Quantitative electronic structure and work-function changes of liquid water induced by solute

Recent advancement in quantitative liquid-jet photoelectron spectroscopy enables the accurate determination of the absolute-scale electronic energetics of liquids and species in solution. The major objective of the present work is the determination of the absolute lowest-ionization energy of liquid water, corresponding to the 1b(1) orbital electron liberation, 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. Our results reveal considerably different behavior of the liquid water 1b(1) binding energy in each case. In the NaI(aq) solutions, the 1b(1) energy increases by about 0.3 eV upon increasing the salt concentration from very dilute to near-saturation concentrations, whereas for TBAI the energy decreases by about 0.7 eV upon formation of a TBAI surface layer. The photoelectron spectra also allow us to quantify the solute-induced effects on the solute binding energies, as inferred from concentration-dependent energy shifts of the I- 5p binding energy. For NaI(aq), an almost identical I- 5p shift is found as for the water 1b(1) binding energy, with a larger shift occurring in the opposite direction for the TBAI((aq)) solution. We show that the evolution of the water 1b(1) energy in the NaI(aq) solutions can be primarily assigned to a change of water's electronic structure in the solution bulk. In contrast, apparent changes of the 1b(1) energy for TBAI((aq)) solutions 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 1b(1) 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 second, 3a(1) orbital liquid water ionization feature which is a sensitive signature of water-water hydrogen bond interactions.