Phase transitions in confined water nanofilms

Phase transitions in water are normally classified as first or second order. But in confined quasi-one-dimensional films of water, simulations show that the solid–liquid transition can take place by means of a first-order transition or a continuous one without a distinction between solid and liquid. Bulk water has three phases: solid, liquid and vapour. In addition to undergoing a phase transition (of the first order) between them, liquid and vapour can deform continuously into each other without crossing a transition line—in other words, there is no intrinsic distinction between the two phases. Hence, the first-order line of the liquid–vapour phase transition should terminate at a critical point. In contrast, the first-order transition line between solid and liquid is believed to persist indefinitely without terminating at a critical point1. In recent years, however, it was reported that inside carbon nanotubes, freezing of water may occur continuously as well as discontinuously through a first-order phase transition2. Here we present simulation results for water in a quasi-two-dimensional hydrophobic nanopore slit, which are consistent with the idea that water may freeze by means of both first-order and continuous phase transitions. Our results lead us to hypothesize the existence of a connection point at which first-order and continuous transition lines meet3,4.