Anomalous Increase of Sum-frequency Signal From Cationic Langmuir Monolayer Upon Salt Addition

Ions in water near the interface creates an electrical double layer, in which counterions are attracted while co-ions are repelled from the interface. Adding salt to subphase water is always expected to decrease the electric field in the electric double layer as increased counterions screen the surface charge more effectively. We discovered a phenomenon contrary to this common belief from cationic Langmuir monolayer upon addition of fluoride salts. Langmuir monolayer consisting of DPTAP (1,2-dipalmitoyl-3-trimethylammonium-propane chloride) at different concentrations of fluoride salts (LiF, NaF, and KF) in the subphase water were investigated by sum-frequency vibrational spectroscopy. The OH signal from interfacial water molecules increased upon addition of NaF and LiF, reached a maximum signal (twice larger than that from pure water) at ~ 100 μM, and decreased afterwards. This phenomenon was explained by the action of Li⁺ and Na⁺ co-ions that binds effectively with the ubiquitous bicarbonate (HCO₃⁻) ions at neutral pH and remove these counterions that initially screen the positively charged surface of DPTAP molecules. By contrast, the sum-frequency signal decreased monotonically with KF addition, indicating the K⁺ co-ion cannot capture the bicarbonate counterions effectively.