Acid−Base Equilibrium at an Aqueous Interface: pH Spectrometry by Heterodyne-Detected Electronic Sum Frequency Generation

We applied interface-selective heterodyne-detected electronic sum frequency generation (HD-ESFG) to the pH spectrometry of an air/cationic surfactant/water interface in order to obtain insight into an acid−base equilibrium at the interface. We used an indicator molecule adsorbed at the interface to probe local pH and local effective polarity. We obtained unprecedentedly high quality spectral data of the interfacial pH spectrometry, which clearly indicates that this interface has higher pH than the bulk owing to the positive charge of the head group of the surfactant. In addition, we found that the air/surfactant/water interface and the micelle interface of the same surfactant are essentially equivalent in local pH and local effective polarity.