Molecular Structure of Surface-Active Salt Solutions:  Photoelectron Spectroscopy and Molecular Dynamics Simulations of Aqueous Tetrabutylammonium Iodide^†

We report photoelectron measurements and molecular dynamics (MD) simulations with a polarizable force field of surface-active tetrabutylammonium iodide (TBAI) in aqueous solution. Photoemission is studied for a photon energy of 100 eV, using a 6-μm-diameter liquid jet. Surfactant activity of the TBAI salt at the solution surface is proved by a dramatic (×70) increase of the I^-(4d) signal, as compared to that of a NaI aqueous solution for identical salt concentrations. Completion of the segregation monolayer is suggested through the growth of the iodide photoelectron emission signal, as a function of the salt concentration. Our experiments reveal identical electron binding energies of iodide in TBAI and NaI aqueous solutions, which are independent of the salt concentration. Zero or very small spectral shifts of any feature, including the low-energy cutoff, suggest that no dipole is formed by TBA⁺ and I^- ion pairs perpendicular to the surface, which is in accord with the simulated ionic density profiles. Both cations and anions exhibit strong surfactant activity, thus failing to form a strong electric double layer. While the cations are surface-bound due to hydrophobic interactions, iodide is driven to the vacuum/water interface by its large polarizability. MD simulations also allow characterization of the thermally averaged geometries of the surface-active cations, in particular the orientations of the butyl chains with respect to the water surface.