Surface Tension and Viscosity of Binary Mixtures of the Fluorinated and Non-fluorinated Ionic Liquids [PFBMIm][PF 6 ] an
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Surface Tension and Viscosity of Binary Mixtures of the Fluorinated and Non‑fluorinated Ionic Liquids [PFBMIm][PF6] and [C4C1Im][PF6] by the Pendant Drop Method and Surface Light Scattering Thomas M. Koller, et al. [full author details at the end of the article] Received: 19 June 2020 / Accepted: 18 July 2020 © The Author(s) 2020
Abstract Mixtures of fluorinated and non-fluorinated ionic liquids (ILs) show a distinct structural organization in the bulk and at the surface. To understand how such microscopic effects influence the macroscopic bulk and surface properties of IL mixtures, knowledge of corresponding thermophysical properties including viscosity and surface tension is required yet lacking. With the intention of investigating surface enrichment effects of the fluorinated IL [PFBMIm][PF6] (3-methyl-1-(3,3,4,4,4pentafluorobutyl)imidazolium hexafluorophosphate) in mixtures with the structurally similar, non-fluorinated IL [C4C1Im][PF6] (1-butyl-3-methylimidazolium hexafluorophosphate) observed with angle-resolved X-ray photoelectron spectroscopy (ARXPS), the pendant drop method and surface light scattering (SLS) were applied in the present study to determine surface tension and dynamic viscosity between (293 and 368) K. By adding small amounts of [PFBMIm][PF6] up to 9 mol %, a distinct increase in the viscosity and decrease in the surface tension of the mixtures relative to the properties of pure [C4C1Im][PF6] was found. This behavior reflects the nanosegregated structure in the bulk and at the surface of the binary IL mixtures. Using the results about the pronounced surface enrichment of the fluorinated chain of [PFBMIm][PF6] quantified by ARXPS, a linear mixing rule for the surface tension of the IL mixtures based on the surface tensions of the pure ILs and the surface concentration of their most surface-active groups is suggested. Keywords Ionic liquids · Mixtures · Pendant drop method · Surface light scattering · Surface tension · Viscosity
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1076 5-020-02720-w) contains supplementary material, which is available to authorized users.
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International Journal of Thermophysics
(2020) 41:144
1 Introduction Ionic liquids (ILs) are interesting working fluids in different fields of chemistry and engineering such as electrolysis [1, 2], catalysis [3, 4], or separation technology [5, 6]. Often, ILs consist of an inorganic anion and an organic cation with at least one alkyl chain of varying length attached to the charged head group of the cation [7]. Within the last years, ILs featuring fluorinated alkyl chains in the anion or cation, so-called fluorinated ILs, received considerable attention due to their, for example, high chemical stabilities and large solubilities for gases [8, 9]. By using, e.g., mixtures of fluorinated and non-fluorinated ILs, their properties can be tailored for specific applications [10–12]. Therefore, it is necessary to know the thermophys
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