High-viscosity reference liquids at high pressures

Encontro realizado no Porto a 31 de agosto de 201

Density of the reference Ionic liquid [C6mim][NTf2] at high pressures

Workshop realizado em Lisboa de 2-3 Fevereiro de 2012Density of the ionic liquid [C6mim][NTf2] – a reference ionic liquid for thermophysical properties, as proposed by IUPAC8 was measured using an Anton Paar density meter DMA HP. The basic experimental setup was described previously, and the modifications introduced in order to avoid water contamination of the sample are described in the presentation. The measurements were performed in the temperature range (283.15 ≤ T ≤ 353.15) K and at pressures (0.1 ≤ p ≤ 65) MPa. The water content of the samples was measured by Karl-Fisher coulometry, both before and after the measurements. As the viscosity of that ionic liquid is much higher than the viscosity of the fluids used in the calibration of the densimeter, a correction of the results became necessary, to account for the consequent error in the period of oscillation of the U-tube. For this purpose, the viscosity effect on the density measurements on the Anton Paar DMA HP was quantified using the viscosity standard specimen (200GW) from ZMK, GmbH. The experimental determination of the viscosity correction to the measured density is described and also the preliminary results of the density of [C6mim][NTf2] are compared with literature values

Viscosity measurements on Ionic liquids : a cautionary tale

The vibrating-wire viscometer has proven to be an exceedingly effective means of determining the viscosity of liquids over a wide range of temperature and pressure. The instrument has a long history but a variety of technological and theoretical developments over a number of years have improved its precision and most recently have enabled absolute measurements of high accuracy. However, the nature of the electrical measurements required for the technique has inhibited its widespread use for electrically conducting liquids so that there have been only a limited number of measurements. In the particular context of ionic liquids, which have themselves attracted considerable attention, this is unfortunate because it has meant that one primary measurement technique has seldom been employed for studies of their viscosity. In the last 2 years systematic efforts have been made to explore the applicability of the vibrating-wire technique by examining a number of liquids of increasing electrical conductivity. These extensions have been successful. However, in the process we have had cause to review previous studies of the viscosity and density of the same liquids at moderate temperatures and pressures and significant evidence has been accumulated to cause concern about the application of a range of viscometric techniques to these particular fluids. Because the situation is reminiscent of that encountered for a new set of environmentally friendly refrigerants at the end of the last decade, in this paper the experimental methods employed with these liquids have been reviewed which leads to recommendations for the handling of these materials that may have consequences beyond viscometric measurements. In the process new viscosity and density data for 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [C6mim][NTf2], 1-ethyl-3-methylimidazolium ethyl sulfate [C2mim][EtSO4], and 1-ethyl-3-methylpyridinium ethyl sulfate [C2mpy][EtSO4] have been obtained

Mutual diffusivity in binary mixtures of n-heptane with n-hexane isomers

This 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

On the mobility of dialkyl adipates by PFGSE NMR, computer simulations and other property measurement methods

Encontro realizado em Alacalá de Henares (Madrid), de 22 a 25 de Setembro de 2014Dialkyl adipates are a class of esters materials used as components of lubricants and plasticizers. Diffusion of plasticizers in polymers is a complex process that is difficult to study [1]. As a contribution to understand this phenomenon, the self-diffusion coefficients of five dialkyl adipates (dimethyl, diethyl, dipropyl, dibuthyl and bis(2-ethylhexyl)) were measured by the PFGSTE method in the temperature range 20 to 60ºC. It was shown that these compounds follow the Stokes-Einstein equation [2]. In effect, the self-diffusion coefficients change linearly with T.η-1(η is the viscosity). The dependence of the activation energies for self-diffusion is discussed in terms of size and branching of the alkyl chains of the adipates. Molecular dynamics (MD) simulations were done for these sytems in order to predict the diffusion coefficients as a function of temperature [3]. Acknolwedgements: This work was supported by the Strategic Projects PEst-OE/QUI/UI0100/2011 and PEst-OE/QUI/ UI0100/2013, both funded by Fundação para a Ciência e a Tecnologia (FCT, Portugal). We thank also FCT for funding the NMR facility through Project RECI/QEQ-QIN/0189/2012. References: [1] Rahman M.; Brazel C. S., Prog. Polym. Sci. 2004, 29, 1223–1248. [2] Price W.S, NMR Studies of Translation Motion, Cambridge University Press, Cambridge, 2009. [3] Pereira, L. A. M.; Martins, L. F. G.; Ascenso, J. R; Morgado, P.; Ramalho, J. P. P.; Filipe, E. J. M., J. Chem. Eng. Data 2014, accepted for publication

Solid-liquid phase equilibrium: in search of suitable PCMs for low temperature energy storage

Centro de Química Estrutural is a Research Unit funded by Fundação para a Ciência e Tecnologia through projects UIDB/00100/2020 and UIDP/00100/2020. • Institute of Molecular Sciences is an Associate Laboratory funded by FCT through project LA/P/0056/2020. • M.C.M. Sequeira acknowledges the PhD grant funded by FCT ref. UI/BD/152239/2021.N/

Viscosity measurements of compressed liquid refrigerant blend R-507A, using a vibrating-wire technique

The refrigerant blend R-507A (50 wt % HFC-143a, 50 wt % HFC-125) is an azeotropic mixture of hydrofluorocarbon refrigerants, 1,1,1-trifluoroethane (HFC-143a) and pentafluoroethane (HFC-125). The paper reports viscosity measurements, performed with a vibrating-wire viscometer, of the refrigerant blend R-507A,at five temperatures in the range (253 to 293) K. The measurements were carried out at pressures from slightly above saturation up to 10 MPa, except for the isotherms at 253.26 K where the maximum pressure was 7.52 MPa and at 263.23 K where the maximum pressure was 7.09 MPa. The overall uncertainty of these measurements is estimated to be ( 1.0 %. The data obtained were correlated by means of a modified hard-sphere based correlation technique. The root-mean-square deviation, rmsd, of the experimental results from the correlation equations is 0.23 %, and their bias is not significant. This correlation method has also been used to interpolate and extrapolate the present results to enable comparisons with measurements performed by other authors of the viscosity of liquid R-507A at different temperatures and pressures

Viscosity reference liquids for industrial applications

Conferência realizada no Porto de 31 de agosto a 4 de setembro de 201

Low temperature energy storage PCM systems: phase equilibrium studies

IMS - LA/P/0056/2020N/

Di-Alkyl adipates as new phase change material for low temperature energy storage

This work is a contribution to the thermal characterization of a selected binary system of two di-n-alkyl adipates that can be used as phase change material for thermal energy storage at low temperatures. The construction of the solid–liquid phase diagram using differential scanning calorimetry (DSC), complemented with Raman Spectroscopy studies for the system composed by diethyl and dibutyl adipates is presented. The solidus and liquidus equilibrium temperatures were determined by DSC for the pure components and 30 binary mixtures at selected molar compositions were used to construct the corresponding solid–liquid phase diagram. The binary system of diethyl and dibutyl adipates presents eutectic behaviour at low temperatures. The eutectic temperature was found at 240.46 K, and the eutectic composition was determined to occur at the molar fraction xdibutyl = 0.46. Additionally, the system shows a polymorphic transition, characteristic of dibutyl adipate, occurring at ca. 238 K, confirmed by optical microscopy. To the best of our knowledge, no reference to the phase diagram of the present system could be found in the literature. Raman spectroscopy was essential to complement the construction of the phase equilibrium diagram, enabling the identification of the solid and liquid phases of the system. Finally, the liquidus curve of the phase diagram was also successfully predicted using a suitable fitting equation, being the root mean square deviation of the data from the correlation equal to 0.54 K. In addition, this fitting operation enabled a correct prediction of the eutectic composition of the system.info:eu-repo/semantics/publishedVersio