Computer simulation of polymer networks: swelling by binary Lennard-Jones mixtures

The swelling of regular, tightly meshed model networks is investigated by a molecular-dynamics-Monte Carlo hybrid technique. The chemical equilibrium between two simulation boxes representing the gel phase and a solvent bath, respectively, is obtained by subjecting the Lennard-Jones particles of a binary mixture, serving as explicit solvent, to the particle transfer step of Gibbs ensemble-Monte Carlo. The swelling behavior, especially preferential absorption of a single component, whose dependence on temperature, pressure, and fluid composition is studied, also depends significantly on the size of the central simulation cell. These finite-size effects correlate well with those exhibited by the density of solvent-free (dry) networks. A theoretical expression, whose derivation is based on network elasticity (of dry networks) yields finite-size scaling behavior in good accord with simulation results for both dry networks and gels in contact with solvent baths. This expression can be used to extrapolate the swelling behavior of simulated finite systems to infinite system size.     

Physiochemical properties of condensed phosphoric acid mixtures

The condensational growth of phosphorus acid is measured as a function of relative humidity and aging time.     

Computer simulation studies of anisotropic systems XIV. Binary mixtures of liquid crystals

We have simulated the behaviour of a model, binary mixture of nematogens composed of cylindrically symmetric particles using the Monte Carlo technique. The characteristics of the model mixture were chosen to be in accord with most of the assumptions made in the Humphries-James-Luckhurst theory of liquid crystalline mixtures. The results of the simulation experiments allow us to test, for the first time, the validity of the molecular field approximation in this theory. In addition to the second rank long range orientational order parameters for both components of the mixture we have also determined certain orientational pair correlation functions. These enable us to investigate the ability of one component to enhance the order of the other component or the same component in its vicinity.     

Adsorption of gas mixtures on solid surfaces,theory and computer simulation

A treatment of the thermodynamics of mixed gas adsorption is presented in which the gas-solid interface is three dimensional. Such a treatment yields an additional term as compared to two dimensional approaches. This additional term has significant consequences for the derivation of adsorbed solution theories, particularly at higher temperatures.Results are presented for a Grand Canonical Monte Carlo study of a model methane-ethane mixture in a carbonaceous slit pore. Comparison of single component and mixture results provides an unambiguous means of testing theories of adsorbed solutions and bears out the thermodynamic treatment presented in the previous section of the paper.     

Computer simulation of the dartmouth process for separation of dilute Ethanol/Water mixtures

High energy costs are associated with the recovery of ethanol from fermentation broths. This paper discusses a computer simulation of the Dartmouth Process, which aims to reduce these costs by the use of IHOSR distillation, extensive heat integration, and extractive distillation using a salt. To resolve the uncertainty in modeling alcohol-water-salt vapor-liquid equilibrium, a new and more accurate activity coefficient model was used. An Aspen™ model was used to generate capital and energy costs for a range of ethanol concentrations in the feed. Simulation results show that the Dartmouth Process offers substantial economic advantages over benzene azeotropic distillation, particularly at low feed concentrations.     

Mobilities of mixtures of ion isotopes in gas mixtures

It is shown how the mobility of a mixture of isotopes of the same ion species in a gas mixture can be obtained directly from mobility data of a single isotope of the ion and one isotope of each neutral species, for any gas temperature and ratio of the electrostatic field strength to the gas number density. This combination of the “aliasing” procedure for ion isotopes, Blanc’s Law for gas mixtures at low fields, and the extended Blanc’s Law at intermediate and high fields is tested by comparing values calculated from the mobilities of ⁷⁹Br^? in ²⁰Ne with those determined both experimentally and ab initio for various mixtures. It is so accurate that it obviates the necessity in future work of making measurements or calculations for more than one isotope of each ion and each neutral.     

Self-association of carbon tetrachloride in gas and condensed phase

The paper suggests the model of carbon tetrachloride self-association in gas and condensed phases, based on comparative analysis of IR findings for carbon-, silicon-, and germanium tetrachloride, obtained at different temperatures in gas, as well as in liquid, solid state, and noble gas solutions.

     

Ignition of gas mixtures containing natural gas and oxygen

Gases released during the thermal treatment of a coal-gas suspension exhibit a strong inhibiting effect on the self-ignition of natural gas but have a minor influence on the combustion of hydrogen.     

Diffusion in mercury-argon gas mixtures

Ab initio quantum-mechanical calculations of potential of interactions U(R) are performed for Hg-Ar. Using equations from the molecular kinetic theory of rarefied gases, a new statistical correlation is found between data on the potential of interaction (molecular beams, molecular spectroscopy, and potential U(R)) and experimental data on the mutual diffusion coefficient (MDC) of mercury-argon gas mixtures. Calculated reference data on the MDC of mercury-argon gas mixtures in the temperature range of 300 to 600 K are offered as a possible standard for calibrating instruments that measure MDCs of liquid vapors and inert gases using the Stefan method.     

Ionization coefficients in gas mixtures

We have tested the application of the common $E/N$ (E—electric field, N—gas number density) or Wieland approximation [Van Brunt, R.J., 1987. Common parametrizations of electron transport, collision cross section, and dielectric strength data for binary gas mixtures. J. Appl. Phys. 61 (5), 1773–1787.] and the common mean energy (CME) combination of the data for pure gases to obtain ionization coefficients for mixtures. Test calculations were made for $\mathrm{Ar}–{\mathrm{CH}}_4$, $\mathrm{Ar}–{\mathrm{N}}_2$, He–Xe and ${\mathrm{CH}}_4–{\mathrm{N}}_2$ mixtures. Standard combination procedure gives poor results in general, due to the fact that the electron energy distribution is considerably different in mixtures and in individual gases at the same values of $E/N$. The CME method may be used for mixtures of gases with ionization coefficients that do not differ by more than two orders of magnitude which is better than any other technique that was proposed [Marić, D., Radmilović-Rađenović, M., Petrović, Z.Lj., 2005. On parametrization and mixture laws for electron ionization coefficients. Eur. Phys. J. D 35, 313–321.].     

Computer simulation studies of surfactant monolayer mixtures at the water/oil interface: charge distribution effects

Charge distribution effects on polar head groups for a mixture of amphiphilic molecules at the water/oil interface were studied. For this purpose a model which allowed us to investigate the charge effects exclusively was created. As a molecular model we used the structure of sodium dodecyl sulfate. Then we prepared molecules with the same molecular structure but with different charge distributions in order to have one cationic and one nonionic molecule. So, in this way, we were able to focus only in the charge effects. The monolayer mixtures were composed of anionic/nonionic and cationic/nonionic surfactants. Simulations of these systems show that the location of the different surfactants at the interface is determined by the interaction and the charge distribution of the molecules. Due to the difference in the charge distribution of the surfactant monolayers, the water molecules present distinct orientations in the mixture. Finally, it was found that the electrostatic potential difference across the interface depended on the interactions (charge distribution) of the anionic, cationic, and nonionic molecules in the mixture.     

Computer simulation of liquid crystals

Summary We review recent progress in the computer simulation of liquid crystals, with special emphasis on hard particle models. Surprisingly, the simplest molecular models, taking account only of molecular size and shape, are sufficient to generate a wide variety of liquid crystalline phases, closely analogous to those observed in real life. Thermodynamic stability of different phases is very sensitive to shape, and presumably will also be sensitive to further details of intermolecular interactions as they are incorporated into the model. Realistic atom-atom potential models of liquid crystals are available, but the associated simulations are quite expensive. Thus, while idealized models may be used to study quite general, fundamental properties of mesophases, the modelling of specific liquid crystal systems in a realistic way remains a great challenge. Progress continues to be made on both these fronts.     

Computer simulation of cluster assembling

Density functional theory is used to investigate the assembling of metallic clusters to yield stable or metastable cluster solids. Motivated by the observed high stability of the Al₁₃H cluster, which has a substantial highest occupied and lowest unoccupied molecular orbitals (HOMO–LUMO) gap, we have modeled the assembling of those clusters. For a favorable relative orientation of each cluster with respect to all its neighbors, a cluster solid is predicted and its structure appears to be stable at least up to 150 K, which is the highest temperature in our simulations. We have also studied the chemical bonding in the stoichiometric solid alloys PbA, where A is one of the alkali elements Na, K, Rb, or Cs. Those crystals exist in an ordered phase formed by tetrahedral Pb₄ clusters surrounded by the alkali atoms. The study of this family of natural cluster compounds reveals the coating role played by the cations, providing further insight into the favorable conditions required for the formation of cluster solids. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Computer simulation of polyolefin interfaces

Interfaces, thin films and amorphous cells of several hydrocarbon polymers are studied with atomistic computer simulation. Solubility parameters, surface tensions and interfacial tensions are evaluated from the computed structures. These structures are constructed using Molecular Dynamics and Molecular Mechanics techniques. The various components of the energetic interactions (torsional, vdW, etc.) are examined in detail in order to gain an insight into the nature of the regular and anomalous interactions between these films.The polymers studied in this article are all are atactic linear hydrocarbons and have approximately 200 carbons in the backbone chain. These include polypropylene (PP), head to head polypropylene (hhPP), a polymer which is termed P78 (a random copolymer composed of 22% of a linear (–CH₂CH₂CH₂CH₂–) monomer and 78% of a branched (–CH₂CH(C₂H₅)–) monomer), polyethylenepropylene (PEP), and PE2P2 (an alternating copolymer consisting of two ethylene (E) and two propylene (P) monomers in the repeating pattern (–(EEPP)_n–)). The primary difference between the different polymers studied is the type of side group (ethyl or methyl) and the location, frequency and spacing of that group.Interfaces formed with the same polymer type (PP–PP, PEP–PEP, P78–P78) yield a surface tension close to zero as expected. Attractive interactions between some of the pairs of films is found in the simulation. The distribution among the energetic terms seems to indicate that this attractive interaction is brought about by favorable local intramolecular energetic terms rather than simply by van der Waals interactions alone.