Ion adsorption and surface basicity of alcohol monolayer/water interface studied by sum-frequency vibrational spectroscopy
Yu-Chieh Wen1,2,3*, Chuanshan Tian3, Y. Ron Shen2,3
1Institute of Physics, Academia Sinica, Taipei, Taiwan
2Department of Physics, University of California, Berkeley, California, USA
3Department of Physics, Fudan University, Shanghai, China
* presenting author:Yu-Chieh Wen,
Specific ion adsorption to aqueous interfaces involving organic molecules is relevant to biology and chemistry, but incompletely understood. Here we study ion adsorption and structure of a long-chain alcohol monolayer/water interface, as a model of non-ionic organic interface, by phase-sensitive sum-frequency vibrational spectroscopy. We show that hydronium, hydroxide, and halide ions can adsorb to this interface formed by the neutral molecules, inducing reorientation order of the interfacial water. The interface ion excess for a given bulk concentration is determined to have the following ranking order: OH- > I- > Cl- ~ H3O+. Particularly, Cl-, I-, and H3O+ have their surface concentrations comparable to these at the water/vapor interface, whereas OH- has much more excess at the alcohol/water interface. It is characterized with surface pOH ~5.2 (surface pH ~6.4) for bulk pH 7, suggesting that the alcohol/pure water interface is preferentially basic.

Keywords: Specific ion adsorption, sum-frequency vibrational spectroscopy, surface pH