Resonant Terahertz Excitation of Confined Water in Carbon Nanotubes

Terahertz (THz) radiation provides an effective means to probe and manipulate collective molecular dynamics in confined water. In this work, nonequilibrium molecular dynamics simulations are employed to investigate the frequency-dependent thermal response and microscopic structure of water confined in armchair-type double-walled carbon nanotubes (DWCNTs). Water encapsulated in (3, 3)@(13, 13), (3, 3)@(18, 18), and (3, 3)@(23, 23) DWCNTs is subjected to linearly polarized THz electric fields applied either parallel or perpendicular to the nanotube axis, and the resulting temperature jump, Δ T, is analyzed over a broad THz frequency range. In comparison with bulk water, confined water exhibits substantially enhanced and highly frequency-selective heating, with a pronounced dependence on field polarization. Under axial polarization, water confined in the narrowest (3, 3)@(13, 13) DWCNT shows a strong resonant response, yielding a maximum temperature increase of Δ T ∼ 370 K, approximately 3.3 times that of bulk water. This enhancement weakens systematically with increasing outer nanotube diameter and higher initial temperature. Analyses of oxygen–oxygen radial distribution functions and transverse density profiles reveal that nanoscale confinement induces pronounced molecular ordering and spatial localization, characterized by ring-like density distributions within the nanotube cross section. These confinement-induced structural features indicate a strong coupling between collective molecular motions and external THz fields, demonstrating that geometric confinement and field polarization jointly regulate resonant energy absorption in nanoscale aqueous systems.