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

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.