Full-spectrum high resolution modeling of the dielectric function of water

TL;DR

This paper provides a comprehensive, high-resolution model of water's dielectric function across the full electromagnetic spectrum, crucial for understanding water's physical interactions and phase behavior.

Contribution

It introduces a detailed parameterization of water's dielectric function over a wide frequency range, including ice-cold water, with implications for van der Waals interactions and phase stability.

Findings

Micron-scale ice layers are stabilized on water surfaces.

Van der Waals interactions favor complete freezing over premelting layers.

Density extrapolation is effective in microwave but not IR frequencies.

Abstract

In view of the vital role of water in chemical and physical processes, an exact knowledge of its dielectric function over a large frequency range is important. In this article we report on currently available measurements of the dielectric function of water at room temperature (25$^{\circ}$C) across the full electromagnetic spectrum: microwave, IR, UV and X-ray (up to 100 eV). We provide parameterisations of the complex dielectric function of water with two Debye (microwave) oscillators and high resolution of IR and UV/X-ray oscillators. We also report dielectric parameters for ice-cold water with a microwave/IR spectrum measured at $0.4^\circ$C, while taking the UV spectrum from 25$^{\circ}$C (assuming negligible temperature dependence in UV). We illustrate the consequences of the model via calculations of van der Waals interactions of gas molecules near water surfaces, and an assessment of the thickness of water films on ice and ice films on water. In contrast to earlier models of ice-cold water, we predict that a micron-scale layer of ice is stabilised on a bulk water surface. Similarly, the van der Waals interaction promotes complete freezing rather than supporting a thin premelting layer of water on a bulk ice surface. Density-based extrapolation from warm to cold water of the dielectric function at imaginary frequencies is found to be satisfactory in the microwave but poor (40% error) at IR frequencies.