This White Paper demonstrates the capabilities of modern molecular dynamics simulations illustrated by the description of phase transitions. In particular, the identification of technically relevant parameters is shown by the example of the transition of methane from the liquid to the solid phase. These parameters have technical relevance for a variety of industrial processes, such as air conditioning and refrigeration technology or crystal growth.
In thermodynamics a phase transition is the conversion of one or more phases in other phases. In particular, the classification of the transitions between the gaseous, liquid and solid phases is shown in figure 1.)
There are a number of further complex phase transitions such as the metal-insulator transition.
In thermodynamics, the Ehrenfest classification is often used. In this case, thermodynamic parameters (enthalpy, volume) are considered as a function of a variable (such as temperature). If the (n-1) th derivative of this function is continuous, then it is a phase transition of n-th order.
Thus, for example melting, evaporation or solidification are first order phase transitions. Using the Ehrenfest representation, important parameters of the phase transition can be extracted. For example the critical temperature and the heat of evaporation will be obtained for the evaporation, using enthalpy-temperature diagram.
Phase transitions play an important role in nature and technology. Thus, for example refrigerants which have a particularly high heat of vaporization are used in compression refrigeration machines (refrigerators, air conditioners). Another example is the heat-transfer fluids which are used in solar systems. Their melting temperature is very low (<-20 ° C) and the boiling point is relatively high (> 100 ° C).