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,Julian Mehler,Hans-Peter Steinrück,Andreas P. Fröba,Florian Maier,Michael H. Rausch,Frances D. Lenahan,Tobias Klein,Peter Wasserscheid,Patrick S. Schmidt

doi:10.1007/s10765-020-02720-w

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-(3,3,4,4,4-pentafluorobutyl)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.

Highlights

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]
To probe whether the observed surface enrichment effects are reflected in the surface tension data and in the dynamics of the surface fluctuations, we focused on the influence of small amounts of [PFBMIm][PF6] up to 9 mol % on the surface tension and viscosity of the IL mixtures between (293 and 368) K
Our work was motivated by the lack of detailed information about the thermophysical properties of such mixtures [25], and by the question how the macroscopic properties viscosity and surface tension are influenced by the distinct structural organization in the bulk and at the surface of ILs containing fluorinated and nonfluorinated alkyl chains [17, 28, 30]

Summary

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]. 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]. E.g., mixtures of fluorinated and non-fluorinated ILs, their properties can be tailored for specific applications [10,11,12]. It is necessary to know the thermophysical properties associated with the bulk and the surface of such IL mixtures. The viscosity and especially the surface tension as representative bulk and surface properties are of main interest. The viscosity as one key transport property describes momentum transfer [13] and the surface tension is relevant in connection with, for example, mass transport across phase boundaries and wetting [14]

Methods

Results

Discussion

Conclusion