Edvaldo Sabadini

Worm-like micelles of CTAB and sodium salicylate under turbulent flow.

Polymers with high molecular weight and worm-like micelles are drag-reducing agents under turbulent flow. However, in contrast to the polymeric systems, the worm-like micelles do not undergo mechanical degradation due to the turbulence, because their macromolecular structure can be spontaneously restored. This very favorable property, together with their drag-reduction capability, offer the possibility to use such worm-like micelles in heating and cooling systems to recirculate water while expending less energy. The formation, growth, and stability of worm-like micelles formed by cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) were investigated using the self-fluorescence of salicylate ions and the ability of the giant micelles to promote hydrodynamic drag reduction under turbulent flow. The turbulence in solutions of CTAB-Sal was produced within the double-gap cell of a rotational rheometer. Detailed diagrams were obtained for different ratios of Sal and CTAB, which revealed transitions associated with the thermal stability of giant micelles under turbulent flow.

Firefighting foam stability: the effect of the drag reducer poly(ethylene) oxide

Abstract The presence in fluids of very small amounts of high molecular weight polymers produces high levels of drag reduction in the fluid flow. This phenomenon, often termed the Toms Effect, can be used in firefighting, mainly due to the reduction in the energy necessary to pump water. The use of one of the most efficient drag reducing agents—poly(ethylene oxide) (PEO)—dissolved in the firefighting foam concentrate can significantly increase foam flow. This paper describes studies of the stability of foam generated from a commercial firefighting foam concentrate with added PEO. These studies were based on the lifetime of foams generated with and without small concentrations of PEO. It was observed that the presence of PEO increases the lifetime of the foam. This result is attributed to changes in the drainage rate due to the adsorption of the polymer at the liquid–air foam interface. The adsorption is probably a consequence of the polar interaction of the ether group of PEO and the polar head of the surfactant.

Molecular Variations in Aromatic Cosolutes: Critical Role in the Rheology of Cationic Wormlike Micelles

Wormlike micelles formed by the addition to cetyltrimethylammonium bromide (CTAB) of a range of aromatic cosolutes with small molecular variations in their structure were systematically studied. Phenol and derivatives of benzoate and cinnamate were used, and the resulting mixtures were studied by oscillatory, steady-shear rheology, and the microstructure was probed by small-angle neutron scattering. The lengthening of the micelles and their entanglement result in remarkable viscoelastic properties, making rheology a useful tool to assess the effect of structural variations of the cosolutes on wormlike micelle formation. For a fixed concentration of CTAB and cosolute (200 mmol L(-1)), the relaxation time decreases in the following order: phenol > cinnamate> o-hydroxycinnamate > salicylate > o-methoxycinnamate > benzoate > o-methoxybenzoate. The variations in viscoelastic response are rationalized by using Mulliken population analysis to map out the electronic density of the cosolutes and quantify the barrier to rotation of specific groups on the aromatics. We find that the ability of the group attached to the aromatic ring to rotate is crucial in determining the packing of the cosolute at the micellar interface and thus critically impacts the micellar growth and, in turn, the rheological response. These results enable us for the first time to propose design rules for the self-assembly of the surfactants and cosolutes resulting in the formation of wormlike micelles with the cationic surfactant CTAB.

Poly(ethylene glycol) or Poly(ethylene oxide)?: Magnitude of end-group Contribution to the Partitioning of Ethylene Oxide Oligomers and Polymers between Water and Organic Phases

PEO partitioning from water to CH2Cl2 and CHCl3 increases with its molar mass, leveling off at ca. 3 000 g mol-1. Such a behaviour is related to PEO end-group contributions, suggesting a polyglycol to polyether transition at ca. 3 000 g mol-1.

Aquecimento em forno de microondas/ Desenvolvimento de alguns conceitos fundamentais

The microwave oven became a common domestic equipment, due mainly to the short time spent to heat foods. One of the most interesting characteristics of the microwave oven is the selective heating. Different from the conventional oven, where the heating is not selective, the heating by microwave depends on the chemical nature of the matter. Many Students of Chemistry have no knowledge of the principles involved in this selective heating, in spite of the daily microwave oven use. The heating by microwave is feasible for chemistry courses. In discussions about the microwave absorption by the matter it is possible to explore chemical properties like: heat capacity, chemical bound, molecular structure, dipole moments, polarization and dielectric constant. This paper presents the basic principles involved in the microwave heating. It is proposed a simple and inexpensive experiment that could be developed in general chemistry courses, to illustrate the relationship between heating and the chemical properties of some solvents. Experiments to check the power of the microwave oven are also proposed.

Bis-Urea-Based Supramolecular Polymer: The First Self-Assembled Drag Reducer for Hydrocarbon Solvents

The hydrodynamic drag reduction phenomenon, also termed the Toms effect, is an unusual case involving macromolecules in solution in which the resistance to flow is reduced comparatively to that of the pure solvent. Although the effect is relatively well characterized, it is still unclear from the molecular viewpoint. The presence of some amount of a polymer with high molecular weight can produce large levels of drag reduction in turbulent flow as a result of the interactions of the long structures with the small vortices developed during the flow. For this reason, the effect is very attractive in the pumping process because a significant amount of energy can be saved. In aqueous systems, giant micelles can be spontaneously formed, driven by the hydrophobic effect, and are effective drag reducers. Giant micelles are interesting in promoting drag reduction because the noncovalent and reversible aggregation of the surfactant molecules avoids mechanical degradation, which typically occurs with classical polymers, due to irreversible scission of the backbone. In this letter, we present the first hydrodynamic drag reducer for hydrocarbons based on a self-assembled polymer formed from the reversible aggregation of bis-urea monomers. This system forms two competitive polymeric structures--the tube (T) and the filament (F) forms--which are in equilibrium with each other. Our rheology results in octane and toluene are fully consistent with calorimetry data and show that only the longest form, T, is able to promote the drag reduction effect.

Stoichiometry of poly(ethylene glycol) in water and benzene by excess volume

In this work, we studied the excess volume of solutions of low molecular weight poly(ethylene glycol)s (200, 400, and 600) in water and in benzene. We noted that there is a higher contraction of the volume for water solutions in all compositions. Solutions of these polymers in benzene also undergo a volume contraction. However, the magnitude of this effect is lower for benzene solution than for water. Moreover, the stoichiometry of the system at the composition corresponding to the maximum volume contraction is different in each case: one water molecule per two oxyethyelene (EO) units, and one benzene molecule per four EO units. The results are compared with the mixing enthalpy measured by Lakanpal et al. As reported, they observed that while the system PEG/water is exothermic in all compositions, the system PEG/benzene is endothermic, which shows that although in both solvents there is volume contraction, the driving forces for these dissolutions are different.

Use of Water Spin−Spin Relaxation Rate to Probe the Solvation of Cyclodextrins in Aqueous Solutions

1H spin-spin relaxation rate constant, R2, of water was measured by using the Carr-Purcell-Meiboom-Gill sequence in aqueous solutions of native cyclodextrins (alpha, beta, and gamma-CD) and chemically modified CDs in order to probe the structuring of the water surrounding these cyclic carbohydrate molecules. R2 values for water in solutions containing glucose and dextran were also measured for comparison. A two-site model for bonded and free water molecules was used to fit the results for the dependence of R2 on the solute concentrations. The order of relaxation rates for water in aqueous solution at a fixed specific hydroxyl group concentration is glucose>dextran congruent with CDs. No significant difference was observed for R2 of water in solutions containing native CDs, which indicates that the size and nature of the cavity has a small effect on the spin-spin relaxation times of water. The lower relaxation rate for water in CD solutions was attributed to the intramolecular hydrogen bonding formed between the secondary hydroxyl groups that line the rim of the CDs. For comparison, the relaxation rates for water in solutions of two chemically modified CDs were also studied.

Study of secondary relaxations of polyethylene by photoluminescence technique

Abstract Secondary γ and β relaxations in polyethylene have been studied by photoluminescence of molecular probes dispersed in the polymer. The temperature of the γ relaxation of low density polyethylene has been determined as −130°C by the phosphorescence emission of benzophenone. The temperature of the β relaxation of the same polymer has been found to be −40°C by the fluorescence emission of anthracene. From the differences in the quenching processes for the phosphorescence and fluorescence of the molecular probes, it should be possible to discuss the mechanisms of both the γ and β relaxations depending on the size of the macromolecular segments involved in each process

More on Polypseudorotaxanes Formed between Poly(ethylene glycol) and α-Cyclodextrin

A new interpretation for the mechanism associated with the spontaneous threading of α-CD, onto a PEG chain followed by the supramolecular hydrogel formation, is described. Beyond a specific stoichiometry, the complexation of α-CD and PEG results in the formation of a two-phase system. Besides the phase separation, for PEG with a molecular weight higher than 6000 Da, part of the polymer chains are unthreaded by the α-CD, leading to the formation of a supramolecular hydrogel. The kinetics for the complexation and the determination of the yield for equilibrated systems consisting of PEG (linear and star) are used to investigate the number of α-CD threaded before and after the phase separation. The results are compared with the prediction obtained from the application of the Poisson distribution and reveal the ratio between α-CD and PEG in each step of the process. Additionally, the kinetics for the hydrogel formation and its inner structure are investigated by using the proton NMR spin-spin relaxation of water.