Vapour–liquid interfacial properties of square-well chains from density functional theory and Monte Carlo simulation

Abstract

The statistical associating fluid theory for attractive potentials of variable range (SAFT-VR) density functional theory (DFT) developed by [Gloor et al., J. Chem. Phys., 2004, 121, 12740-12759] is used to predict the interfacial behaviour of molecules modelled as fully-flexible square-well chains formed from tangentially-bonded monomers of diameter σ and potential range λ = 1.5σ. Four different model systems, comprising 4, 8, 12, and 16 monomers per molecule, are considered. In addition to that, we also compute a number of interfacial properties of molecular chains from direct simulation of the vapour-liquid interface. The simulations are performed in the canonical ensemble, and the vapour-liquid interfacial tension is evaluated using the wandering interface (WIM) method, a technique based on the thermodynamic definition of surface tension. Apart from surface tension, we also obtain density profiles, coexistence densities, vapour pressures, and critical temperature and density, paying particular attention to the effect of the chain length on these properties. According to our results, the main effect of increasing the chain length (at fixed temperature) is to sharpen the vapour-liquid interface and to increase the width of the biphasic coexistence region. As a result, the interfacial thickness decreases and the surface tension increases as the molecular chains get longer. The interfacial thickness and surface tension appear to exhibit an asymptotic limiting behaviour for long chains. A similar behaviour is also observed for the coexistence densities and critical properties. Agreement between theory and simulation results indicates that SAFT-VR DFT is only able to predict qualitatively the interfacial properties of the model. Our results are also compared with simulation data taken from the literature, including the vapour-liquid coexistence densities, vapour pressures, and surface tension.

Vapor−Liquid Interface of the Lennard-Jones Truncated and Shifted Fluid: Comparison of Molecular Simulation, Density Gradient Theory, and Density Functional Theory

“…In some of these studies, also two of the methods were applied and compared. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] The results show that the methods agree qualitatively.…”

Enrichment at vapour–liquid interfaces of mixtures: establishing a link between nanoscopic and macroscopic properties

“…The influence of the molecular parameters on density profiles and other interfacial properties has been studied several times in the literature. Results are available for the influence of the size and energy parameters of the pure components [10,11,13,14,24,25,27,52,133,136,137,142], the cross interactions [10,11,13,14,24,25,27,133,142], and the chain length of the components [18,32,98,102,132,143,144]. Some studies also investigated the influence of associating components [17,79,106,109] on vapour-liquid interfacial properties.…”

Random surface statistical associating fluid theory: Adsorption of n-alkanes on rough surface

“…The general idea is modification of known bulk SAFT Helmholtz free energy using certain version of DFT as was done previously [2,4,5]. As we discussed above in our study VR-SAFT plays the role of the basic model corresponding to homogeneous limit.…”

“…Indeed, in literature one can find a variety of surface phenomena which was described in terms of DFT. Despite the fact, that description of simple fluids using DFT has a long history and now is a standard practice, the extension to the case of SAFT-EOS is an actual problem [2,4,5]. Rigorous way to account for chain structure of molecules in DFT approach is developed in work [6].…”

“…where χ is correction pre-factor which depends on density of fluid (we have used expression from[3]), K HS = kT (∂ρ/∂P ) T is isothermal compressibility of reference system (HS fluid), ρ and P are the number segment density and the pressure of HS fluid, respectively. Parameter K HS = K HS (φ) is function of dimensionless density φ =4 3 πρd 3 and can be calculated from the Carnahan Starling compressibility[16] …”