Study On The Effect Of Electric Field On The Change Of Limited Water Structure Under High Pressure

Water confined within nanoscale geometries under an external electric field has many interesting and unusual properties which is very important for its application in biological processes and industrial engineering. Despite considerable effort and progress has been made in the field of formation of ordered ice nanotubes(NTs) inside carbon nanotubes, the effect of electric fields on the phase transformation of water confined within a carbon nanotube (CNT) at high pressure is still insufficient. In this paper, using molecular dynamics simulations(MD), we take an open-ended single-walled (15,3) CNT immersed in a water box as a model to research the influence of electric fields on the structure phase transition of water inside and around the CNT at pressure 1GPa and higher 10GPa. Some meaningful results reveal that:(1) Water confined within the CNT under different conditions can freeze into three kinds of stable ice tube structures, including (7,0), (7, 0)+1D and (7,0)+1L ice NTs, while the (7,0)+1 D structure is heptagonal ice nanotube encapsulating a water chain and it is a new ice phase.(2) As for the inside space of (15,3) CNT, increasing the pressure can only let the confined water structure transform from tube (6,0) to (7,0) single-walled NT which has a larger tube radius. Simply raising the pressure can not produce a water chain inside the NT, it must be combined with the axial electric field, and the premise condition is the ice tube radius should be greater than or equal to that of (7,0) tube structure.(3) When the solid-liquid phase transition occurs, the order degree of electric polarization of the confined water will increase, while the order degree of structure is reduced, the electric polarization order is not beneficial to the formation of the ordered structure, which may be the basic reason why solid-liquid phase transition temperature rises under high axial electric field, since it needs higher temperature to reduce the electric polarization order and improve the order degree of structure.(4) With the different pressure and axial electric field, it is found that water can exhibit a first-order freezing transition to heptagonal NT, and a continuous solid-liquid phase transition into a heptagonal NT with a linear-like water chain inside or a helical ice nanotube encapsulating a helical water line. Increasing the pressure tends to occur continuous phase transformation and can also enhance the solid-liquid phase transition temperature. Compared with the influence of increasing pressure, applying electric field is more effective to raise that temperature.(5) Under a high external electric field (2.0 V/nm), the water molecules around CNT form two different obvious structures, the outer water around CNT shows a continuous liquid-solid phase transition to an ABCD stacking structure at pressure 1GPa, and at higher 10GPa pressure it displays a first-order phase transformation to an ABC stacking structure.(6) The phase transition of the outer water around CNT could be continuous or discontinuous. Compared with the result for the water confined within the CNT, the structure order and electric polarization order of the outer water around CNT are not mutually exclusive since the space is unrestricted, these of the ice phase are both higher than that of liquid phase, and the axial electric field has little influence on solid-liquid phase transition temperature.