E. A. Oliveira

A new soap/detergent/water lyotropic liquid crystal with a biaxial nematic phase

Abstract A new lyotropic liquid crystal, potassium laurate, decylammonium chloride and water, which has two uniaxial phases and a biaxial nematic phase, is reported. A surface of the phase diagram and X-ray diffraction studies are presented. The chemical stability of this mixture is compared with the potassium laurate/decanol/water mixture. Optical measurements of the birefringence and X-ray studies indicate that this new mixture is more stable than the usual mixtures with alcohol.

Gaussian deconvolution: a useful method for a form-free modeling of scattering data from mono- and multilayered planar systems

A new method for analysis of scattering data from lamellar bilayer systems is presented. The method employs a form-free description of the cross-section structure of the bilayer and the fit is performed directly to the scattering data, introducing also a structure factor when required. The cross-section structure (electron density profile in the case of X-ray scattering) is described by a set of Gaussian functions and the technique is termed Gaussian deconvolution. The coefficients of the Gaussians are optimized using a constrained least-squares routine that induces smoothness of the electron density profile. The optimization is coupled with the point-of-inflection method for determining the optimal weight of the smoothness. With the new approach, it is possible to optimize simultaneously the form factor, structure factor and several other parameters in the model. The applicability of this method is demonstrated by using it in a study of a multilamellar system composed of lecithin bilayers, where the form factor and structure factor are obtained simultaneously, and the obtained results provided new insight into this very well known system.

On-line digestion in a focused microwave-assisted oven for elements determination in orange juice by inductively coupled plasma optical emission spectrometry

Um sistema de fluxo acoplado a um forno de microondas focalizadas foi utilizado para digestao em linha de amostras de suco de laranja para a determinacao de Ca, Cu, Fe, K, Mg, Mn, Na, P e Zn por espectrometria de emissao otica com plasma indutivamente acoplado. Uma bobina de reacao de PTFE (4,0 m de comprimento e 1,6 mm de diâmetro interno) foi posicionada no interior da cavidade de vidro do forno de microondas focalizadas. Aliquotas de 500 mL de amostra e de 1000 mL de reagente (80% v/v HNO3) foram misturadas em uma confluencia e conduzidas para a bobina de reacao utilizando ar como carregador. O desvio padrao relativo para cinco determinacoes da mesma amostra usando o metodo proposto foi inferior a 5,0%. Boas recuperacoes variando de 91 a 111% foram obtidas para adicoes de quantidades conhecidas dos elementos de interesse. Os resultados obtidos, utilizando o sistema de digestao proposto, foram concordantes a um nivel de confianca de 95% com aqueles obtidos por digestao total. Com este metodo de digestao foi possivel processar 12 amostras h-1, minimizando contaminacoes, consumo de amostra e reagentes e gerando pequena quantidade de residuos.

Monte Carlo method applied to modeling copper transport in river sediments

Monte Carlo simulation (MCS) methodology has been applied to explain the variability of parameters for pollutant transport and fate modeling. In this study, the MCS method was used to evaluate the transport and fate of copper in the sediment of the Tibagi River sub-basin tributaries, Southern Brazil. The statistical distribution of the variables was described by a dataset obtained for copper concentration using sequential extraction, organic matter (OM) amount, and pH. The proposed stochastic spatial model for the copper transport in the river sediment was discussed and implemented by the MCS technique using the MatLab 7.3™ mathematical software tool. In order to test some hypotheses, the sediment and the water column in the river ecosystem were considered as compartments. The proposed stochastic spatial model makes it possible to predict copper mobility and associated risks as a function of the organic matter input into aquatic systems. The metal mobility can increase with the OM posing a rising environmental risk.

Liquid-Crystal Alignment on Anisotropic Homeotropic-Planar Patterned Substrates

ABSTRACT The development of patterned surfaces for liquid crystal (LC) alignment has attracted much attention because it is applicable to bistable displays allowing the complete control of LC alignment. In this work we propose a method of preparing patterned substrates for LC alignment combining two surface treatments that lead to alternating homeotropic–planar alignment potentials. By superimposing two similar patterned substrates with crossed directions of treatment, a twisted nematic texture was observed in the regions of planar alignment in both surfaces, indicating the possibility of technological applications.

Surface Treatments for Lyotropic Liquid Crystals Alignment

For nematic lyotropic liquid crystals it has been experimentally observed that in nontreated glass cells the director in the surface layer can be reoriented by a magnetic field in any direction parallel to the surface, and the final state is stable. In this paper we will focus on the equilibrium configuration of the director in the surface layer in the presence of a magnetic field, using cells with treated surfaces to introduce an easy axis. The surface treatments employed in this study are: rubbing of glass plates coated with polymer (PMMA), rubbing of bare glass plates, and unidirectional deposition of Teflon. All these surface treatments induce a preferential orientation to the liquid crystal. From AFManalysis we observe microchannels in the polymer films that are more homogeneous for the Teflon film. The experimental results indicate that the anchoring energy depends on the surface treatment and is larger for the rubbed glass plates.

Multilamellar-to-unilamellar transition induced by diphenylalanine in lipid vesicles

In the present work, we investigate the effect of two short phenylalanine-based peptides on lipid membranes. A simplified model membrane composed of lecithin vesicles was used to incorporate different amounts of the two amino acid sequences, the dimmer l,l-diphenylallanine (FF) and the trimmer cysteine-diphenylallanine (CFF). Spectroscopic and scattering techniques were applied to probe in detail the structural behavior of lipid membranes in the presence of the peptides. The experimental results demonstrate that both peptides are located mainly at the interface of the membrane interacting with phosphate groups modifying membrane thickness and flexibility. The multilamellar structure of the vesicles is preserved with inclusion of small amounts of FF, accompanied by changes in membrane thickness and elasticity. Finally, a multi- to unilamellar transition is observed as a result of peptide self-association into a crystalline structure onto the membrane interface.

Inscription of local surface relief gratings with a Scanning Near-field Optical Microscope on an azo-polymer film

A Scanning Near-field Optical Microscope (SNOM) was used to inscribe locally limited Surface Relief Gratings (SRGs) on pDR1M, which is an azo-polymer film. The influence of parameters such as the line scanning time and the line spacing on the amplitude and the period of the gratings is discussed. SRGs have been used to orient Liquid Crystals (LCs), where the LC alignment takes place due to the elastic distortions of the nematic medium close to the surface. In addition, the photoalignment of the azo-chromophores was also used to induce LC alignment. In this work we investigate the use of local SGRs together with the photoalignment to get a local control of the LC alignment properties. The advantage of this method is that it is reversible and the gratings can easily be erased.

Surface effects on lyotropic liquid crystals

Liquid crystals are very sensitive to surface effects. In fact, these effects are very useful in designing eletro-optical devices. We present a review of the theoretical models that describe the surface interactions in liquid crystals, focusing on lyotropic systems. Experimental results will be presented and compared to theoretical predictions.