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

Viscosity measurements of compressed ionic liquid EMIM OTf

1ª edição do CQE Days, organizada pelo Centro de Química Estrutural, realizada na Academia das Ciências de Lisboa, de 30-31 de maio de 2019 - https://cqe.tecnico.ulisboa.pt/cqedaysIonic liquids have attracted considerable interest in recent years, as they can be used for multiple aims, namely, as antistatic agents, electrolytes, solvents, lubricants, and CO2 absorbents [1]. The use of ionic liquids in industrial processes require their thermophysical properties, in particular, the viscosity and the thermal conductivity. However, transport properties are scarce due to the difficulty of the measurements, particularly at pressures higher than the atmospheric pressure. Our group has developed a programme of measurements aiming at obtaining rigorous results for the viscosity of ionic liquids using the vibrating wire method. This technique, although very accurate for molecular, non-conducting liquids, could have some difficulties with ionic liquids due to their electrical conductivity [2]. As we were planning to use the vibrating wire method in the forced mode of oscillation, the method requires the acquisition of the frequency response of the wire in a range of frequencies containing the velocity resonance for the transverse oscillations of the wire. Therefore, it is important to verify if the ionic liquid sample is a good electrolytic conductor in the range of frequencies that matter for the measurement of viscosity. The problematic of measuring the viscosity of ionic liquids both in general and in particular, using the vibrating wire technique was studied [3]. Pardal et al. [4] have used 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIMOTf) mixed with water as the electrolyte to successfully reduce CO2 at high pressure. The objective of this work is to contribute with viscosity data in the pressure and temperature range of the work performed by those authors. Therefore, we present new ionic viscosity results for temperatures between 298 K and 347 K and pressures up to 50 MPa.N/

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

On capillary viscosity measurements: how far do surface tension effects go?

Viscosity is a fundamental thermophysical property of liquids making it very important particularly in the industry. Capillary viscometers have been widely used for viscosity measurements in different applications, the most relevant being the definition of viscosity standards, traceable to the primary water viscosity standard, by metrological institutions and industrial applications, mostly for quality control. Practical viscometry is based on the internationally accepted primary standard value for the kinematic viscosity of water at 20ºC and atmospheric pressure, which has been measured using capillary viscometers [1]. However, due to the water surface tension, viscosity measurements which have been related to water as a primary standard, can be significantly affected. It is difficult to rigorously assess the surface tension effects on capillary viscometers, and the practical way to avoid this problem is to use long capillaries, which are not appropriate for routine measurements [1-3]. After several experimental studies, using different types of viscometers, the usual procedure to correct surface tension effects in capillary viscosity measurements adopted by different authors, is to employ an empirical expression [1-4]. Additionally, other types of problems exist as the need to perform a kinetic energy correction which must also be taken into consideration [1]. The main goal of this work was to perform the calibration of a suspended-level, or Ubbelohde, capillary viscometer, which is not a long capillary viscometer, as well as the study of corrections to be used for the measurements performed with it. The experimental work covers the calibration of that Ubbelohde capillary viscometer, the evaluation of the uncertainty of the corresponding viscometer constant and the overall uncertainty of the measurements performed with it. This study includes the evaluation of the necessary corrections for kinetic energy and surface tension effects and, finally, the analysis of the case of a set of measurements performed with n-tetradecane. The ultimate purpose of this work is to obtain the lowest uncertainty for the Ubbelohde capillary viscometer 541 01/Ia, and to understand the need for the corrections that must be considered when using capillary viscometers and how they should be applied.N/

Studying the PEG family

1ª edição do CQE Days, organizada pelo Centro de Química Estrutural, realizada na Academia das Ciências de Lisboa, de 30-31 de maio de 2019 - https://cqe.tecnico.ulisboa.pt/cqedaysThe main goal of this line of research is the realisation of experimental measurements of thermophysical properties of a homologous series of ethylene and polyethylene glycols [H(OCH2CH2)nOH], and the development of correlation methods, with an accuracy adequate for the applications. Ethylene glycols and poly (ethylene) glycols (PEG) are widely used in many industrial applications as green solvents and as components of important sustainable processes as they are considered environmentally acceptable compounds [1,2]. Liquid Poly(ethyleneglycols) [PEGs] are in general considered as green solvents. They are non-volatile; their toxicity is very low, such that they are being used as food additives [3]. PEGs have been found to be biodegradable by bacteria in soil or sewage, but the ability of bacteria to biodegrade PEG decreases with increasing molecular weight [3]. The study of this series of compounds is important in many respects, not only because it is aimed at the study of PEGs which have innumerable practical applications but also because this study can be useful to monitor the degree of polymerization in the production of PEGs, themselves. In the present work, the viscosity of three ethylene glycols, namely diethylene, triethylene and tetraethylene glycols [4] and PEG 400 were measured with high accuracy using the vibrating wire technique at moderately high pressures. Complementary experimental density, surface tension and rheological behavior were obtained for the same liquids. One of the aims of the work is to analyse the relation of the present results with those obtained before for CO2 saturated PEG400 mixturesProject UID/QUI/00100/2013 and Project UID/QUI/00100/2019 funded by Fundação para a Ciência e a Tecnologia (FCT), Portugal.N/

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

Exploratory study on the viscosity of Poly(ethyleneglycols) and their mixtures with CO2

Encontro realizado em Erlangen, a 31 de agosto de 2019.Project UID/QUI/00100/2013 and Project UID/QUI/00100/2019 funded by Fundação para a Ciência e a Tecnologia (FCT), Portugal.N/

Solid-liquid phase equilibrium: alkane systems for low temperature energy storage

The incessant and increasing need for energy requires urgent and effective solutions. The supply of renewable energies is mainly intermittent, which often leads to a gap between the availability of energy and its consumption. Therefore, energy storage becomes imperative for increasing the efficiency in the use of the produced energy. Concerning this, the use of phase change materials (PCMs) has taken a very important role, especially for active cooling systems. In particular, eutectic systems can be designed with characteristics that make them suitable for each application, allowing significant energy savings, which is very relevant in what concerns economic and environmental sustainability [1]. This work aims to the characterization of selected systems that can be used for energy storage at low temperatures, starting by the construction of solid-liquid phase diagrams using differential scanning calorimetry (DSC) and Raman Spectroscopy results. Phase diagrams are crucial to characterize the behaviour of these systems and reveals the robustness of the PCM, and for new PCMs, equilibrium studies are essential to achieve the desired melting temperature [2]. This work is focused on binary systems composed by normal alkanes, in particular, the binary mixtures C8-C10 and C10-C12. These systems revealed to behave as eutectic systems at low temperatures, showing a promising applicability for thermal energy storage for low temperature applications. The eutectic points are around -61ºC and -35ºC, respectively. Additionally, a fitting equation for the experimental liquidus lines of the phase equilibrium diagrams is proposed for each system.N/