Surface Composition and Stability of Ionic Liquids

In the past decade, ionic liquids have become the subject of academic research and as a result, a high number of publications have been produced in this field.

A partial explanation for this increase is the fact that these unique materials hold potential for use in a wide variety of applications. This new field has only recently drawn the attention of an increasing number of surface scientists who are interested in studying the interactions at the liquid/solid and liquid/gas interfaces.

The composition and structure of the liquid/gas interface are of particular interest as this is where the absorption and adsorption of gases occur. Gas absorption and adsorption play major roles in processes such as gas distillation, gas separation and heterogeneous catalysis. In addition, the surface analysis of ionic liquids helps researchers in developing a comprehensive understanding of these unique materials.

In this article, a common ionic liquid is analyzed to demonstrate the ways that the structure and chemistry of the liquid/gas interface can be investigated through the use of a high-performance X-ray photoelectron spectrometer.

Experimental

X-Ray photoelectron spectroscopy (XPS) was used to analyze the surfaces of an ionic liquid. The liquid was analyzed with the AXIS Supra, using monochromatic Al Ka X-rays.

Using literature methods, the ionic liquid [EMIM][NTf₂] was prepared at high purity. For analysis, a single drop of each sample was pipetted onto a copper sample stub, which was pumped for more than 3 hours before being introduced into the analysis chamber. This enabled all remaining solvents to be pumped away.

Results

The survey spectrum collected for the ionic liquid [EMIM][NTf₂] is shown in Figure 1, allowing for the accurate identification of the elemental composition of the surface region. This spectrum reveals the presence of all of the elements attributed to the ionic liquid: (C, O, F, S and N), in the surface region. Moreover, any contaminant possibly remaining in the liquid can be observed.

Figure 1. Survey spectrum of [EMIM][NTf₂].

The software automatically generates a quantification report to accurately determine the relative atomic concentration of the near surface region (Table 1).

Table 1. Quantification report from survey scan for [EMIM][NTf₂]

The next step was the collection of the narrow region spectra for the relevant elements in order to examine the surface chemistry and the different electronic environments present.

The narrow region spectra for both the N 1s and C 1s orbitals are shown in Figure 2. As observed, the two peaks observed in the N 1s spectra can be attributed to the two different nitrogen environments in the molecule. The higher binding energy peak is assigned as the [EMIM] cation, whereas the lower peak is assigned to the [NTf₂] anion.

The relative stoichiometry of these peaks also shows good agreement with this conclusion. Several interesting features are exhibited by the C 1s spectrum. The peak at 293.0 eV can be assigned to the [NTf₂] anion’s fluorine bonded carbon atoms.

Figure 2. N 1s (left) and C 1s (right) spectrum of [EMIM][NTf₂].

The lower binding energy feature can be peak-fitted in accordance with the different chemical environments of the [EMIM] cation. The photoelectron emission angle can be varied by changing the angle of the sample, enabling the user to acquire non-destructive depth information of the surface region.

Surface concentration data was obtained by acquiring the C 1s spectra at different emission angles (Figure 3). A change in shape can be clearly seen, with increasing intensity of the feature at 285.3 eV. This peak was earlier attributed to the aliphatic chain carbon, indicating an enrichment of the surface region with the aliphatic chain.

This result is in good agreement with earlier studies where the aliphatic carbon is considered to govern the chemistry of the liquid surface, with the chain orientated away from the liquid towards the vacuum.

Figure 3 also displays the change in intensity of the various chemical environments of the carbon peak, corresponding to emission angle; showing the orientation away from the liquid towards the vacuum.

Figure 3. ARXPS of [EMIM][NTf₂].

Conclusion

This article has demonstrated the analysis of the carbon chemistry and elemental composition of a common ionic liquid [EMIM][NTf₂], using the X-ray photoelectron spectroscopy.

In this study, the XPS technique was used to explore the chemical composition of the surface and the various chemical states of the nitrogen and carbon atoms. Angle-resolved experiments reveal that the aliphatic carbon chain is orientated away from liquid towards the vacuum.

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This information has been sourced, reviewed and adapted from materials provided by Kratos Analytical Ltd.

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