First-order transition in confined water between high-density liquid and low-density amorphous phases

Supercooled water and amorphous ice have a rich metastable phase behaviour. In addition to transitions between high- and low-density amorphous solids1,2, and between high- and low-density liquids3,4,5,6,7,8, a fragile-to-strong liquid transition has recently been proposed9,10, and supported by evidence from the behaviour of deeply supercooled bilayer water confined in hydrophilic slit pores11. Here we report evidence from molecular dynamics simulations for another type of first-order phase transition—a liquid-to-bilayer amorphous transition—above the freezing temperature of bulk water at atmospheric pressure. This transition occurs only when water is confined in a hydrophobic slit pore12,13,14 with a width of less than one nanometre. On cooling, the confined water, which has an imperfect random hydrogen-bonded network, transforms into a bilayer amorphous phase with a perfect network (owing to the formation of various hydrogen-bonded polygons) but no long-range order. The transition shares some characteristics with those observed in tetrahedrally coordinated substances such as liquid silicon15,16, liquid carbon17 and liquid phosphorus18.