Probing Surface and Interfacial Tension of Ionic Liquids in Vacuum with the Pendant Drop and Sessile Drop Method

Abstract

We report on the surface and interface tension measurements of the two ionic liquids (ILs) [C8C1Im][PF6] and [m(PEGn)2Im]I (n = 2, 4, 6) in a surface science approach. The measurements were performed in a newly developed and unique experimental setup, which allows for surface tension (ST) measurements using the pendant drop method and for contact angle measurements using the sessile drop method under the well-defined conditions of a high vacuum (from 10−7 mbar). The setup also allows for in vacuum transfer to an ultrahigh vacuum system for surface preparation and analysis, such as in angle-resolved X-ray photoelectron spectroscopy. For [C8C1Im][PF6], we observe a linear decrease in the surface tension with increasing temperature. The ST measured under high vacuum is consistently found to be larger than under ambient conditions, which is attributed to the influence of water uptake in air by the IL. For [m(PEGn)2Im]I (n = 2, 4, 6), we observe a decrease in the ST with increasing polyethylene glycol chain length in a vacuum, similar to very recent observations under 1 bar Argon. This decrease is attributed to an increasing enrichment of the PEG chains at the surface. The ST data obtained under these ultraclean conditions are essential for a fundamental understanding of the relevant parameters determining ST on the microscopic level and can serve as a benchmark for theoretical calculations, such as molecular dynamic simulations. In addition to the ST measurements, proof-of-principle data are presented for sessile drop measurements in HV, and a detailed description and characterization of the new setup is provided.

Understanding the Buoy Effect of Surface‐Enriched Pt Complexes in Ionic Liquids: A Combined ARXPS and Pendant Drop Study**

“…Also shown is the γ value of a solution of the equivalent complex without fluorinated side chains 2 with 5 % mol concentration (green data point) at 298 K. The corresponding full set of temperature‐dependent surface tension measurements is shown in Figure S5. Note that due to careful characterization and calibration procedures of our PD setup, the uncertainty in γ values relative to each other for different IL solutions and at different temperatures is ±0.1 % (±0.04 mN/m), [15b] and thus, lies within the height of the solid squares shown in Figures 5 and S5. For the surface tension of the neat solvent [C 4 C 1 Im][PF 6 ], we observe a value 43.4 mN/m, which is slightly higher than values reported in literature also using the PD method under ambient conditions (42.9 mN/m at 298 K, [16] 41.6 mN/m at 295 K [17] ).…”

Direct Correlation of Surface Tension and Surface Composition of Ionic Liquid Mixtures—A Combined Vacuum Pendant Drop and Angle-Resolved X-ray Photoelectron Spectroscopy Study

“…Thereafter, sample degassing was performed in the fast entry load-lock of the vacuum chamber for at least 12 h. The ARXPS measurements were performed under ultra-high vacuum (UHV) at temperatures from 363 K down to around 240 K (and slightly below), where the onset of sample charging in ARXPS indicated solidification (see Table S4 ) [ 49 ]. The ST measurements were carried out under high vacuum (HV) conditions of ~10 −6 mbar background pressure in a separate chamber [ 46 ], where the ILs and IL mixtures were first carefully degassed at a temperature of about 373 K for 15 h and then measured also from 363 K down to room temperature.…”

“…Surface tension measurements were performed in a high vacuum pendant drop (PD) system, which was recently developed by our group. The properties and calibration of the setup were described in a recent publication [ 46 ]. The applied stainless steel cannula has an outer diameter of 2.02 mm.…”

“…The ST was determined from each droplet shape using the axisymmetric drop shape analysis—(ADSA) and the Young--Laplace equation solution—software package SCA 22/15 (version 5.0.35) from DataPhysics (V.5.0.35build5035, DataPhysics Instruments GmbH, Filderstadt, Germany); for further details on the setup and evaluation procedure, see Ref. [ 46 ].…”

“…In the present work, we now extend this concept to IL mixtures with very different anions and cations. The corresponding measurements of the temperature-dependent ST were performed under ultraclean vacuum conditions with an apparatus recently developed in our lab [ 46 ], thereby minimizing any risk of possible surface contamination affecting the ST. The surface composition was determined from temperature-dependent ARXPS in an ultra-high vacuum [ 26 , 47 , 48 , 49 ].…”