Conceptual study on the thermal equilibrium state of nanoscale systems

According to classical thermodynamics, a state of equilibrium signifies that in a system the thermophysical properties must be equal regardless where the measurement is taken. However, some recent studies have reported that there is a temperature difference over a liquid–vapor interface even though the system is in a saturated equilibrium state when reduced to nanoscale size. Those conclusions lead to questions as to whether the definition of a thermal equilibrium state should be altered for nanoscale systems. In this study, we carefully investigated the temperature profile over the liquid–vapor interface using the molecular dynamics (MD) simulation and found that the temperature discontinuities between a liquid phase and a vapor phase, over an interface, resulted from the methodological difference of the temperature calculation. In other words, the results of this study indicate that there are no sufficient proofs that the classical definition of a thermal equilibrium state should be changed even if the system of interest would be reduced to nanoscale size.