Influence of electric fields on the structure and structure transition of water confined in a carbon nanotube

Abstract

Our previous work demonstrated that liquid water can freeze continuously into either pentagonal or helical solid-like ice nanotubes in a single-walled carbon nanotube (SWCNT) with a tube diameter of 1.2 nm, depending on the strengths of an external electric (E) field applied along the tube axis. In this study, the structure and the structure transition behavior of water confined in a wider SWCNT (diameter = 1.31 nm) under the influence of E-fields are investigated by molecular dynamics simulations using the TIP4P model for water at atmospheric pressure. We find that confined water can freeze into three different polygonal (including hexagonal, heptagonal, and mixed hexagonal-heptagonal) ice nanotubes through a first-order phase transition at lower E (<0.75 V/nm), while form a helical ice nanotube encapsulating a helical water nanoline through a continuous phase transition at higher E (1.0 < E < 2.0 V/nm), different from the phase transition behavior of water in a SWCNT with a diameter = 1.2 nm. The populations of the three different polygonal ice nanotubes are modulated by both temperature and electric field. In addition, an E-induced discontinuous solid-solid phase transition between polygonal and helical ice nanotubes is observed at low temperature (T < 230 K) with a significant electric hysteresis loop of 1.0 V/nm. Finally, we present a rich phase diagram of confined water as a function of temperature and electric field, in which the boundaries of the first-order phase transition at lower E and the continuous phase transition at higher E are connected by a connecting line which corresponds to the hysteresis region.