RESUMO
The statistical associating fluid theory for attractive potentials of variable range (SAFT-VR) density functional theory (DFT) developed by [Gloor et al., J. Chem. Phys., 2004, 121, 12740-12759] is used to predict the interfacial behaviour of molecules modelled as fully-flexible square-well chains formed from tangentially-bonded monomers of diameter σ and potential range λ = 1.5σ. Four different model systems, comprising 4, 8, 12, and 16 monomers per molecule, are considered. In addition to that, we also compute a number of interfacial properties of molecular chains from direct simulation of the vapour-liquid interface. The simulations are performed in the canonical ensemble, and the vapour-liquid interfacial tension is evaluated using the wandering interface (WIM) method, a technique based on the thermodynamic definition of surface tension. Apart from surface tension, we also obtain density profiles, coexistence densities, vapour pressures, and critical temperature and density, paying particular attention to the effect of the chain length on these properties. According to our results, the main effect of increasing the chain length (at fixed temperature) is to sharpen the vapour-liquid interface and to increase the width of the biphasic coexistence region. As a result, the interfacial thickness decreases and the surface tension increases as the molecular chains get longer. The interfacial thickness and surface tension appear to exhibit an asymptotic limiting behaviour for long chains. A similar behaviour is also observed for the coexistence densities and critical properties. Agreement between theory and simulation results indicates that SAFT-VR DFT is only able to predict qualitatively the interfacial properties of the model. Our results are also compared with simulation data taken from the literature, including the vapour-liquid coexistence densities, vapour pressures, and surface tension.
RESUMO
We have obtained the interfacial properties of short rigid-linear chains formed from tangentially bonded Lennard-Jones monomeric units from direct simulation of the vapour-liquid interface. The full long-range tails of the potential are accounted for by means of an improved version of the inhomogeneous long-range corrections of Janecek [J. Phys. Chem. B 110, 6264-6269 (2006)] proposed recently by MacDowell and Blas [J. Chem. Phys. 131, 074705 (2009)] valid for spherical as well as for rigid and flexible molecular systems. Three different model systems comprising of 3, 4, and 5 monomers per molecule are considered. The simulations are performed in the canonical ensemble, and the vapor-liquid interfacial tension is evaluated using the test-area method. In addition to the surface tension, we also obtain density profiles, coexistence densities, critical temperature and density, and interfacial thickness as functions of temperature, paying particular attention to the effect of the chain length and rigidity on these properties. According to our results, the main effect of increasing the chain length (at fixed temperature) is to sharpen the vapor-liquid interface and to increase the width of the biphasic coexistence region. As a result, the interfacial thickness decreases and the surface tension increases as the molecular chains get longer. The surface tension has been scaled by critical properties and represented as a function of the difference between coexistence densities relative to the critical density.
RESUMO
El artículo publicado por Taylor y cols., en junio de 2010 en el American Journal of Hypertension, puso en duda la eficacia que tendría la reducción de la ingesta de sal en la dieta como medida para la prevención y tratamiento de la hipertensión arterial y de otras patologías de causa cardiovascular. La publicación de este artículo ha generado una gran controversia, y las respuestas no se hicieron esperar tanto desde sociedades médicas como científicas. Éstas han criticado fuertemente los resultados de este metaanálisis, señalando serias falencias metodológicas. En el presente artículo presentamos críticamente la evidencia experimental que muestra la importancia del aporte dietario de sal y su papel como determinante de la presión arterial. Describimos muy brevemente el paradigma que explica el papel de la ingesta de sal en la presión arterial (hipótesis y modelo de Guyton) y mencionamos evidencia experimental que lo sustenta. Comentamos brevemente los estudios clásicos que indican que la ingesta de sal (NaCl) contribuye directamente al desarrollo de hipertensión arterial y daño de tejidos diana. Finalmente, exponemos brevemente los datos experimentales que motivan la controversia en cuanto al papel de la sal (NaCl) o el sodio como agentes prohipertensivos.
The study by Taylor et al published in June 2010 in the American Journal of Hypertension questions the effectiveness of reducing salt intake in the diet in the prevention and treatment of high blood pressure and other cardiovascular conditions. The publication of this article has lead to great controversy and medical associations and learned societies responded promptly. The response criticized the results of the meta-analysis and pointed out its methodological shortcomings. In this review we critically appraise the experimental evidence that shows the importance of diet salt intake and its role as a determinant of blood pressure. We briefly describe the paradigm that explains the role of salt intake in contributing in the regulation of blood pressure (Guyton hypothesis and model) and we mention the experimental evidence that supports this. We briefly comment on the classical studies that indicate that salt intake (NaCl) contributes directly to the development of high blood pressure and target tissues. Finally, we briefly mention the experimental data that is related with the controversy on the role of salt (NaCl) or sodium as prohypertensive agents.
