The Interfacial Tension module is intended to model the thermodynamic properties of an explicit vapor-liquid interface, and in particular, compute the interfacial tension at various conditions. The molecules in the system are modeled with a square-well intermolecular potential, and the system size, temperature, and number of molecules can be varied over a range of values. In addition, surfactant molecules (with variable intermolecular parameters) can be added to the system, and these species can be tailored to increase or decrease the interfacial tension.
Although a single module is used, the simulations are typically performed in two distinct stages. In the first stage, the molecules are equilibrated and stabilized at a fixed density and temperature in the simulation cell, with conditions corresponding to a liquid phase. In the second stage, after the liquid has equilibrated, the box is immediately expanded along the horizontal direction, which then permits the formation of the vapor phase. During the expansion, surfactant molecules are added to the simulation cell according to the specified amount, which can range from zero (no surfactants) up to relatively high concentrations. Once the system has further equilibrated, two well-established vapor-liquid interfaces are created (one on each side of the simulation cell), along with a specified number of surfactant molecules.
The output from the simulation is analyzed in a number of ways. The system configuration can be viewed, in order to confirm the interfacial stability and system characteristics. Also, thermodynamic properties, such as the chemical potential profile (as a function of the horizontal coordinate), the virial profile, the surface tension profile, the density profile, the surfactant orientation, and the total system energy can be charted during the course of the simulation. These properties are important for characterizing the interface and for tracking the system equilibration. Cumulative average thermodynamic properties, as well as standard deviations of the measured properties, are also tabulated.
Overall, the module provides a good theoretical and practical interpretation of the interfacial tension of a vapor-liquid interface. With an accurate intermolecular potential, these types of simulations provide information that can be compared against experimental and other theoretical treatments or approximations. Furthermore, the module introduces the concept of interfacial modifications (via surfactants) and thermal effects on interfacial properties. In this way, there is a direct connection established between the specific intermolecular interactions in the system and the emergent thermodynamic properties of a vapor-liquid interface.