When Heating Enhances Dielectric Screening: Thermal Disordering of Surface-Templated Antiferroelectric Order Reverses the Temperature Response of Interfacial Water

The in-plane dielectric constant (ε∥) of nanoconfined water plays an essential role in temperature-gated nanofluidics and reaction kinetics at electrochemical interfaces. Water monolayers with antiferroelectric rhombic order on fcc(100) surfaces have been shown, in our previous work, to exhibit an anomalously low ε∥, but how this dielectric response evolves with temperature is unexplored. Using molecular dynamics simulations, we find that ε∥ of water on the fcc(100) surface increases monotonically with temperature, in stark contrast to the monotonic decrease observed on fcc(111) surfaces and in bulk water. This anomaly originates from the progressive thermal disordering of the antiferroelectric rhombic structure, releasing previously suppressed dipolar fluctuations. Consequently, the enhancement of the Kirkwood factor GK overcomes both the conventional 1/T thermal suppression and the concurrent loss of hydrogen bonds. By constructing a phase diagram of d(ln (ε∥ – 1))/d(ln T) in the temperature–surface interaction strength plane, we identify three distinct regimes governed by the surface–water interaction: monotonic increase, nonmonotonic behavior, and monotonic decrease. These findings establish a molecular mechanism for the dielectric temperature response of ordered water monolayers and provide a predictive framework for designing thermally programmable nanofluidic systems, with mechanistic insights relevant to electrochemical interfaces.