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We investigated the dynamic properties of alkali halide solutions (NaCl, NaF, NaBr, NaI, LiCl, and KCl) using molecular dynamics simulations and several non-polarisable, polarisable, and scaled-charge models. The concentration dependence of shear viscosity was obtained with low statistical uncertainties to allow for calculation of the viscosity Jones-Dole B-coefficients. No prior values are available for the B-coefficients from molecular simulations of fully atomistic models for electrolyte solutions. In addition, we obtained diffusion coefficients with rigorous finite-size corrections to access ion mobilities; these provide insights on single ion hydration behaviour. We find that all models studied, even polarisable and scaled-charge models, quantitatively over-predict water structuring but qualitatively follow the experimentally determined Hofmeister series. All ion models considered are kosmotropes based on their calculated B-coefficient and diffusion coefficients, even for ions experimentally found to be chaotropes. These observations indicate that the water-ion interactions in these models are not adequately represented; additional interactions such as charge transfer must be incorporated in future models in order to better represent electrolyte solution properties. [ABSTRACT FROM AUTHOR]

We report measurements of the vapour–liquid coexistence densities and surface tension of fully flexible Lennard–Jones chain molecules ranging in length from 4 to 60 beads. We demonstrate that the surface tension for all chain lengths collapses to a single master curve when plotted according to the universal parachor correlation. We find a universal parachor exponent 3.79 ± 0.05 for conditions close to the critical point, with a deviation observed for the longest chains far below the critical point. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were employed for the calculation of diffusion coefficients of pure CO2and of H2O in CO2over a wide range of temperatures (298.15 K

We have performed atomistic molecular dynamics simulations coupled with thermodynamic integration to obtain the excess chemical potential and pressure-composition phase diagrams for CO2in poly(ethylene oxide) oligomers. Poly(ethylene oxide) dimethyl ether, CH3O(CH2CH2O)nCH3(PEO for short) is a widely applied physical solvent that forms the major organic constituent of a class of novel nanoparticle-based absorbents. Good predictions were obtained for pressure–composition–density relations for CO2+ PEO oligomers (2 ≤n≤ 12), using the Potoff force field for PEO [J. Chem. Phys. 136, 044514 (2012)] together with the TraPPE model for CO2[AIChE J. 47, 1676 (2001)]. Water effects on Henry’s constant of CO2in PEO have also been investigated. Addition of modest amounts of water in PEO produces a relatively small increase in Henry’s constant. Dependence of the calculated Henry’s constant on the weight percentage of water falls on a temperature-dependent master curve, irrespective of PEO chain length. [ABSTRACT FROM AUTHOR]

We examine the self-organization scenarios of star-shaped AB-block copolymers, consisting of a solvophilic A-block and a solvophobic B-block, in which f such blocks are chemically anchored on a common centre on their A-parts, leaving the B-blocks exposed on their exterior. We employ a lattice model and we perform Grand Canonical Monte Carlo simulations for the case f = 6, varying thereby the percentage of attractive monomers as well as the concentration of stars. In agreement with previous studies on the low-functionality case f = 3 [F. Lo Verso, A.Z. Panagiotopoulos, and C.N. Likos, Phys. Rev. E 79, 010401(R) (2009)], we find that when the majority of monomers in the star are attractive, macrophase separation between two fluid phases at different concentrations of stars occurs below a system-dependent critical temperature. When the majority of monomers is repulsive, novel forms of self-organization arise, which include not only well-defined spherical micelles but also the coexistence of a multiply-connected percolating cluster with smaller micelles having a well-defined size. The morphological characteristics and the sizes of the ensuing aggregates are quantitatively analysed and a critical comparison with the case f = 3 and f = 10 is presented. [ABSTRACT FROM AUTHOR]

Grand canonical Monte Carlo simulations with histogram reweighting are used to examine the micellization and phase separation of a lattice model for binary amphiphile mixtures in an implicit solvent. The model amphiphiles consist of a neutral, rigid head group that occupies seven lattice sites and solvophobic tail chains of varying length and number. It is found that the critical micelle concentration of the mixtures closely matches theoretical values obtained by assuming that the components form an ideal mixture. The solubility of amphiphiles which undergo a first-order transition to form a disordered liquid phase can be increased by adding an amphiphile that forms micelles, and the phase transitions in these mixtures are between a dense liquid state and a low concentration phase that contains aggregates. In both micelles and dense liquids, the amphiphiles mix indiscriminately and the micelle size and shape depends on the concentration ratio of the two species. [ABSTRACT FROM AUTHOR]

A review of simulation studies of phase equilibria and free energies for systems dominated by coulombic interactions is presented. Phase transitions occur for ionic systems in the strong-coupling limit realized in low-dielectric constant solvents, at low temperatures, or for high charge valences. The majority of simulation results to date are for primitive models that treat the solvent as a uniform dielectric continuum. Transitions involving fluid and solid phases for such models have been studied extensively in the past decade. There is now strong evidence that the vapour-liquid transition is in the Ising universality class. For highly charged colloids the vapour-liquid transition becomes metastable with respect to the fluid-solid transition and the behaviour matches that of charged hard plates. Phase transitions of charged chains illustrate sensitivity of the phase behaviour to the charge pattern. Studies of salt solubilities using models with explicit solvent suggest that reasonable agreement with experiment can be achieved with existing force fields, but there is considerable room for improvement. Areas of future research needs are briefly discussed. [ABSTRACT FROM AUTHOR]

The phase behaviour of polyampholyte chains has been investigated by grand canonical Monte Carlo simulations. First, the effects of chain length and charge sequence on the critical parameters and phase diagrams of fully-charged but overall neutral polyampholytes up to a chain length of 16 beads were determined. As the chain length increases, the critical temperature increases while the critical volume fraction decreases. The critical temperatures of the diblock polyampholytes are much higher and increase more quickly with increasing chain length than the critical temperatures of random polyampholytes, consistent with current experimental observations and theoretical predictions. This difference is attributed to the localization of like charges in a diblock chain which leads to stronger attraction and aggregation between chains. Cluster size distributions also reveal a stronger tendency of diblock polyampholytes to form larger aggregates than random chains for the same conditions. The phase behaviour of end-charged polyampholytes up to a chain length of 18 beads was also investigated, and both the critical temperature and volume fraction were observed to decrease with increasing chain length. The formation of micelles was not observed up to the longest chain length considered. Instead, the chains form clusters which eventually aggregate into a percolating gel-like structure through chain bridges spanning across different clusters. The sol–gel density–temperature locus has also been determined for the longest chain length considered, and the vapour–liquid transition is found to be between two gel phases. [ABSTRACT FROM AUTHOR]

We modify Larson's lattice model [Larson, R.G., Scriven, L.E. and Davis, H.T. (1985). J. Chem. Phys., 83, 2411-2420] and use it to study formation of spherical micelles in a supercritical solvent by large-scale Monte Carlo (MC) simulations and by the multicomponent solution model. Carbon dioxide and perfluoroalkylpoly(ethylene oxide) serve as prototypes for the solvent and surfactant, respectively. Larson-model type parameters for carbon dioxide and perfluoroalkylpoly(ethylene oxide) are obtained using experimental values of critical parameters and solubility along with a modified Berthelot combining rule. We perform canonical MC simulations at a supercritical temperature and low surfactant concentrations, varying the number of surfactant head and tail segments and the solvent density. Various properties such as the critical micelle concentration, the aggregate size distribution and the size of the micelles is evaluated. The multicomponent solution model and the simulation results for the aggregate size distribution are then combined to determine the standard state chemical potential for the spherical micelles and the intermicellar interaction; we present a novel approach to model this standard state chemical potential. The implications of these results for the thermodynamics of the formation of the spherical micelles in supercritical solvents are explored. [ABSTRACT FROM AUTHOR]

The micellization properties and phase behaviour of model ternary surfactant–oil–water systems were studied by grand canonical Monte Carlo simulations assisted by histogram reweighting techniques. Larson's lattice model was used for a symmetrical H[sub 4]T[sub 4 ]surfactant, where H represents hydrophilic and T hydrophobic groups. In contrast to earlier studies, oil chains of variable length, from T to T[sub 4], were studied and the effect of added oil on the critical micelle concentration (cmc) was determined quantitatively. It was found that the cmc decreases as the volume fraction of oil in the aqueous phase is increased. Longer oil chains have a stronger effect than shorter chains for the same volume fraction. The oil, tail and head density profiles were obtained for typical micelles. Simulation results for the density profiles indicate that long chain oils are almost exclusively located within the micellar aggregates, in agreement with single-chain mean-field theory predictions. Ternary phase diagrams were obtained both by simulations and by quasichemical theory. The results suggest that the critical micelle concentration line is directed towards the oil-lean corner of the three-phase triangle, if there is one present in the system. It was also found that the amount of oil that can be dissolved in the ternary system increases sharply at the surfactant critical micelle concentration, indicating that micellization greatly enhances oil solubility. [ABSTRACT FROM AUTHOR]

A methodology is presented for Monte Carlo simulation of fluids in a new ensemble that can be used to obtain phase coexistence properties of multicomponent systems from a single computer experiment. The method is based on performing a simulation simultaneously in two distinct physical regions of generally different densities and compositions. Three types of perturbations are performed, a random displacement of molecules that ensures equilibrium within each region, an equal and opposite change in the volume of the two regions that results in equality of pressures, and random transfers of molecules that equalize the chemical potentials of each component in the two regions. The method is applied to the calculation of the liquid-gas coexistence envelope for the pure Lennard-Jones (6, 12) fluid for several reduced temperatures from the vicinity of the triple point to close to the critical point (T = 0.75 to T = 1.30). Good overall agreement with previously available literature results is obtained, with some deviations at the extremes of this temperature range. [ABSTRACT FROM AUTHOR]

Vapour-liquid coexistence densities near the critical point are reported for polarizable water models, determined by a series of grand canonical Monte Carlo simulations with the histogram reweighting method. Previously proposed point charge models rescaled to match the experimental permanent dipole moment in the gas phase were used. Isotropic polarizability was added to account for three-body and higher order interactions in dense phases. The coexistence density, vapour pressure and heat of vaporization of the models were calculated. The models used in this study, including a recently introduced polarizable model that gives good agreement with the experimental structure, do not give quantitative agreement with the experimental coexistence properties. The potential parameters of the models were varied in an attempt to improve agreement with experimental values for the coexistence properties. [ABSTRACT FROM AUTHOR]

Valleau's thermodynamic scaling Monte Carlo conceptis combined with the Gibbs ensemble method for phase coexistence calculations, and phase co-existence properties of several Hamiltonians (potential models) at many temperatures are obtained in a single simulation. The theoretical formulation of the method is presented in detail. An application to the (n - 6) Lennard-Jones family of potentials is demonstrated. The combined method is found to be more efficient than the conventional Gibbs ensemble method when properties of related potential models at many thermodynamic states are of interest. Possible extensions of the combined methodology to mixtures, intermolecular potential model development, and multisegment systems that require configurational-bias sampling are discussed. [ABSTRACT FROM AUTHOR]