Abstract

We present results of a systematic investigation of the structural and single particle dynamical properties at the free surface of imidazolium-based room temperature ionic liquids by applying intrinsic analysis methods both at the molecular and atomic levels. Besides assessing the effect of the potential model and temperature, we focus in particular on the effect of changing the anion type, and, hence, their shape and size. In doing so, we try to elucidate the relative roles of electrostatic interactions, hydrophobic (or, strictly speaking, apolar) effects and steric restrictions on the properties of the interface. Further, we also address the role of the length of the cation alkyl chains, known to protrude into the vapor phase, on the surface dynamics of the ions.

Concerning the surface structure, we see no evidence for the existence of a double-layer-type arrangement of the ions, nor for their self-association at the surface of the liquid. Instead, our results show that cation chains associate into apolar domains that protrude into the vapor phase, while charged groups form domain that are embedded in this apolar environment at the surface. However, the apolar chains largely obscure the cation groups to which they are bound, while the smaller and more mobile anions can more easily access the free surface, leading to a somewhat counterintuitive net excess of negative charge at the interface. Importantly, this excess charge could only be identified by applying intrinsic analysis. [1] Our results also show that this surface can be viewed as a superposition of two “interfaces”, one between a hydrophobic layer of cation alkyl chains and the vapor phase, and another between that hydrophobic layer and an ionic fluid composed of polar groups of the cations and anions. Remarkably, the properties of this ionic surface are practically independent of the cation alkyl chain length, suggesting they are a universal feature of imidazolium-based RTILs. [2]

Further, we observe that the surface dynamics of ionic liquids, being dominated by strong electrostatic interactions, are about two orders of magnitude slower than that for common molecular liquids. Furthermore, the free energy driving force for exposing apolar chains to the vapor phase “pins” the cations at the surface layer for much longer than anions, allowing them to perform noticeable lateral diffusion at the liquid surface during their stay there. On the other hand, anions, accumulated in the second layer beneath the liquid surface, stay considerably longer here than in the surface layer. The ratio of the mean surface residence time of the cations and anions depends on the relative size of the two ions: larger size asymmetry typically corresponds to larger values of this ratio. We also find, in a clear contrast with the bulk liquid phase behavior, that anions typically diffuse faster at the liquid surface than cations. Finally, our results show that the surface dynamics of the ions is largely determined by the apolar layer of the cation alkyl chains at the liquid surface, as in the absence of such a layer, cations and anions are found to behave similarly with respect to their single particle dynamics. [3]

References

[1] H. Tóth Ugyonka, Gy. Hantal, I. Szilágyi, A. Idrissi, M. Jorge, P. Jedlovszky, J. Coll. Interface Sci., 676, 989 (2024)

[2] H. Tóth Ugyonka, Gy. Hantal, I. Szilágyi, A. Idrissi, M. Jorge, P. Jedlovszky, J. Phys. Chem. Letters 16, 1873 (2025)

[3] H. Tóth Ugyonka, Gy. Hantal, I. Szilágyi, A. Idrissi, M. Jorge, P. Jedlovszky, J. Phys. Chem. B 129, 579 (2025).