Charge Inversion by Monovalent Hydroxide Ions

The interaction between the hydroxide ion (OH^–) and the headgroup of a model cationic lipid (DPTAP, 1,2-dipalmitoyl-3-trimethylammonium-propane chloride) was investigated for different concentrations of NaOH solutions by using sum-frequency vibrational spectroscopy. The OH signal (3000–3700 cm^–1) of the interfacial water under the Langmuir monolayer of DPTAP decreased with increasing NaOH concentration, due to screening of the surface charge by OH^– counterions. Surprisingly, after reaching a minimum at 5 mM NaOH, the OH signal steadily increased. The phase-sensitive spectra revealed a sign change in the OH stretch band, indicating the overcompensation of the surface charge by OH^–. By contrast, for a DODAB (didodecyldimethylammonium bromide) monolayer (a cationic surfactant without ester groups), the OH stretch signal decreased monotonically with NaOH addition. This charge inversion behavior is driven by the specific interaction between the ester moiety of DPTAP and the OH^– that overcomes the Coulomb repulsion between the adsorbed ions.