Abstract
Interfacial properties such as interfacial profiles, surface activity, wetting transitions, and interfacial tensions along the three-phase line are described for a Type IIIa binary mixture. The methodological approach combines the square gradient theory coupled to the statistical associating fluid theory for Mie potentials of variable range, and coarse-grained molecular dynamics simulations using the same underlying potential. The water + n-hexane mixture at three-phase equilibrium is chosen as a benchmark test case. The results show that the use of the same molecular representation for both the theory and the simulations provides a complementary picture of the aforementioned mixture, with an excellent agreement between the molecular models and the available experimental data. Interfacial tension calculations are extended to temperatures where experimental data are not available. From these extrapolations, it is possible to infer a first order wetting transition at 347.2 K, where hexane starts to completely wet the water/vapor interface. Similarly, the upper critical end point is estimated at 486.3 K. Both results show a very good agreement to the available experimental information. The concentration profiles confirm the wetting behavior of n-hexane along with a strong positive surface activity that increases with temperature, contrasting the weak positive surface activity of water that decreases with temperature.
Highlights
According to the van Konynenburg and Scott [1] classification of phase equilibria, Type III mixtures are characterized by a branch of the critical line ending in an upper critical end point (UCEP) of a three-phase line, while the other branch diverges to the high-pressure range
For n = 2 to 26, the critical line connecting the critical point of the n-alkane ends in an UCEP characterized by a vapor–liquid critical point and an aqueous-rich liquid phase; this behavior is classified as Type IIIa
In Equation (5), a0 is the Helmholtz energy density of the homogenous system, which is given by the SAFT-VR Mie EoS for non-associating chain fluids [30], μi 0 is the chemical potential of species i evaluated at the phase equilibrium conditions, calculated from its definition in the canonical ensemble μi 0 = (∂a0 / ∂ρi )T,V,ρj
Summary
According to the van Konynenburg and Scott [1] classification of phase equilibria, Type III mixtures are characterized by a branch of the critical line ending in an upper critical end point (UCEP) of a three-phase line, while the other branch diverges to the high-pressure range. To SGT, the self-consistent field theory has been used to describe both three-phase equilibrium and interfacial behavior in Type III mixtures We select the water + n-hexane binary mixture along the three-phase line to exemplify the interfacial properties for Type IIIa systems In this case, the available tensiometry data [11] only cover the temperature range from 293.15 K to 333.15 K. The main two advantages of this force field are: (i) its capacity of describing simultaneously bulk phases and interfaces and (ii) the possibility of using it in both MD and SGT coupled to the statistical associating fluid theory for Mie potentials of variable range (SAFT-VR Mie) EoS [30] to obtain a complementary picture of the experimentally inaccessible interfacial phenomena.
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