| Abstract
| - We report photoelectron measurements and molecular dynamics (MD) simulations with a polarizable forcefield of surface-active tetrabutylammonium iodide (TBAI) in aqueous solution. Photoemission is studied fora photon energy of 100 eV, using a 6-μm-diameter liquid jet. Surfactant activity of the TBAI salt at thesolution surface is proved by a dramatic (×70) increase of the I-(4d) signal, as compared to that of a NaIaqueous solution for identical salt concentrations. Completion of the segregation monolayer is suggestedthrough the growth of the iodide photoelectron emission signal, as a function of the salt concentration. Ourexperiments reveal identical electron binding energies of iodide in TBAI and NaI aqueous solutions, whichare independent of the salt concentration. Zero or very small spectral shifts of any feature, including thelow-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 surfactantactivity, thus failing to form a strong electric double layer. While the cations are surface-bound due tohydrophobic interactions, iodide is driven to the vacuum/water interface by its large polarizability. MDsimulations also allow characterization of the thermally averaged geometries of the surface-active cations, inparticular the orientations of the butyl chains with respect to the water surface.
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