Sculpting the Oil−Water Interface to Probe Ion Solvation

Abstract

Solvation of ions at oil-water interfaces is so important in cell wall and enzyme function, colloidal chemistry, fuel cells, and other areas that substantial effort has been made to understand the process via molecular-scale simulations of well-specified model systems. Here we report a series of experiments probing ion transport and solvation in composite films that have a geometrical specificity that rivals what theory is routinely able to employ. Proton/hydronium transport across the water-organic interface has been studied using a novel "soft-landed" ion technique that allows precision tailoring of the interfaces (including initial ion position) and sensitive monitoring of the ion motion via the electric potential generated by the ions. There are two main findings. First, the ion solvation by water at the organic-aqueous interface is probed continuously for various monolayers of water present in simple and complex sandwiched structures. The potential trap created by the solvation can be systematically overwhelmed by the collective electric field of the ions, giving us some unique information about both the trap depth and shape. A simple Born model for the trap reproduces some, but not all, of the details experimentally observed. Second, ion transport in several organic glasses is well-predictable by continuum viscosity models at electric fields up to approximately 10 8 V/m.

Hydronium ion motion in nanometer 3-methyl-pentane films

“…To estimate the thickness of the films L dosed with multiples of that time ͑8.2 s here͒ required to complete the first monolayer, we previously 11,12,15,21,28 had assumed that the absolute density of molecules in this first monolayer was the same as that for hexane, 31 and assumed that the density of the 3MP was the same as liquid 3MP at 30 K ͑extrapolated from 98 to 293 K data 32 ͒. This gave for a dose of ͑in ML͒ a thickness= * ͑3.35 Å / ML͒.…”

“…We looked for such effects and found no evidence for this down to the 1% level. 11,12 D. Electric field part 1 38 This would be favored by high temperature annealing or growth and evidenced by severely impeded ion motion.…”

“…Computer simulations have explored the structure and dynamics of ion transfer across a fully atomistic liquid/liquid interface, 5,7,10 but there are little experimental data on the ion transfer process itself. 11,12,[14][15][16] We completely avoid complications associated with counterions 17,18 by using only a single ionic species. Using thin films with ions placed at specific locations, we probe highly nonequilibrium events, probing the structure of the liquid and interfaces.…”

“…11,12,19 Presented here are hydronium and cesium ion motions in 3-methyl-pentane ͑3MP͒ films supported on a Pt͑111͒ crystal, constructed with an ion soft-landing technique. 11,12,19 Presented here are hydronium and cesium ion motions in 3-methyl-pentane ͑3MP͒ films supported on a Pt͑111͒ crystal, constructed with an ion soft-landing technique.…”

Macroscopic interface self-motion of an oil/water liquid membrane

“…1 As one of artificial membranes, the liquid membrane was widely studied and even could be used to understand chemical sensors in the future. [2][3][4][5][6] In the system of oil/water liquid membrane, the investigations of the relationship between the potential oscillation and the interfacial tension change is quite important, [7][8][9][10][11][12][13][14] because of the new information that it could produce. Unfortunately, relatively few papers are devoted to the study of mechanism analysis, [15][16][17][18][19] although the selfmotion is very common in biological system, for example, heart beat.…”