Water + 1-alkanol systems: Modeling the phase, interface and viscosity properties

A combined density gradient theory with equation of state model for the study of surface tension of refrigerant fluids

The leading order terms in a curvature expansion of surface tension, the Tolman length (first order), and rigidities (second order) have been shown to play an important role in the description of nucleation processes. This work presents general and rigorous expressions to compute these quantities for any nonlocal density functional theory (DFT). The expressions hold for pure fluids and mixtures and reduce to the known expressions from density gradient theory (DGT). The framework is applied to a Helmholtz energy functional based on the perturbed chain polar statistical associating fluid theory (PCP-SAFT) and is used in an extensive investigation of curvature corrections for pure fluids and mixtures. Predictions from the full DFT are compared to two simpler theories: predictive DGT, which has a density and temperature dependent influence matrix derived from DFT, and DGT, where the influence parameter reproduces the surface tension predicted from DFT. All models are based on the same equation of state and predict similar Tolman lengths and spherical rigidities for small molecules, but the deviations between DFT and DGT increase with chain length for alkanes. For all components except water, we find that DGT underpredicts the value of the Tolman length but overpredicts the value of the spherical rigidity. An important basis for the calculation is an accurate prediction of the planar surface tension. Therefore, further work is required to accurately extract Tolman lengths and rigidities of alkanols because DFT with PCP-SAFT does not accurately predict surface tensions of these fluids.

Interfacial tension and adsorption in the binary system ethanol and carbon dioxide: Experiments, molecular simulation and density gradient theory

Ionic liquids have attracted a large amount of interest in the past few years. One approach to better understand their peculiar nature and characteristics is through the analysis of their surface properties. Some research has provided novel information on the organization of pure ionic liquids at the vapor-liquid interface; yet, a systematic study on the surface properties of mixtures of ionic liquids and their organization at the surface has not previously been carried out in the literature. This work reports, for the first time, a comprehensive analysis of the surface organization of mixtures of ionic liquids constituted by 1-alkyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquids, [C(n)mim][NTf(2)]. The surface tension of mixtures composed of [C(4)mim][NTf(2)] + [C(n)mim][NTf(2)] (n = 1, 2, 5, 6, 8, and 10) was experimentally determined, at 298.2 K and atmospheric pressure, in the whole composition range. From the experimental data, the surface tension deviations and the relative Gibbs adsorption isotherms were estimated showing how the surface composition of an ionic liquid mixture differs from that of the liquid bulk and that the surface is enriched by the ionic liquid with the longest alkyl chain length. Finally, the soft-SAFT equation of state coupled with the density gradient theory (DGT) was used, for the first time, to successfully reproduce the surface tension experimental data of binary mixtures of ionic liquids using a molecular-based approach. In addition, the DGT was used to compute the density profiles of the two components across the interface, confirming the experimental results for the components distribution at the bulk and at the vapor-liquid interface.

Measurement and modelling of the interfacial tension of triglyceride mixtures in contact with dense gases

The density gradient theory was combined with a modern equation of state based on statistical mechanics, the perturbed chain-statistical association fluid theory, or with the well-known Peng−Robinson equation of state, and is applied to compute the surface tension of various binary mixtures made up of nonpolar substances. Additionally, some new data for the system methane + hexane and methane + heptane were measured using the pendant drop method equipped with a high-pressure cell. Both equations of state can be used to predict the surface tension of mixtures very close to experimental data. The surface tension of pure ethane could also be predicted using the surface tension data of other n-alkanes. Additionally, the surface tensions for mixtures as a function of liquid composition, of temperature, and of pressure were calculated for systems, where no experimental data are available. If the mixture contains a high volatile component, strong relative enrichment effects in the interface could be observed.

The interfacial properties of real systems consisting of complex mixtures are not easily accessible through experimentation. The Density Gradient Theory (DGT) coupled with the Peng-Robinson equation of state (PR-EOS) is applied here to predict the interfacial properties of the mixture, such as interfacial tension, interfacial thickness, and density profile within the interface, both far from and near the critical region. A modified algorithm has been developed to solve DGT equations without the need to select the reference fluid or estimate the domain size by explicitly introducing the time-dependent molar density term. The approach accurately describes the interfacial tensions of mixtures compared to the available experimental data. Furthermore, based on the obtained density profiles and interfacial thicknesses, the transition between the vapor-liquid two-phase and single-phase of the multicomponent system is quantified using the Knudsen number criterion under high-pressure conditions.

Modeling interfacial tension in N2/n-alkane systems using corresponding state theory: Application to gas injection processes

A collocation method for surface tension calculations with the density gradient theory

Modeling interfacial tension and minimum miscibility pressure in paraffin-nitrogen systems: Application to gas injection processes

Molecular dynamics (MD) simulations of the N2+hexane+H2O system (two-phase) were conducted at 343–443 K and 60–150 MPa. The MD results agreed reasonably well with the density gradient theory (DGT) results based on the CPA EoS. The interfacial tensions (IFTs) were found to increase with pressure and decrease with temperature. An important finding is that the IFTs only slightly decreased with increasing N2 mole fraction in the N2/hexane-rich phase (xN2). In general, N2 shows a positive surface excess, and hexane shows a negative surface excess. The increase in the IFT with pressure indicates that the IFT behavior is dominated by the negative surface excess of hexane. MD simulations of the corresponding N2+hexane+H2O+silica (hydrophilic) system showed that the water contact angles (CAs) are not greatly affected by pressure or temperature. Importantly, the water CAs slightly decreased with increasing xN2, and the adhesion tensions increased with increasing xN2. MD simulations of the N2+hexane+brine system were also conducted (salt concentration (cs) up to 5.4 mol/kg NaCl). The MD results agreed reasonably well with the DGT results based on the CPA EoS with the Debye–Hückel contribution. Here, Na+ and Cl– were excluded from the interfacial regions. The solubility of N2 in the H2O-rich phase decreased with increasing cs, because of the salting-out effect. The IFTs increased linearly with increasing cs. MD simulations of the corresponding N2+hexane+brine+silica (hydrophilic) system showed that the water CAs increase with increasing cs. Our previous studies showed that the CO2+hexane and CO2+hexane+silica (hydrophilic) systems in the presence of water or brine gave generally similar results. However, for example, the adhesion tensions of the CO2+hexane+H2O+silica (hydrophilic) system decreased with increasing xCO2.

Studies of Polymer, Copolymer, and Associating Liquids by Melt Surface Tension Methods and Cahn-Hilliard Density-Gradient Theory

Molecular dynamics simulations were performed to study the bulk and interfacial properties of methane + n-decane, carbon dioxide + n-decane, and methane + carbon dioxide + n-decane systems under geological conditions. In addition, theoretical calculations using the predictive Peng-Robinson equation of state and density gradient theory are carried out to compare with the simulation data. A key finding is the preferential dissolution in the decane-rich phase and adsorption at the interface for carbon dioxide from the methane/carbon dioxide mixture. In general, both the gas solubility and the swelling factor increase with increasing pressure and decreasing temperature. Interestingly, the methane solubility and the swelling of the methane + n-decane system are not strongly influenced by temperature. Our results also show that the presence of methane increases the interfacial tension (IFT) of the carbon dioxide + n-decane system. Typically, the IFT of the studied systems decreases with increasing pressure and temperature. The relatively higher surface excess of the carbon dioxide + n-decane system results in a steeper decrease in its IFT as a function of pressure. Such systematic investigations may help to understand the behavior of the carbon dioxide-oil system in the presence of impurities such as methane for the design and operation of carbon capture and storage and enhanced oil recovery processes.

Molecular dynamics simulations and theoretical analysis were carried out to study the bulk and interfacial properties of carbon dioxide-methane-water and carbon dioxide-methane-brine systems under geological conditions. The density gradient theory with the bulk phase properties estimated using the cubic-plus-association (CPA) equation of state (EoS) can well describe the increase in the interfacial tension (IFT) of the CO2-water system in the presence of methane. The theoretical estimates of species mole fractions in the carbon dioxide-methane-water system are in good quantitative agreement with the experimental results. Furthermore, simulations of carbon dioxide-methane-brine system show that the IFT of the CaCl2 case is generally higher than that of the NaCl case. This is probably due to the stronger hydration of Ca2+ ions and their stronger repulsion from the interface compared to Na+. While the overall shape of the ionic profiles is not much affected by the ion type, the water profiles show a local enrichment at the interface in the system with CaCl2. In contrast to the case of NaCl, the slopes of the plots of IFT vs CaCl2 concentration are dependent on temperature. Species mole fractions in the carbon dioxide-methane-brine system predicted by combining the CPA EoS with the Debye-Hückel electrostatic term are in good agreement with simulation results.

Pitfalls of using the geometric-mean combining rule in the density gradient theory