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We present the second part of a two-part paper series intended to address a gap in computational capability for coarse-grain particle modeling and simulation, namely, the simulation of phenomena in which diffusion via mass transfer is a contributing mechanism. In part 1, we presented a formulation of a dissipative particle dynamics method to simulate interparticle mass transfer, termed generalized energy-conserving dissipative particle dynamics with mass transfer (GenDPDE-M). In the GenDPDE-M method, the mass of each mesoparticle remains constant following the interparticle mass exchange. In part 2 of this series, further verification and demonstrations of the GenDPDE-M method are presented for mesoparticles with embedded binary mixtures using the ideal gas (IG) and van der Waals (vdW) equation-of-state (EoS). The targeted readership of part 2 is toward practitioners, where applications and practical considerations for implementing the GenDPDE-M method are presented and discussed, including a numerical discretisztion algorithm for the equations-of-motion. The GenDPDE-M method is verified by reproducing the particle distributions predicted by Monte Carlo simulations for the IG and vdW fluids, along with several demonstrations under both equilibrium and non-equilibrium conditions. GenDPDE-M can be generally applied to multi-component mixtures and to other fundamental EoS, such as the Lennard-Jones or Exponential-6 models, as well as to more advanced EoS models such as Statistical Associating Fluid Theory.

An extension of the generalized energy-conserving dissipative particle dynamics method (GenDPDE) that allows mass transfer between mesoparticles via a diffusion process is presented. By considering the concept of the mesoparticles as

Water in different physical states has been attracting great interest for decades for its strong association with the origin of life, the biophysical behaviors of biomacromolecules, and the transpo...

We present a generalised, energy-conserving dissipative particle dynamics (DPDE) method appropriate for the non-isothermal simulation of particle interaction force fields that are both density- and temperature-dependent. A detailed derivation is formulated in a bottom-up manner by considering the thermodynamics of small systems with the appropriate consideration of the fluctuations. Connected to the local volume is a local density and corresponding local pressure, which is determined from an equation-of-state based force field that depends also on a particle temperature. Compared to the original DPDE method, the formulation of the generalised DPDE method requires a change in the independent variable from the particle internal energy to the particle entropy. As part of the re-formulation, the terms dressed particle entropy and the corresponding dressed particle temperature are introduced, which depict the many-body contributions in the local volume. The generalised DPDE method has similarities to the energy form of the smoothed dissipative particle dynamics method, yet fundamental differences exist, which are described in the manuscript. The basic dynamic equations are presented along with practical considerations for implementing the generalised DPDE method, including a numerical integration scheme based on the Shardlow-like splitting algorithm. Demonstrations and validation tests are performed using analytical equation-of-states for the van der Waals and Lennard-Jones fluids. Particle probability distributions are analysed, where excellent agreement with theoretical estimates is demonstrated. As further validation of the generalised DPDE method, both equilibrium and non-equilibrium simulation scenarios are considered, including adiabatic flash heating response and vapour-liquid phase separation.

An angular momentum conservative pure bulk viscosity term for smoothed particle hydrodynamics (SPH) is proposed in the present paper. This formulation permits independent modeling of shear and bulk viscosities, which is of paramount importance for fluids with large bulk viscosity in situations where sound waves or large Mach numbers are expected. With this aim a dissipative term proportional to the rate of change of the volume is considered at the particle level. The equations of motion are derived from the minimization of a Lagrangian combined with an appropriate dissipation function that depends on this rate of change of particle volume, in analogy with the corresponding entropy production contribution in fluids. Due to the Galilean invariance of the formulation, the new term is shown to exactly conserve linear momentum. Moreover, its invariance under solid-body rotations also ensures the conservation of angular momentum. Two verification cases are proposed: the one-dimensional propagation of a sound pulse and a two-dimensional case, modeling the time decay of an accelerating-decelerating pipe flow. The SPH solutions are compared to exact ones, showing that the newly proposed term behaves indeed as a viscosity associated only with the local expansion-compression of the fluid. In view of these considerations, we conclude that the method presented in this paper allows for setting up a bulk viscosity independently of the shear one and as large as any particular problem may require. At the same time, together with the prescribed momentum conservation to reproduce the Navier-Stokes equation, the new term also keeps the angular momentum conservation required to properly model free interfaces or overall rotations of the bulk fluid.

A combined experimental and theoretical study of the phase behavior of mixtures of gelatin and maltodextrin is presented. This system, at neutral pH and 50C, exhibits coexistence of three equilibrium phases: two liquid phases and a third, which is a solid-like precipitate that resembles a coacervate. This phase behavior completely changes at lower pH, where only two phases remain. Flory-Huggins solution theory is used to model the observed phase behavior. The optimal fits of the associated Flory-Huggins mixing parameters are obtained using a novel non-linear regression method based on neural networks. The neural network was trained using as input only the number of phases present, without knowledge of the specific compositions of the coexisting phases. The optimal Flory-Huggins parameters result in phase diagrams which are consistent with our experimental data. A comparison with the experimental results indicates that the method can produce relevant fits of the model parameters in situations where the information is limited. Moreover, the formation of the solid-like precipitate can be explained within the standard Flory-Huggins framework, despite the fact that important local and specific interactions or induced structural changes occur in the system to produce such a precipitate. In conclusion, the model obtained for this system allows a straightforward exploration, prior to the experimental validation, of the appropriate conditions for future practical applications of this system., The authors acknowledge the Spanish Ministry of Science and Innovation, the Agencia Estatal de Investigación and the European Regional Development Fund for CTQ2014-52687-C3-1-P and CTQ2017-84998-P projects, the Generalitat de Catalunya (2017 SGR 1610) and the University Rovira i Virgili (2018PFR-URV-B2-52). F.M. also acknowledges the Universitat Rovira i Virgili for the Martí i Franquès scholarship 2016 MF-PIPF-27. J.E. and Y.B. acknowledge the Marie Sklodowska Curie Initial Training Networks (FP7-PEOPLE-2013-ITN-606713, BIBAFOODS project), and support from Generalitat de Catalunya (2014SGR1655 and 2017 SGR 1778).

The morphology of sphere-forming block copolymers assembled in thin films on patterned surfaces is theoretically analyzed. The patterns on the lower surface are alternating bands of a given width distinctively attracting or repelling a given block. We find that long- range order can be achieved, and it depends on the commensurability of the characteristic length of the block domains with both band periodicity and slit thickness. The comparison of the simulation results with experimental data shows a very good agreement. Furthermore, we show that the proper selection of the band periodicity and, consequently, of the film thickness permits the system to switch from hexagonal packing to body-centered orthohedra. Therefore, we show that it exists a way to control the formation of long-range ordered structures of different types in this kind of system.

DOI: 10.1103/PhysRevE.96.023101Filiació URV: SI Stokes' hypothesis, the zeroing of the bulk viscosity in a Newtonian fluid, is discussed in this paper. To this aim, a continuum macroscopic fluid domain is initially modeled as a Hamiltonian system of discrete particles, for which the interparticle dissipative forces are required to be radial in order to conserve the angular momentum. The resulting system of particles is then reconverted to the continuum domain via the framework of the smoothed particle hydrodynamics (SPH) model. Since an SPH-consistent approximation of the Newtonian viscous term in the momentum equation incorporates interparticle radial as well as nonradial terms, it is postulated that the latter must be null. In the present work it is shown that this constraint implies that first and second viscosities are equal, resulting in a positive value for the bulk viscosity, in contradiction to the cited Stokes' hypothesis. Moreover, it is found that this postulate leads to bulk viscosity coefficients close to values found in the experimental literature for monoatomic gases and common liquids such as water.

The exchange of surfactants in micelles involves several processes that are difficult to characterize experimentally. Microscopic simulations have the potential to reveal some of the key activities that occur when a surfactant spontaneously exits a micelle. In this work, we present a quantitative analysis of the kinetic exchange process over a large range of time. This study is based on a dynamic version of single-chain mean-field theory using a coarse-grained model for poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer systems. The kinetics described in our simulations involves three different regimes. After a fast initial rearrangement of the labeled chains, the system undergoes a logarithmic relaxation, which has been experimentally observed. Contrary to what has been reported in previous analyses, our simulations indicate that this regime is caused by the intrinsic physical behavior of the system and is not due only to the polydispersity of the samples. Finally, the terminal regime is characterized by an exponential decay. The exit rates predicted by our simulations are in good agreement with the values reported experimentally. In addition, we address the sequence of microscopic conformational changes undergone by the surfactants when leaving the micellar aggregates. We found a subtle variation in the radius of gyration of the hydrophobic block, which challenges the image of either a complete collapse or a full stretching commonly accepted in the current theoretical and experimental literature.

Keplerates are a family of anionic metal oxide spherical capsules containing up to 132 metal atoms and some hundreds of oxygen atoms. These capsules holding a high negative charge of −12 coordinate both mono-anionic and di-anionic ligands thus increasing their charge up to −42, even up to −72, which is compensated by the corresponding counter-cations in the X-ray structures. We present an analysis of the relative importance of several energy terms of the coordinate bond between the capsule and ligands like carbonate, sulphate, sulphite, phosphinate, selenate, and a variety of carboxylates, of which the overriding component is contributed by solvation/de-solvation effects.

Experimental measurements of the relaxation kinetics of copolymeric surfactant exchange for micellar systems unexpectedly show a peculiar logarithmic decay. Several authors use polydispersity as an explanation for this behavior. However, in coarse-grained simulations that preserve microscopic details of the surfactants, we find evidence of the same logarithmic behavior. Since we use a strictly monodisperse distribution of chain lengths such a relaxation process cannot be attributed to polydispersity, but has to be caused by an inherent physical process characteristic of this type of system. This is supported by the fact that the decay is specifically logarithmic and not a power law with an exponent inherited from the particular polydispersity distribution of the sample. We suggest that the degeneracy of the energy states of the hydrophobic block in the core, which is broken on leaving the micelle, can qualitatively explain the broad distribution of energy barriers, which gives rise to the observed nonexponential relaxation.

The viscoelastic response of multilayer polymeric films (sandwiches) of linear low-density polyethylene (LLDPE) and ethylene vinyl alcohol (EVOH) copolymer has been experimentally analyzed. We propose an equation to predict the mechanical response of the sandwich films from the data experimentally obtained for the monomaterial films. The equation is based on the hypothesis that the layers independently contribute to the final response and that there are no interfacial effects. The predictive character of the equation permits us to conclude that the response of the sandwich does not significantly depend on the individual layer thickness but only on the volume fraction of the EVOH in the system. More important, we find a strong sensitivity of the response to the processing conditions, which we quantitatively describe. POLYM. ENG. SCI., 55:1960–1968, 2015. © 2014 Society of Plastics Engineers

The dynamics of water encapsulated in the well-known polyoxomolybdate nanocapsules of the type Mo-132 is studied through molecular dynamics simulations. Two different ligands, namely, formate and sulfate ligands, are considered as decoration of the inner surface of the capsule. In both cases it is found that 172 water molecules are trapped inside, 72 of which are coordination water molecules, and the remaining 100 form a water nanodrop whose properties are studied. We find that the dynamic behavior of the nanodrop is significantly different between the two types of capsules considered as they have different interiors. We argue that the commensurability of the sulfate ligand nanocapsule imposes a high degree of tetrahedrality that confers rigidity to the three-dimensional water network affecting the whole water nanodrop in the cavity. The formate ligand capsule, instead, permits additional degrees of freedom that confer the water nanodrop a more liquid-like behavior. The overall size of the inner cavity is of the order of 1 nm which is comparable to the crossover length observed for effects related to the rigidity of water layers in contact with hydrophobic molecules.(1) The results could stimulate the investigation of encapsulated water in the presently used capsule, the interior of which can be widely tuned (even from hydrophilic to hydrophobic) in context with the fact that knowledge about water under confined conditions is of extreme importance.

The structure and dynamics of water confined inside a polyoxomolybdate molecular cluster [{(Mo)Mo(5)O(21)(H(2)O)(6)}(12){Mo(2)O(4)(SO(4))}(30)](72-) metal oxide nanocapsule have been studied by means of molecular dynamics simulations under ambient conditions. Our results are compared to experimental data and theoretical analyses done in reverse micelles, for several properties. We observe that the characteristic three-dimensional hydrogen bond network present in bulk water is distorted inside the cavity where water organizes instead in concentric layered structures. Hydrogen bonding, tetrahedral order, and orientational distribution analyses indicate that these layers are formed by water molecules hydrogen bonded with three other molecules of the same structure. The remaining hydrogen bond donor/acceptor site bridges different layers as well as the whole structure with the hydrophilic inner side of the cavity. The most stable configuration of the layers is thus that of a buckyball with 12 pentagons and a variable number of hexagons. The geometrical constraints make it so that the bridges between the layers display a significant degree of frustration. The main modes of motion at short times are correlated fluctuations of the entire system with a characteristic frequency. Switches of water molecules between layers are rare events, due to the stability of the layers. At long times, the system shows a power law decay (pink noise) in properties like the fluctuations in the number of molecules in the structures and the total dipole moment. Such behavior has been attributed to the complex relaxation of the hydrogen bond network, and the exponents found are close to those encountered in bulk water for the relaxation of the potential energy. Our results reveal the importance of the competition between the confinement and the long-range structure induced in this system by the hydrogen bond network.

A single chain mean field theory is used to quantitatively describe the micellization process of the nonionic polyethylene oxide alkyl ether, C(n)E(m) class of surfactants at 25 °C. An explicit but simple microscopic model with only three interaction parameters is shown to be able to reproduce with high accuracy the critical micelle concentrations of a wide range of head and tail surfactant lengths. In addition, the aggregation number of the micelles is studied, the effect of the number of the hydrophobic and hydrophilic segments on CMC and aggregation number of the micelles are discussed and volume fraction profiles are given.

This study is aimed at developing an explicit solvent model for highly charged species such as polyoxometalates. The model includes solvent molecules in the first solvation shell explicitly, and long-range bulk effects and counter-ions as a set of single point charges. The model strongly stabilizes the electronic structure of the Keggin anion. The energies of the Kohn–Sham orbitals obtained using our model lie very close to those computed using the COSMO continuum solvent model; moreover, the total solvation energy evaluated with our model compares well to the value calculated by COSMO.

International audience; Molecular dynamics simulations of CH4, CO2, H2S and their mixtures, chosen as representative of acid gas mixtures, have been performed. Static properties (pressure or density) and dynamic properties (shear viscosity) have been computed. A comparison has been performed between results from three different models: a simple Lennard-Jones (LJ) model, an isotropic multipolar (IMP) one and the usual Lennard-Jones plus point charge (LJP). For pure fluids, a reasonable agreement is found between all three models and experiments except for CO2, for which the LJ model suffers from its lack of electrostatic contributions. Concerning CH4-H2S mixtures, results obtained using the three molecular models are consistent with each other for static and dynamic properties. In addition, no significant differences between the results obtained using both, Lorentz-Berthelot and Kong combining rules were noticed. For the CH4-H2S-CO2 mixture, the situation is different: a strong dependence of the pressure on the molecular models as well as on the combining rules, in the case of the LJP model, has been noted. Shear viscosity was found to be less dependent on the choice of models and combining rules. Thanks to simulations on H2S-CO2 mixtures, it has been found that the way cross interactions are treated between these two compounds explains the discrepancies for CH4-H2S-CO2 mixtures. For the systems studied, the IMP approximation seems to be the best option for engineering calculations of physical properties since it is quick to compute, ensures accuracy and is weakly dependent on the combining rules employed. In addition, it is shown that a van der Waals one fluid model combined with an accurate LJ Equation of State together with a correlation on viscosity is able to provide results consistent with simulations (on both LJ and IMP models). Such a result makes this correlative scheme a good alternative for industrial applications.

We have employed an anisotropic united atom model of benzene (R. O. Contreras, Ph.D. thesis, Universitat Rovira i Virgili 2002) that reproduces the quadrupolar moment of this molecule through the inclusion of seven point charges. We show that this kind of interaction is required to reproduce the solvation of these molecules in supercritical water. We have computed self-diffusion coefficient and Maxwell-Stefan coefficients as well as the shear viscosity for the mixture water-benzene at supercritical conditions. A strong density and composition dependence of these properties is observed. In addition, our simulations are in qualitative agreement with the experimental evidence that, at medium densities (0.6 g/cm(3) and 673 K), almost half of the benzene molecules have one hydrogen bond with water molecules. We also observe that these bonds are longer lived than the corresponding hydrogen bonds between water molecules. Similarly, we obtain an important reduction of the dielectric constant of the mixture with the increment of the amount of benzene molecules at medium and high densities.

The single chain mean field theory (SCMF) with importance sampling methodology has been applied to study homopolymer adsorption onto flat interfaces. A comparison with recent Monte Carlo data reveals that the excluded volume correlations along the chain are crucial in quantitatively reproducing the observed structure of the layer. The SCMF approach introduces a sampling of the self-avoiding one-chain conformations in a self-consistent mean-field, that allows for the local swelling of the chain. Unlike classical self-consistent field approaches, based on ideal chain statistics, the correct spatial as well as bulk density dependence of the profiles and structure are recovered by the SCMF approach.

In this work we have introduced an importance sampling methodology in the framework of the single chain mean field theory (SCMF), to describe the adsorption of moderately long polymers onto solid surfaces. Since the method is based on the generation of self-avoiding chain conformations, excluded volume correlations along the chain backbone are maintained, leading to the proper scaling behavior in the properties depending on one-chain statistics, unlike the self-consistent field theories based on ideal (Markovian) chains. The formulation presented is general and is expected to be very useful for future applications. Because of its mean field nature, the methodology does not require large computer capabilities. As an application of the methodology given here, we present novel results on the profiles of the end-monomer distributions for two chain lengths and different bulk concentrations.

The nonideal behavior of the transport properties of water–dimethyl sulfoxide (DMSO) mixtures has been studied through equilibrium and nonequilibrium molecular dynamic simulations. The shear viscosity and thermal conductivity of the mixture has been analyzed and compared with available experimental data at ambient conditions. The enhancement of shear viscosity at molar fractions xW=0.65 of water has been quantitatively reproduced in our simulations. In agreement with this fact, we have found an increase in the rigidity of the system reflected by an increase in the decay time of the survival probability of the H bonds. In addition, we compute the tetrahedral order parameter of water molecules in the solution at different molar fractions. This parameter indicates a reduction in the local tetrahedral order of water when the solute concentration is increased, followed by a clear minimum at the equimolar concentration near the locus of the maximum density of the mixture, probably due to the formation of water–...

Using the Kierlik and Rosinberg fundamental measure theory, we test the density functional theory method for determination of pore size distributions from adsorption data for porous glasses. The glasses chosen for study are model glasses prepared by a quench molecular dynamics method that mimics the experimental synthesis process and are completely characterized at the molecular level. The density functional method involves two approximations: (a) the glasses can be regarded as made up of a distribution of nonconnected pores of simple geometry, which we refer to as the independent pore model, and (b) the adsorption isotherms for these nonconnected pores can be described by the density functional theory. Using simulated adsorption isotherm data for the glasses and adsorption isotherms for the pores of simple geometry calculated by the density functional theory, a regularization method is used to determine the pore size distribution from the adsorption data. These calculated pore size distributions, as well as the adsorption isotherms for the materials, are compared with the exact geometric pore size distributions for the material and with the simulated isotherms. Both slit-shaped and cylindrical pores are used in the density functional theory method. It is found that a unique geometry is not able to accurately describe the whole adsorption isotherm. The use of slit-shaped pores gives overall better results, although the low-pressure regime is more accurate when cylindrical pores are used; reasons for this are discussed. The pore size distributions from the density functional theory are in reasonable agreement with the geometrical ones, giving the same shape and mean pore width and similar porosities in the four materials. Since it is known that the density functional theory gives excellent results for the adsorption isotherms (approximation b above), this comparison tests the independent pore model directly.

A new non-equilibrium molecular dynamics algorithm is presented based on the original work of Muller-Plathe, (1997, J. chem. Phys., 106, 6082), for the non-equilibrium simulation of heat transport maintaining fixed the total momentum as well as the total energy of the system. The presented scheme preserves these properties but, unlike the original algorithm, is able to deal with multicomponent systems, that is with particles of different mass independently of their relative concentration. The main idea behind the new procedure is to consider an exchange of momentum and energy between the particles in the hot and cold regions, to maintain the non-equilibrium conditions, as if they undergo a hypothetical elastic collision. The new algorithm can also be employed in multicomponent systems for molecular fluids and in a wide range of thermodynamic conditions.

The local structure and the dynamical behavior of water have been analyzed in two different regimes (the dense isochore of 0.995 g/cm2 and the supercritical isotherm of 673 K) through four rigid nonpolarizable models of water. An important change in the slope of temperature dependence of the self-diffusion coefficient at ρ=0.995 has been observed at T≈450 K, showing two main regions that are related to a change on the activation energy of the process (originated for a change of the structure of the first solvation shell from a tetrahedral to dodecahedral arrangement). The local orientational structure of water has been analyzed through the use of tetrahedral order parameters q. A direct relation between q and D has been observed for all models showing some kind of master curve up to 450 K at the 0.995 g/cm3 isochore. The structure of the system at short and large radial distances has been analyzed through a decomposition of the site–site radial distribution functions in terms of spherical harmonics, and a...

A reversible algorithm [enforced energy conservation (EEC)] that enforces total energy conservation for microcanonical simulations is presented. The key point is the introduction of the discrete-gradient method to define the forces from the conservative potentials, instead of the direct use of the force field at the actual position of the particle. We have studied the performance and accuracy of the EEC in two cases, namely Lennard-Jones fluid and a simple electrolyte model. Truncated potentials that usually induce inaccuracies in energy conservation are used. In particular, the reaction field approach is used in the latter. The EEC is able to preserve energy conservation for a long time, and, in addition, it performs better than the Verlet algorithm for these kinds of simulations.

Simulation results of the thermal conductivity ${\cal L}$ of Dissipative Particle Dynamics model with Energy Conservation (DPDE) are reported. We also present an analysis of the transport equations and the transport coefficients for DPDE based on a local equilibrium approximation. This approach is valid when the particle-particle thermal conductivity $\lambda$ and the friction coefficient $\zeta$ are large. A qualitative derivation of the scaling form of the kinetic contribution of the transport of energy is derived, yielding two different forms for the kinetic contribution to the heat transport, depending on the value of $\lambda$. We find agreement between the theoretically predicted value for ${\cal L}$ and the simulation results, for large $\lambda$ and many particles interacting at one time. Significant differences are found for small number of interacting particles, even with large $\lambda$. For smaller values of $\lambda$, the obtained macroscopic thermal conductivity is dominated by diffusive transport, in agreement with the proposed scaling form.

The stochastic differential equations for a model of dissipative particle dynamics, with both total energy and total momentum conservation at every time step, are presented. The algorithm satisfies detailed balance as well as the fluctuation–dissipation theorems that ensure that the proper thermodynamic equilibrium can be reached. The Fokker–Planck equation for the evolution of the probability distribution for the system is given. Macroscopic equilibrium probability distributions as well as equations of state for the model are also derived, and an appropriate definition of the free energy of the system is consistently proposed. Several simulations, results of equilibrium as well as transport properties, including heat transport and thermal convection in a box, are shown as proof of the internal consistency of the model.

The kinetics of irreversible adsorption of spherical particles onto a flat surface is theoretically studied. Previous models, in which hydrodynamic interactions were disregarded, predicted a power-law behavior $t^{-2/3}$ for the time dependence of the coverage of the surface near saturation. Experiments, however, are in agreement with a power-law behavior of the form $t^{-1/2}$. We outline that, when hydrodynamic interactions are considered, the assymptotic behavior is found to be compatible with the experimental results in a wide region near saturation., Comment: 4 pages, 1 figures, Phys. Rev. Lett. (in press)

In this paper we address the problem of consistently constructing Langevin equations to describe fluctuations in nonlinear systems. Detailed balance severely restricts the choice of the random force, but we prove that this property, together with the macroscopic knowledge of the system, is not enough to determine all the properties of the random force. If the cause of the fluctuations is weakly coupled to the fluctuating variable, then the statistical properties of the random force can be completely specified. For variables odd under time reversal, microscopic reversibility and weak coupling impose symmetry relations on the variable-dependent Onsager coefficients. We then analyze the fluctuations in two cases: Brownian motion in position space and an asymmetric diode, for which the analysis based in the master equation approach is known. We find that, to the order of validity of the Langevin equation proposed here, the phenomenological theory is in agreement with the results predicted by more microscopic models. (c) 1995 The American Physical Society

A general closed form for the mobility tensor of a sphere moving in a fluid in stationary homogeneous flow is derived using the induced force method up to the first order in the square root of the Reynolds number based on the velocity gradient of the unperturbed flow. The closed form for the mobility tensor is valid for the time-dependent case as well as for the stationary case. As a special case, we calculate it explicitly for a simple shear flow. The result for the x, z-component for the stationary case, which gives the lift force, agrees with the value calculated by Saffman.

10.1021/la204132c We present a microscopic analysis of shape transitions of micelles of model linear nonionic surfactants. In particular, symmetric H4T4 and asymmetric H3T6 surfactants have been chosen for the study. In a previous work, it has been observed that symmetric surfactants have a strong tendency to prefer spherical micelles over a wide range of chemical potentials, while asymmetric surfactants undergo shape transitions between a spherical micelle at low concentration to other forms, mainly finite cylindrical micelles. This study combines the application of a two-dimensional single-chain mean-field theory (SCMFT) with Monte Carlo (MC) simulations of exactly the same systems. On the one hand, the characteristics of the SCMFT make this method suitable for free energy calculations, especially for small surfactants, due to the incorporation of relevant microscopic details in the model. On the other hand, MC simulations permit us to obtain a complete picture of the statistical mechanical problem, for the purpose of validation of the mean-field calculations. Our results reveal that the spherical shape for the symmetric surfactant is stable over a large range of surfactant concentrations. However, the asymmetric surfactant undergoes a complex shape transition that we have followed by calculating the standard chemical potential as a function of the aggregation number. The results indicate that the system forms prolate spheroids prior to developing short capped cylinders that gradually grow in length, with some oscillations in the energy of formation. The most important result of our work is the evidence of a bifurcation where, together with the elongated objects, the system can develop oblate aggregates and finally a torus shape similar to a red blood cell.

Mass diffusion coefficients of CO(2)/brine mixtures under thermodynamic conditions of deep saline aquifers have been investigated by molecular simulation. The objective of this work is to provide estimates of the diffusion coefficient of CO(2) in salty water to compensate the lack of experimental data on this property. We analyzed the influence of temperature, CO(2) concentration,and salinity on the diffusion coefficient, the rotational diffusion, as well as the electrical conductivity. We observe an increase of the mass diffusion coefficient with the temperature, but no clear dependence is identified with the salinity or with the CO(2) mole fraction, if the system is overall dilute. In this case, we notice an important dispersion on the values of the diffusion coefficient which impairs any conclusive statement about the effect of the gas concentration on the mobility of CO(2) molecules. Rotational relaxation times for water and CO(2) increase by decreasing temperature or increasing the salt concentration. We propose a correlation for the self-diffusion coefficient of CO(2) in terms of the rotational relaxation time which can ultimately be used to estimate the mutual diffusion coefficient of CO(2) in brine. The electrical conductivity of the CO(2)-brine mixtures was also calculated under different thermodynamic conditions. Electrical conductivity tends to increase with the temperature and salt concentration. However, we do not observe any influence of this property with the CO(2) concentration at the studied regimes. Our results give a first evaluation of the variation of the CO(2)-brine mass diffusion coefficient, rotational relaxation times, and electrical conductivity under the thermodynamic conditions typically encountered in deep saline aquifers.

Numerical integration schemes based upon the Shardlow-splitting algorithm (SSA) are presented for dissipative particle dynamics (DPD) approaches at various fixed conditions, including a constant-enthalpy (DPD-H) method that is developed by combining the equations-of-motion for a barostat with the equations-of-motion for the constant-energy (DPD-E) method. The DPD-H variant is developed for both a deterministic (Hoover) and stochastic (Langevin) barostat, where a barostat temperature is defined to satisfy the fluctuation-dissipation theorem for the Langevin barostat. For each variant, the Shardlow-splitting algorithm is formulated for both a velocity-Verlet scheme and an implicit scheme, where the velocity-Verlet scheme consistently performed better. The application of the Shardlow-splitting algorithm is particularly critical for the DPD-E and DPD-H variants, since it allows more temporally practical simulations to be carried out. The equivalence of the DPD variants is verified using both a standard DPD fluid model and a coarse-grain solid model. For both models, the DPD-E and DPD-H variants are further verified by instantaneously heating a slab of particles in the simulation cell, and subsequent monitoring of the evolution of the corresponding thermodynamic variables as the system approaches an equilibrated state while maintaining their respective constant-energy and constant-enthalpy conditions. The original SSA formulated for systems of equal-mass particles has been extended to systems of unequal-mass particles. The Fokker-Planck equation and derivations of the fluctuation-dissipation theorem for each DPD variant are also included for completeness.

By the deliberate choice of the internal ligands of the porous nanocapsules [{(Mo)Mo(5)}(12){Mo(2)(ligand)}(30)](n-), the respective cavities' shells can be differently sized/functionalized. This allows one to trap the same large number of water molecules, that is, 100 in a capsule cavity with formate ligands having a larger space available, as well as in a cavity containing sulfates and hypophosphites, that is, with less space. Whereas the 100 molecules fill the space completely in the second case in which they are organized in three shells, a four-shell system with underoccupation and broken hydrogen bonds is observed in the other case. This is an unprecedented result in terms of the structurally well defined special forms of "higher and lower density" water molecule assemblies. Precisely, by replacing the larger ligands in the mentioned nanocapsule type by formates, voids in the capsule cavity of (HC(NH(2))(2))(22)[{(HC(NH(2))(2))(20)+(H(2)O)(100)} subset{(Mo)Mo(5)O(21)(H(2)O)(6)}(12){Mo(2)O(4)(HCO(2))}(30)]ca. 200 H(2)O are generated that get filled with water molecules concomitant with an expansion of the three to four shell {H(2)O}(100) cluster. The water shells in both capsules containing different ligands are organized in the form of dodecahedra (partly with underoccupation) and a strongly distorted rhombicosidodecahedron spanned by a {H(2)O}(60)={(H(2)O)(5)}(12) aggregate. The well-defined water shells only emerge if cations cannot enter into the capsules, which is achieved by closing the pores with plugs/guests such as formamidinium cations. The work is based on the syntheses of two new compounds, related single-crystal X-ray diffraction studies, and molecular dynamics simulations, which show remarkably that water molecule shell structuring occurs in the capsules due to the confined conditions even in the case of open pores and at room temperature if cation uptake is prevented.

Dissipative particle dynamics simulations for the dewetting of a stratified thin film, composed of two immiscible fluids of different viscosity, reveal a nontrivial dewetting kinetics depending on the location of the more viscous liquid with respect to the solid substrate. Our simulations show that when the layer of higher viscosity is in contact with the substrate, the energy loss through viscous dissipation is concentrated near the contact line. The kinetics of dewetting is then inversely proportional to the viscosity of the lower layer eta_{A} (with respect to eta_{B} for the upper layer). However, when the liquid of higher viscosity is on the upper layer, a new dynamics appears with a dewetting velocity V approximately eta_{B};{-0.44}eta_{A};{-0.56}. The difference between these two scenarios lies in the different routes through which the interfacial energy is converted into heat by viscous dissipation in the rim.

Monte Carlo and molecular dynamics simulations have been used in order to test the ability of a three center intermolecular potential for carbon dioxide to reproduce literature experimental thermophysical values. In particular, both the shear viscosity under supercritical conditions and along the phase coexistence line, as well as the thermal conductivity under supercritical conditions, have been calculated. Together with the already reported excellent agreement for the phase coexistence densities, the authors find that the agreement with experimental values is, in general, good, except for the thermal conductivity at low density. Although extended versions of the model were employed, which include an explicit account of bending and vibrational degrees of freedom, a significant difference was still found with respect to the reported experimental value.

The Keggin anion, PW12O40(3-), is one of the most representative polyoxometalates (POMs). In recent years increasing theoretical work focused on this family of compounds has explained or even predicted some of their properties using quantum mechanics methods. In this report we applied for the first time molecular dynamics (MD) to the title compound to analyze its interactions with water. We used three atomic charge definitions (Mulliken, ChelpG, and formal charges) to carry out MD simulations. The results show that the terminal oxygens of the cluster are invariably most effectively solvated by water because of their prominent position within the framework. On the other hand, bridging oxygens, which are confined in more internal positions, concentrate a smaller portion of the whole solvation. We investigated the hydrogen bonds existing between water and the cluster, confirming that the terminal positions form more contacts with H2O than any other site of the cluster. In the end, the lifetime of such contacts is longer with bridging oxygens, presumably due to their higher atomic charge.

The Keggin anion, PW12O403-, is one of the most representative polyoxometalates (POMs). In recent years increasing theoretical work focused on this family of compounds has explained or even predicted some of their properties using quantum mechanics methods. In this report we applied for the first time molecular dynamics (MD) to the title compound to analyze its interactions with water. We used three atomic charge definitions (Mulliken, ChelpG, and formal charges) to carry out MD simulations. The results show that the terminal oxygens of the cluster are invariably most effectively solvated by water because of their prominent position within the framework. On the other hand, bridging oxygens, which are confined in more internal positions, concentrate a smaller portion of the whole solvation. We investigated the hydrogen bonds existing between water and the cluster, confirming that the terminal positions form more contacts with H2O than any other site of the cluster. In the end, the lifetime of such contacts...

The histogram reweighting technique, widely used to analyze Monte Carlo data, is shown to be applicable to dynamic properties obtained from Molecular Dynamics simulations. The theory presented here is based on the fact that the correlation functions in systems in thermodynamic equilibrium are averages over initial conditions of functions of the trajectory of the system in phase-space, the latter depending on the volume, the total number of particles and the classical Hamiltonian. Thus, the well-known histogram reweighting method can almost straightforwardly be applied to reconstruct the probability distribution of initial states at different thermodynamic conditions, without extra computational effort. Correlation functions and transport coefficients are obtained with this method from few simulation data sets., Comment: 4 pages, 3 figures

In this work we have implemented the fundamental-measure density functional theory due to Kierlik and Rosinberg to describe the adsorption of Lennard-Jones molecules in cylindrical pores. The accuracy of the theory in predicting adsorption isotherms and particle density profiles is checked by comparison with grand canonical Monte Carlo simulations for a wide range of pore sizes, showing very good agreement in all cases. In addition, the theory has been applied to the adsorption in slitlike pores to study the influence of the pore geometry on this property. The results indicate that the confinement of the cylindrical geometry introduces significant differences in the shape of the adsorption isotherms and density profiles. These differences are relevant for the characterization of porous materials., The authors thank Lev D. Gelb and Keith E. Gubbins for useful discussions. Alexander V. Neimark, Peter I. Ravikovitch, and Aleksey Vishnyakov are gratefully acknowledged for providing data for Fig. 10. This work was supported by the Spanish Government Ministerio de Ciencia y Tecnología (Grant Nos. PPQ2000-2888-E, PPQ2001-0671) and by URV (Project No. 2000PIR-21). S.F-G. thanks URV (Spain) and CONACyT (Me ́xico) for financial support. F.J.B. thanks the Universidad de Huelva and the Junta de Andalucía for financial support

Polymer adsorption has been widely investigated in the context of self-consistent mean-field theories. As a further simplification, the “ground state dominance approximation” is often made, treating the polymer chains as infinitely long. For short polymers, or not so concentrated polymer solutions, corrections to ground state dominance may be important, however. In this work, we discuss analytical solutions to the full self-consistent field equations, valid for any chain length, in the limit of weak adsorption. We show how the resulting equations may be put into a free energy functional formalism, in analogy to the de Gennes–Lifshitz free energy for infinitely long polymer chains. Analytical expressions are derived for polymer density profiles, surface tension and the interaction potential between two planar, polymer-adsorbing surfaces. Particular attention is paid to the distal ordering of the polymer coils that shows up as oscillations in the polymer density profile and interaction potential at the scal...

Asymmetric membranes have a wide field of application because of its unique morphology. Its high selectivity is assured when a dense layer alternates with a porous one. Immersion precipitation is one of the processes generally used for the membrane production. In this process, a polymeric solution placed on the top of a glass plate is immersed in a bath of a non-solvent. The membranes are formed as long as the solvent leaves the polymeric layer and the non-solvent takes its place. The driving force for the membrane formation is the difference of chemical potential between the bath and the polymeric layer.

Ubiquitous water is apart from its anomalies, for several reasons the most mysterious compound. According to its basic role in life processes it has even been considered as a biomolecule especially in context of the important role of confined “high and low density water”. C. Bo, A. Muller et al. report on page 1844 ff., confined higher and lower density water assemblies encapsulated in the same type of porous capsule that exhibit differently functionalized internal shells (see picture).

We analyze the light-scattering spectrum of a suspension in a viscoelastic fluid under density and velocity gradients. When a density gradient is present, the dynamic structure factor exhibits universality in the sense that its expression depends only on the reduced frequency and the reduced density gradient. For a velocity gradient, however, the universality breaks down. In this last case we have found a transition point from one to three characteristic frequencies in the spectrum, which is governed by the value of the external gradient. The presence of the viscoelastic time scales introduces a shift in the ``critical¿¿ point.

Molecular-dynamics simulation results on thermodynamic and transport properties of pure H2S under conditions of practical interest are presented. Our data are in very good quantitative agreement with the scarce experimental data and estimates on thermophysical properties of this substance. Our results serve as a test of the validity of the intermolecular potential used in the simulations as well as the consistency of the existing data in the studied range. New simulation data on thermal conductivity at low temperature as well as in supercritical states are also reported. Furthermore, we present a comparative analysis between the local order in the liquid phase of pure hydrogen sulfide and water, due to the molecular analogies between both substances, and its relation with the formation of H=S bonds. Our results indicate that under the same corresponding thermodynamic states, H2S is a much less structured substance, with a first solvation shell with a dodecahedral order instead of the tetrahedral order observed in water.

A single-chain mean-field theory is used to predict the properties of binary surfactant solutions including the critical micelle concentration (cmc). In particular, the cmc of two symmetric nonionic amphiphiles is calculated as a function of temperature in order to analyze the validity of the ideal mixing assumption, often employed in the mass action model. On comparing against literature Monte Carlo results for the same lattice model we find that although it is applicable at low temperatures and hence cmcs at low amphiphile concentrations, at higher temperatures it becomes necessary to correct for the nonideal mixing of the free chain-free chain bulk interaction. We find that a simplistic model taking into account only the repulsive interaction is sufficient to restore the excellent quantitative agreement found between a single-chain mean-field theory calculations and literature molecular simulation results at the low temperature limit.

The correlation function for the exit of poloxamer copolymers from equilibrated micelles is found to show up to four regimes depending on the chain flexibility: an initial fast reorganization, a logarithmic intermediate regime, followed by an exponential intermediate regime, and a final exponential decay. The logarithmic intermediate regime has been observed experimentally and attributed to the polydispersity of the polymer samples. However, we present dynamic single-chain mean-field theory simulations with chains of variable flexibility which show the same logarithmic relaxation but with strictly monodisperse systems. In agreement with our previous studies, we propose that this logarithmic response arises from a degeneracy of energy states of the hydrophobic block in the micelle core. For this to occur, a sufficiently large number of degenerate conformational states are required, which depend on the polymer flexibility and therefore should not be present for rigid polymers. Experimental results for monodisperse polymeric samples claiming the absence of such a logarithmic response may also lack a sufficient number of hydrophobic blocks for the required number of configurational states for this type of response to be seen. The insight gained from analyzing the simulation results allows us to propose a modified Eyring equation capable of reproducing the observed dynamic behavior. On scaling experimental results from different sources and systems according to this equation, we find a unique master curve showing a universal nature of the intermediate regimes: the logarithmic regime together with the secondary exponential decay. The terminal exponential regime at long times proposed by the standard Halperin and Alexander model is beyond the range of the data analyzed in this article. The universality observed suggests an entropic origin of the short-time dynamic response of this class of systems rather than the polydispersity.

The kinetics of the deposition of colloidal particles onto a solid surface is analytically studied. We take into account both the diffusion of particles from the bulk as well as the geometrical aspects of the layer of adsorbed particles. We derive the first kinetic equation for the coverage of the surface (a generalized Langmuir equation) whose predictions are in agreement with recent simulation results where diffusion of particles from the bulk is explicitly considered., 4 pages