  The equilibrium silica liquid-liquid interface between the high-density
liquid (HDL) phase and the low-density liquid (LDL) phase is examined using
molecular-dynamics simulation. The structure, thermodynamics, and dynamics
within the interfacial region are characterized in detail and compared with
previous studies on the liquid-liquid phase transition (LLPT) in bulk silica,
as well as traditional crystal-melt interfaces. We find that the silica HDL-LDL
interface exhibits a spatial fragile-to-strong transition across the interface.
Calculations of dynamics properties reveal three types of dynamical
heterogeneity hybridizing within the silica HDL-LDL interface. We also observe
that as the interface is traversed from HDL to LDL, the Si/O coordination
number ratio jumps to an unexpectedly large value, defining a thin region of
the interface where HDL and LDL exhibit significant mixing. In addition, the
LLPT phase coexistence is interpreted in the framework of the traditional
thermodynamics of alloys and phase equilibria.
