title: Surface-Induced Changes in the Thermochromic Transformation of an Ionic Liquid Cobalt Thiocyanate Complex.
abstract: We demonstrate that a thermodynamic complex equilibrium within an ionic liquid film can be significantly influenced by the presence of the liquid-vacuum interface. Using surface-sensitive X-ray photoelectron spectroscopy, we find that the temperature-driven transition from the blue-colored tetrahedral [Co(II) (NCS)4]2-to the red-colored octahedral [Co(II) (NCS)6]4-complex already occurs within the outermost nanometers at around +4 °C as compared with -25 °C in the bulk. This thermochromic transformation in the near-surface region goes along with a loss in preferential surface orientation of free [SCN]-anions and with a pronounced decrease in the complex density; both effects are attributed to the formation of a weakly bound solvation shell around the [Co(II) (NCS)6]4-anion, leading to an effective complex dilution. Our results are not only relevant for high-surface area thin film systems, such as in sensor and catalysis applications, but also shed light on the role of ionic liquid surfaces in particular and liquid surfaces in general.
authors.affiliation: Lehrstuhl Physikalische Chemie II, FAU Universität Erlangen-Nürnberg , Egerlandstraße 3, D-91058 Erlangen, Germany.
authors.forename: Benjamin
authors.initials: B
authors.lastname: May
authors.affiliation: Lehrstuhl Physikalische Chemie II, FAU Universität Erlangen-Nürnberg , Egerlandstraße 3, D-91058 Erlangen, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Hönle
authors.affiliation: Lehrstuhl Physikalische Chemie II, FAU Universität Erlangen-Nürnberg , Egerlandstraße 3, D-91058 Erlangen, Germany.
authors.forename: Bettina
authors.initials: B
authors.lastname: Heller
authors.affiliation: Lehrstuhl Physikalische Chemie II, FAU Universität Erlangen-Nürnberg , Egerlandstraße 3, D-91058 Erlangen, Germany.
authors.forename: Francesco
authors.initials: F
authors.lastname: Greco
authors.affiliation: Lehrstuhl Physikalische Chemie II, FAU Universität Erlangen-Nürnberg , Egerlandstraße 3, D-91058 Erlangen, Germany.
authors.forename: Radha
authors.initials: R
authors.lastname: Bhuin
authors.affiliation: Lehrstuhl Physikalische Chemie II, FAU Universität Erlangen-Nürnberg , Egerlandstraße 3, D-91058 Erlangen, Germany.
authors.forename: Hans-Peter
authors.identifier: http://orcid.org/0000-0003-1347-8962
authors.initials: HP
authors.lastname: Steinrück
authors.affiliation: Lehrstuhl Physikalische Chemie II, FAU Universität Erlangen-Nürnberg , Egerlandstraße 3, D-91058 Erlangen, Germany.
authors.forename: Florian
authors.identifier: http://orcid.org/0000-0001-9725-8961
authors.initials: F
authors.lastname: Maier