Fermi Resonant Interaction of the Tailed Methyl Groups of Langmuir Monolayer at the Air/Water Interface during Phase Transition

Abstract

Molecular insight into the interactions of two-dimensional (2D) materials at the interface is essential to understand the functionality of interfacial molecular devices. Yet it still remains elusive so far. Fermi resonant interaction is highly sensitive to the total molecular interactions. In this study, we used lipid 1,2-dimyristoyl-<i>sn</i>-glycero-3 -phospho-(1′-rac-glycerol) (sodium salt) (DMPG) monolayer as a model, and performed a systematic study to investigate the Fermi resonant interactions of 2D materials at the interface during liquid-expanded (LE) to liquid-condensed (LC) phase transition using multiplexed-polarization sum frequency generation vibrational spectroscopy (SFG-VS). It is found that the ratio (<i>R</i>₁) between Fermi resonance and symmetric stretching mode of the tailed methyl groups sharply decreases during the phase transition. The sharp drop of <i>R</i>₁ originates from the nonsynchronous change of the tail and head groups of the lipid. The tailed CH₃ groups of DMPG locally accumulate at the air/water interface during LE–LC phase transition while the head glycerol groups do not. The local aggregation of the methyl groups strengthens the van der Waals (vdW) interaction, leading to the decrease of the total intermolecular interactions and the drop of the ratio of <i>R</i>₁. However, such phenomena are not observed at the air/KCl solution (0.3M) interface