Water at low frequencies: Vibrational decomposition reveals spectral fingerprints of surface charge and dynamic surfaces

The low-frequency vibrational dynamics of water (i.e., 0-1000cm-1) encodes rich details about its local environment, hydrogen bonding, and collective motion that are essential to water's various unique properties that are incredibly important for life in a manner that is easily comparable between experiments and simulations. This study uses molecular dynamics to perform a comprehensive mode-resolved analysis of the low-frequency spectrum of water, decomposing it into individual translational and rotational components, and investigates how ionic concentration, confinement, surface charge, and surface oscillations modulate the spectral intensity of the peaks and their corresponding frequencies. The reported results reveal a distinct spectroscopic signature associated with the sign of surface charge polarity that suggests a potential practical route for experimentally inferring surface charge identity from interfacial experimental spectroscopic measurements. It also demonstrates that surface-driven perturbations-particularly at or near 50cm-1-couple most strongly to the translational modes, while their additional energy dissipates quickly along translational and librational modes. This paper increases the fundamental understanding of the dynamics of water in various local environments.