Characterization of the Acetonitrile Aqueous Solution/Vapor Interface by Liquid-Jet X-ray Photoelectron Spectroscopy

We report photoelectron spectroscopy measurements from binary acetonitrilewater solutions, for a wide range of acetonitrile mole fractions (x(CH)3CN = 0.011-0.90) using a liquid microjet. By detecting the nitrogen and carbon 1s photoelectron signal of CH3CN from aqueous surface and bulk solution, we quantify CH3CNs larger propensity for the solution surface as compared to bulk solution. Quantification of the strong surface adsorption is through determination of the surface mole fraction as a function of bulk solution, x(CH3)CN, from which we estimate the adsorption free energy using the Langmuir adsorption isotherm model. We also discuss alternative approaches to determine the CH3CN surface concentration, based on analysis of the relative amount of gas- versus liquid-phase CH3CN, obtained from the respective photoelectron signal intensities. Another approach is based on the core-level binding energy shifts between liquid- and gas-phase CH3CN, which is sensitive to the change in solution surface potential and thus sensitive to the surface concentration of CH3CN. Gibbs free energy of adsorption values are compared with previous literature estimates, and we consider the possibility of CH3CN bilayer formation. In addition, we use the observed changes in N 1s and C 1s peak positions to estimate the net molecular surface dipole associated with a complete CH3CN surface monolayer, and discuss the implications for orientation of CH3CN molecules relative to the solution surface.