Description of self-diffusion coefficients of gases,liquids and fluids at high pressure based on molecular simulation data

The purpose of this work is to evaluate the potential of modeling the self-diffusion coefficient (SDC) of real fluids in all fluid states based on Lennard–Jones analytical relationships involving the SDC, the temperature, the density and the pressure. For that, we generated an equation of state (EOS) that interrelates the self-diffusion coefficient, the temperature and the density of the Lennard–Jones (LJ) fluid. We fit the parameters of such LJ–SDC–EOS using recent wide ranging molecular simulation data for the LJ fluid. We also used in this work a LJ pressure–density–temperature EOS that we combined with the LJ–SDC–EOS to make possible the calculation of LJ–SDC values from given temperature and pressure. Both EOSs are written in terms of LJ dimensionless variables, which are defined in terms of the LJ parameters ɛ and σ. These parameters are meaningful at molecular level. By combining both EOSs, we generated LJ corresponding states charts which make possible to conclude that the LJ fluid captures the observed behavioral patterns of the self-diffusion coefficient of real fluids over a wide range of conditions. In this work, we also performed predictions of the SDC of real fluids in all fluid states. For that, we assumed that a given real fluid behaves as a Lennard–Jones fluid which exactly matches the experimental critical temperature T_c and the experimental critical pressure P_c of the real fluid. Such an assumption implies average true prediction errors of the order of 10% for vapors, light supercritical fluids, some dense supercritical fluids and some liquids. These results make possible to conclude that it is worthwhile to use the LJ fluid reference as a basis to model the self-diffusion coefficient of real fluids, over a wide range of conditions, without resorting to non-LJ correlations for the density–temperature–pressure relationship. The database considered here contains more than 1000 experimental data points. The database practical reduced temperature range is from 0.53 to 2.4, and the practical reduced pressure range is from 0 to 68.4.     

On the properties of methylbenzoate/n-hexane mixed solvents: a theoretical and experimental study

This paper reports on an experimental and theoretical study of methylbenzoate/n-hexane mixed solvents as a function of pressure and temperature in the whole composition range. We have measured the pressure-volume-temperature (PVT) behavior of these fluids over wide temperature and pressure ranges; from the experimental data, relevant derived coefficients required for the fluid's characterization were calculated. The structure of mixed fluids was analyzed from macroscopic data according to excess and mixing properties. The statistical associating fluid theory (SAFT) and perturbed chain (PC)-SAFT molecularly based equations of state were used to predict the PVT behavior with model parameters for pure fluids fitted from correlation of available saturation literature data. The results provided by the PC-SAFT equation of state were clearly superior. Using the fitted PC-SAFT parameters, the global phase behavior of the mixture was predicted, and a type I pattern was inferred according to the van Konynenburg systematic. The molecular level structure was studied through classical molecular dynamics simulations in the NPT ensemble using the optimized potential for liquid simulations (all atom version) (OPLS-AA) force field. Molecular dynamics provides, on one hand, theoretical values of thermophysical properties, which are compared with the experimental ones to check the quality of simulations, and, on the other hand, valuable molecular level structural and dynamic information. Based on both macroscopic and microscopic studies, fluid structure was inferred.     

A stepwise approximation for modeling of the wall-fluid potential of a mesoscopic pore

A common computational method for the characterization of porous materials is to calculate the adsorption isotherm of fluids in the materials from the pre-assumed wall-fluid potential. If the wall-fluid potential is unknown, the common computational method becomes invalid. In a realistic experiment, however, it is common to know the experimental adsorption isotherm of nitrogen and not to know the wall-fluid potential. Here we propose a stepwise approximation for modeling wall-fluid potential under conditions where only the adsorption isotherm of nitrogen is measured experimentally. Based on the modeled wall-fluid potential, we can characterize the porous materials and predict the adsorption of other adsorbates on the materials. It is expected that the approach would provide a powerful means for the characterization of novel materials under conditions where only the experimental adsorption isotherm is available.     

Water PMF for predicting the properties of water molecules in protein binding site

Water is an important component in living systems and deserves better understanding in chemistry and biology. However, due to the difficulty of investigating the water functions in protein structures, it is usually ignored in computational modeling, especially in the field of computer‐aided drug design. Here, using the potential of mean forces (PMFs) approach, we constructed a water PMF (wPMF) based on 3946 non‐redundant high resolution crystal structures. The extracted wPMF potential was first used to investigate the structure pattern of water and analyze the residue hydrophilicity. Then, the relationship between wPMF score and the B factor value of crystal waters was studied. It was found that wPMF agrees well with some previously reported experimental observations. In addition, the wPMF score was also tested in parallel with 3D‐RISM to measure the ability of retrieving experimentally observed waters, and showed comparable performance but with much less computational cost. In the end, we proposed a grid‐based clustering scheme together with a distance weighted wPMF score to further extend wPMF to predict the potential hydration sites of protein structure. From the test, this approach can predict the hydration site at the accuracy about 80% when the calculated score lower than ?4.0. It also allows the assessment of whether or not a given water molecule should be targeted for displacement in ligand design. Overall, the wPMF presented here provides an optional solution to many water related computational modeling problems, some of which can be highly valuable as part of a rational drug design strategy. © 2012 Wiley Periodicals, Inc.     

A discrete complex compliance spectra model of the nonlinear viscoelastic creep and recovery of microcellular polymers

The present work reports a discrete stress‐dependent, complex compliance spectra method that may be used to predict the mechanical response of nonlinear viscoelastic polymers during creep and recovery processes. The method is based on the observation that the real and imaginary parts of a discrete complex compliance frequency spectra obtained from creep and recovery measurements are smooth, easily fit functions of stress. The new method is applied to a set of microcellular polycarbonate materials with differing relative density. The nonlinear viscoelastic characteristics of a microcellular polycarbonate material system are very sensitive to relative density and therefore, this material system is a particularly difficult modeling challenge. However, the present model was able to exhibit excellent quantitative agreement with the basis creep and recovery measurements at all experimental stress levels for each of the experimental relative density material types. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 691–697, 2000     

The environmental stress cracking of linear ethylene copolymers

A new method of measuring the environmental stress cracking (ESC) of polymers at constant strain is proposed in which an occurrence of fracture is detected automatically. The new method showed a very good reproducibility within 10% compared with a few hundred percent by conventional methods. The ESC values obtained by the new method was found to be proportional to those by the conventional Bell Telephone Laboratory method in which the ESC was determined by the occurrence of small cracks, with a proportionality contant of 2.8. From the fact that the difference of both ESCs were also proportional to the ESC determined by the BTL method, it was concluded that the value of the ESC was proportional to the velocity of crack propagation. These conclusions were supported by the observation of the test pieces by a scanning electron microscope. The study of the blended polymers revealed that the additivity of logarithmical ESC with weight fraction holds for a wide range of polymes, which enables estimation of ESCs up to millions of hours. Using this technique, the ESCs of a wide range of molecular weights and a number of short chain branches were studied. It was found that the branches in the high molecular weight polymer were much more effective on the ESC than those in the low molecular weight polymer. This makes it possible to design a good resin with a good ESC.     

Experimental investigation and modeling of the mechanical behavior of PC/ABS during monotonic and cyclic loading

The purpose of this work is to characterize the mechanical behavior of blends of polycarbonate (PC) and acrylonitrile-butadiene-styrene (ABS) during monotonic and cyclic loading. Compression experiments were performed using a SHIMADZU universal testing machine (10⁻⁴ to 10⁻² s⁻¹) and a split Hopkinson pressure bar (1600–5000 s⁻¹), with, the test temperatures ranging from 293 to 353 K. The influence of the rate and temperature on the deformation of PC/ABS is discussed in detail. Based on the investigation of numerous constitutive models, a phenomenological model called DSGZ was chosen to describe the compression behavior of PC/ABS. This model could not accurately reproduce the deformation of polymers at high strain rates when utilizing the same material coefficients for the low and high strain–rate deformations. In addition, this model was unable to capture the deformation features during unloading and subsequent reloading when adopting the original stress–strain updating algorithm. Hence, some improvements to the model have been implemented to better predict the deformation. Finally, the model predictions are shown to be consistent with the experimental results.     

Quantum-chemical calculations of the enthalpies and entropies of gas-phase reactions possible in chemical vapor deposition of Group III-Group V element binary compounds

The results of quantum-chemical modeling of possible gas-phase reactions in chemical vapor deposition of Group III-Group V element binary compounds are generalized. Modern computational methods are shown to be capable of obtaining the thermodynamic characteristics of gas-phase reactions in agreement with experimental data. Quantum-chemical methods can be used to estimate the possibility and determine the temperature conditions of formation of oligomeric intermediates during deposition, predict the feasibility of synthesizing new rod-like inorganic polymers, and develop the concept of precursors for the controlled synthesis of III–V composites.     

Improvement of environmental stress cracking resistance of polycarbonate by silicone coating

To explore the possibility of using surface coating to reduce environmental stress cracking (ESC) of transparent polycarbonate (PC) parts, silicone coated and SiO₂ coated PC were tested in a self-made three-point bending apparatus in the presence of ethanol. The variation of stress with time was recorded, and the surface cracking was observed to evaluate the ESC resistance of samples. Slower stress relaxation rates and fewer surface cracks indicated that silicone coating improved the ESC resistance of PC, but SiO₂ coated PC was found to be no better than that of uncoated PC. Silicone coating reduced the absorption of ethanol in PC, weakening the surface plasticization, thus hindering the formation and development of cracks in PC. Nanoindentation test results showed that the mechanical properties such as hardness and elastic modulus of silicone coating are a better match for PC than SiO₂ coating. This allows the silicone coating to have a favorable effect in providing continuous protection for PC under the combined action of ethanol and stress.     

Insights into Tris‐(2‐Hydroxylethyl)methylammonium Methylsulfate Aqueous Solutions

We report herein a combined experimental–computational study on tris‐(2‐hydroxylethyl)methylammonium methylsulfate in water solutions, as a representative ionic liquid of the aqueous‐solution behavior of hydroxylammonium‐based ionic liquids. Relevant thermophysical properties were measured as a function of mixture composition and temperature. Classical molecular dynamics simulations were performed to infer microscopic structural features. The reported results for ionic liquid in water‐rich solutions show that it behaves as isolated non‐interacting ions solvated by water molecules, through well‐defined solvation shells, exerting a disrupting effect on the water hydrogen bonding network. Nevertheless, as ionic liquid concentration increase, interionic association increases, even for diluted water solutions, evolving from the typical behavior of strong electrolytes in solution toward large interacting structures. For ionic‐liquid‐rich mixtures, water exerts a minor disrupting effect on the fluid’s structuring because it occupies regions around each ion (developing water–ion hydrogen bonds) but without significantly weakening anion–cation interactions.     

Coherent thermodynamic model for solid,liquid and gas phases of elements and simple compounds in wide ranges of pressure and temperature

Thermodynamic modeling of fluids (liquids and gases) uses mostly series expansions which diverge at low temperatures and do not fit to the behavior of metastable quenched fluids (amorphous, glass like solids). These divergences are removed in the present approach by the use of reasonable forms for the “cold” potential energy and for the thermal pressure of the fluid system. Both terms are related to the potential energy and to the thermal pressure of the crystalline phase in a coherent way, which leads to simpler and non diverging series expansions for the thermal pressure and thermal energy of the fluid system. Data for solid and fluid argon are used to illustrate the potential of the present approach.     

Crazing of glassy polymers in alcohols and hydrocarbons

In the crazing of glassy amorphous polymers, wetting ability and penetration of the fluid are the important practical parameters governing the activity of the fluid. Higher molecular weight and the presence of polar groups in the fluids result in an increase in the critical stress for craze initiation in polystyrene and polycarbonate. The Eyring treatment of the craze process can describe fairly well the temperature and strain rate dependence of the critical stress. The parameters involved in the Eyring theory suggest that the crazing takes place by a molecular motion of lower energy than does macroscopic yielding.     

Polymer modeling: where has it been and where is it going?

The development of the area of polymer modeling often referred to as molecular modeling has been reviewed from its early beginnings to the present day. Key forces influencing the development include computational power, algorithmic advances and access to computational resources. The desire to apply modeling techniques to predict the properties of increasingly complex polymer-containing systems, taken in conjunction with a number of current limitations discussed in this brief review, is expected to define in part some essential future developments.     

The influence of sub-Tg annealing on environmental stress cracking resistance of polycarbonate

To explore the effect of physical aging on environmental stress cracking (ESC) behavior of polycarbonate (PC), sub-T_g annealing was utilized as a method for accelerated aging. Injection molded samples were annealed at 130 °C for different time varying up to 96 h. A three point bending apparatus was used to evaluate critical stress for crazing and to record the variation of stress with immersion time at constant strain. The ESC results indicated that the critical stress for crazing initiation of PC in ethanol is increased by sub-T_g annealing. However, the resistance of annealed PC to ESC with immersion time during the stress relaxation test depends on the level of initial stress. When a relatively low initial stress was used, a short time (24 h) of sub-T_g annealing reduced the stress relaxation rate and decreased the number of cracks on the surface of PC. However, under higher initial stress, the stress relaxation rate of PC had a slight change only when the annealing time was prolonged about threefold (72 h). This can be explained by the formation of cohesional entanglement sites during the sub-T_g annealing process, which was demonstrated by the thermal and dynamic mechanical tests.     

Computational electrochemo-fluid dynamics modeling in a uranium electrowinning cell

A computational electrochemo-fluid dynamics model has been developed to describe the electrowinning behavior in an electrolyte stream through a planar electrode cell system. Electrode reaction of the uranium electrowinning process from a molten-salt electrolyte stream was modeled to illustrate the details of the flow-assisted mass transport of ions to the cathode. This modeling approach makes it possible to represent variations of the convective diffusion limited current density by taking into account the concentration profile at the electrode surface as a function of the flow characteristics and applied current density in a commercially available computational fluid dynamics platform. It was possible to predict the conventional current–voltage relation in addition to details of electrolyte fluid dynamics and electrochemical variables, such as the flow field, species concentrations, potential, and current distributions throughout the galvanostatic electrolysis cell.     

A new perspective on the coarse-grained dynamics of fluids

A computational methodology is presented that is designed to model, at a coarse-grained level, the mesoscale dynamics of fluids and potentially other forms of soft matter. Within a molecular dynamics simulation, "ghost" particles of a specific size, corresponding to the fundamental length-scale of coarse-graining, are used as micro-probes designed to respond to local mesoscale fluid flows and stress gradients. A subsequent coarse-grained model is then developed that incorporates both the coarse-grained mesoscale dynamics and isothermal compressibility of the original microscopic system. The method is applied to water and methanol. A contrast with dissipative particle dynamics (DPD) is also presented.     

Cubic equation-of-state correlation of the solubility of some anti-inflammatory drugs in supercritical carbon dioxide

The accurate experimental determination of solid drug solubility in supercritical fluids (SCFs) and its correlation are crucially important to the development of supercritical technologies for the pharmaceutical industry. In this work, the solubilities of flurbiprofen, ketoprofen, naproxen and ibuprofen in supercritical carbon dioxide (scCO₂) were correlated using the Peng–Robinson (PR), Soave–Redlich–Kwong (SRK) and Patel–Teja–Valderrama (PTV) equations-of-state with van der Waals (vdW), Panagiotopoulos–Reid (mrPR) and Mukhopadhyay–Rao (MR) mixing rules. Several modeling and correlation computational programs were developed in Mathematica^®, a powerful symbolic computational language. Correlations were compared and discussed on the basis of the employed equation-of-state and mixing/combining rules. The importance of the estimation methods used for the determination of critical and other drug physical properties (namely drug's sublimation pressure) was also underlined in the discussion of the correlation results quality. A potential helpful procedure (based on simultaneous correlation of one of the critical or physicochemical properties) is suggested, in order to satisfactorily choose property estimation methods to use.     

Simple yield stress fluids

Recent years have seen a flurry of research on yield stress fluids, approached from different perspectives (physicochemical, rheological and fluid mechanical), considering different length scales and timescales, using a range of tools: experimental, computational and analytical. In this context, it has been common to denigrate the simple models of yield stress fluids for their various acknowledged deficiencies and proffer improved models. Here, we push back, in defence of the century-old simplicity, exploring what is new and useful and what these models have given in the past few decades.