“On the Journey of Classical Density Functional Theory: Adsorption and Interfacial Tension”
Abstract
Classical Density Functional Theory (DFT) has shown to be a consistent framework for the study of surface thermodynamics. Two of the most common applications of this theory are the calculation of adsorption and interfacial tension.
For these applications the microscopic, or local density distribution, is non uniform. Classical DFT is based on statistical mechanics concepts used in molecular simulations, and with the definition of a Helmholtz free energy functional of the particle density, it allows the implementation of the already well-known knowledge of bulk equations of state into the study of inhomogeneous fluids.
This locates classical DFT in an advantageous position, as it allows the determination of density profiles and surface properties with accuracy similar to molecular simulations, but with only requiring a modest amount of time.
However, when compared to current models such as Density Gradient Theory for the calculation of interfacial tension, and to Multicomponent Potential Theory of Adsorption, a single DFT calculation for systems containing real fluids takes one order of magnitude more in terms of computational time.
In this work, the Perturbed Chain Statistical Association Fluid Theory (PC-SAFT) equation of state is used to define the Helmholtz free energy functional within the classical DFT framework. Which enables the important feature that it is not required to fit new pure component parameters and only those from bulk fluids are used.
The PC-SAFT DFT implementation is used for the calculation of adsorption and surface tension of pure components and multicomponent systems of non-associating and associating fluids of interest for the chemical and petroleum industries.