples points located on the Spanish coast, since their organic matter content was similar. Finally, the determination of seawater BOD without the use of inoculum may lead to an underestimation of BOD.

Simple Equation for Suitability of Heat Pump Use in Distillation

Abstract A distillation column can be considered as a heat engine that produces separation instead of work. The heat is provided in the reboiler and collected degraded at a lower temperature at the condenser. The energy collected at low temperature at the top of the column can be upgraded back to higher temperatures by means of a heat pump and reused to heat a lower column stage.This can bring saving in terms of the overall amount of energy required. However, the energy required to increase the pressure is of higher quality and price than hot services. The aim of the present paper is to provide an easy way to check, in the early stages of design, when the use of a heat pump can provide a more sustainable distillation process decreasing its energy requirements. After several simplifications, it can be stated that it depends mainly on the efficiency of Carnot. When the efficiency of Carnot is evaluated for the industrial systems where the heat pumps are used, it is concluded that all have Carnot efficiency around 0.1 or lower. Therefore, in the early design of a new distillation column, this criterion is useful to decide when a heat pump is worth to be included in the more rigorous simulations.