On the adsorption of n-butane on alkyl imidazolium ionic liquids with different anions using a new molecular beam setup

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 ([CnC 1 Im][Tf 2 N]; 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 ([CnC 1 Im][PF 6 ]; 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.

n‐Butane, iso‐Butane and 1‐Butene Adsorption on Imidazolium‐Based Ionic Liquids Studied with Molecular Beam Techniques

“…If all molecules are scattered from the surface (that is, S 0 =0), no change in pressure is observed (Figure 1d). Note that in this case in our experiment a small apparent pressure drop occurs, because of an experimental artifact caused by our sample cooling system, for which we apply a correction in our data evaluation procedure [33b] …”

“…The molecular volumes were calculated from the IL densities at 298.15 K, 101.325 kPa, for which an average over the values published in the ILThermo database from NIST [39] were used. Some of the values for E des are already published [33b] . The error bars indicate the estimated uncertainty, by assuming an identical prefactor for all ILs (for details see text).…”

“…In the transition region, the residence time of the n ‐butane molecules is in the order of the measurement time required to determine S 0 net . We developed a model, [33b] based on the reasonable assumption that thermal accommodation occurs always with the same probability S 0 LT , which can be measured at low temperature, where desorption is negligible. S 0 net is then given by Equation …”

“…Note that in this case in our experiment a small apparent pressure drop occurs, because of an experimental artifact caused by our sample cooling system, for which we apply a correction in our data evaluation procedure. [33b] …”

“…In the case of the room temperature solids [C 4 C 1 Im]Br and [C 4 C 1 Im]Cl, the sample plate was gently heated to melt the ILs to ensure homogenous spreading. The samples were introduced via a loadlock into the ultra‐high vacuum system (base pressure≈1 ⋅ 10 −10 mbar), which was already described in detail elsewhere [33b] . A type K thermocouple inserted into the sample plate was used to measure the sample temperature, which was controlled by liquid nitrogen cooling and radiation heating with a filament mounted at the back of the sample.…”