Ciências e Tecnologia | Artigos em revistas internacionais / Papers in international journals
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- Mutual diffusivity in n-Heptane + n-Hexane isomersPublication . Lopes, M. L. Matos; Castro, C. A. Nieto de; Oliveira, Carla Padrel deA preliminary study of the effect of branching in the binary diffusion of hexane isomer + n-heptane systems is presented. Measurements have been performed with an instrument modeled for the Taylor dispersion technique, at several compositions, at 297 K, for the n-hexane + n-heptane and 2,2-dimethylbutane + n-heptane binary mixtures. The accuracy is estimated to be 1%. The influence of branching is discussed. It was found that the rough hard sphere model for binary diffusion can reproduce the experimental data within 6%. This model was used to predict the binary diffusion coefficients in the 2-methylpentane + n-heptane and 3-methylpentane + n-heptane mixtures, with an estimated accuracy of 5%.
- Mutual diffusivity in binary mixtures of n-heptane with n-hexane isomersPublication . Oliveira, Carla Padrel de; Fareleira, João M. N. A.; Castro, C. A. Nieto deThis paper presents a study of the influence of branching in the binary diffusion coefficients of n-heptane+n-hexane isomers, in the liquid state. The measurements have been made with the Taylor dispersion technique, at several compositions, at 283 and 298 K, for the X+n-heptane mixtures, where X=n-hexane, 3-methylpentane, 2, 3-dimethylbutane, and 2, 2-dimethylbutane. The results show a very interesting behavior of the composition dependence of the binary diffusion coefficients, presenting a maximum, for compositions about a molar fraction of n-heptane of 0.5, which increases with the increase in the degree of branching, suggesting the possibility of order~tisorder effects caused by stereochemically favored packing in the liquid phase and energetically favored segment interaction in the liquid mixtures. An attempt to apply the van der Waals model to these data could not predict the experimental binary diffusion coefficients of these systems within the experimental accuracy.
- The 9-Anthroate chromophore as a fluorescent probe for waterPublication . Costa, Fernando Pestana da; Maçanita, António L.; Costa, Sílvia B.; Melo, Eurico C.; Santos, HelenaWater quenches the fluorescence of methyl 9-anthroate with a rate constant showing little dependence on solvent viscosity or polarity. In dioxane, at 20ºC the value of the rate constant is 9.6 X 10^6 M^(-1) s^(-1) , and the activation energy found for the process is 14.1 kJ mol^(-1). The quenching process is entropy-controlled and is likely to involve a hydrogen-bonded complex as an intermediate. Since the fluorescence lifetime of methyl 9-anthroate does not depend on the solvent properties other than its hydrogen-bonding ability, the concentration of nearby water can be estimated directly. Values of 3, 54, and 14 M were obtained for the solubilization site of methyl 9-anthroate in micelles of Triton X-100, sodium dodecyl sulfate (SDS), and dodecyltrimethylammonium chloride (DTAC), respectively. From the ring current effect of the anthroate group on the 'H NMR chemical shifts of the surfactant protons, it is concluded that the anthroate fluorescent probe is preferentially located in the surface region of the SDS and DTAC micelles; however, in Triton X-100, it resides in the micelle interior near the phenoxy groups of the surfactant molecule.
- Vibrating-wire viscometry on liquid hydrocarbons at high pressurePublication . Oliveira, Carla Padrel de; Assael, Marc J.; Papadaki, Maria; Richardson, Stephen M.; Wakeham, William A.A vibrating-wire viscometer for the measurement of the viscosity of liquids at elevated pressures is described. The instrument is one of a pair, developed collaboratively in different laboratories, that operate in a relative manner according to a complete theory of the instrument. The instrument described here has been used for measurements at pressures up to 80 MPa in toluene, and in n-heptane along two isotherms.
- The viscosity of five liquid hydrocarbons at pressures up to 250 MPaPublication . Oliveira, Carla Padrel de; Wakeham, William A.This paper reports new measurements of the viscosity of toluene, n-pentane, n-hexane, n-octane, and n-decane at pressures up to 250 MPa in the temperature range 303 to 348 K. The measurements were performed with a vibrating-wire viscometer and with a relative method of evaluation. Calibration of the instrument was carried out with respect to reference values of the viscosity of the same liquids at their saturation vapour pressure. The viscosity measurements have a precision of _0.1% but the accuracy is limited by that of the calibration data to be +0.5%. The experimental data have been represented by polynomial functions of pressure for the purposes of interpolation. The data are also used as the most precise test yet applied to a representation of the viscosity of liquids based upon hard-sphere theory.
- Vibrating-wire viscometers for liquids at high pressuresPublication . Oliveira, Carla Padrel de; Assael, Marc J.; Papadaki, Maria; Wakeham, William A.The design and operation of two independent vibrating-wire viscometers are described. The instruments are intended for operation in the liquid phase at pressures up to 300 MPa and have been designed specifically for this purpose using the detailed theory of the device. Extensive evidence is adduced to demonstrate that the operation of the viscometers is consistent with the theory. Although the instruments attain a precision in viscosity measurements of __+1% when used in an absolute mode the accuracy that can be achieved is no better than __+3%. However, if the instrument is calibrated for two welldefined instrumental parameters, the uncertainty in the reported viscosity is improved to _+0.5%. The results of measurements of the viscosity of normal heptane in the temperature range 303 to 348 K at pressures up to 250 MPa made with one of the viscometers are reported. The results are shown to be totally consistent with measurements reported earlier using the instrument designed for lower pressures.
- The viscosity of R32 and R125 at saturationPublication . Oliveira, Carla Padrel de; Wakeham, William A.This paper reports new measurements of the viscosity of R32 and R125, in both the liquid and the vapor phase, over the temperature range 220 to 343 K near the saturation line. The measurements in both liquid and vapor phases have been carried out with a vibrating-wire viscometer calibrated with respect to standard reference values of viscosity. It is estimated that the uncertainty of the present viscosity data is one of 0.5-1%, being limited partly by the accuracy of the available density data. The experimental data have been represented by polynomial functions of temperature for the purposes of interpolation.
- The viscosity of liquid R134aPublication . Oliveira, Carla Padrel de; Wakeham, William A.The paper reports new measurements of the viscosity of liquid R134a over the temperature range 235 to 343 K and pressures up to 50MPa. The measurements have been carried out in a vibrating-wire viscometer calibrated with respect to the viscosity of several liquid hydrocarbons. It is estimated that the uncertainty in the viscosity data reported is +0.6%. The data therefore have a lower uncertainty than that of earlier measurements of the viscosity of this environmentally acceptable regrigerant. The viscosity data have been represented as a function of density by means of a formulation based upon the rigid, hard-sphere theory of dense fluids with a maximum deviation of _+0.3%. This representation allows the present body of experimental data to be extended to regions of thermodynamic state not covered by the measurements.