Interactions of Divalent Cations with the Carboxylate Moiety : A Vibrational Sum Frequency Study

The interactions between the divalent salts CaCl2, MnCl2 and NiCl2 with the carboxylicacid groups in an arachidic acid Langmuir monolayer are studied using vibrational sumfrequency spectroscopy (VSFS). At low salt concentrations the charging of the monolayer is shown to behave in accordance with classical theories of the electrical doublelayer. However, deviations from classical theories are readily apparent at concentrations starting from 1 μM and shown to differ depending on the nature of the cation, indicating an ion specific effect. From the analysis of the carboxylate symmetric vibration at least two different types of ion pair interactions between the divalent cations and the carboxylicacid moiety are detected: a solvent-separated and two (or more) possible contact ion-pairs and/or complexes. By using different VSFS polarization schemes the molecular orientation of the vibration is elucidated and further insight into the state of the carboxylic acidis gained. The ordering of water molecules in the diffuse layer as a result of the surfacepotential of the charged monolayer is also probed and found, at low ionic strengths, to bein agreement with the behaviour predicted by the Gouy-Chapman model. At high concentrations the hydration within the Stern layer can be detected and found to differ with the nature of the electrolyte. Additionally, the importance of the purity grade of the salts used for correctly interpreting the results is shown theoretically and verified experimentally,where trivalent ions affect the surface at concentrations several orders of magnitude below those from mono and divalent ions. Finally, diffusion is shown to play a major rolein the kinetics of deprotonation of the monolayer at submicromolar concentrations, while higher valency cations are also shown to deprotonate the surface at a quicker rate through electroflux