Abstract
The adsorption of reactants is an elementary step in the interaction of molecules with liquid or solid surfaces. We recently reported on the trapping of n-butane on the frozen surfaces of ionic liquids (ILs), namely, 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ILs ([CnC1Im][Tf2N]; n = 1, 2, 3, and 8). To study the influence of the anion, we now present results concerning the trapping of n-butane on 1-alkyl-3-methylimidazolium hexafluorophosphate ILs ([CnC1Im][PF6]; n = 2, 4, and 8), that is, ILs with a smaller anion. The adsorption energies close to zero coverage are determined from the temperature dependence of the initial trapping probability using a novel approach. For both groups of ILs, the binding energy is dominated by the interaction of n-butane with the alkyl chain of the cation, whereas the ionic headgroups contribute only weakly. Comparing ILs with different alkyl chains at the IL cation, we find that the adsorption strength of n-butane increases with increasing length of the alkyl chain. In addition, detailed information on the new setup and the data analysis is provided.
Highlights
The dynamic adsorption process of a gas molecule on a surface followed by possible physical and chemical transformations is of fundamental interest
Since 2005/2006, ionic liquids (ILs) are in scitation.org/journal/jcp the focus of surface science studies, and this new field of “ionic liquid surface science”50 has developed a detailed understanding of the steady state or static properties of the vacuum/IL interface, such as surface composition, surface enrichment, and orientation effects
In case B [Fig. 3(b)], the pressure drops initially (Δp0) but rises afterward until it reaches the level it had before opening the sample flag (p = ΔpMB). We attribute this effect to partial desorption from the sample surface, yielding a steady state between adsorbing and desorbing molecules. This becomes apparent by looking at the pressure response on closing the sample flag (t ≈ 60 s): The pressure rises above the level it had before opening the sample flag (p > ΔpMB) because in addition to the molecules being scattered from the sample flag, the quadrupole mass spectrometer (QMS) detects molecules desorbing from the sample surface while the adsorption rate has dropped to zero
Summary
The dynamic adsorption process of a gas molecule on a surface followed by possible physical and chemical transformations is of fundamental interest. Detailed information on the adsorption/desorption process, preferred orientation and bonding of the molecules to the surface, surface diffusion, bond activation, and chemical reactions has been obtained.1–19 Understanding these processes at the fundamental level is imperative for several applications such as heterogeneous catalysis, thin film growth, or separation technology.. Sophisticated instrumentation is necessary to apply UHV-based molecular beam methods to explore the dynamics at the surfaces of conventional aqueous or organic liquids that are volatile under these conditions.. Due to the small number of studies, the understanding of gas adsorption on IL surfaces and the subsequent interaction, diffusion, and transport is rather limited This motivated us to build a novel UHV chamber combining a supersonic molecular beam, a rotatable mass spectrometer, and facilities for in situ X-ray photoelectron spectroscopy (XPS) to investigate the interaction of gases with non-volatile liquids. We provide details on the instrumental setup and its characterization
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