An ion moving across an oil-water interface experiences strong solvation changes. We have directly measured the solvation potential from 0.4 to 4 nm for Cs~+ ions approaching the oil-water interface from the oil side ("oil" = 3-methylpentane). The interfaces were built at 30 K using molecular beam epitaxy. Ions were precisely placed within the film during its growth using a soft-landing ion beam. The ion's collective electric field was progressively increased (by adding more ions) until it balanced the individual ion's solvation potential slope. As the samples were slowly warmed, near 90 K the ions began moving, as measured by a Kelvin probe. Their motion precisely determines the local slope of the solvation potential, which was integrated to get the potential. The potential is Born-like for z > 0.4 nm away from the oil-water interface. Our method could provide important tests of theoretical estimates of ion motion at biological interfaces and in atmospheric aerosols.
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