Quantitative Predictions and Experimental Validation of Liquid-Vapor Interfacial Tension in Binary and Ternary Mixtures of Alkanes Using Molecular Dynamics Simulations

Liquid-vapor interfacial properties of alkane mixtures present a challenge for experimental determination, especially under conditions relevant to the energy industry processes. Molecular dynamics (MD) simulations can accurately predict interfacial tensions (IFTs) for complex alkane mixtures under virtually any conditions, thereby alleviating the need for difficult and costly experiments. MD simulations with the CHARMM force field and empirical corrections for the IFT and pressure were used to obtain the IFT for three binary mixtures of ethane (with n-pentane, n-hexane, and n-nonane) and a ternary system (ethane/n-butane/n-decane) under a variety of conditions. The results were thoroughly validated against experimental data from the literature, and new original IFT data were collected using the pendant drop method. The simulations are able to reproduce the experimental IFT to better than 0.5 mN/m or 5% on average and within 1 mN/m or 10% in the worst case. IFTs for the studied three binary and ternary alkane mixtures were predicted for wide ranges of conditions with no known experimental data. Finally, using the MD simulation data, the reliability of the widely used empirical parachor model for predicting IFT was reaffirmed, and the significance of the empirical parameters examined to establish an optimal balance between the accuracy and broad applicability of the model.

The phase behavior study of human antibody solution using multi-scale modeling

Phase transformation in antibody solutions is of growing interest in both academia and the pharmaceutical industry. Recent experimental studies have shown that, as in near-spherical proteins, antibodies can undergo a liquid-liquid phase separation under conditions metastable with respect to crystallization. However, the phase diagram of the Y-shaped antibodies exhibits unique features that differ substantially from those of spherical proteins. Specifically, antibody solutions have an exceptionally low critical volume fraction (CVF) and a broader and more asymmetric liquid-liquid coexistence curve than those of spherical proteins. Using molecular dynamics simulation on a series of trimetric Y-shaped coarse-grained models, we investigate the phase behavior of antibody solutions and compare the results with the experimental phase diagram of human immunoglobulin G (IgG), one of the most common Y-shape typical of antibody molecules. With the fitted size of spheres, our simulation reproduces both the low CVF and the asymmetric shape of the experimental coexistence curve of IgG antibodies. The broadness of the coexistence curve can be attributed to the anisotropic nature of the inter-protein interaction. In addition, the repulsion between the inner parts of the spherical domains of IgG dramatically expands the coexistence region in the scaled phase diagram, while the hinge length has only a minor effect on the CVF and the overall shape of the coexistence curve. We thus propose a seven-site model with empirical parameters characterizing the exclusion volume and the hinge length of the IgG molecules, which provides a base for simulation studies of the phase behavior of IgG antibodies.

Desalination by dragging water using a low-energy nano-mechanical device of porous graphene

We propose a dragging nano-structured suction system based on graphene sheets for water desalination processes.

Phase Segregation at the Liquid-Air Interface Prior to Liquid-Liquid Equilibrium

Binary systems with partial miscibility segregate into two liquid phases when their overall composition lies within the interval defined by the saturation points; out of this interval, there is one single phase, either solvent-rich or solute-rich. In most systems, in the one-phase regions, surface tension decreases with increasing solute concentration due to solute adsorption at the liquid-air interface. Therefore, the solute concentration at the surface is higher than in the bulk, leading to the hypothesis that phase segregation starts at the liquid-air interface with the formation of two surface phases, before the liquid-liquid equilibrium. This phenomenon is called surface segregation and is a step toward understanding liquid segregation at a molecular level and detailing the constitution of fluid interfaces. Surface segregation of aqueous binary systems of alkyl acetates with partial miscibility was theoretically demonstrated by means of a thermodynamic stability test based on energy minimization. Experimentally, the coexistence of two surface regions was verified through Brewster's angle microscopy. The observations were further interpreted with the aid of molecular dynamics simulations, which show the diffusion of the acetates from the bulk toward the liquid-air interface, where acetates aggregate into acetate-rich domains.

Development and application of a particle-particle particle-mesh Ewald method for dispersion interactions

For inhomogeneous systems with interfaces, the inclusion of long-range dispersion interactions is necessary to achieve consistency between molecular simulation calculations and experimental results. For accurate and efficient incorporation of these contributions, we have implemented a particle-particle particle-mesh Ewald solver for dispersion (r(-6)) interactions into the LAMMPS molecular dynamics package. We demonstrate that the solver's O(N log N) scaling behavior allows its application to large-scale simulations. We carefully determine a set of parameters for the solver that provides accurate results and efficient computation. We perform a series of simulations with Lennard-Jones particles, SPC/E water, and hexane to show that with our choice of parameters the dependence of physical results on the chosen cutoff radius is removed. Physical results and computation time of these simulations are compared to results obtained using either a plain cutoff or a traditional Ewald sum for dispersion.

Chemical potential perturbation: extension of the method to lattice sum treatment of intermolecular potentials

The recently developed chemical potential perturbation (CPP) method [S. G. Moore and D. R. Wheeler, J. Chem. Phys. 134, 114514 (2011)] is extended to the lattice (Ewald) sum treatment of intermolecular potentials. The CPP method predicts chemical potentials for a range of composition points using the local (position-dependent) pressure tensor of an inhomogeneous system. When computing the local pressure tensor, one can use the Irving-Kirkwood (IK) or Harasima (H) contours of distributing the pressure. We compare these two contours and show that for a planar interface, the homogeneous pressure and resulting chemical potential can be approximated with the CPP method using either the IK or the H contour, though with the lattice sum method the H contour has much greater computational efficiency. The proposed methods are validated by calculating the chemical potentials of the Lennard-Jones fluid and extended simple point-charge (SPC/E) water, and results show a high level of agreement with respective equations of state.

Chemical potential perturbation: a method to predict chemical potentials in periodic molecular simulations

A new method, called chemical potential perturbation (CPP), has been developed to predict the chemical potential as a function of density in periodic molecular simulations. The CPP method applies a spatially varying external force field to the simulation, causing the density to depend upon position in the simulation cell. Following equilibration the homogeneous (uniform or bulk) chemical potential as a function of density can be determined relative to some reference state after correcting for the effects of the inhomogeneity of the system. We compare three different methods of approximating this correction. The first method uses the van der Waals density gradient theory to approximate the inhomogeneous Helmholtz free energy density. The second method uses the local pressure tensor to approximate the homogeneous pressure. The third method uses the Triezenberg-Zwanzig definition of surface tension to approximate the inhomogeneous free energy density. If desired, the homogeneous pressure and Helmholtz free energy can also be predicted by the new method, as well as binodal and spinodal densities of a two-phase fluid region. The CPP method is tested using a Lennard-Jones (LJ) fluid at vapor, liquid, two-phase, and supercritical conditions. Satisfactory agreement is found between the CPP method and an LJ equation of state. The efficiency of the CPP method is compared to that for Widom's method under the tested conditions. In particular, the new method works well for dense fluids where Widom's method starts to fail.