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

Abstract

Low-temperature properties of nanopore hydroxylamine (HA) aqueous solutions with HA mole-fractions (\(x_{\text{HA}}\)s) 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 \(x_{\text{HA}} = 0.03\) and that the maximum or transition temperature (\(T_{\text{trs}}\)) increased with increasing the \(x_{\text{HA}}\). 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 \(T_{\text{trs}}\) showed its peak at around \(x_{\text{HA}} = 0.12\). 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.