Size-dependent effect of nano-confinement of water in an ionic liquid matrix at low temperature

One of the leading hypotheses explaining water's anomalies is a metastable liquid-liquid phase transition (LLPT) at high pressure and low temperatures, which remains experimentally elusive due to homogeneous nucleation. Infrared spectroscopy experiments have shown that adding hydrazinium trifluoroacetate to water induces a sharp, reversible LLPT at ambient pressure, potentially originating from the same underlying mechanism as in pure water. In a previous work, we demonstrated that this transition can be attributed to the behavior of pure water only when nanosegregation of the aqueous component is brought into play. Here, by means of molecular dynamics simulations and the structural order parameter ζ, we explicitly analyze the effect of the ionic compound on the structure of liquid water at low temperature, both in a mixed solution and nanoconfined in spherical clusters of varying size. Our findings indicate that the ions surrounding the water induce structural perturbations that disrupt the water hydrogen-bond network up to a depth of approximately 0.70-0.75 nm from the surface toward the center of the sphere. This suggests that, in order to preserve a low-density liquid state within this ionic matrix, and more in general highly ionic matrices, water must be confined within pockets with radii greater than approximately 0.70-0.75 nm.