Calculation of surface properties of pure fluids using density gradient theory and SAFT-EOS

Aqueous solutions of alcohols are interesting because of their anomalous behavior that is believed to be due to the molecular structuring of water and alcohol around each other in solution. The interfacial structuring and properties are significant for application in alcohol purification processes and biomolecular structure. Here we study aqueous mixtures of short alcohols (methanol, ethanol, 1-propanol, and 2-propanol) at a hydrophobic surface using interfacial statistical associating fluid theory which is a perturbation density functional theory. The addition of a small amount of alcohol decreases the interfacial tension of water drastically. This trend in interfacial tension can be explained by the structure of water and alcohol next to the surface. The hydrophobic group of an added alcohol preferentially goes to the surface preserving the structure of water in the bulk. For a given bulk alcohol concentration, water mixed with the different alcohols has different interfacial tensions with propanol having a lower interfacial tension than methanol and ethanol. 2-propanol is not as effective in decreasing the interfacial tension as 1-propanol because it partitions poorly to the surface due to its larger excluded volume. But for a given surface alcohol mole fraction, all the alcohol mixtures give similar values for interfacial tension. For separation of alcohol from water, methods that take advantage of the high surface mole fraction of alcohol have advantages compared to separation using the vapor in equilibrium with a water-alcohol liquid. © 2013 AIP Publishing LLC.

Section: Introductionmentioning

confidence: 99%

Hydrophobic and hydrophilic interactions in aqueous mixtures of alcohols at a hydrophobic surface

Ballal

Chapman

2013

“…The square gradient treatment 11,12,20,38 -40 has also been employed with the SAFT free energy to examine the interfacial properties of a number of real molecules. [41][42][43][44] Though the square-gradient SAFT approach has been shown to provide a good representation of the surface tension of a number of pure fluids and mixtures, this requires the use of empirical adjustable parameters ͑the so-called ''influence'' parameters͒ which limits the predictive capabilities of the method.…”

Section: Introductionmentioning

confidence: 99%

An accurate density functional theory for the vapor-liquid interface of associating chain molecules based on the statistical associating fluid theory for potentials of variable range

Gloor

Jackson

Blas

et al. 2004

119

134

A Helmholtz free energy density functional is developed to describe the vapor-liquid interface of associating chain molecules. The functional is based on the statistical associating fluid theory with attractive potentials of variable range ͑SAFT-VR͒ for the homogenous fluid ͓A. Gil-Villegas, A. Galindo, P. J. Whitehead, S. J. Mills, G. Jackson, and A. N. Burgess, J. Chem. Phys. 106, 4168 ͑1997͔͒. A standard perturbative density functional theory ͑DFT͒ is constructed by partitioning the free energy density into a reference term ͑which incorporates all of the short-range interactions, and is treated locally͒ and an attractive perturbation ͑which incorporates the long-range dispersion interactions͒. In our previous work ͓F. J. Blas, E. Martín del Río, E. de Miguel, and G. Jackson, Mol. Phys. 99, 1851 ͑2001͒; G. J. Gloor, F. J. Blas, E. Martín del Río, E. de Miguel, and G. Jackson, Fluid Phase Equil. 194, 521 ͑2002͔͒ we used a mean-field version of the theory ͑SAFT-HS͒ in which the pair correlations were neglected in the attractive term. This provides only a qualitative description of the vapor-liquid interface, due to the inadequate mean-field treatment of the vapor-liquid equilibria. Two different approaches are used to include the correlations in the attractive term: in the first, the free energy of the homogeneous fluid is partitioned such that the effect of correlations are incorporated in the local reference term; in the second, a density averaged correlation function is incorporated into the perturbative term in a similar way to that proposed by Toxvaerd ͓S. Toxvaerd, J. Chem. Phys. 64, 2863 ͑1976͔͒. The latter is found to provide the most accurate description of the vapor-liquid surface tension on comparison with new simulation data for a square-well fluid of variable range. The SAFT-VR DFT is used to examine the effect of molecular chain length and association on the surface tension. Different association schemes ͑dimerization, straight and branched chain formation, and network structures͒ are examined separately. The surface tension of the associating fluid is found to be bounded between the nonassociating and fully associated limits ͑both of which correspond to equivalent nonassociating systems͒. The temperature dependence of the surface tension is found to depend strongly on the balance between the strength and range of the association, and on the particular association scheme. In the case of a system with a strong but very localized association interaction, the surface tension exhibits the characteristic ''s shaped'' behavior with temperature observed in fluids such as water and alkanols. The various types of curves observed in real substances can be reproduced by the theory. It is very gratifying that a DFT based on SAFT-VR free energy can provide an accurate quantitative description of the surface tension of both the model and experimental systems.

“…This interesting analysis was then extended to the interfacial behaviour exhibited by type I, type II and type V binary mixtures of equal sized Lennard-Jones molecules using a more realistic DGT + soft-SAFT treatment 42,43 , including a comparison with molecular simulation. Kahl and Enders 44,45 were the first to combine a DGT treatment with the SAFT EOS to study the interfacial tension of mixtures of alkanes and of alkanes with methanol. In a series of subsequent studies, Enders and co-workers compared the performance of DGTs based on the PR, Sanchez-Lacombe (SL), SAFT, and PC-SAFT EOSs for mixtures of carbon dioxide and polystyrene…”

mentioning

confidence: 99%

Classical density functional theory for the prediction of the surface tension and interfacial properties of fluids mixtures of chain molecules based on the statistical associating fluid theory for potentials of variable range

Llovell

Galindo

Blas

et al. 2010

The SAFT-VR DFT Helmholtz free energy functional developed by Gloor et al., [J. Chem. Phys. 121, 12740 (2004)] is revisited and generalised to treat mixtures. The functional, which is based on the statistical associating fluid theory for attractive potentials of variable range (SAFT-VR) for homogeneous fluids, is constructed by partitioning the free energy density into a reference term (which incorporates all of the short-range interactions and is treated locally) and an attractive perturbation (which incorporates the long-range dispersion interactions). In this work, two different functionals are compared. In the first, one uses a mean-field version of the theory to treat the long-range dispersive interaction, incorporating an approximate treatment of the effect of the correlations on the attractive energy between the segments by introducing a short range attractive contribution in the reference term. In the second, one approximates the correlation function of the molecular segments in the inhomogeneous system with that of a homogeneous system with an average density of the two positions, following the ideas 2