Dissipative Particle Dynamics Beads Research Articles

Phase Diagram Study of Catanionic Surfactants Using Dissipative Particle Dynamics.

Dissipative particle dynamics (DPD) simulations has been performed to study the phase transition of a mixture of cationic and anionic surfactants in an aqueous solution as a function of the total concentration in water and the relative ratio of surfactants. The impact of the relative difference between the tail lengths of the cationic and anionic surfactants on the phase diagram has been simulated by tuning the number of DPD beads in the simulation model. This research also discusses the impact of the frequently used values of the parameters associated with the harmonic bonds among the bonded DPD beads on the obtained self-assemblies. We find remarkable differences in the resultant self-assemblies based on different choices of harmonic bond parameters. The performed simulations show an enhanced spectrum of self-assemblies with augmented tail lengths and disparate harmonic bond parameters. The obtained self-assemblies are quite unique and can potentially be used in the future for various applications. We also compare the simulation results of the vesicle structures obtained by modeling the electrostatic interaction in the simulation among the charged beads by explicitly introducing charges with a long-range interaction with those obtained by tuning the implicit electrostatic interaction without the long-range interaction. The effects of the chain length of the model and the harmonic bond parameters on the internal density of DPD beads and stress profiles within the vesicles are examined closely. These results are a significant contribution to understanding the stability of the phases and tailoring of the desired vesicles.

Heavy crude oil mesoscopic modeling to predict interfacial tension

Heavy crude oils represent a significant percentage of oil reserve available in the world, part of which are located in Venezuela. Nevertheless, they have a high level of complexity due to the asphaltenes, heavy metals, and heteroa-toms presence which converts these crudes into highly viscous fluids. These properties make difficult its theoretical and experimental study. Despite the computational power today, it is not feasible to represent and study these sys-tems, expressing their principal constituents at the atomistic level. Considering this fact, the study of a heavy crude from Venezuela was carried out, at a mesoscopic scale, based on theoretical and experimental data, modeled by ap-plying Dissipative Particle Dynamics (DPD), and the coarse-graining technique. DPD beads correspond to the heavy crude oil main fractions, for which a super- grain of each fraction was made, implying a new approach in a heavy crude oil system representation. The models' validation was done by studying the behavior of the systems at the interface, evaluating the interfacial tension evolution, density profiles, and images obtained from the dynamic performed, according to the variations of the concentrations of the heavy crude oil, their fractions, and solvents used. The results successfully reproduced the behavior and trends reported for these systems, under similar condi-tions, and support the validity and advantage of the approach of the present study, as an alternative for the analysis, prediction and understanding of the behavior in the interface of this type of systems.

Translation of Chemical Structure into Dissipative Particle Dynamics Parameters for Simulation of Surfactant Self-Assembly.

Dissipative particle dynamics (DPD) can be used to simulate the self-assembly properties of surfactants in aqueous solutions, but in order to simulate a new compound, a large number of new parameters are required. New methods for the calculation of reliable DPD parameters directly from chemical structure are described, allowing the DPD approach to be applied to a much wider range of organic compounds. The parameters required to describe the bonded interactions between DPD beads were calculated from molecular mechanics structures. The parameters required to describe the nonbonded interactions were calculated from surface site interaction point (SSIP) descriptions of molecular fragments that represent individual beads. The SSIPs were obtained from molecular electrostatic potential surfaces calculated using density functional theory and used in the SSIMPLE algorithm to calculate transfer free energies between different bead liquids. This approach was used to calculate DPD parameters for a range of different types of surfactants, which include ester, amide, and sugar moieties. The parameters were used to simulate the self-assembly properties in aqueous solutions, and comparison of the results for 27 surfactants with the available experimental data shows that these DPD simulations accurately predict critical micelle concentrations, aggregation numbers, and the shapes of the supramolecular assemblies formed. The methods described here provide a general approach to determining DPD parameters for neutral organic compounds of arbitrary structure.

Coarse-grained simulation of the translational and rotational diffusion of globular proteins by dissipative particle dynamics.

With simplified interactions and degrees of freedom, coarse-grained (CG) simulations have been successfully applied to study the translational and rotational diffusion of proteins in solution. However, in order to reach larger lengths and longer timescales, many CG simulations employ an oversimplified model for proteins or an implicit-solvent model in which the hydrodynamic interactions are ignored, and thus, the real kinetics are more or less unfaithful. In this work, we develop a CG model based on the dissipative particle dynamics (DPD) that can be universally applied to different types of proteins. The proteins are modeled as a group of rigid DPD beads without conformational changes. The fluids (including solvent and ions) are also modeled as DPD beads. The electrostatic interactions between charged species are explicitly considered by including charge distributions on DPD particles. Moreover, a surface friction between the protein and fluid beads is applied to control the slip boundary condition. With this model, we investigate the self-diffusion of a single globular protein in bulk solution. The translational and rotational diffusion coefficients of the protein can be tuned by the surface frictional constant to fit the predictions of the Stokes-Einstein (SE) relation. We find that both translational and rotational diffusion coefficients that meet with the prediction of the SE relation based on experimental results of the hydrodynamic radius are reached at almost the same frictional constant for different types of proteins. Such scaling behavior indicates that the model can be applied to simulate the translational and rotational diffusion together for various types of proteins.

Inertial migration of a rigid sphere in plane Poiseuille flow as a test of dissipative particle dynamics simulations.

After reviewing and organizing the literature on the problem of inertial cross-stream migration of rigid spheres in various geometries including tubes and channels, we use Dissipative Particle Dynamics (DPD) simulations to study the simplest case of migration of a single neutrally or non-neutrally buoyant sphere with diameter 20% of the gap in plane Poiseuille flow and assess the potential and limitations of DPD simulations for this and similar problems. We find that the neutrally buoyant sphere lags by up to 6% behind the surrounding fluid and is focused at a position around 50% of the distance between the channel center and the wall. With Re increasing from around 100 to 500, the sphere migrates closer to the channel center. With flow driven by gravity, a much denser non-neutrally buoyant sphere leads the surrounding fluid and is focused at a position closer to the wall, around 60% the distance from the channel center to the wall, in qualitative agreement with previous work. The lower values of the Schmidt number Sc in DPD simulations relative to real fluids, due to the relatively large diffusivity of DPD beads, are shown to not significantly affect the consistency of our DPD results with literature results although they make results noisy at low Re (i.e., 50). However, the increase in Ma and Wi with increasing Re leads to compressible flow effects and in some cases viscoelastic effects at high Re depending on the DPD parameters chosen. Even for optimally chosen parameters, we require to avoid strong compressibility effects. Thus, the relative simplicity of the DPD method for complex fluid flows is offset by the need to control the effects of unphysically high values of other parameters, such as Ma and Wi, which seriously limits the range of conditions under which DPD simulations give valid results in fluid transport problems.

Simulations of Interfacial Tension of Liquid-Liquid Ternary Mixtures Using Optimized Parametrization for Coarse-Grained Models.

In this work, liquid-liquid systems are studied by means of coarse-grained Monte Carlo simulations (CG-MC) and Dissipative Particle Dynamics (DPD). A methodology is proposed to reproduce liquid-liquid equilibrium (LLE) and to provide variation of interfacial tension (IFT), as a function of the solute concentration. A key step is the parametrization method based on the use of the Flory-Huggins parameter between DPD beads to calculate solute/solvent interactions. Parameters are determined using a set of experimental compositional data of LLE, following four different approaches. These approaches are evaluated, and the results obtained are compared to analyze advantages/disadvantages of each one. These methodologies have been compared through their application on six systems: water/benzene/1,4-dioxane,water/chloroform/acetone, water/benzene/acetic acid, water/benzene/2-propanol, water/hexane/acetone, and water/hexane/2-propanol. CG-MC simulations in the Gibbs (NVT) ensemble have been used to check the validity of parametrization approaches for LLE reproduction. Then, CG-MC simulations in the osmotic (μsoluteNsolventP zzT) ensemble were carried out considering the two liquid phases with an explicit interface. This step allows one to work at the same bulk concentrations as the experimental data by imposing the precise bulk phase compositions and predicting the interface composition. Finally, DPD simulations were used to predict IFT values for different solute concentrations. Our results on variation of IFT with solute concentration in bulk phases are in good agreement with experimental data, but some deviations can be observed for systems containing hexane molecules.

Effectively parameterizing dissipative particle dynamics using COSMO-SAC: A partition coefficient study.

Dissipative Particle Dynamics (DPD) provides a tool for studying phase behavior and interfacial phenomena for complex mixtures and macromolecules. Methods to quickly and automatically parameterize DPD greatly increase its effectiveness. One such method is to map predicted activity coefficients derived from COSMO-SAC onto DPD parameter sets. However, there are serious limitations to the accuracy of this mapping, including the inability of single DPD beads to reproduce asymmetric infinite dilution activity coefficients, the loss of precision when reusing parameters for different molecular fragments, and the error due to bonding beads together. This report describes these effects in quantitative detail and provides methods to mitigate much of their deleterious effects. This includes a novel approach to remove errors caused by bonding DPD beads together. Using these methods, logarithm hexane/water partition coefficients were calculated for 61 molecules. The root mean-squared error for these calculations was determined to be 0.14-a very low value-with respect to the final mapping procedure. Cognizance of the above limitations can greatly enhance the predictive power of DPD.

Modeling Biofilm Formation on Dynamically Reconfigurable Composite Surfaces.

We augment the dissipative particle dynamics (DPD) simulation method to model the salient features of biofilm formation. We simulate a cell as a particle containing hundreds of DPD beads and specify p, the probability of breaking the bond between the particle and surface or between the particles. At the early stages of film growth, we set p = 1, allowing all bonding interactions to be reversible. Once the bound clusters reach a critical size, we investigate scenarios where p = 0, so that incoming species form irreversible bonds, as well as cases where p lies in the range of 0.1-0.5. Using this approach, we examine the nascent biofilm development on a coating composed of a thermoresponsive gel and the embedded rigid posts. We impose a shear flow and characterize the growth rate and the morphology of the clusters on the surface at temperatures above and below Tc, the volume phase transition temperature of a gel that displays lower critical solubility temperature (LCST). At temperatures above Tc, the posts effectively inhibit the development of the nascent biofilm. For temperatures below Tc, the swelling of the gel plays the dominant role and prevents the formation of large clusters of cells. Both these antifouling mechanisms rely on physical phenomena and, hence, are advantageous over chemical methods, which can lead to unwanted, deleterious effects on the environment.

Dissipative Particle Dynamics Simulations of Anion Exchange Membranes

Anion exchange membranes (AEM) are currently under investigation for their application in energy and conversion devices.1,2 The main challenges of the AEMs are the low stability and ionic conductivity. Rather than increasing the ion exchange capacity, a realistic strategy to enhance the conductivity is to use phase segregated AEMs.3 Hence, the morphology of the AEM polymer is very important to achieve high performance. However, there is still a lack of fundamental understanding of stability and transport in AEMs.4 Recently, researchers have reported elastomeric AEMs exhibiting satisfactory chemical stability.5 These AEMs are based on the triblock copolymer, polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS), and functionalized with various cationic groups. In this research, dissipative particle dynamics (DPD) simulations were chosen to model the morphology of SEBS systems functionalized with tetra alkyl ammonium groups. DPD, a meso-scale simulation technique, enables relatively large time and length scales to be examined,6,7 and therefore permits the morphology of the polymer system in acceptable time and computational expenses. The SEBS polymer was coarse-grained in to DPD beads, balancing both chemical distinction and fine structural variants. The optimized structure and the interaction parameters of these beads were calculated by using a recently developed methodology employing DFT based electronic structure calculations.8 ,9 The morphology of the aforementioned AEMs were simulated at different hydration levels. The result shows that the morphology changes with the hydration levels. The effects of the alkyl chain tethered to the cation group, the ion exchange capacity, and the copolymer composition were also investigated.

Morphology of Elastomeric Anion Exchange Membranes: A Dissipative Particle Dynamics Study

The design and optimization of stable and highly conductive anion exchange membranes (AEMs) is essential to the development of energy and conversion technologies.1 , 2 There has been an increased interest and research to address the low stability and ionic conductivity of polymeric AEMs. However, a fundamental understanding of stability and transport in AEMs is still in its infancy3 and the effects of the choice of the cationic functional group and polymer architecture are still not known. Recently developed elastomeric AEMs4 exhibiting satisfactory chemical stability are investigated in this study. These AEMs are based on the elastomeric triblock copolymer, polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS), that are functionalized with various cationic groups. Dissipative particle dynamics (DPD) is a meso-scale simulation technique5 , 6 which enables the simulation of large time- and length-scales with reasonable computational expense has been selected to investigate the morphology and dynamical behavior of such systems. The system of hydrated AEM is coarse-grained to a degree that the consideration of both chemical distinction and fine structural variants are satisfied. The structures of the DPD beads were optimized using first principles electronic structure calculations and the interaction parameters were determined using a recently developed methodology.7 The effects of several microstructural parameters including the length of the tether chains, type of the cationic group (trimethylammonium [(CH3)3NH]+; dimethylimidazolium [DMIm+]; and triphenylphosphonium [(C6H5)3PH]+ ), and the polymer architecture on the morphology and transport of anions in the aforementioned elastomeric AEMs were investigated. The effect of water content in the membrane and the effect of ion exchange capacity (IEC) were also studied. The results are compared with the widely studied proton exchange membrane Nafion.

Parametrization of Chain Molecules in Dissipative Particle Dynamics.

This paper presents a consistent strategy for parametrization of coarse-grained models of chain molecules in dissipative particle dynamics (DPD), where the soft-core DPD interaction parameters are fitted to the activities in solutions of reference compounds that represent different fragments of target molecules. The intercomponent parameters are matched either to the infinite dilution activity coefficients in binary solutions or to the solvent activity in polymer solutions. The respective calibration relationships between activity and intercomponent interaction parameter are constructed from the results of Monte Carlo simulation of the coarse-grained solutions of reference compounds. The chain conformation is controlled by the near neighbor and second neighbor bond potentials, which are parametrized by fitting the intramolecular radial distribution functions of the coarse-grained chains to the respective atomistic molecular dynamics simulations. The consistency, accuracy, and transferability of the proposed parametrization strategy is demonstrated drawing on the example of nonionic surfactants of the poly(ethylene oxide) alkyl ether (CnEm) family. The lengths of tail and head sequences are varied (n = 8-12 and m = 3-9), so that the critical micelle concentration ranges from 10 to 0.1 mM. The surfactants are modeled at different coarse-graining levels using DPD beads of different diameters. We found consistent agreement with experimental data for the critical micelle concentration and aggregation number, especially for surfactants with relatively long hydrophilic segments. Depending on the system, we observed surfactant aggregation into spheroidal, elongated, or core-shell micelles, as well as into irregular agglomerates. Using the models at different coarse-graining levels for the same molecules, we found that the smaller the bead size the better is agreement with experimental data.

Dissipative Particle Dynamics interaction parameters from ab initio calculations

Dissipative Particle Dynamics (DPD) is a commonly employed coarse-grained method to model complex systems. Presented here is a pragmatic approach to connect atomic-scale information to the meso-scale interactions defined between the DPD particles or beads. Specifically, electronic structure calculations were utilized for the calculation of the DPD pair-wise interaction parameters. An implicit treatment of the electrostatic interactions for charged beads is introduced. The method is successfully applied to derive the parameters for a hydrated perfluorosulfonic acid ionomer with absorbed vanadium cations.

Meso-Scale Study of Hydrated Nafion with Diffused Vanadium Cations and Sulfuric Acid

Recently, there has been an increased interest in redox flow batteries (RFBs) in order to transpire the substitution of fossil fuels with renewable energy sources such as solar and wind. Having the power and energy decoupled, these batteries can store electrical energy in the form of chemical energy on a multi-megawatt scale. Particularly, all-vanadium redox flow batteries (VRFBs) are of interest for grid-scale electrical energy storage due to long lifespan, fast response, and no inherent cross-contamination [1]. The VRFB has vanadium cations at four different oxidation states as its electro-active species. The four vanadium cations are solvated in a concentrated sulfuric acid solution as the supporting electrolyte. The V3+/V2+ couple at the negative electrolyte tank and the VO2+/VO2 +couple at the positive tank. They are separated by an ion exchange membrane that only allows the permeation of one type of ion (i.e., cation or anion) in order to complete the electrical circuit.Membranes used in these systems should exhibit low permeability to vanadium cations, high proton or sulfate conductivity, good thermal and chemical stability, good mechanical integrity, and low cost [1, 2]. One of the most widely used cation exchange membranes for VRFBs is perfluorosulfonic acid (PFSA) membrane, Nafion. In the presence of water Nafion exhibits a phase separated morphology with the water and ions confined into domains surrounded by the hydrophobic polymer matrix. The water domains are lined with sulfonate groups which facilitate cation transfer [3-6]. In a VRFB environment protons and vanadium cations can permeate through the water domains. Although, the mobility of the vanadium cations is much lower than that of a proton, over time the crossover of vanadium cations results in capacity loss and electrolyte contamination [7]. In addition, the unexpected diffusion of sulfuric acid into Nafion is reported [8, 9]. Hence, there is ongoing research to decrease solvent and vanadium cation transport across the membrane. And since Nafion is one of the most stable membranes for VRFBs it is important to understand the effect of vanadium and sulfuric acid diffusion on the hydrophobic and hydrophilic domains of the membrane as well as its morphology. In this work dissipative particle dynamics (DPD) simulations were undertaken to understand the hydrated membrane morphology in the presence of absorbed vanadium cations and sulfuric acid molecules. Distinct DPD beads were constructed based on our previous work on hydrated PEMs and recent studies on the hydration structure of vanadium cations in water and the effect of sulfuric and triflic acids [10-12]. Since the sulfonic acid group is very likely to interact with vanadium cations through their primary hydration shell, the sulfonic acid and hydrated vanadium cations are considered as separate kinds of DPD beads. The structures of the DPD beads were optimized using first principles electronic structure calculations. The interaction parameters were determined by mapping the interaction energies obtained from the ab initiocalculations to the DPD conservative force, while matching the compressibility with that of the actual system. In order to maintain the principle of electroneutrality upon the addition of vanadium cations to the system, the acidity was decreased through the removal of hydrated protons.The effect of equivalent weight, molecular weight, hydration level, and the ratio of absorbed species to the sulfonate groups on the morphology are examined. The radial distribution functions (RDFs) were generated and used to characterize the simulation results. The root mean square end to end distance for the ionomer was obtained and used to characterize backbone aggregation. The hydrated morphologies were further investigated by obtaining two dimensional contour plots detailing the density distribution of each bead type.

A Mesoscopic Model for an Asphaltene and Complex Mixtures of Asphaltenes

A representation for an asphaltene system and asphaltene mixtures is proposed based on experimental data reported by Sócrates et al. These were modeled at the mesoscopic level by applying dissipative particle dynamics (DPD), where DPD beads correspond to the main structure that constitutes the real asphaltene fraction. The experimental solubility parameters were used to obtain the Flory–Huggins parameter, based on which the repulsive parameters aij, were calculated using the methodology developed by Groot and Warren (1997). Finally, the obtained asphaltene model, it presents the evolution of the Interfacial Tension in DPD unit values, according to the solvents used. It also describes how to obtain the asphaltene solubility parameters.

Molecular fragment dynamics study on the water-air interface behavior of non-ionic polyoxyethylene alkyl ether surfactants

Molecular Fragment Dynamics (MFD) is a mesoscopic simulation technique based on Dissipative Particle Dynamics (DPD). Whereas DPD beads in general may not necessarily be identified with chemical compounds at all the MFD variant uses specific molecules or molecular fragments as its basic coarse-grained interacting entities (rather than the fine-grained atom types of Molecular Mechanics). MFD can be used to study formulations of drugs and active agents in oil, water and emulsions. MFD simulations of the nonionic polyoxyethylene alkyl ether surfactants C6E6, C10E6, C12E6 and C16E6 at the water-air interface are performed to study their nanoscale structures and surface properties. The simulations of the self-aggregation of the polyoxyethylene alkyl ether surfactants lead to equilibrium nanoscale structures and computationally determined surface tensions which are in agreement with experimental data for different surfactant concentrations [1]. Figure 1

Scaling behaviour of different polymer models in dissipative particle dynamics of unentangled melts

We present a dissipative particle dynamics (DPD) study of scaling behaviour for three polymer models. The scaling behaviour is explored for the conformational and dynamic properties of unentangled polymer melts. DPD employs a bead–spring model together with an aggressive coarse-graining to represent polymers at the mesoscale. The first model studied utilises a simple soft repulsion potential for the bead–bead interactions together with a harmonic spring potential to connect beads into a polymer chain. The second model differs from the first model by replacing the harmonic spring with a finitely extensible nonlinear elastic spring. The third model uses realistic coarse-grain potentials for the bead–bead, spring and bending interactions based on the iterative Boltzmann inversion procedure and it corresponds to a mesoscopic model of polyethylene. We systematically vary the chain length and spring constant (in the case of the first and second models), and simulate the conformational properties such as the end-to-end distance or radius of gyration, and dynamic properties such as the centre-of-mass self-diffusion coefficient or viscosity. The scaling of the conformational and dynamic properties with chain length (scaling laws) is compared with the Rouse theory, which is considered as a standard theory for unentangled polymer melts. The comparison shows that simulated scaling laws typically agree with the Rouse scaling laws for the DPD polymer models with more than 10 DPD beads. For the shorter DPD polymers, deviations from the Rouse theory exist and become significant for the dynamic properties, especially for the viscosity of the polymer melts.

Critical micelle concentration of an ammonium salt through DPD simulations using COSMO‐RS–based interaction parameters

In order to determine the critical micelle concentration (CMC) of aqueous dodecyltrimethylammonium chloride (DTAC), a screening of the DTAC gathering process at different molalities by performing dissipative particle dynamics (DPD) simulations over mesomolecules whose beads interact via repulsive conservative forces was performed. Conductor‐like screening model for real solvent quantum methodology, applied to molecular segments that describe the DTAC chemistry and were mapped onto the DPD beads, allows us the computing of activity coefficients at infinite dilution to obtain thermodynamic Flory–Huggins interaction parameters, from which we calculated the maxima repulsive conservative forces, that is, the DPD interaction parameters. Results indicate that at room temperature the CMC is 0.0217 mol/kg, the aggregation number ranges from 46 to 54 molecules, and the aggregate radius varies from 19.58 to 22.02 Å; all values are in excellent agreement with literature reported experimental ones of 0.0213 mol/kg, 47 5 molecules, and 20.1 Å. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4413–4423, 2013

Interfacial tension in oil-water-surfactant systems: on the role of intra-molecular forces on interfacial tension values using DPD simulations.

We have computed interfacial tension in oil-water-surfactant model systems using dissipative particle dynamics (DPD) simulations. Oil and water molecules are modelled as single DPD beads, whereas surfactant molecules are composed of head and tail beads linked together by a harmonic potential to form a chain molecule. We have investigated the influence of the harmonic potential parameters, namely, the force constant K and the equilibrium distance r0, on the interfacial tension values. For both parameters, the range investigated has been chosen in agreement with typical values in the literature. Surprisingly, we observe a large effect on interfacial tension values, especially at large surfactant concentration. We demonstrate that, due to a subtle balance between intra-molecular and inter-molecular interactions, the local structure of surfactants at the oil-water interface is modified, the interfacial tension is changed and the interface stability is affected.

Dielectric response of nanoscopic spherical colloids in alternating electric fields: a dissipative particle dynamics simulation

We study the response of single nanosized spherical colloids in electrolyte solution to an alternating electric field (AC field) by computer simulations. We use a coarse-grained mesoscopic simulation approach that accounts in full for hydrodynamic and electrostatic interactions as well as for thermal fluctuations. The solvent is modeled as a fluid of single dissipative particle dynamics (DPD) beads, and the colloidal particle is modeled as a rigid body made of DPD beads. We compute the mobility and the polarizability of a single colloid and investigate systematically the effect of amplitude and frequency of the AC fields. Even though the thickness of the Debye layer is not ‘thin’ compared to the radius of the colloid, and the thermal fluctuations are significant, the results are in good agreement with the theoretical prediction of the Maxwell–Wagner–O’Konski theory, especially for uncharged colloids.