Cristina Garza

A dynamic light scattering investigation of the nucleation and growth of thaumatin crystals

Abstract The aim of this contribution is to show some important physical–chemical parameters that must be taken into account during the crystallization of proteins. For this purpose, we have calculated the overall free energy, the chemical potential, the entropy, and the enthalpy for a model protein. Dynamic light scattering techniques were used in order to study how far the system was from the equilibrium after obtaining single crystals. Additionally, we show the solubility plot of the model protein in order to explain the plausible crystal growing zone. We have also presented the first experimental approach to infer the equilibrium state by using light scattering techniques. Classical thermodynamic measurements were obtained and compared with our approach. It has also been possible to explain the role of ionic strength and temperature in the crystallization of thaumatin. Finally, we have demonstrated how our crystal quality predictions obtained from dynamic light scattering and X-ray diffraction analyses can be compared with classical predictions.

Rice husks as a source of high purity silica

Abstract Silica originating from rice husks has been proposed for use in the manufacture of solar cells. By measuring the concentration of impurities (Fe, Zn, Ca, Mn) found in the washing solutions, the conditions necessary for achieving high purity, including the optimum time, number of washing cycles and the temperature and concentration of the washing solutions ( HCl + H 2 O ), were chosen. Following this procedure, 99.98% pure silica as obtained. The major impurity was Ca with a concentration of 100 ppm.

Rheology and DWS microrheology of concentrated suspensions of the semiflexible filamentous fd virus

Microrheology measurements were performed on suspensions of bacteriophage fd with diffusive wave spectroscopy in the concentrated regime, at different values of ionic strength. Viscosity vs. shear rate was also measured, and the effect of bacteriophage concentration and salt addition on shear thinning was determined, as well as on the peaks in the viscosity vs. shear curves corresponding to a transition from tumbling to wagging flow. The influence of concentration and salt addition on the mean square displacement of microspheres embedded in the suspensions was determined, as well as on their viscoelastic moduli up to high angular frequencies. Our results were compared with another microrheology technique previously reported where the power spectral density of thermal fluctuations of embedded micron-sized particles was evaluated. Although both results in general agree, the diffusive wave spectroscopy results are much less noisy and can reach larger frequencies. A comparison was made between measured and calculated shear modulus. Calculations were made employing the theory for highly entangled isotropic solutions of semiflexible polymers using a tube model, where various ways of calculating the needed parameters were used. Although some features are captured by the model, it is far from the experimental results mainly at high frequencies.

The mutual diffusion coefficient for the van der Waals binary mixtures of types II, III, IV, and V

In the framework of the mean‐field kinetic variational theory, a numerical study is presented to understand the concentration dependence of the mutual diffusion coefficient in terms of molecular sizes and interaction parameters for the van der Waals binary mixtures of types II, III, IV, and V, in the scheme of Scott and van Konynenburg. This work is an extension to the study for systems of type I presented by us quite recently. In addition, the behavior of the mutual diffusion coefficient of the van der Waals mixture is compared with that of the hard‐sphere mixture and for the case of systems of type II, with experimental data of actual systems: water/n‐propanol, n‐hexane/acetone, and n‐heptane/acetone. The mutual diffusion coefficients for the last two systems were determined by us with the Taylor dispersion technique. The mutual diffusion coefficients for the systems n‐hexane/acetone and n‐heptane/acetone are reported here at 298.15 and 303.15 K, respectively, along all the concentration range. The expl...

Temperature dependence of the mutual diffusion coefficients in aqueous solutions of alkali metal chlorides

Measurements of mutual diffusion coefficients at infinite dilution in aqueous solutions of alkali-metal chlorides have been made with the Taylor dispersion technique. Data were obtained for the series LiCl/H2O, NaCl/H2O, KCl/H2O, RbCl/H2O, and CsCl/H2O, at five temperatures between 298.15 and 318.15 K. A linear dependence with temperature was found. This technique is very convenient in comparison with other more time-consuming techniques.

The mutual diffusion coefficient for the van der Waals binary mixture of type I

In the framework of the mean‐field kinetic variational theory, the explicit dependence of the mutual diffusion coefficient of the van der Waals binary mixture with composition and interaction parameters is obtained. The different kinds of behavior shown by this coefficient can be classified according to the scheme of van Konynenburg and Scott devised to describe the global phase diagram of this model mixture. A numerical study to understand the concentration dependence of the mutual diffusion coefficients for mixtures of type I is presented here, in terms of molecular masses, sizes, and interaction parameters. Moreover, the behavior of the mutual diffusion coefficient of the van der Waals mixture is compared with that of a hard‐sphere mixture. In addition, a comparison is made between our calculations and experimental data of binary systems classified as belonging to type I: H2O/D2O, hexane/heptane, toluene/hexane, and benzene/hexane. From the explicit model presented here, one can obtain semiquantitative...

Worm-like micelles in water solutions of 1, 4 poly (1, 3-butadiene)-polyethylene oxide diblock copolymer.

The main purpose of this study is to determine for the first time the structure of the self-assembled aggregates in the system made of 1,4 poly(1,3-butadiene)-polyethylene oxide diblock copolymer (IUPAC name: poly(but-2-ene-1,4-diyl)-block-polyoxyethylene) and water, and the rheological behavior of the solution. The degree of polymerization of the polybutadiene and polyethylene oxide blocks is 37 and 45, respectively. The diblock copolymer concentration was limited to be

Atomic force microscopy images of lyotropic lamellar phases.

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

2.5 wt% to avoid phase separation. Small X-ray scattering revealed that the diblock copolymer self-assembles in worm-like micelles with a diameter of ∼ 12 nm. This system does not closely follow the rheological behavior of worm-like micelle solutions made of typical surfactants. The system steadily shear thins reaching very low viscosity values at large shear rates, however there are not shear-thickening peaks. In thixotropic loops, the micellar solution does not present hysteresis. The viscoelastic spectra do not follow the Maxwell model at low and intermediate frequencies. This uncommon behavior for a worm-like micellar system is explained by the slow dynamics of the self-assembly. The extremely high hydrophobicity of the polybutadiene block does not allow any micellar rearrangementGraphical abstract

Measuring adhesion on rough surfaces using atomic force microscopy with a liquid probe

For the very first time, atomic force microscope images of lamellar phases were observed combined with a freeze fracture technique that does not involve the use of replicas. Samples are rapidly frozen, fractured, and scanned directly with atomic force microscopy, at liquid nitrogen temperature and in high vacuum. This procedure can be used to investigate micro-structured liquids. The lamellar phases in Sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/water and in C12E5/water systems were used to asses this new technique. Our observations were compared with x-ray diffraction measurements and with other freeze fracture methods reported in the literature. Our results show that this technique is useful to image lyotropic lamellar phases and the estimated repeat distances for lamellar periodicity are consistent with those obtained by x-ray diffraction.