Variation of Critical Crystallization Pressure for the Formation of Square Ice in Graphene Nanocapillaries

Abstract

Two-dimensional square ice in graphene nanocapillaries at room temperature is a fascinating phenomenon and has been confirmed experimentally. Instead of the temperature for bulk ice, the high van der Waals pressure becomes an all-important factor to induce the formation of square ice and needs to be studied further. By all-atom molecular dynamics simulations of water confined between two parallel graphene sheets, which are changed in size (the length and the width of the graphene sheets) over a wide range, we find that the critical crystallization pressure for the formation of square ice in the nanocapillary strongly depends on the size of the graphene sheet. The critical crystallization pressure slowly decreases as the graphene size increases, converging to an approximately macroscopic crystallization pressure. The unfreezable threshold for graphene size is obtained by estimating the actual pressure, and it is difficult to form the square ice spontaneously in practice when the graphene sheet is smaller than the threshold. Moreover, the critical crystallization pressure fluctuates when the graphene size is minuscule, and the range of oscillation narrows as the sheet size increases, converging to the macroscopic behavior of a single critical icing pressure for large sheets. The graphene size also affects the stability and crystallization time of the square ice.

Correlation between 2D Square Ice and 3D Bulk Ice by Critical Crystallization Pressure

“…In this study, all the results of critical crystallization pressure are obtained in three approaches: (1) The transition is accompanied by a sudden drop of the potential energy, so the potential energy curve can be used as an intuitive estimation of identifying phase transitions. (2) The square icing parameters, MCV 1 and MCV 2 , are calculated to quantitatively distinguish between square ice structures and liquid water, 43,51 and the same results of the critical crystallization pressure as the first method are obtained. (3) All the results of critical crystallization pressure are verified by observing the ice formation from simulation snapshots frame by frame.…”

“…This effect has been discussed in detail in our previous research. 51 In a finite system, the phase transition may be smeared over a pressure region, and the center of the pressure region may also be shifted, corresponding to the "rounding exponent" and the "shift exponent". 53 In this study, the variations of the critical crystallization pressure correspond to the "shift exponent", and the oscillations of the critical crystallization pressure correspond to the "rounding exponent".…”