Quantifying Double-Layer Potentials at Liquid-Gas Interfaces from Vibrational Sum-Frequency Generation.

Vibrational sum-frequency generation (SFG) spectroscopy is demonstrated as a fast method to quantify variations of the electric double-layer potential ϕ0 at liquid-gas interfaces. For this, mixed solutions of nonionic tetraethyleneglycol-monodecylether (C10 E4 ) and cationic hexadecyltrimethylammonium bromide (C16 TAB) surfactants were investigated using SFG spectroscopy and a thin-film pressure balance (TFPB). Derjaguin-Landau-Verwey-Overbeek analysis of disjoining pressure isotherms obtained with the TFPB technique provides complementary information on ϕ0 , which we apply to validate the results from SFG spectroscopy. By using a single ϕ0 value, we can disentangle χ(2) and χ(3) contributions to the O-H stretching modes of interfacial water molecules in the SFG spectra. Having established the latter, we show that unknown double-layer potentials at the liquid-gas interface from solutions with different C16 TAB/C10 E4 mixing ratios can be obtained from an analysis of SFG spectra and are in excellent agreement with the complementary results from the TFPB technique.