Specific Heat Capacity and Coordination Number of Nano-Confined Transcritical Water

In the field of nano energy, investigating the specific heat capacity and coordination number of nano-confined water is highly significant for gaining a better understanding of the energy and microstructure of confined water. In this work, we employed the method of molecular dynamics (MD) simulation to calculate the specific heat capacity at constant volume and coordination number of water molecules confined in carbon nanotubes (CNTs) under different conditions (T=600–700 K, P=21.776 and 25 MPa, CNT diameter=0.949–5.017 nm). The results showed that near the critical point, the specific heat capacity at constant volume of confined water was lower than that of bulk water, and the energy fluctuation showed a trend of first increasing and then remaining unchanged with the increase of temperature and CNT diameter. Among them, the saturation point of temperature is 650 K (reduced pressure P_r=1) and 660 K (P_r=1.15), and the saturation point of CNT diameter is 2.034 nm. Additionally, the pseudo-critical temperature of confined water was the same as bulk water, and it increased with the increase of critical pressure. Moreover, with the increase of CNT diameter, the coordination number of confined water increased rapidly, and reaches the saturation state when the CNT diameter is 2.034 nm. This investigation revealed the mass and energy characteristics of nano-confined water near the critical point, which could provide guidance for the critical phase transition of nano-confined water.