Jorge Raúl Palacios Delgado

Diffusing wave spectroscopy in Maxwellian fluids

We present a critical assessment of the diffusing wave spectroscopy (DWS) technique for obtaining the characteristic lengths and for measuring the loss and storage moduli of a reasonable well-known wormlike micelle (WM) system. For this purpose, we tracked the Brownian motion of particles using DWS embedded in a Maxwellian fluid constituted by a wormlike micellar solution made of cetyltrimethylam-monium bromide (CTAB), sodium salicylate (NaSal), and water. We found that the motion of particles was governed by the viscosity of the solvent at short times and by the stress relaxation mechanisms of the giant micelles at longer times. From the time evolution of the mean square displacement of particles, we could obtain for the WM solution the cage size where each particle is harmonically bound at short times, the long-time diffusion coefficient, and experimental values for the exponent that accounts for the broad spectrum of relaxation times at the plateau onset time found in the 〈Δr2(t)〉 vs. time curves. In addition, from the 〈Δr2(t) vs. time curves, we obtained G′(ω) and G″(ω) for the WM solutions. All the DWS microreological information allowed us to estimate the characteristic lengths of the WM network. We compare our DWS microrheological results and characteristic lengths with those obtained with mechanical rheometers at different NaSal/CTAB concentration ratios and temperatures.

Control of the electrochemical reduction of horminone by pH imposition in acetonitrile

Abstract An electrochemical study using transient techniques of a quinone-type natural product, horminone, has been performed in acetonitrile in the presence of two buffer solutions of pH = 17.2 and pH = 15.5. Using linear sweep voltammetry, it was found that at both pH values the reduction mechanism of horminone involves a monoelectronic charge-transfer step, followed by a protonation step and homogeneous charge-transfer step due to disproportionation of the protonated intermediate. At pH = 15.5 the mechanism for the homogeneous charge-transfer step was found to be of the DISP1 type (first order disproportionation) from the results of double step chronoamperometry experiments. At pH = 17.2 the latter mechanism was found to be of the DISP2 type (second order disproportionation) since at this pH value the protonation step is minimized, thus causing the disproportionation reaction to be the controlling step of the mechanism.

Flow Velocity Profiles and Shear Banding Onset in a Semidilute Wormlike Micellar System under Couette Flow

Velocity profiles in Couette flow are measured in a wormlike micellar solution made of cetyltrimethylammonium bromide (CTAB), sodium salicylate (NaSal), and water, at R (= [NaSal]/[CTAB]) = 2 and at R = 4; [CTAB] = 100 mM. Velocity profiles were obtained by using a two-incident beam laser Doppler technique. Profiles reveal that one of the micellar solutions (R = 2) becomes heterogeneous a long time after flow inception, even at very low imposed shear rates. However, profiles do not correspond to what is expected for gradient shear banding, because the fluid splits in one section close to the moving cylinder where the local mean velocity depends linearly on the gap position and in a second section presenting important velocity fluctuations. Close to the static cylinder, there is a third section where the fluid does not flow; it behaves like a slipping block. On the other hand, at high imposed shear rates, the former slipping block flows and presents a linear profile. Here, velocity profiles are consistent with gradient shear banding. The onset of shear banding was observed. The picture of two stable shear bands separated by a thin steady interface is not always valid. Inhomogeneous flow could be observed, although it cannot be classified as shear banding. In addition, conditions can be found where, as shear rate is increased and before shear banding appears, instead of a thin interface, a fluctuating intermediate band can be observed. On the contrary, for the R = 4 solution, the flow never becomes heterogeneous even at high shear rates. Flow curves were measured in a Couette cell under shear rate control in two cases, when stress is sensed with the moving cylinder and when it is sensed with the static cylinder of the cell. Differences between the flow curves can be explained by using the velocity profiles for both solutions.

A survey of the phases and the metastable phases in the ternary systems of divalent metal (bis-2-ethylhexyl)-sulphosuccinate/iso-octane/water

This paper surveys the phases for three systems: Co(AOT)2/iso-octane/water, Ni(AOT)2/iso-octane/water, and Cu(AOT)2/iso-octane/water, over a wide region of the phase diagram prism (T = 5–80 ◦ C; α = 0.50, 0.75, and 0.90; γ = 0.02–0.22). At low surfactant concentration, these systems present Winsor progressions (¯ 2� –3� –2� ) without the addition of salt. We found a variety of phases at higher surfactant concentration: the one-phase microemulsion 1� , which is probably bicontinuous, and several examples of two coexisting phases: coloured transparent phases coexisting with a transparent or with an opaque birefringent phase, and transparent birefringent phases coexisting with excess oil. Three coexisting phases where also found, where an iso-octane-rich phase is coexisting with a coloured isotropic transparent phase, and an opaque birefringent phase.