  Recent advancement in quantitative liquid-jet photoelectron spectroscopy
enables the accurate determination of the absolute-scale electronic energetics
of liquids and species in solution. Our major objective is the determination of
the absolute lowest-ionization energy of liquid water which is found to vary
upon solute addition, and depends on the solute concentration. We discuss two
prototypical aqueous salt solutions, NaI(aq) and tetrabutylammonium iodide,
TBAI(aq), with the latter being a strong surfactant. Considerably different
behavior is revealed: In the NaI(aq) solutions, water's 1b1 energy increases by
300 meV upon increasing the concentration near-saturation concentrations,
whereas for TBAI the energy decreases by about 0.7 eV upon formation of a
surface layer. The solute-induced effects on the solute binding energies are
quantified as well, as inferred from concentration-dependent energy shifts of
the I- 5p peak energy. For NaI(aq), an almost identical I- 5p shift is found as
for the water 1b1 binding energy, with a larger shift occurring in the opposite
direction for the TBAI(aq) solution. We show that this result in NaI(aq) can be
primarily assigned to a change of water's electronic structure in the solution
bulk. In contrast, apparent changes of the 1b1 energy for TBAI(aq) can be
related to changes of the solution work function which could arise from surface
molecular dipoles. Furthermore, for both of the solutions studied here, the
measured water 1b1 binding energies can be correlated with the extensive
solution molecular structure changes occurring at high salt concentrations,
where in the case of NaI(aq), too few water molecules exist to hydrate
individual ions and the solution adopts a crystalline-like phase. We also
comment on the concentration-dependent shape of the 3a1 orbital of liquid water
which is a sensitive signature of water-water hydrogen bond interactions.
