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

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][PF 6 ] (3-methyl-1-(3,3,4,4,4pentafluorobutyl)imidazolium hexafluorophosphate) in mixtures with the structurally similar, non-fluorinated IL [C 4 C 1 Im][PF 6 ] (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][PF 6 ] 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 [C 4 C 1 Im][PF 6 ] 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][PF 6 ] 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.

Surface Structure of Alkyl/Fluoroalkylimidazolium Ionic–Liquid Mixtures

“…At this qualitative level, our results reinforce previous observations of a generally stronger surface preference for fluoroalkyl chains over alkyl chains. − The conformity to the “universal” functional form of the deviation from ideality for other types of IL mixtures noted previously for [C n mim] (1– x ) [C 8 mimF 13 ] x [Tf 2 N] mixtures with n = 8 is also now shown here to extend at least to n = 12 …”

“…It increases again for longer alkyl chains, which become dominant at the surface. Most recently, Heller et al have investigated the surfaces of mixtures of varying mole fractions of butyl and 3,3,4,4,4-pentafluorobutylimidazolium cations combined with a common [PF 6 ] − anion using ARXPS and through ST and surface light-scattering measurements. , The topmost layers were found to be enriched in pentafluorobutyl chains, with the deviation from stoichiometry largest for the most dilute mixture examined (10% pentafluorobutylimidazolium). The enrichment was enhanced at lower temperatures, while the system remained liquid.…”

Diffusivities in Binary Mixtures of n-Hexane or 1-Hexanol with Dissolved CH4, Ne, Kr, R143a, SF6, or R236fa Close to Infinite Dilution

“…This comes at the expense of computational cost. Given the reliable predictions observed for both dynamic and equilibrium properties for linear and branched alkanes and alcohols, including aromatic compounds, , the AA optimized potentials for liquid simulations (OPLS) FF , and its extension to long-chained alkanes and alcohols, L-OPLS, , have been chosen in this work. Originally developed for biological applications, the OPLS-based FFs have been shown to provide sound predictions of thermophysical properties at states close to 298.15 K. ,− However, stronger deviations are observed for both dynamic and equilibrium properties with increasing temperature. , Therefore, a recent in-house development introduces a temperature correction to the L-OPLS FF up to 573.15 K by adding a temperature dependency to the description of the Lennard-Jones potential .…”

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

“…Here, the analysis of σ is based on the comparison between the recorded shape of an axisymmetric PD and its theoretical description by the Young-Laplace equation [13]. In our previous studies [14,18,19], the PD method was used to determine σ for ILs and IL mixtures as a function of temperature with typical expanded uncertainties (coverage factor k = 2) of 2 %. An alternative to conventional methods is surface light scattering (SLS).…”