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 routinely
determined. These IEs are referenced to the local vacuum level, which is the
appropriate quantity for condensed matter with associated surfaces, including
liquids. Here, we connect this newly accessible energy level to another
important surface property, namely, the solution work function, eΦliq.
We lay out the prerequisites for and unique challenges of determining eΦ
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 measured spectra of aqueous solutions to the Fermi level and
quantitatively assign surfactant concentration-dependent spectral shifts to
competing work function and electronic-structure effects, the latter
determining, e.g., (electro)chemical reactivity. We describe the extension of
liquid-jet PES to quantitatively access concentration-dependent surface
descriptors that have so far been restricted to solid-phase measurements. These
studies thus mark the beginning of a new era in the characterization of the
interfacial electronic structure of aqueous solutions and liquids more
generally.Comment: Main manuscript: 26 pages, 7 figures. Supporting information: 5
pages, 5 figure