Combined Surface Light Scattering and Pendant-Drop Experiments for the Determination of Viscosity and Surface Tension of High-Viscosity Fluids Demonstrated for Ionic Liquids

Abstract

The present study provides a strategy for the determination of the viscosity and surface tension of high-viscosity fluids in the form of ionic liquids (ILs) at equilibrium conditions by combining surface light scattering (SLS) and the pendant-drop (PD) method within one experimental setup. Through the study of the same sample under identical conditions by both methods inside a closed system, the surface tension determined via the PD method can be directly used to evaluate the dynamics of surface fluctuations of ILs with an overdamped behavior probed by SLS for accessing their viscosity. In connection with the SLS experiments, variations in the applied detection geometries in reflection and transmission direction and in the probed wave vectors down to relatively small values were also addressed. The reliability and self-consistency of SLS and the PD method applied within the same sample cell has been proven by investigating the reference fluids tris(2-ethylhexyl) trimellitate (TOTM) and n-dodecane featuring relatively high and low viscosities. For the two studied model ILs of opaque to non-transparent color, i.e., the hydrophobic 1-methyl-3-octylimidazolium hexafluorophosphate ([OMIM][PF6]) and the hydrophilic 1,3-bis(2-(2-methoxyethoxy)ethyl)imidazolium iodide ([(mPEG2)2Im]I), the combination of PD measurements and SLS experiments in reflection direction performed at ambient pressure between (303 and 373) K allowed access to the viscosity and surface tension with typical relative expanded uncertainties of (4 and 2) %. These results agree well with own viscosity data from capillary viscometry and experimental data in the literature, demonstrating the performance of the novel approach for the contactless in-situ measurement of viscosity and surface tension of fluids with relatively high-viscosity such as ILs.

High-temperature measurements on viscosity and surface tension of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf2]) up to 473 K

“…More details to these schemes are reported in Ref. [22]. By averaging over the individual uncertainties within the overdamped and oscillatory region, expanded (k = 2) uncertainties of 2.3 % for η L in the overdamed case as well as of 4.0 % and 3.0 % for η L and, respectvely, in the oscillatory case were obtained.…”

“…Combined SLS and PD measurements were performed within a setup speci cally designed for this task. A detailed description of the SLS-PD setup can found in our previous study [22]. Prior to the measurements, the sample cell made of stainless steel was lled with the pure and dried IL sample, and ushed with Ar by adjusting a pressure of around 0.1 MPa measured by a pressure transducer.…”

“…This is particularly bene cial for ILs with their commonly large viscosities, where the evaluation of the SLS signals requires information on the surface tension in order to determine the viscosity. Under such conditions, the determination of viscosity and surface tension by the combination of SLS and the PD method could be realized for ILs and IL mixtures at gas pressures between (0.02 and 10) MPa and temperatures between (303 and 393) K [22][23][24].…”

Evolution of Surface Tension and Hansen Parameters of a Homologous Series of Imidazolium-Based Ionic Liquids

“…To investigate the impact of water on the surface tension of ILs, pendant-drop measurements were conducted for binary mixtures of the three ILs ([C 4 C 1 Im]­[NTf 2 ], [C 4 C 1 Im]­[I], and [(mPEG 2 ) 2 Im]­[I]). Mixtures with various mole fractions of water ( x (H 2 O)) at an ambient temperature of (294 ± 1) K were studied using a setup described by Zhai et al Pure ILs were taken as a reference. Prior to the measurements, all three ILs were degassed at pressures below 0.5 Pa and at 343 K for at least 8 h to minimize the initial water content.…”

“…). Mixtures with various mole fractions of water (x(H 2 O)) at an ambient temperature of (294 ± 1) K were studied using a setup described by Zhai et al 72 Pure ILs were taken as a reference. Prior to the measurements, all three ILs were degassed at pressures below 0.5 Pa and at 343 K for at least 8 h to minimize the initial water content.…”