Intrinsic structure and interfacial tension of the HDL/LDL water interface.

The coexistence line between the high-density liquid (HDL) and the low-density liquid (LDL) phases of water lies in the supercooled, high pressure region of the phase diagram where water is metastable with respect to ice, relaxation is slow, and the low free-energy cost of forming interfaces between the HDL and LDL phases can lead to soft, fluctuating, and morphologically complex domains. These conditions make both the structure and the thermodynamics of the interface difficult to access not only experimentally but also in computer simulations. Here, we study explicit HDL-rich/LDL-rich coexistence in long simulations at constant temperature, volume, and number of water molecules in elongated cells that promote interface stability. Using a high 1:1:8 aspect ratio proved key to obtaining two stable planar interfaces that persist for the full simulation time of about 1.35 μs. These interfaces bound an HDL-rich slab containing transient LDL-rich droplets, while the surrounding LDL-rich phase contains a fluctuating population of ice-like crystallites. This complex morphology makes the pressure anisotropy inadequate for measuring the interfacial tension of the HDL/LDL boundary. An analysis based on capillary wave theory, including the intrinsic interfacial broadening, reveals instead a very soft boundary, with γ = 4.25 ± 0.25 mN/m. These results point to an HDL/LDL interface that is weak, structured, and easily distorted by the competing liquid and crystalline fluctuations present in this region of the phase diagram.