Collective Vibration Decoupling of Confined Water in Membrane Channels

We previously reported that the asymmetric IR absorption of monolayer water confined within two-dimensional nanochannels is capable of nonthermally inducing a unidirectional flow [Zhang, Q. L. <i>Phys. Rev. Lett.</i> 2024, 132, 184003], while the reason for the difference in the collective vibration IR spectrum between the confined water (CW) and bulk water is still not fully understood. Here, using molecular dynamics simulations, we systematically demonstrated that the CW in narrow graphene membrane channels will appear as a predominant fingerprint-peak and a subpeak in the collective vibration spectrum band. A comparison with the calculated IR spectrum for the CW in the channels with different interlayer spacings revealed that the double-peaked pattern originates from the decoupling of the CW's collective vibration. The highlight spectral intensity of the fingerprint-peak is attributed to the low-cost out-of-plane vibration (wag mode) of the CW molecules. These findings help us understand the physical origins of the unique IR spectra of CW in nanochannels, thereby providing a robust theoretical support for the regulation of the CW's structure and dynamics properties by a remote terahertz stimulation.