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Intermolecular Interactions at the Interface Quantified by Surface-Sensitive Second-Order Fermi Resonant Signals

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journal contribution
posted on 23.07.2015, 00:00 by Kangzhen Tian, Baixiong Zhang, Shuji Ye, Yi Luo
Accurate determination of intermolecular interaction forces at the surface and the interface is essential to identify the nature of interfacial phenomena such as absorption, interfacial assembly, and specific ion effect, but it still represents a major technical challenge. In this study, we proposed a novel method to deduce the interfacial interaction forces by using surface-sensitive second-order Fermi resonant signals, generated in sum frequency generation vibrational spectroscopy (SFG-VS). By investigating the influence of lipid chain length and intermolecular distance on the Fermi resonant signals of phospholipid monolayer at the air/CaF2 surface and the air/water interface, a linear correlation between the Fermi resonant intensity ratio and the dominated interactions in the lipid monolayer has been observed. It implies that the amplitude of the intensity ratio can be used as an effective in situ vibrational optical ruler to characterize the total intermolecular interaction forces at the surface and the interface. Such a relationship further enables us to elucidate the specific ion effects on the interfacial interactions, allowing us to identify different contributions from van der Waals, electrostatic, and hydration interactions. This study clearly demonstrates the power of the second-order Fermi resonant signals for evaluating the interfacial interaction forces in situ and in real time.