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

[Image: see text] Vibrational sum-frequency generation (SFG) spectroscopy is demonstrated as a fast method to quantify variations of the electric double-layer potential ϕ(0) at liquid–gas interfaces. For this, mixed solutions of nonionic tetraethyleneglycol-monodecylether (C(10)E(4)) and cationic hexadecyltrimethylammonium bromide (C(16)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 ϕ(0), which we apply to validate the results from SFG spectroscopy. By using a single ϕ(0) 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 C(16)TAB/C(10)E(4) mixing ratios can be obtained from an analysis of SFG spectra and are in excellent agreement with the complementary results from the TFPB technique.