How to measure work functions from aqueous solutions

The recent application of concepts from condensed-matter physics to photoelectron spectroscopy (PES) of volatile, liquid-phase systems has enabled the measurement of electronic energetics of liquids on an absolute scale. Particularly, vertical ionization energies, VIEs, of liquid water and aqueous solutions, both in the bulk and at associated interfaces, can now be accurately, precisely, and routinely determined. These IEs are referenced to the local vacuum level, which is the appropriate quantity for condensed matter with associated surfaces, including liquids. In this work, we connect this newly accessible energy level to another important surface property, namely, the solution work function, e & phi;liq. We lay out the prerequisites for and unique challenges of determining e & phi; of aqueous solutions and liquids in general. We demonstrate - for a model aqueous solution with a tetra-n-butylammonium iodide (TBAI) surfactant solute - that concentration-dependent work functions, associated with the surface dipoles generated by the segregated interfacial layer of TBA+ and I- ions, can be accurately measured under controlled conditions. We detail the nature of surface potentials, uniquely tied to the nature of the flowing-liquid sample, which must be eliminated or quantified to enable such measurements. This allows us to refer aqueous-phase spectra to the Fermi level and to quantitatively assign surfactant-concentration-dependent spectral shifts to competing work function and electronic-structure effects, where the latter are typically associated with solute-solvent interactions in the bulk of the solution which determine, e.g., chemical reactivity. The present work describes the extension of liquid-jet PES to quantitatively access concentration-dependent surface descriptors that have so far been restricted to solid-phase measurements. Correspondingly, these studies mark the beginning of a new era in the characterization of the interfacial electronic structure of aqueous solutions and liquids more generally. Fermi referencing and work-function determination from aqueous solutions is enabled by the control of extrinsic potentials, which are unique to streaming liquids. Concentration-dependent changes in both quantities are described for the first time.

Automated summary

The recent application of condensed-matter physics concepts to photoelectron spectroscopy (PES) of liquid-phase systems has allowed for the measurement of electronic energetics of liquids. The vertical ionization energies (VIEs) of liquid water and aqueous solutions can now be accurately determined and referenced to the local vacuum level. This work discusses the connection between these energies and the solution work function and demonstrates the accurate measurement of concentration-dependent work functions for an aqueous solution.