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

Two-dimensional square ice in graphene nanocapillariesat roomtemperature is a fascinating phenomenon and has been confirmed experimentally.Instead of the temperature for bulk ice, the high van der Waals pressurebecomes an all-important factor to induce the formation of squareice and needs to be studied further. By all-atom molecular dynamicssimulations of water confined between two parallel graphene sheets,which are changed in size (the length and the width of the graphenesheets) over a wide range, we find that the critical crystallizationpressure for the formation of square ice in the nanocapillary stronglydepends on the size of the graphene sheet. The critical crystallizationpressure slowly decreases as the graphene size increases, convergingto an approximately macroscopic crystallization pressure. The unfreezablethreshold for graphene size is obtained by estimating the actual pressure,and it is difficult to form the square ice spontaneously in practicewhen the graphene sheet is smaller than the threshold. Moreover, thecritical crystallization pressure fluctuates when the graphene sizeis minuscule, and the range of oscillation narrows as the sheet sizeincreases, converging to the macroscopic behavior of a single criticalicing pressure for large sheets. The graphene size also affects thestability and crystallization time of the square ice.