We present a corresponding states correlation based on the description of fluid phase properties by means of an Mie intermolecular potential applied to tangentially bonded spheres. The macroscopic properties of this model fluid are very accurately represented by a recently proposed version of the Statistical Associating Fluid Theory (the SAFT-γ version). The Mie potential can be expressed in a conformal manner in terms of three parameters that relate to a length scale, σ, an energy scale, ε, and the range or functional form of the potential, λ, while the nonsphericity or elongation of a molecule can be appropriately described by the chain length, m. For a given chain length, correlations are given to scale the SAFT equation of state in terms of three experimental parameters: the acentric factor, the critical temperature, and the saturated liquid density at a reduced temperature of 0.7. The molecular nature of the equation of state is exploited to make a direct link between the macroscopic thermodynamic parameters used to characterize the equation of state and the parameters of the underlying Mie potential. This direct link between macroscopic properties and molecular parameters is the basis to propose a top-down method to parametrize force fields that can be used in the coarse grained molecular modeling (Monte Carlo or molecular dynamics) of fluids. The resulting correlation is of quantitative accuracy and examples of the prediction of simulations of vapor−liquid equilibria and interfacial tensions are given. In essence, we present a recipe that allows one to obtain intermolecular potentials for use in the molecular simulation of simple and chain fluids from macroscopic experimentally determined constants, namely the acentric factor, the critical temperature, and a value of the liquid density at a reduced temperature of 0.7.

A c c e p t e d M a n u s c r i p t Highlights -A framework to obtain accurate coarse-grained force fields is described -Force field parameters are presented for compounds of interest -A methodology to simulate bubble points of complex mixtures is proposed. -The force fields are shown to be robust and transferrable Highlights (for review) AbstractThe atomistically-detailed molecular modelling of petroleum fluids is challenging, amongst other aspects, due to the very diverse multicomponent and asymmetric nature of the mixtures in question. Complicating matters further, the time scales for many important processes can be much larger that the current and foreseeable capacity of modern computers running fully-atomistic models. To overcome these limitations, a coarse grained (CG) model is proposed where some of the less-important degrees of freedom are safely integrated out, leaving as key parameters the average energy levels, the molecular conformations and the range of the Mie intermolecular potentials employed as the basis of the model. The parametrization is performed by using an analytical equation of state of the statistical associating fluid theory (SAFT) family to link the potential parameters to macroscopically observed thermophysical properties. The parameters found through this top-down approach are used directly in molecular dynamics simulations of multi-component multi-phase systems. The procedure is exemplified by calculating the phase envelope of the methane-decane binary and of two synthetic light condensate mixtures. A procedure based on a discrete expansion of a mixture is used to determine the bubble points of these latter mixtures, with an excellent agreement to experimental data. The model presented is entirely predictive and an abridged table of parameters for some fluids of interest is provided.

A molecular model for simulating the aggregation of asphaltenes and resins in crude oils on a mesoscale is proposed. The asphaltene molecules are treated as discotic seven-center Lennard-Jonesium molecules, the resins are modeled as single spheres, and the surrounding crude oil is modeled as a continuum, characterized by a screening factor, and defined using a combination of its Hamaker and dielectric constants. The parameters for the model are obtained by coarse-graining the potential energy surface obtained from model atomistic simulations of pairs of asphaltenes and resins. Canonical Monte Carlo simulations are performed with this model, and effects of temperature, asphaltene, and resin concentration are studied parametrically. The results agree with experimentally observed tendencies. The asphaltene is seen not to conform to a linear aggregation model, but exhibits a more complex multimodal aggregation pattern. The screening constant of the crude oil, which ultimately controls the aggregation, can itself be related to other measurable quantities such as the refractive index.

Abstract:A methodology for the determination of the solid-fluid contact angle, to be employed within molecular dynamics (MD) simulations, is developed and systematically applied. The calculation of the contact angle of a fluid drop on a given surface, averaged over an equilibrated MD trajectory, is divided in three main steps: (i) the determination of the fluid molecules that constitute the interface, (ii) the treatment of the interfacial molecules as a point cloud data set to define a geometric surface, using surface meshing techniques to compute the surface normals from the mesh, (iii) the collection and averaging of the interface normals collected from the post-processing of the MD trajectory. The average vector thus found is used to calculate the Cassie contact angle (i.e., the arccosine of the averaged normal z-component). As an example we explore the effect of the size of a drop of water on the observed solid-fluid contact angle. A single coarsegrained bead representing two water molecules and parameterized using the SAFT-γ Mie equation of state (EoS) is employed, meanwhile the solid surfaces are mimicked using integrated potentials. The contact angle is seen to be a strong function of the system size for small nano-droplets. The thermodynamic limit, corresponding to the infinite size (macroscopic) drop is only truly recovered when using an excess of half a million water coarse-grained beads and/or a drop radius of over 26 nm.

Molecularly imprinted polymers (MIPs) offer a unique opportunity to significantly advance volatile organic compound (VOC) sensing technologies and a number of other applications. However, the development of these applications using MIPs has been hindered by poor understanding of the microstructure of MIPs, geometry of binding sites, and the details of molecular recognition processes in these materials. This is further complicated by the vast number of optimization parameters such as building components and processing conditions. Computer simulations and molecular modeling can help us understand adsorption and binding phenomena in MIPs on the molecular level and thus provide a route to more efficient MIP design strategies. So far, molecular models have been either oversimplified or severely limited in length scale, essentially focusing on a single binding site. Here, we propose a more general, atomistically detailed model that describes the microstructure of MIPs. We apply this model to investigate adsorption of pyridine, benzene, and toluene in MIPs and demonstrate that it is able to capture a number of essential experimental features. Therefore, this model can serve as a starting point in computational design and optimization of MIPs.

This work is framed within the Ninth Industrial Fluid Properties Simulation Challenge, with the aim of assessing the capability of molecular simulation methods and force fields to accurately predict the interfacial tension of oil + water mixtures at high temperatures and pressures. The challenge focused on predicting the liquid-liquid interfacial tension of binary mixtures of dodecane + water, toluene + water and a 50:50 (wt%) mixture of dodecane:toluene + water at 1.825 MPa (250 psig) and temperatures from 110 to 170 °C. In our entry for the challenge, we employed coarse-grained intermolecular models parametrized via a top-down technique in which an accurate equation of state is used to link experimentally observed macroscopic properties of fluids with 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 terms of the Mie potential is employed here. Interfacial tensions are calculated through a direct method, where an elongated simulation cell is sampled through molecular dynamics in the isobaric-isothermal constant area ensemble (NP zz AT). The coarse-grained nature of the force field allows for the accelerated calculation of relatively large systems. The binary interaction parameters that describe the crossinteractions have been obtained in previous works by fitting to interfacial tensions of the constituent binaries at lower pressures and temperatures; these are taken as constant for all conditions and mixtures studied. After disclosure of the challenge results, we observe that the interfacial properties of the mixtures are described with an error of less than 5 mN/m over the whole range of conditions, demonstrating the accuracy and transferability of the top-down SAFT-γ Mie force field approach.

A coarse-grained (CG) model for atactic polystyrene is presented and studied with classical molecular-dynamics simulations. The interactions between the CG segments are described by Mie potentials, with parameters obtained from a top-down approach using the SAFT-γ methodology. The model is developed by taking a CG model for linearchain-like backbones with parameters corresponding to those of an alkane and decorating it with side branches with parameters from a force field of toluene, which incorporate an "aromatic-like" nature. The model is validated by comparison with the properties of monodisperse melts, including the effect of temperature and pressure on density, as well as structural properties (the radius of gyration and end-to-end distance as functions of chain length). The model is employed within large-scale simulations that describe the temperature−composition fluid-phase behavior of binary mixtures of polystyrene in n-hexane and n-heptane. A single temperature-independent unlike interaction energy parameter is employed for each solvent to reproduce experimental solubility behavior; this is sufficient for the quantitative prediction of both upper and lower critical solution points and the transition to the characteristic "hourglass" phase behavior for these systems.

An experimental, theoretical, and molecular simulation consolidated framework for the efficient characterization of the adsorption and fluid-phase behavior of multi-component hydrocarbon mixtures within tight shale rocks is presented. Fluid molecules are described by means of a top-down coarse-grained model where simple Mie intermolecular potentials are parametrized by means of the statistical associating fluid theory. A four-component (methane, pentane, decane, naphthalene) mixture is used as a surrogate model with a composition representative of commonly encountered shale oils. Shales are modeled as a hierarchical network of nanoporous slits in contact with a mesoporous region. The rock model is informed by the characterization of four distinct and representative shale core samples through nitrogen adsorption, thermogravimetric analysis, and contact angle measurements. Experimental results suggest the consideration of two types of pore surfaces: a carbonaceous wall representing the kerogen regions of a shale rock, and an oxygenated wall representing the clay-based porosity. Molecular dynamics simulations are performed at constant overall compositions at a temperature of 398.15 K (257 °F) and explore pressures from 6.9 MPa up to 69 MPa (1000–10000 psi). Simulations reveal that it is the organic nanopores of 1 and 2 nm that preferentially adsorb the heavier components, while the oxygenated counterparts show little selectivity between the adsorbed and free fluid. Upon desorption, this trend is intensified, as the fluid phase in equilibrium with a carbon nanopore becomes increasing leaner (richer in light components) and almost completely depleted of the heavy components which remain trapped in the nanopores and surfaces of the mesopores. Oxygenated pores do not contribute to this unusual behavior, even for the very tight pores considered. The results presented elucidate the relative importance of considering both the pore size distribution and the heterogeneous nature of the confining surfaces when theoretically describing adsorption and transport of oil through shale rocks, and they provide a plausible explanation for the abnormal continuous leaning of shale gases seen during field production.