Recently, we noticed the studies on the temperature boundary layer at a liquid-vapor interface, which reported that there exists the temperature difference over a liquid-vapor interface even though the system is at equilibrium state when the size of a system reduces to a micro-scale. The conclusion of those studies leaves a highly attractive question on whether the definition of equilibrium state should be changed in the case that the size of a system reduces to a nano-scale dimension. From the viewpoint of classical thermo -dynamics, if a system is at equilibrium state, the properties of interest should be measured as the same value within an instrument error. As example, the temperatures of a liquid and a vapor region are actually measured to be identical if they are in a saturated state.
Although there are many molecular dynamics (MD) studies to survey the features of a liquid-vapor interface, a solid-liquid interface, or a solid-solid interface, there is no reported study that ascertains the temperature difference over a liquid-vapor interface at equilibrium state.
In regard with the temperature discontinuity over an interface, which is the main focus of the present study, Maruyama et al. firstly observed the temperature jump at the solid-liquid interface that is called Kapitza resistance through the MD simulation when evaporation and condensation occur. In their study, they estimated the quantitative temperature jump at solid-liquid interface and calculated the equivalent liquid heat conduction thickness corresponding to an interface thermal resistance. Therefore, it is needed to confirm the temperature discontinuity over a liquid-vapor interface in an equilibrium two phase state. At present study, I clarified through the molecular dynamics method that the conclusions of the previous studies might be resulted from the misleading method to calculate the temperature profile.
Molecular dynamics simulations compute the motions of individual molecules in models of solids, liquids, and gases. The key idea is motion which describes how positions, velocities, and orientations change with time.
From this study, I suggest that there is no temperature discontinuity over a liquid-vapor interface independent of a system size if the system is in equilibrium state. Even though this research is contrary to other recent MD studies, the definition of an equilibrium state from classical thermodynamics can be still applicable to a nano-scale system if the approach is adopted.