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The gradient theory of fluid interfaces is for the first time applied, without any lumping, to complex mixtures of more than three components, here made up of hydrocarbons and of a high proportion of carbon dioxide, nitrogen, or methane. It is combined with the volume-corrected Peng-Robinson equation of state. No adjustable parameters are used in the influence parameters mixing rule, which allows use of the gradient theory in a predictive manner. It gives very good estimates of the surface tension of the complex mixtures studied. In any case, it is found to be much superior to the traditional parachor method. The gradient theory is also used to compute the density profiles of the mixture components in the interface; it confirms that the low interfacial tensions of the systems studied are principally induced by a local accumulation of carbon dioxide, nitrogen, or methane in the interface.

This work is dedicated to the simultaneous application of the gradient theory of fluid interfaces and Monte Carlo molecular simulations for the description of the interfacial behavior of the methane/water mixture. Macroscopic (interfacial tension, adsorption) and microscopic (density profiles, interfacial thickness) properties are investigated. The gradient theory is coupled in this work with the SAFT-VR Mie equation of state. The results obtained are compared with Monte Carlo simulations, where the fluid interface is explicitly considered in biphasic simulation boxes at both constant pressure and volume (NPT and NVT ensembles), using reliable united atom molecular models. On one hand, both methods provide very good estimations of the interfacial tension of this mixture over a broad range of thermodynamic conditions. On the other hand, microscopic properties computed with both gradient theory and MC simulations are in very good agreement with each other, which confirms the consistency of both approaches. Interfacial tension minima at high pressure and prewetting transitions in the vicinity of saturation conditions are also investigated.

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The Gradient Theory of fluid interfaces is for the first time combined with the SAFT-VR Mie EOS to model the interfacial properties of the water/CO(2) mixture. As a preliminary test of the performance of the coupling between both theories, liquid-vapor interfacial properties of pure water have been determined. The complex temperature dependence of the surface tension of water can be accurately reproduced, and the interfacial thickness is in good agreement with experimental data and simulation results. The water/CO(2) mixture presents several types of interfaces as the liquid water may be in contact with gaseous, liquid, or supercritical CO(2). Here, the interfacial tension of the water/CO(2) mixture is modeled accurately by the gradient theory with a unique value of the crossed influence parameter over a broad range of thermodynamic conditions. The interfacial density profiles show a systematic adsorption of CO(2) in the interface. Moreover, when approaching the saturation pressure of CO(2), a prewetting transition is highlighted. The adsorption isotherm of CO(2) is computed as well in the case of a gas/liquid interface and compared with experimental data. The good agreement obtained is an indirect proof of the consistency of interfacial density profiles computed with the gradient theory for this mixture and confirms that the gradient theory is suitable and reliable to describe the microstructure of complex fluid interfaces.

In a first part, interfacial properties of a pure monoatomic fluid interacting through the Mie n-6 potential (n=8, 10, 12, and 20) have been studied using extensive molecular simulations. Monte Carlo and molecular dynamics simulations have been employed, using, respectively, the test area approach and the mechanic route. In order to yield reference values, simulations have been performed with a cutoff radius equal to 10sigma, which is shown to be sufficient to avoid long range corrections. It is shown that both approaches provide results consistent with each other. Using the molecular simulations results, it is demonstrated that a unique scaling law is able to provide an accurate estimation of the surface tension whatever the repulsive exponent n, even far from the critical point. Furthermore, it is shown that the surface tension of the Mie n-6 fluid is as well accurately described by a unique Parachor's law. Density profiles are shown to be well represented by the tanh mean field profile, with slight deviations for the lowest temperatures and the smallest n. In addition, the interfacial width is shown to increase when n decreases (for a given reduced temperature) and to follow the usual scaling behavior for not too low temperature. In a second part, interfacial properties of the Mie n-6 fluid computed by the gradient theory, coupled with an equation of state based on the Barker-Henderson perturbation theory, have been compared with those obtained by molecular simulations. It is demonstrated that, even far from the critical point, the gradient theory is efficient to compute surface tensions and density profiles of this model fluid, provided the equation of state accurately model the phase behavior of the fluid involved (which is not the case for n=8 in this study).

Hydroquinone (HQ) forms organic clathrates in the presence of various gas molecules in specific thermodynamic conditions. For some systems, clathrate phase equilibrium and occupancy data are very scarce or inexistent in literature to date. This work presents experimental results obtained for the CO 2 −HQ, CH 4 −HQ, and N 2 − HQ clathrates, in an extended range of temperature from about 288 to 354 K. Formation/dissociation pressures, and occupancies at the equilibrium clathrate forming conditions, were determined for these systems. Experiments showing the influence of the crystallization solvent, and the effect of the gas pressure on HQ solubility, were also presented and discussed. A good agreement is obtained between our experimental results and the already published experimental and modeling data. Our results show a clear dependency of the clathrate occupancy with temperature. The equilibrium curves obtained for CO 2 −HQ and CH 4 −HQ clathrates were found to be very close to each other. The results presented in this study, obtained in a relatively large temperature range, are new and important to the field of organic clathrates with potential impact on gas separation, energy storage, and transport.