Specificity and variation of length scale over which monovalent halide ions neutralize a charged interface

We report on halide ion (Cl, Br, I) adsorption from the subphase water to a cationic Langmuir monolayer consisting of 1,2 dipalmitoyl 3 trimethylammonium propane (DPTAP) molecules at the air/water interface. Reductions in the water OH signal of sum frequency spectra and the surface pressure of the π A isotherm follow the order of the anion size, indicating preferable adsorption of the larger anions to better screen the surface charge of the DPTAP monolayer. Complementary X ray fluorescence measurements of DPTAP on Cl and I reveal that the integrated number of adsorbed ions within the probing depth (6 8 nm) is the same for both ions. Incorporating the above outcomes leads to the contrasting adsorption structures that the larger halide anions (I) are directly adsorbed to the headgroup strata, while the Cl ions form a more diffusive distribution contiguous to the monolayer. Our study shows that the length scale over which ions neutralize a charged interface varies significantly and specifically even for monovalent ions.