Nematic-isotropic transition in a density-functional theory for hard spheroidal colloids

We introduce a density-functional formalism based on the Parsons-Lee and the generalized van der Waals theories in order to describe the thermodynamics of anisotropic particle systems with steric interactions. For ellipsoids of revolution, the orientational distribution function is obtained by minimizing the free energy functional and the equations of state are determined. The system exhibits a nematic-isotropic discontinuous transition, characterized by a phase separation between nematic and isotropic phases at finite as well low packing fractions. The model presents a phase behavior which is in good agreement with Monte-Carlo simulations for finite aspect ratios.Comment: 12 pages, 5 figure

Density functional theory for dense nematics with steric interactions

The celebrated work of Onsager on hard particle systems, based on the truncated second order virial expansion, is valid at relatively low volume fractions for large aspect ratio particles. While it predicts the isotropic-nematic phase transition, it fails to provide a realistic equation of state in that the pressure remains finite for arbitrarily high densities. In this work, we derive a mean field density functional form of the Helmholtz free energy for nematics with hard core repulsion. In addition to predicting the isotropic-nematic transition, the model provides a more realistic equation of state. The energy landscape is much richer, and the orientational probability distribution function in the nematic phase possesses a unique feature: it vanishes on a nonzero measure set in orientational space

Non-Markovian Effects on Overdamped Systems

We study the consequences of adopting the memory dependent, non-Markovian, physics with the memory-less over-damped approximation usually employed to investigate Brownian particles. Due to the finite correlation time scale associated with the noise, the stationary behavior of the system is not described by the Boltzmann-Gibbs statistics. However, the presence of a very weak external white noise can be used to regularize the equilibrium properties. Surprisingly, the coupling to another bath effectively restores the dynamical aspects missed by the over-damped treatment.Comment: 10 page

Comparison of esters determination techniques in cachaça

An analytical comparison of three different techniques for quantitative profile of esters in cachaça is reported. The Ministério da Agricultura Pecuária e Abastecimento (MAPA) recommends the use of GC/FID or volumetry. Despite being laborious and lacking in chemical speciation, the volumetric technique for total ester content shows to be appropriate, reproducible, and accurate for the analysis of cachaça. However, the GC/FID suggested by MAPA, considering only ethyl acetate, shows inaccuracy, underestimating the total ester content by a median factor of 72%, mainly due to the absence of ethyl lactate analysis. On the other hand, the GC/MS technique that comprises the analysis and speciation of nine esters, including ethyl lactate, proved to be reproducible, simple, fast and accurate for the analysis of total ester content in cachaça. Thus, the total ester content results obtained using GC/FID must be considered with precaution.FAPESPCNP

Mean-field model for a mixture of biaxial nematogens and dipolar nanoparticles

We analyze a mean-field model for mixtures involving biaxial nematogens and dipolar nanoparticles, taking into account orientational and isotropic pair interactions between nematogens, but also orientational nematogen-nanoparticle interactions. We determine bulk equilibrium phase diagrams for a wide range of interaction strengths, identifying in each case the effect of the nanoparticles on the stability of nematic phases and on the appearance of multicritical points. Special attention is given to the limit of low concentration of nanoparticles, in which their effect on the temperatures of both the first-order uniaxial-isotropic and the continuous biaxial-uniaxial transitions is investigated in detail

Otimização da produção de biodiesel a partir de óleo de coco babaçu com aquecimento por microondas

As reações sob aquecimento por microondas geralmente apresentam significativa redução no tempo de reação e elevados rendimentos. Portanto, o objetivo desse trabalho foi adaptar um forno de microondas doméstico de forma adequada e segura para realização de reações de transesterificação, e otimizar o processo de produção de biodiesel por microondas usando óleo de coco babaçu como matéria prima. Para este fim, foi usado um planejamento composto central no qual se variou o tempo de irradiação, concentração de KOH e razão óleo:metanol. Análises estatísticas foram feitas para avaliarem a significância do modelo usado. As condições experimentais ótimas foram: razão óleo:metanol, 8,59, concentração de KOH, 2,19%, tempo de irradiação, 70 segundos, dando rendimento de aproximadamente 100%.Reactions under microwave heating present reduced reaction times and larger yields. Therefore, this work is aimed at adapting a domestic microwave oven and optimizing the transesterification reaction used in biodiesel production with microwave heating, using babaçu coconut oil as raw material. It was used a central composite design for varying irradiation time, KOH concentration, and oil:methanol ratio. Statistical analyses were performed in order to assess the significance of the model used. The optimized experimental conditions were: oil:methanol ratio, 8.59; KOH concentration, 2.19 %; and irradiation time, 70 seconds, giving an yield of approximately 100% regarding esters formation.FAPEM