AVS 46th International Symposium
    Surface Science Division Monday Sessions
       Session SS3-MoM

Paper SS3-MoM5
Soft-Landed Ion Study of a Liquid-Liquid-Solid Interface

Monday, October 25, 1999, 9:40 am, Room 612

Session: Water-Surface Interactions
Presenter: J.P. Cowin, Pacific Northwest National Laboratory
Authors: J.P. Cowin, Pacific Northwest National Laboratory
K. Wu, Pacific Northwest National Laboratory
M.J. Iedema, Pacific Northwest National Laboratory
Correspondent: Click to Email
Ion beams with energies of about 1 eV allow one to gently land, without damage, molecular ions important to aqueous/organic liquid chemistry. We have used these soft-landed ions to probe the transfer of hydronium ions from non-aqueous to organic media. Molecular beam epitaxy is used to create aqueous/organic interfaces with monolayer precision, and the motion of the ions is detected with a non-contact work function probe, in UHV environments. Amorphous vapor- deposited films become true liquids above their glass temperatures (135 for water, 85 K for methyl cyclohexane (MCH)). We find hydrating the hydronium strongly effects its ability to enter the organic phase. We also see strong non-linear electric field effects. When a "dry" 30 monolayer MCH film is ramped at 0.2 K/s, we find that the ions traverse a film at about 97 K, under their self-induced field on order of 6 volts/(10.2 nm). We then added water with the ions on top of the MCH film. The temperature for ion transport shifts linearly with added water to 100 K at 0.4 monolayer, and is constant at @DELTA@T of 3K from 0.4 to 1.0 monolayer. This is consistent with an increase of ion size due a large hydration "sphere" (or pancake) being dragged through the organic film. Above 1 monolayer of added water the temperature shifts as much as 15 K, and is consistent with the expected @DELTA@G difference for transferring a solvated ions from this nano-aqueous environment to the organic phase. At electric fields from 0.01 to 0.1 V/angstrom, we see considerable deviations from simple viscosity-based ion transport models.