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

Abstract

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.