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.

It has been reported that an octadecylamine (ODA) Langmuir monolayer becomes unstable at low pH values with no measurable surface pressure at around pH 3.5, suggesting significant dissolution of the ODA molecule into the subphase solution (Albrecht, Colloids Surf. A 2006, 284-285, 166-174). However, by lowering the pH further, ODA molecules reoccupy the surface, and a full monolayer is recovered at pH 2.5. Using surface sum-frequency spectroscopy and pressure-area isotherms, it is found that the recovered monolayer at very low pH has a larger area per molecule with many gauche defects in the ODA molecules as compared to that at high pH values. This structural change suggests that the reappearance of the monolayer is due to the adsorbed Cl(-) counterions to the protonated amine groups, leading to partial charge neutralization. This proposition is confirmed by intentionally adding monovalent salts (i.e., NaCl, NaBr, or NaI) to the subphase to recover the monolayer at pH 3.5, in which the detailed structure of the monolayer is confirmed by sum frequency spectra and the adsorbed anions by X-ray reflectivity.

I n this article, we report the effect of surface modification of NCM80 [Li(Ni0.8Co0.1Mn0.1)O2] cathode material on the performance of all-solid-state batteries (ASSBs) with oxide-based organic/inorganic hybrid solid electrolytes. Ni-rich...