Search

In their seminal monograph ’Molecular Theory of Capillarity’, Rowlinson and Widom describe different possible shapes of density profiles at the vapor-liquid interface of mixtures. They postulated that in some instances, density profiles could possibly be non-monotonic, exhibiting either a maximum and/ or a minimum. This contribution revisits this statement in the light of four decades of posterior research. We summarize the distinct morphologies at the vapor-liquid interface suggested in the literature recognizing that the condition of a single minimum in the profile has not yet been reported. Interfacial density profiles with a single maximum as well as fully monotonic density profiles have been observed and reported extensively. The case of a simultaneous maximum and minimum is more controversial, as it has only been predicted using theoretical approaches such as density gradient theory (DGT). This ambiguity is further investigated in this work using the example of the vapor-liquid interface of cyclohexane + butanol. Both DGT in combination with several distinct equations of state and molecular dynamics simulations are used. The results from the two methods are found to be contradictory: while the DGT results predict a maximum/minimum structure, the computer experiment results indicate only a single maximum in the density profiles. This work thereby emphasizes that results from DGT for highly non-ideal mixtures should not be taken for granted.

This work is framed within AIChE's 10th Industrial Fluid Properties Simulation Challenge, with the aim of assessing the capability of molecular simulation methods and force fields to accurately predict the pressure dependence of the shear viscosity of 2,2,4-trimethylhexane at 293.15 K (20 °C) at pressures up to 1 GPa. In our entry for the challenge, we employ coarse-grained intermolecular models parametrized via a top-down technique where an accurate equation of state is used to link the experimentally-observed macroscopic volumetric properties of fluids to the force-field parameters. The state-of-the-art version of the statistical associating fluid theory (SAFT) for potentials of variable range as reformulated in the Mie incarnation is employed here. The potentials are used as predicted by the theory, with no fitting to viscosity data. Viscosities are calculated by molecular dynamics (MD) employing two independent methods; an equilibrium-based procedure based on the analysis of the pressure fluctuations through a Green-Kubo formulation and a non-equilibrium method where periodic perturbations of the boundary conditions are employed to simulate experimental shear stress conditions. There is an indication that, at higher pressures, the model predicts a solid phase (freezing) which we believe to be an artefact of the simplified molecular geometry used in the modelling. A comparison (made after disclosure of the experimental data) show that the model consistently underpredicts the viscosity by about 30%, but follows the pressure dependency accurately.

1-component Extended Hard-Sphere (1-cEHS) model has been developed recently to predict the viscosity of liquid, n-alkane mixtures. It represents a mixture by a single pseudo-component characterized by an appropriate molecular weight and calculates the viscosity by means of the modified, extended hard-sphere model (EHS) that makes use of a universal function relating reduced viscosity to reduced volume. In this work we have extended the model to also predict the viscosity of mixtures containing alkylbenzene and cycloalkane species. Furthermore, we have developed a new 3-component Extended Hard-Sphere (3-cEHS) model which requires only a knowledge of the overall composition of n-alkane, alkylbenzene and cycloalkane species. Extensive comparison with the available experimental data indicates that both models (1-cEHS and 3-cEHS) predict the viscosity of binary and multicomponent mixtures containing n-alkane, alkylbenzene and cycloalkane species with uncertainty of 5–10%. The proposed models are a precursor of a new family of models that do not require a knowledge of the detailed composition of the mixture, but still take advantage of the underlying molecular description.

Interfacial tension (IFT) between reservoir fluids is an important property in enhanced oil recovery (EOR) and carbon geological storage (CGS). Quantitative knowledge of IFT is needed to support and assist the interpretation of multiphase flow and wetting behaviour in porous media and to facilitate numerical reservoir simulation. Iododecane and iodide-containing brines are common contrast agents in visualisation of multiphase flow in porous media by X-ray CT imaging. The effect of the introduced contrast agents on the IFT was studied in this work by means of pendant-drop experiments and modelling with the density-gradient theory. We report experimental IFTs between N2, 7 mass% KI (aq), and decane-iododecane mixtures with various iododecane mass fractions at temperatures from 298 K to 353 K and pressures from 1 MPa to 30 MPa. The IFTs between N2 and the liquid phases decrease with the increase of either pressure or temperature and increase with the increasing KI molality or iododecane mass fraction. The IFTs between H2O and decane-iododecane mixtures decrease with temperature or iododecane mass fraction and increase slightly with increasing pressure. The IFT data were modelled by means of the density-gradient theory coupled with the volume-translated Peng-Robinson equation of state. Empirical equations were also developed to correlate all of the measured data. A workflow was proposed for estimating the IFTs between gas, brine and the doped hydrocarbon systems based on the experimental and modelling work.

We report new interfacial tension (IFT) measurements of the (H2O + CO2 + H2) and (H2O + H2) systems at pressures of (0.5 to 45) MPa, and temperatures of (298.15 to 448.15) K, measured by the pendant-drop method. The expanded uncertainties at 95% confidence are 0.05 K for temperature, 70 kPa for pressure, 0.017·γ for IFT in the both the binary (H2O + H2) system and the ternary (CO2 + H2 + H2O) system. Generally, the IFT was found to decrease with both increasing pressure and increasing temperature. However, for (H2O + H2) at the lowest two temperatures investigated, the isothermal IFT data were found to exhibit a maximum as a function of pressure at low pressures before declining with increasing pressure. An empirical correlation has been developed for the IFT of the (H2O + H2) system in the full range of conditions investigated, with an average absolute deviation of 0.16 mN m−1, and this is used to facilitate a comparison with literature values. Estimates of the IFT of the (H2O + CO2 + H2) ternary system, by an empirical combining rule based on the coexisting phase compositions and the interfacial tensions of the binary systems, were found to be unsuitable at low temperatures, with an average absolute deviation of 3.6 mN m−1 over all the conditions investigated.

We report measurements of the solubility of hydrogen in pure water and in sodium chloride brine of molality 2.5 mol/kg at temperatures between 323.15 K and 423.15 K and at pressures up to about 40 MPa. The estimated expanded relative uncertainty of the hydrogen solubility at given temperature and pressure is 3%, with a coverage factor of 2. The new results, together with data from the literature for hydrogen solubility in water and brine, are used to construct a simple model to predict hydrogen solubility in water and sodium chloride brines as a function of temperature, pressure and salt molality.

The prediction of the solubility of active pharmaceutical ingredients (APIs) is a significant challenge which is of importance in pharmaceutical applications and solvent selection. Here, we extend the table of group interactions (3 like interactions, 47 unlike interactions) of the SAFT- γ Mie group-contribution equation of state to model the phase behaviour and solubility of mefenamic acid, a nonsteroidal anti-inflammatory drug, in a range of solvents. In addition to mefenamic acid, we also consider its molecular synthons: benzoic acid and diphenylamine. New experimental solubility data are presented for the three molecules in a range of solvents, and three new SAFT- γ Mie functional groups are defined (aCCOOH, aCNHaC and CH 3 CO) and characterised, together with their interactions with solvent groups. Literature data for the vapour pressure, single-phase density, saturation density, vapourisation enthalpy, bubble temperature, dew temperature, and bubble pressure are used to characterise the new group interactions. Solubility data are used to characterise the new group-group interactions only if there are no other experimental data available. The transferability and predictive accuracy of the new models are assessed by comparing the theoretical predictions with the experimental solubility data. Our comparison includes alcohols, ketones, and esters as families of solvents and mixed-solvent solubility predictions.

Interfacial properties of the (CH4 + CO2 + H2O) system are of great importance in many geotechnical engineering applications. In this study, we report the first experimental measurements of the interfacial tensions in the (CH4 + CO2 + H2O) system under both three-phase (VLLE) and biphasic conditions. The measurements were made by the pendant drop method. The compositions of the coexisting phases were obtained from a previous study of the phase behavior and the phase densities were then calculated from an equation of state. IFTs along five isotherms in the VLLE region and along six isotherms in biphasic region are reported. In the VLLE region, the IFT between the water-rich liquid and the gas phase varied between (33 and 39) mN·m−1, with a sharp increase as the pressure increased along an isotherm towards the upper critical end point. In the same region, the IFT between the water-rich and CO2-rich liquids varied between (30 and 33) mN·m−1. The IFT between the gas phase and the CO2-rich liquid phase was too small to measure accurately but an approximate value was obtained which is consistent with Antonov's equality. In the biphasic region, measurements were made at temperatures up to 423 K and at pressures up to 30 MPa. As observed in other water-gas systems, the IFT declines monotonically along isotherms with increasing pressure and decreases with increasing temperature at constant pressure.

The viscosity and density are reported for six synthetic mixtures, composed of up to 13 components, designed to match a light, dead crude oil of API 32° and molar mass of approximately 184 g mol−1. The measurements were made in the liquid region at temperatures between (323 and 398) K and in the pressure range from 1 MPa to 70 MPa. The viscosity was measured with a vibrating-wire viscometer, while the density was measured by means of a vibrating U-tube densimeter. The density and viscosity data have expanded relative uncertainties of 0.12% and 1.2%, respectively with a coverage factor of 2. We have used the measured viscosity data to test the predictive power of the four viscosity models, the extended hard sphere (EHS), one-component EHS (1-cEHS), three-component EHS (3-cEHS) and Vesovic-Wakeham (VW), that have their basis in kinetic theory and the molecular description of the fluid. Two of the models (EHS and VW) require full compositional description of the mixture, while the other two belong to a new family of models which dispense with full compositional characterization, but retain molecular description. On average the EHS and VW models predict the viscosity data with lower deviations than 1-cEHS and 3-cEHS models, but all four models represent the data with uncertainty of 5–10%.