Anomalous Increase of Sum-Frequency Signal from a Cationic Langmuir Monolayer upon Salt Addition

Ions in water near the interface create an electrical double layer, in which counterions are attracted, while co-ions are repelled from the interface. Adding salt to the subphase water is always expected to decrease the electric field in the electrical double layer as increased counterions screen the surface charge more effectively. We discovered a phenomenon contrary to this common belief in a cationic Langmuir monolayer upon addition of fluoride salts. A Langmuir monolayer consisting of DPTAP (1,2-dipalmitoyl-3-trimethylammonium-propane chloride) at different concentrations of alkali-metal fluoride salts (LiF, NaF, KF, RbF, and CsF) in the subphase water was investigated by sum-frequency vibrational spectroscopy (SFVS). The OH signal from interfacial water molecules increased upon addition of NaF and LiF, reached a maximum signal (about twice that from pure water) at ∼100 μM, and decreased afterward. This phenomenon was explained by the action of Li+ and Na+ co-cations that bind effectively with the ubiquitous bicarbonate (HCO3-) anions at neutral pH and remove these counterions that initially screen the positively charged headgroups of the DPTAP molecules. By contrast, the sum-frequency signal decreased almost monotonically with KF addition, indicating that the K+ co-cation cannot capture these bicarbonate counterions effectively, and this behavior is explained in terms of the higher solubility of KHCO3 as compared to LiHCO3 and NaHCO3. This trend toward even heavier alkali cations did not continue as RbF and CsF also showed an increase in the sum-frequency signal, in spite of the monotonic increase of the solubility from LiHCO3 to CsHCO3. This tendency demonstrates that the polarizability and resulting surface excess of large alkali cations become important at the interface, making them more accessible to the bicarbonate counterions.