Finding of a Liquid-Liquid Phase Transition in the Nanopore Water Doped with Hydroxylamine of a Small Amount

Low-temperature properties of nanopore hydroxylamine (HA) aqueous solutions with HA mole-fractions (xHAs) below 0.35 were studied by adiabatic calorimetry. It was found that the heat-capacity maximum at 233 K in pure water changed quickly to a phase transition of the first order by doping HA only of xHA=0.03 and that the maximum or transition temperature (Ttrs) increased with increasing the xHA. The HA molecule was interpreted as acting like an inseparable water-molecular dimer and as functioning to stabilize the low-temperature low-density water phase with a highly developed hydrogen-bond network. The Ttrs showed its peak at around xHA=0.12. This was interpreted to indicate that the HA addition of a large amount leads to a little different hydrogen-bond network from the one formed in pure water.