We report photoelectron measurements and molecular dynamics simulations with a polarizable 
force field of surface-active aqueous tetrabutyl-ammonium iodide (TBAI). Photoemission was 
studied for 100 eV photon energy using a 6 .m diameter liquid jet. For this small jet size, 
water molecules evaporate without collisions, and hence electrons originating from the 
aqueous solution can be directly measured. Surfactant activity of the TBAI salt at the 
solution surface is confirmed by a ca. 70x increase of the I-(4d) signal as compared to 
a NaI aqueous solution, for the identical salt concentration. Completion of the segregation 
monolayer is identified through the evolution of both the iodide and the water photoemission 
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. Furthermore, no spectral shifts due to work function changes are 
observed implying that there is no dipole arising from TBA+ and I- ion pairs for solvated 
TBAI perpendicular to the solution surface. The experimental observations are consistent 
with results of molecular dynamics simulations of slabs of aqueous TBAI. Both cations and 
anions are found to exhibit a strong surfactant activity, failing thus to form an 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. Molecular dynamics 
simulations also allow characterizing the thermally averaged geometries of the 
surface-active cations, in particular the orientations of the butyl chains with respect 
to the water surface.