Centro de Química Estrutural

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Viscosity and density of two 1-alkyl-3-methyl-imidazolium triflate ionic liquids at high pressures: experimental measurements and the effect of alkyl chain length

Publication . Sequeira, Maria Carolina; Avelino, Helena Maria da Nóbrega Teixeira; Caetano, Fernando J. P.; Fareleira, João M. N. A.

New measurements of the viscosity of 1-butyl-3-methyl-imidazolium triflate ([BMIM][OTf]) and 1-hexyl-3-methyl-imidazolium triflate ([HMIM][OTf]) have been carried out at high pressures, using a vibrating-wire technique operated in the forced mode of oscillation. The measurements for [BMIM][OTf] have been performed along six isotherms from 298 to 358 K at pressures up to 50 MPa. The viscosity measurements for [HMIM][OTf] have been carried out along five isotherms from 303 to 358 K at pressures up to 50 MPa. The estimated uncertainty of the measurements is less than U(η) = 0.02·η for viscosities up to 68 mPa·s and less than U(η) = 0.026·η for higher viscosities, with a confidence level of 0.95 (k = 2). For both ionic liquids, complementary density measurements have been performed using an Anton Paar HP densimeter in the same temperature and pressure ranges as those used for the viscosity measurements. The density results have an uncertainty smaller than U(ρ) = 0.002·ρ with a confidence level of 0.95 (k = 2). The viscosity results were correlated with the density data using a previously described hard-sphere-based technique. The individual correlations are able to describe the viscosity results for each liquid with an uncertainty smaller than the estimated uncertainty of the experimental data. The effect of alkyl substituents on the viscosity and the density of these ionic liquids has been analyzed. For this purpose, previously published results for the viscosity and density of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][OTf]) have been considered in addition to the data obtained in the present work for [BMIM][OTf] and [HMIM][OTf].

2021-03Journal article

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Viscosity measurements of compressed ionic liquid EMIM OTf

Publication . Sequeira, Maria Carolina; Avelino, Helena Maria da Nóbrega Teixeira; Caetano, Fernando J. P.; Fareleira, João M. N. A.

Ionic 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.

2019-05-30Conference object

Open access

Viscosity measurements of poly(ethyleneglycol) 400 [PEG 400] at temperatures from 293 K to 348 K and at pressures up to 50 MPa using the vibrating wire technique

Publication . Sequeira, Maria Carolina; Pereira, Marta F. V.; Avelino, Helena Maria da Nóbrega Teixeira; Caetano, Fernando J. P.; Fareleira, João M. N. A.

The article reports new measurements of the viscosity of Poly(ethyleneglycol) 400 [PEG 400] in the range (293–348) K and pressures up to 50 MPa. Complementary measurements of the density of the same sample of PEG 400 have been made covering the ranges of temperature and pressure, (293–353) K and (0.1–50) MPa, respectively. The viscosity measurements were performed using the vibrating wire technique in the forced mode of oscillation and the density measurements were carried out with an Anton Paar vibrating U-tube densimeter. The density raw data were corrected for viscosity effects. The overall uncertainty of the viscosity measurements is estimated to be less than ±2% for viscosities up to 68 mPa s and less than ±2.6% for higher viscosities. The densities have an estimated overall uncertainty of ±0.2%. The rheological behaviour of Poly(ethylene Glycol) 400 has also been studied, using a cone-plate Brookfield viscometer, in a temperature range between (293 and 333) K. The measurements were carried out at shear rates up to 20 s−1 and shear stresses up to 2.20 Pa and have evidenced Newtonian behaviour. The viscosity data obtained were correlated by means of a modified hard-sphere based correlation technique. The relative root mean square, rms, deviation of the experimental results from the correlation equations is 0.54%, and their bias is practically zero. The density data obtained were correlated using a Tait-type equation. As a complement of the present study, the surface tension of PEG 400 was measured by the pendant drop method. This study aims to be useful for viscosity measurements using capillary viscometers. As far as the authors are aware, the present viscosity measurements are the first results to be published for PEG 400 at pressures higher than atmospheric pressure.

2019-05-25Journal article

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

Publication . Sequeira, Maria Carolina; Pereira, Marta; Avelino, Helena Maria Nóbrega Teixeira; Caetano, Fernando J. P.; Fareleira, João M. N. A.

2019-08-31Conference object

Open access

Viscosity measurements of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM OTf) at high pressures using the vibrating wire technique

Publication . Sequeira, Maria Carolina; Avelino, Helena Maria da Nóbrega Teixeira; Caetano, Fernando J. P.; Fareleira, João M. N. A.

The goal of the present work is to contribute to the characterization of ionic liquids by measuring their viscosity at high pressures. As 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM OTf) has been used as a solvent in CO2 capture processes, the temperature and pressure ranges of the measurements cover the intervals used in those processes. Measurements of the viscosity of EMIM OTf along five isotherms in the range (298–358) K and at pressures up to 50 MPa, have been performed using the vibrating wire technique in the forced mode of operation. As far as the authors are aware, these are the first measurements of this ionic liquid at pressures higher than 0.1 MPa, to be published. The viscosity results were correlated with the molar volume, using a modified hard-spheres model. The root mean square (σ) deviation of the data from the correlation is less than 0.5% The expanded uncertainty of the present viscosity data is estimated as ±2.0% at a 95% confidence level. As a complement, the pressure-viscosity coefficient has been calculated within the temperature range of the present results. Previous studies of the influence of the electric conductivity of ionic liquids, including EMIM OTf, in the vibrating wire method, have been taken into account for the present work. Complementary measurements of the density have been performed along seven isotherms in the temperature range from (298–363) K and pressures from (0.1–70) MPa. The density measurements were carried out with an Anton Paar vibrating U-tube densimeter and the raw data were corrected for viscosity effects. The density results were correlated with the temperature and pressure using a modified Tait equation. The expanded uncertainty of the present density data is estimated as ±0.2% at a 95% confidence level.

2020-02Journal article

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