Charge Inversion at the Surface by Adsorption of Trivalent Cations on a Langmuir Monolayer

초록

IR-visible sum-frequency generation (SFG) spectroscopy was used to study cation adsorption and charge inversion behavior at the water/lipid interface of a 1, 2-dimyristoyl-sn-glycero-3-phosphatidic acid (DMPA) monolayer. At low subphase salt (LaCl3) concentrations, the sum-frequency signal from interfacial water molecules decreased with increasing salt concentration of the subphase water. At around similar to 1 mu M of bulk LaCl3 concentration, the SFG signal was at its minimum, suggesting charge neutrality at this specific salt concentration. With further increase in the salt concentration, a small SFG signal from the interfacial water molecules was again observed. This phenomenon was explained by an overcompensation of the surface charge by trivalent cations causing an inversion of surface charge, which caused an alignment of the interfacial water molecules at high salt concentrations.

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