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Context: To determine the miscibility of liquids at high temperatures using the concept of Hildebrand solubility parameter δ , the current practice is to examine the difference in δ between two liquids at room temperature, assuming that δ is not sensitive to temperature. However, such an assumption may not be valid for certain polymer blends and solutions. Therefore, a knowledge of the δ values of the liquids of interest at high temperatures is desirable. The determination of δ at high temperatures, especially for high-molecular-weight polymers, is impossible, as polymers have vapor pressures of zero. To this end, molecular dynamics (MD) simulations provide a practical means for determining δ over a wide range of temperatures. In this work, we study the temperature dependence of δ of five hydrocarbon polymers: polyethylene (PE), isotactic and atactic polypropylene (i-PP and a-PP), polyisobutylene (PIB), and polyisoprene (PI) in five hydrocarbon solvents: n-pentane, n-hexane, n-dodecane, isobutene, and cyclohexane. The polymers are modeled as monodisperse chains with 100 repeat units. The average δ values of PE, i-PP, a-PP, PIB, and PI at 300 K are determined as 18.6, 14.9, 14.6, 14.3, and 16.4 MPa1/2, respectively, in a good agreement with experimental data. The δ values of these polymers at various temperatures are also determined. The temperature dependence of δ is fitted to two linear equations, one above and the other below the polymer's glass transition temperature Tg. The δ values are more sensitive to temperature at T ≥ Tg. The Tg values of the polymers, determined based upon their specific volumes and δ values agree with the experiment qualitatively. The determination of the temperature dependence of δ has a great potential for industrial applications, such as determining miscibility, developing polymeric organogelators as flocculants and oil spill treating agents, and identifying potential solvents and ideal processing temperatures. Methods: The MD simulations are performed using the GROMACS 2022.3 package with optimized potential for liquid simulations-all atom (OPLS-AA) force field parameters. All polymers are built as extended chains using CHARMM-GUI Polymer Builder. [ABSTRACT FROM AUTHOR]

We used molecular dynamics simulation to study the size distribution and coalescence of free volume around an amphiphilic molecule (nonyl ethoxylate (NE)) at a concentration of 0.5 wt% in blends of linear and branched polyethylene that contained a small four‐arm alkane (7,12 hexyl octadecane). The branched polyethylene chains had 10 and 82 hexyl branches/1000 backbone carbons. The coalescence dynamics (fluctuation of free volume in time) was quantified by the number of Fourier frequencies and the corresponding power (amplitude). In our previous work, we hypothesized that cavitation, the first step of environmental stress cracking observed experimentally, starts from the free volume coalescence at the interface between NE and polyethylene molecules and showed that hexyl branches in the branched polyethylene molecules suppress such coalescence, suggesting that cavitation, a much longer time scale process, could also be slowed down. The current work showed that the behavior of hexyl branches in branched polyethylene in a blend with linear polyethylene was similar to that in pure branched polyethylene, but that the hexyl branches in the 7,12 hexyl octadecane exhibited the opposite behavior. In particular, they intensified free volume coalescence, especially around the hydrophilic ethylene oxide segments of NE. The addition of 7,12 hexyl octadecane to branched polyethylene alone or blended with linear polyethylene does not seem to slow down free volume coalescence (cavitation), leading us to conclude that the effect of hexyl branches on the free volume coalescence around NE depends on the size of the backbone to which the branches are attached. Highlights: Hexyl branches reduce free volume coalescence activities.The longer the backbone is, the stronger the hexyl branch effect.More coalescence activities occur on the hydrophilic segment of NE. [ABSTRACT FROM AUTHOR]

Using a many-chain system of Gaussian chains interacting with each other through the Lennard-Jones (LJ) potential, we demonstrate that there exists a connection between polymer fractional free volume (f) and the entanglement chain length (Ne). Here, f is determined by the generic van der Waals (GvdW) equation of state using the intermolecular radial distribution function [g(r)] of the Gaussian chain generated by the polymer reference interaction sites model while Ne by the polymer mode-coupling theory (PMCT) for strongly coupled macromolecules. The key concept in PMCT is that intermolecular forces experienced by a bead in a chain surrounded by other chains are strongly coupled to the site specific intramolecular radial distribution function. The calculated Gp and η of polyethylene with chain lengths from 500 to 2000 interacting with one another through the LJ potential agree well with experiment and molecular dynamics simulation. [ABSTRACT FROM AUTHOR]

We carried out molecular dynamics (MD) simulations on polyethylene (PE) with linear, ring, and four‐arm symmetrical star structures at various molecular weights (420 − 5700 g ⋅ mol−1) and temperatures (410−450 K). We then analyzed the MD data using the technique of proper orthogonal decomposition (POD) to obtain the time correlation functions of different eigenmodes, thereby calculating the viscosity of the aforementioned macromolecules. The time correlation functions show that the ring and star PEs relax much faster than their linear counterpart. Free volume size distributions and the mean fractional free volume of the equilibrated PE melts were determined by Voronoi tessellation (VT). The mean fractional free volume and the corresponding effective pressure were then used to calculate viscosity using a polymer free volume theory recently developed in our lab. The POD and VT approaches yielded similar viscosity values. Furthermore, they both successfully predicted the crossover in the molecular weight dependence of viscosity. It is interesting to note that the difference in the free volume parameters (ϕ+ − F) of various structures always fall within the range of 0.02−0.06. Here, F and ϕ+ signify the probability of a bead having enough free volume for the activation of diffusive motion or momentum transfer and the minimum required fraction of such beads, respectively. The temperature dependence of viscosity as obtained from the POD and VT approaches gave comparable apparent activation energy (Eaapp) values of 5.8−6.4 and 5.4−6.4 kcal ⋅ mol−1, respectively, for the three structures. [ABSTRACT FROM AUTHOR]

Relaxation dynamics of unentangled polymer melts (with a chain length below N = 100 for polyethylene) can be studied by performing either Rouse mode or relaxation mode analysis, such as proper orthogonal decomposition. The former mode analysis can be derived analytically from the linear equation of motion of polymers with the inclusion of inertia term, whereas the latter is designed for the case when the numerical solution to a highly nonlinear equation of motion is known, such as MD simulation data. This article presents a summary of both approaches and their applications in the evaluation of zero-shear viscosity of polymer melts. [ABSTRACT FROM AUTHOR]

• Obtain crystal and amorphous layered double oxides (LDOs) using molecular dynamics. • Overall CO 2 physisorption on amorphous LDO is higher than that on crystalline LDO. • CO 2 form stable adsorption layer near crystalline LDO, preventing further adsorption. • CO 2 shows ordered arrangement on crystalline LDO, but random on amorphous LDO. Understanding the atomic structure and the corresponding adsorption mechanism of CO 2 adsorbents is the key to develop efficient CO 2 adsorption materials. Here, we studied hydrotalcite-derived oxides or layered double oxides (LDOs), one of the most promising candidates for the inexpensive and effective CO 2 adsorption in the intermediate temperature regime, using atomistic simulation techniques. We first constructed the equilibrium structure of an LDO and showed that this oxide was crystalline with an MgO-like structure. In particular, Mg ions configured with the FCC arrangement of oxygen ions and most of the Al3+ ions occupied octahedral positions by substituting Mg2+ ions and the rest (∼10%) occupied the tetrahedral positions. We then compared the CO 2 physisorption properties of the amorphous and crystalline LDOs that respectively represented LDOs form at lower and higher limits of hydrotalcite calcination. Static and dynamic adsorption analyses revealed that the amorphous LDO had overall higher physisorption capacity. CO 2 molecules exhibited a random arrangement in the adsorption layer of the amorphous LDO but an ordered arrangement in the case of crystalline LDO. As a result, CO 2 molecules were able to adsorb to and to desorb from the amorphous LDO surface dynamically while the stable adsorption layer near the crystalline LDO surface acted like a contamination preventing other CO 2 molecules to adsorb, thereby lowering the overall adsorption capacity of the crystalline LDO. Depending on the level of crystallinity, the physisorption behavior of the mixed oxide forming during the hydrotalcite calcination is expected to change between two structures studied here. [ABSTRACT FROM AUTHOR]

Steady state pressure driven flow of liquid argon through a finite length cylindrical nanopore was investigated numerically by classical Navier-Stokes (NS) hydrodynamic models and nonequilibrium molecular dynamics (MD) simulations. In both approaches, the nanopore was nominally 2.2 nm in diameter and 6 nm long. For the MD simulations, the intermolecular properties of the walls were specified independently from the liquid. Comparisons between the approaches were made in terms of the gross feature of total flow rate through the nanopore, as well as the more refined considerations of the spatial distributions of pressure, density, and velocity. The results showed that for the NS equations to predict the same trends in total flow rate with increasing pressure difference as the MD simulation, submodels for variations in density and viscosity with pressure are needed to be included. The classical NS boundary conditions quantitatively agreed with the flow rate predictions from MD simulations only under the condition of having a neutral-like solid-liquid interaction. Under these conditions, the NS and MD models also agreed well in streamwise distributions of pressure, density, and velocity, but not in the radial direction. [ABSTRACT FROM AUTHOR]

Fluid transport through a nanopore in a membrane was investigated by using a novel molecular dynamics approach proposed in this study. The advantages of this method, relative to dual-control-volume grand-canonical molecular dynamics method, are that it eliminates disruptions to the system dynamics that are normally created by inserting or deleting particles from control volumes, and that it functions well for dense systems due to the number of particles being fixed in the system. Using the proposed method, we examined liquid argon transport through a nanopore by performing nonequilibrium molecular dynamics (NEMD) simulations under different back pressures. Validation of the code was performed by comparing simulation results to published experimental data obtained under equilibrium conditions. NEMD results show that constant pressure difference across the membrane was readily achieved. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations using a modified Drieding 2.21 force field were carried out to study the coalescence behavior of nanometer-sized water droplets in vacuum and in n-heptane. The coalescence mechanisms of the water droplets in the above-noted environments are fairly similar in a sense that the water droplets form a bridge linking the droplets before they merge. However, in the latter situation, due to the presence of n-heptane molecules in between the water droplets, the coalescence was observed to be slowed down considerably, especially in the first 10 ps of the process. However, once the bridge is formed, the water droplets, in both situations, spend about the same amount of time to form a single droplet. The maximum distance between the droplets above which coalescence does not occur was found to be 10 Å. In terms of the dynamics, the diffusion coefficient of n-heptane in the emulsion system was very close to its value in the pure liquid form. This may be because n-heptane is the continuous phase. Nonetheless, the dynamic behavior of water in n-heptane is different from that of pure water during and after the coalescence. In particular, the self-diffusion coefficient of water molecules in n-heptane is about 20% higher than the experimental value of pure water. Due to the lack of strong attraction forces between water and n-heptane molecules, the n-heptane molecules were observed to orient themselves perpendicularly to the water/n-heptane interfaces so that the contacting area is minimized. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Inclusion of the inertia term gives the velocity time correlation function. • TCFs of different normal modes can be either overdamped or underdamped. • The rotational time correlation function isalways overdamped. Abstract In this paper, with the inclusion of inertia term in the equation of motion, the expression of the velocity time correlation of a Rouse chain, for which hydrodynamic interactions are neglected, is analytically derived. For long time scale mode of motion, the velocity time correlation function is an exponential decay function of time, whereas for short time scale mode of motion, it is a damped oscillatory function of time. This was confirmed by our numerical simulation results. [ABSTRACT FROM AUTHOR]

We used isobaric–isothermal molecular dynamics simulation to study the diffusivity of a hydrophobic drug Cucurbitacin B (CuB) in polymer matrices made up of a single polyethylene oxide- b -polycaprolactone (PEO- b -PCL) chain and different numbers of water molecules at body temperature (310 K) and ambient pressure (1 bar). Four PEO- b -PCL models were used. Three of them had the same total molecular weight of 3750 g/mol but different PCL/PEO weight ratios at 0.5, 1 and 2. The remaining model had a total molecular weight of 5000 g/mol and a PCL/PEO weight ratio of 1. The data showed that self-diffusion coefficients of CuB decreased with increasing PCL/PEO weight ratio and was insensitive to the water concentration at fixed PCL/PEO weight ratios. The first observation was attributed to a high number of hydrogen bonds formed between the oxygen atoms on CuB and the activated hydrogen atoms on the PCL block as the PCL/PEO weight ratio increased. It was found that degree of swelling and effects of the mobility of the PEO and PCL blocks on the diffusivity of CuB were minor and were overshadowed by the PCL block-CuB hydrogen bonds. [ABSTRACT FROM AUTHOR]

We used molecular dynamics (MD) simulation to investigate whether an environmental stress cracking (ESC) agent would help the cavitation process in linear polyethylene (LPE) and branched polyethylene (BPE) by examining the free volume hole size and spatial distributions at the [PE]-[ESC agent] interfaces. The BPE models used contained 10 and 82 butyl/hexyl branches/1000 backbone carbons and the ESC agent used was nonyl ethoxylate (NE) at concentrations 0.001–1.4 w t %. The results show that there exist no significant changes in the packings of LPE and BPE chains, as characterized by their radial distribution functions (RDFs), in the presence of NE regardless its concentration. However, molecular packings at the [PE]-[NE] interfaces vary with NE concentration. In particular, ethylene (E) segments of NE, compared to the ethylene oxide (EO) segments, exhibit stronger association with LPE and BPE chains. We used the Voronoi tessellation method to determine the size and spatial distributions of free volume holes at the [PE]-[NE] interfaces and found that more and larger free volume holes formed around the E-segments than the EO-segments. And such effect was found to be more pronounced in the case of LPE chains than the BPE chains. We further investigated the free volume holes coalescence dynamics using the Fourier mode analysis. The number of frequency modes and the corresponding intensities of the E-segments were significantly higher than those of EO-segments, suggesting that E-segments exhibited a higher tendency to form more and larger free volume holes. This suggests that [PE]-[E-segments] interfaces have a higher chance to form larger voids (i.e., cavitation, a feature of brittle failure that is experimentally observed in ESC). The tendency intensified with increasing NE concentration but was reduced by the presence of short chain branches. The high mobility of terminal carbons in the short chain branches is believed to suppress the formation of large free volume holes at the [PE]-[E-segments] interfaces by absorbing the newly formed free volume by that of the terminal carbons. [Display omitted] • Free volume holes coalescence occurs at the hydrophobic part of an ESC agent. • Free volume holes coalescence likely causes cavitation. • Short chain branches suppress free volume holes coalescence. [ABSTRACT FROM AUTHOR]

The conformation and dynamics of precisely branched polyethylene, i.e., polyethylene molecules containing regularly spaced, short chain branches along its linear backbone, were studied using molecular dynamics simulation. Models with branch content varying from 0 to 47.6 branches per 1000 backbone carbons were studied over a temperature range of 273–550 K and under a constant pressure of 1 bar. Two types of models were built, one of which contained ethyl branches while the other contained hexyl branches. The results indicated that at a given temperature, the global orientation order parameter decreased almost linearly with increasing branch content up to a value of approximately 38.5 and increased considerably and unexpectedly at a branch content of 47.6. The order parameter was insensitive to the branch length except at high branch contents. The computed packing lengths and activation energy for diffusion are consistent with the above observations. [ABSTRACT FROM AUTHOR]

Isobaric–isothermalmolecular dynamics simulation was usedto study the diffusion of a hydrophobic drug Cucurbitacin B (CuB)in pseudomicelle environments consisting of poly(ethylene oxide-b-caprolactone) (PEO-b-PCL) swollen byvarious amounts of water. Two PEO-b-PCL configurations,linear and branched, with the same total molecular weight were used.For the branched configuration, the block copolymer contained onelinear block of PEO with the same molecular weight as that of thePEO block used in the linear configuration but with one end connectingto three PCL blocks with the same chain length, hereafter denotedPEO-b-3PCL. Regardless of the configuration, thesimulation results showed that the diffusivity of CuB was insensitiveto the water concentration up to ∼8 wt % while that of waterdecreased with an increasing water concentration. The diffusivityof CuB (10–8cm2/s) was 3 orders of magnitudelower than that of water (10–5cm2/s).This is attributed to the fact that CuB relied on the wiggling motionof the block copolymers to diffuse while water molecules diffusedvia a hopping mechanism. The rates at which CuB and water diffusedinto PEO-b-PCL were twice those in PEO-b-3PCL because the chain mobility and the degree of swelling are higherand there are fewer intermolecular hydrogen bonds in the case of PEO-b-PCL. The velocity autocorrelation functions of CuB showthat the free volume holes formed by PEO-b-3PCL aremore rigid than those formed by PEO-b-PCL, makingCuB exhibit higher-frequency collision motion in PEO-b-3PCL than in PEO-b-PCL, and the difference in frequencyis insensitive to water concentration. [ABSTRACT FROM AUTHOR]

Abstract: We used molecular dynamics simulation together with the OPLS-AA force field along with a high temperature equilibration protocol proposed by Belmares et al. to calculate selected thermodynamic properties of poly(vinyl alcohol) (PVA) with different tacticities. The results showed that the OPLS-AA force field was able to reproduce specific volumes, thermal expansion coefficients, glass transition temperatures and solubility parameters of the PVAs over a wide range of temperatures (200–550 K). PVA with different tacticities in the amorphous phase showed different solubility parameters but possessed similar specific volumes, thermal expansion coefficients and glass transition temperatures. For heat capacities, 300% overestimations were obtained. Such overestimations were reduced significantly to about 30% by applying the quantum correction method of Berens et al. We have also applied the newly developed two-phase thermodynamic (2 PT) approach in an attempt to further improve the results but failed. [Copyright &y& Elsevier]

Abstract: Molecular dynamics simulation was used to study the diffusivity of water at low concentrations in two types of amorphous polyurethanes PU1 and PU2. PU1 was made up of castor oil and 4,4′-diphenylmethano-diisocyanate (MDI), a polyurethane that is not swollen by water, and PU2 was made up of polyethylene glycol (PEG) with MDI, a polyurethane exhibiting a high degree of swelling by water. PU2 also has the ability to form a considerable amount of hydrogen bonds with water molecules. To mimic the non-swollen and swollen behavior, canonical (i.e., NVT) and isothermal-isobaric (i.e., NPT) ensembles were used for PU1 and PU2, respectively. Simulation results showed that the diffusivity of water in PU1 was insensitive to its concentration at low concentrations but decreased with increasing concentration for PU2. Further data analysis revealed that in the case of PU1, both the mean number of hops of water molecules from one free volume hole to another and the mean hopping distance did not vary with water concentration. In other words, at low concentrations, hopping of water molecules is not affected by the presence of other water molecules, suggesting that there is no blocking effect. In the case of PU2, the mean number of hops did decrease drastically in the concentration range of 0–2 wt% and leveled off at higher concentrations while the mean hopping distance exhibited a slight decrease. It was found that the observed behavior is mainly attributed to the hydrogen bonds formed between PU2 and water molecules. However, it is the swelling behavior (i.e., high below 2 wt% but almost constant above 2 wt%) leads to the concentration dependence profile of diffusion coefficient. Compared with the theory of Barrie and Platt, the computed diffusivity is much lower than that is predicted by the theory. This is because the theory does not include effects from the hydrogen bonds formed between PU2 and water molecules and the swelling behavior of PU2. [Copyright &y& Elsevier]

A novel algorithm is proposed to study fluid properties in the isothermal-isobaric (NPT) ensemble. The major feature of this approach is that the constant pressure in the NPT ensemble is created by two auto-adjusting boundaries that allow the system volume to fluctuate. Relative to other methods used to create the NPT ensemble, this approach is simpler to perform since no additional variables are introduced into the simulation system. To test this method, two systems with the same constant target pressure and temperature but different thermostats (Nose-Hoover and Berendsen) were performed by using a commonly used cut-off distance (i.e. rc = 2.5σ). The simulation results show that the proposed method works well in terms of creating spatially uniform mean temperature, pressure and density while still allowing appropriate levels of instantaneous fluctuations for observable quantities. The fluctuations of the system volume produced by this method were compared with that calculated by the theoretical equation. To test the reliability of the proposed method, additional simulations were carried out at eight different thermodynamic states but with the use of a longer cut-off distance (rc = 4.5σ). The results were compared with those obtained using the Nose-Hoover barostat with an rc of 4.5σ, as well as with experiments. The comparison shows that the results using the algorithm proposed in this article agree well with those obtained using other methods. [ABSTRACT FROM AUTHOR]

The effect of cut-off distance used in molecular dynamics (MD) simulations on fluid properties was studied systematically in both canonical (NVT) and isothermal-isobaric (NPT) ensembles. Results show that the cut-off distance in the NVT ensemble plays little role in determining the equilibrium structure of fluid if the ensemble has a high density. However, pressures calculated in the same NVT ensembles strongly depend on the cut-off distance used. In the NPT ensemble, cut-off distance plays a key role in determining fluid equilibrium structure, density and self-diffusion coefficient. The characteristic of the radial distribution function of fluid in NPT ensembles depending on the cut-off distance used in MD simulations means that the WCA theory (a perturbation theory developed by Weeks, Chandler and Andersen) is not suitable for NPT ensembles because the assumption (the effect of the attractive force in determining the liquid structure is negligible) used in the WCA theory is not valid. The dependence of fluid properties on the cut-off distance also indicates that using the WCA potential (the repulsive part of the intermolecular potential proposed in the WCA theory) to calculate fluid transport in heterogeneous systems could lead to significant errors or incorrect results. [ABSTRACT FROM AUTHOR]

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Conformational energies of models of polyethylene (PE), isotactic-polypropylene (i-PP), atactic-polypropylene (a-PP) and polyisobutylene (PIB) with different chain lengths in their unperturbed liquid state (El), unperturbed isolated state (Eiu) and perturbed isolated state (Eip) at 190 °C were computed using molecular dynamics simulation. The computed El, Eiu and Eip were then used to calculate the solubility parameters of the polymers (δ). Our results indicate that there exists a cutoff chain length (∼20 backbone carbons) above which the differences between the solubility parameters (Δδ) computed based on (Eiu−El) and (Eip−El), respectively, become significant. In addition, it was found that the higher the degree of shrinkage of the polymer in vacuum is, the larger the Δδ is. Since PE and PIB exhibited considerable shrinkage in vacuum, their Δδ are much higher than those of the PPs. It seems that, at 190 °C, vacuum acts more or less like a Θ solvent for both types of polypropylenes but a bad solvent for PE and PIB. Our results also suggest that the characteristic ratio of the polymer has little effect on Δδ and that it was the long range attractive interactions between the monomers distant along the backbone contour of the same molecule that led to the formation of fairly compact globule in the cases of PE and PIB. Since δ of the high molecular weight models computed from (Eiu−El) agree with experiment better than those from (Eip−El), it implies that δ obtained from indirect measurements correspond to a hypothetical vaporization process where the conformations of polymer coils do not undergo significant changes. In other words, δ of a polymer that is determined by the currently available indirect methods may deviates significantly from their ‘true’ value if the polymer exhibits a high degree of swelling or shrinkage in vacuum at the temperature of the experiment. [Copyright &y& Elsevier]

Molecular dynamics simulation used to study the binary polymer blend of poly (ether imide) (PEI) and polycarbonate (PC) showed that these polymer blends are immiscible. The Flory-Huggins interaction parameter, χ, calculated from simulation reached a minimum value at 80 wt% PEI. The simulation results showed that the concentration dependence of χ was mainly due to electrostatic interaction and van der Waals force. The simulation results were supported by differential scanning calorimetry (DSC) measurements. The DSC measurements showed that there are two distinct glass transition temperatures for all the blends'' concentrations. However, at 80wt % PEI, the Tg of PEI-rich phase reached a minimum while that of the PC-rich phase was comparable to its pure form indicating that there is some partial miscibility of PC in the PEI rich phase, but no PEI is incorporated in the PC rich phase. From simulations, the χ versus concentration plot shows the same trend as the experimentally measured glass transition temperature versus concentration plot. [Copyright &y& Elsevier]

• Presenting force field parameters for description of millerite (NiS) within the frame work of molecular dynamics simulation. • Estimation of bulk and surface properties including surface energy and contact angle. • Investigating the effect of oxidization on the surface wettability. • Investigating the effect of strong alkaline solution on the affinity of collector molecules towards the charged surfaces. The present study aims at developing force field parameters for describing the millerite structure and subsequently for investigation of its interfacial properties under varying conditions within the framework of molecular dynamics simulations. The first set of calculations were performed to obtain the required force field parameters, validated through reproduction of the bulk properties. This was followed by estimation of the surface energy and evaluation of the collector physisorption onto the non-polar (100) and (110) surfaces. Collector molecules exhibited higher affinity towards the former, with the branched alkyl chain favoring the interactions with either of the surfaces. Further simulations were conducted to examine the changes in the contact angle of the intrinsically hydrophilic (110) surface upon oxidization. Observations through transition from the neutral surface fully covered by elemental sulfur to a charge-bearing surface containing sulfur and polysulfides were compared against the theoretical predictions made by the Lippmann equation, and the discrepancies were attributed to the significance of line tension at the atomic scale. Finally, propensity of collector molecules towards the positively and negatively charged (1 1 0) surface was examined in the absence and presence of excess calcium hydroxide. In both cases, the collector-surface interactions were impeded in the strong alkaline solution. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations were carried out on single chain models of linear low-density polyethylene in vacuum to study the effects of branch length, branch content, and branch distribution on the polymer’s crystalline structure at 300 K. The trans/gauche (t/g) ratios of the backbones of the modeled molecules were calculated and utilized to characterize their degree of crystallinity. The results show that the t/g ratio decreases with increasing branch content regardless of branch length and branch distribution, indicating that branch content is the key molecular parameter that controls the degree of crystallinity. Although t/g ratios of the models with the same branch content vary, they are of secondary importance. However, our data suggests that branch distribution (regular or random) has a significant effect on the degree of crystallinity for models containing 10 hexyl branches/1,000 backbone carbons. The fractions of branches that resided in the equilibrium crystalline structures of the models were also calculated. On average, 9.8% and 2.5% of the branches were found in the crystallites of the molecules with ethyl and hexyl branches while C[sup 13] NMR experiments showed that the respective probabilities of branch inclusion for ethyl and hexyl branches are 10% and 6% [Hosoda et al., Polymer 1990, 31, 1999–2005]. However, the degree of branch inclusion seems to be insensitive to the branch content and branch distribution. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A non‐linear Brownian dynamics (BD) model based upon the Rouse model is developed for unentangled polymer melts. The essence of the model is that it includes an equilibrium spring length b, a non‐linear force term that is not present in the equation of motion of the Rouse model. The equation of motion is solved numerically with the technique of proper orthogonal decomposition (POD) to obtain the dynamics information. To illustrate that the BD model can be readily implemented on polymers with different molecular structures, it is applied to polyethylene with linear, ring, and star structures. For comparison purpose, the corresponding molecular dynamics (MD) simulation is also carried out on molecules with the same sizes (N = 30–73). To characterize the dynamics, time correlation functions of the end‐to‐end vector, the m‐to‐n vector, and the arm vector of the linear, ring, and star polyethylene are determined respectively. It is found that the longest relaxation times (τ1s), the relaxation times of the vectors (τvs), as well as zero‐shear viscosity (η0s) obtained from the BD and MD simulations agree well with each other. The time correlation functions can be reasonably described using the eigenmodes. [ABSTRACT FROM AUTHOR]

The early-stage competitive co-adsorption of interfacially active naphthenic acids (NAs) and polyaromatic (PA) molecules to a toluene-water interface from the bulk toluene phase was studied using molecular dynamics (MD) simulation. The NA molecules studied had the same polar functional group but different cycloaliphatic nonpolar tails, and a perylene bisimide (PBI)-based molecule was used as a representative PA compound. The results from our simulations suggest that the size and structural features of NA molecules greatly influence the interfacial activity of PA molecules and partitioning of NA molecules at the toluene-water interface. At low concentrations of PA (∼2.3 wt %) and NA (∼0.4 wt %) molecules, NA molecules containing large cycloaliphatic rings (e.g., four rings) or with a very long aliphatic tail (e.g., carbon chain length of 14) were observed to impede the migration of PA molecules to the interface, whereas small NA molecules containing two cycloaliphatic rings had little effect on the adsorption of PA molecules at the toluene-water interface. At high NA concentrations, the adsorption of PA molecules (∼5.75-17.25 wt %) was greatly hindered by the presence of small NA molecules (∼1.6-4.8 wt %) due to the solvation of PA nanoaggregates in the bulk. Adsorption mechanisms of PA and NA molecules at toluene-water interfaces were clarified through a detailed analysis on the interactions among different species in the system. The results obtained from this work provide insights into designing appropriate chemical demulsifiers or co-demulsifiers for breaking water-in-oil emulsions of great industrial applications. [ABSTRACT FROM AUTHOR]

Abstract: Molecular dynamics simulation was used to study the potential of using a block copolymer containing three poly(ɛ-caprolactone) (PCL) blocks of equal length connected to one end of a poly(ethylene oxide) (PEO) block, designated as PEO-b-3PCL, to encapsulate two classes of hydrophobic drugs with distinctively different molecular structures. In particular, the first class of drugs consisted of two cucurbitacin drugs (CuB and CuI) that contain multiple hydrogen bond donors and acceptors evenly distributed on their molecules while the other class of drugs (fenofibrate and nimodipine) contain essentially only clustered hydrogen bond acceptors. In the case of cucurbitacin drugs, the results showed that PEO-b-3PCL lowered the Flory–Huggins interaction parameters (χ) considerably (i.e., increased the drug solubility) compared to the linear di-block copolymer PEO-b-PCL with the same PCL/PEO (w/w) ratio of 1.0. However, the opposite effect was observed for fenofibrate and nimodipine. Analysis of the intermolecular interactions indicates that the number of hydrogen bonds formed between the three PCL blocks and cucurbitacin drugs is significantly higher than that of the linear di-block copolymer. On the other hand, owing to the absence of hydrogen bond donors and the clustering of the hydrogen bond acceptors on the fenofibrate and nimodipine molecules, this significantly reduces the number of hydrogen bonds formed in the multi-PCL block environment, leading to unfavourable χ values. The findings of the present work suggest that multi-hydrophobic block architecture could potentially increase the drug loading for hydrophobic drugs with structures containing evenly distributed multiple hydrogen bond donors and acceptors. [Copyright &y& Elsevier]

Abstract: Molecular dynamics (MD) simulation was used to investigate the solubility of two hydrophobic drugs Cucurbitacin B (CuB) and Cucurbitacin I (CuI) in poly(ethylene oxide)-b-poly(α-benzyl carboxylate ɛ-caprolactone) (PEO-b-PBCL) block copolymers with different tacticities. In particular, di-block copolymer with three different tacticities viz. PEO-b-iPBCL, PEO-b-sPBCL, and PEO-b-aPBCL were used. The solubility was quantified by calculating the corresponding Flory–Huggins interaction parameters (χ) using random binary mixture models with 10wt% of drug. The tacticity of the di-block copolymer was found to influence significantly the solubility of two drugs in it. In particular, based on MD simulation results, only PEO-b-sPBCL exhibited solubility while the other two did not. Given the fact that the drugs were shown to be soluble in PEO-b-PBCL experimentally, it is predicted that the tacticity of the di-block copolymer synthesized in experiment is syndiotactic. This predication matches well with the dominant ring opening polymerization of cyclic lactones to syndiotactic polymers by stannous octoate as catalyst used to prepare PEO-b-PBCL block copolymers in our previous experiments. The simulation results showed that the solubility of the drugs in PEO-b-sPBCL is attributed to the favorable intra-molecular interaction of the di-block copolymer and favorable intermolecular interaction between the di-block copolymer and the drugs. Radial distribution function analysis provides useful insights into the nature and type of the intermolecular interactions. [Copyright &y& Elsevier]

Stability of water/toluene interfaces saturated with adsorbed naphthenic acids with various sizes at room temperature was studied by molecular dynamics simulation. In direct comparison with our recent results on similar systems involving n-heptane as the oil phase, it has been found that toluene significantly enhances the stability of the water/oil interface, as suggested by the considerably reduced diffusion coefficients of the interface-bound naphthenic acids. The oil phase dependence may be rationalized in terms of the oil–naphthenic acid interactions, both electronically and sterically, and the intermolecular interactions between the molecules in the oil phase. Consistent with our previous studies, mobility of naphthenic acid has been found to be dependent on its size, with naphthenic acid featuring a longer carbon chain in between the carboxylic and cycloaliphatic groups showing lower mobility, leading to a more stable interface. However, such size dependence is much more pronounced for the water/toluene than the water/n-heptane interface. This may be understood in terms of the structural influence of toluene on the compactness of the liquid crystalline mesophase formed by naphthenic acids adsorbed at the interface. [Copyright &y& Elsevier]

The effects of branching characteristics of low-density polyethylene (LDPE) on its melt miscibility with high-density polyethylene (HDPE) were studied using molecular simulation. In particular, molecular dynamics (MD) was applied to compute Hildebrand solubility parameters ( δ ) of models of HDPE and LDPE with different branch contents at five temperatures that are well above their melting temperatures. Values computed for δ agreed very well with experiment. The Flory–Huggins interaction parameters ( χ ) for blends of HDPE and different LDPE models were then calculated using the computed δ values. The level of branch content for LDPE above which the blends are immiscible and segregate in the melt was found to be around 30 branches/1000 long chain carbons at the chosen simulation temperatures. This value is significantly lower than that of butene-based linear low-density polyethylene (LLDPE) (40 branches/1000 carbons) in the blends with HDPE computed by one of the authors (polymer 2000; 41:8741). The major difference between LDPE and LLDPE models is that each modeled LDPE molecule has three long chains while each modeled LLDPE molecule had only one long chain. The present results together with those of the LLDPE/HDPE blends suggest that the long chain branching may have significant influence on the miscibility of polyethylene blends at elevated temperatures. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulation was used to study the wettability of the hydrophilic sphalerite (110) surface chemically modified by butylthiols made up of normal and branched alkyl tails, referred to as n-butylthiol and i-butylthiol hereafter, at different adsorption site coverages. Butylthiol molecules were grafted onto the adsorption sites of the surface in two different distributions--ordered and random. The results showed that for a given butylthiol at a given site coverage, random surface distribution yielded a slightly larger contact angle. This observation was attributed to the fact that average distances between the first and second nearest neighbors of butylthiol molecules are shorter in the case of random surface distribution, resulting in smaller patches of bare surface exposed to water molecules compared to those of the ordered surface distribution. Regardless of the tail structure, the random surface distribution exhibited hydrophobic character (i.e., contact angle ≥ 90°) at a relatively low site coverage of about 25%. The test area method and the Kirkwood and Buff approach were adopted to estimate surface energies (γSV) of the bare sphalerite (110) surface and the collector monolayer, respectively. Using the obtained γSV values of these two pure states, the apparent surface energy as a function of surface coverage was determined based on Cassie's law. This allowed us to estimate the corresponding values of solid-liquid apparent interfacial tension (γSL). Both γSV and γSL exhibit a linear inverse dependence on surface coverage with a crossover point at 25% site coverage (about 50% surface coverage), above which γSV falls below γSL, leading to contact angles greater than 90°. The results also revealed that contact angles of the two butylthiols are comparable at site coverages below ∼85%, but above that, they are significantly lower for the branched thiols compared to their normal counterparts. Considering the Lennard-Jones interaction energies between the water cluster and the butylthiols, stronger attractive interactions were present in the case of i-butylthiol due to the presence of two methyl groups in its alkyl chain. This difference was the most intense at site coverages above ~85%. [ABSTRACT FROM AUTHOR]

The geopolymerization process involves the dissolution of aluminosilicates in an alkali solution followed by the polymerization of the dissolved aluminate and silicate oligomers to form an amorphous geopolymer. It is generally accepted that the dissolution determines the nature of the oligomers formed, thereby the properties of the resultant geopolymer. Accordingly, in this work, a series of molecular dynamics (MD) simulations were carried out in the isothermal-isobaric (NPT) ensemble at 298 K and 1 atm to study the initial stage of dissolution process that takes place at the two basal surfaces (partially deprotonated octahedral and tetrahedral) of kaolinite in alkali media. Three different alkali media containing sodium (Na + ), potassium (K + ) and a mixture of 50/50 Na + /K + cations were modeled at low (1 M) and medium (5 M) concentrations. The MD results showed that cations migrated to the vicinity of the deprotonated sites, trigging the dissociation of the nearby surface hydroxyl groups and increasing the interatomic distances between the aluminum atom that were adjacent to the deprotonated site and those attached such aluminum atom, thereby dissolving them in the form of aluminates into the alkali solution. Radial distribution functions and structural analyses indicated that the crystallinity of the model kaolinite decreased as simulation time and alkali solution concentration increased. The distortion occurred mostly on the octahedral surface that was attributed to the change of the coordination of aluminum from 6-fold octahedral to 5- and 4-fold tetrahedral. However, Na + and K + exhibited different dissolution mechanisms. In particular, Na + with a higher charge density induced more dissociation of the surface hydroxyl groups, whereas K + with a lower charge density resulted in more dissociation of aluminum atoms in the form of aluminates. In the case of the solution containing 50/50 Na + /K + , there was a synergistic effect of both cations that the solution led to more surface structure distortion as quantified by the number of 5- and 4-fold aluminum atoms. [ABSTRACT FROM AUTHOR]

We used molecular dynamics (MD) simulation to determine the line tension of two sulfide mineral surfaces, namely galena ( PbS )(001) and sphalerite ( ZnS )(110) at 298 K . Density functional theory (DFT) calculations were used to estimate the partial charges, and the force field parameters were derived based on the nature of the surfaces and the governing potential terms. The computed bulk structures, surface energies and water adsorption energies of the minerals using such partial charges and force field parameters agree well with experiment or DFT results. Contact angles of a series of nano-sized water clusters with different sizes were determined from the MD simulations. Line tensions and macroscopic contact angles of the two mineral surfaces were then determined using the modified Young's equation. The macroscopic contact angles (~ 84 ° and 40 ° for galena and sphalerite surfaces) agree well with the experimental data. The resultant line tensions are on the order of 10 − 11 J / m for both surfaces but with different signs. Here, the line tension for PbS (001) with very low wettability is positive, and for the hydrophilic ZnS (110) surface is negative. Examination of the orientation of the water molecules on the two mineral surfaces showed that the tetrahedron structure present in bulk water (order parameter ( q ) = 0.61) was significantly perturbed on the ZnS (110) surface ( q = 0.11) compared to the PbS (001) surface ( q = 0.47). In particular, the average angle between water dipole moment and the surface normal and the radial distribution functions of the water molecules show that the oxygen in water molecules tends to orient itself to the cationic sites ( Zn atom) in the sphalerite surface, while such orientation to the Pb atom in the galena surface is minimal. The above results explain that the favorable interaction between water molecules and sphalerite causes the microscopic contact angles to be smaller than the macroscopic value, which in turn yields a negative line tension. On the contrary, stronger affinity of water molecules to the bulk region rather than to galena surface results in bigger microscopic contact angles compared to the macroscopic value and a positive line tension. [ABSTRACT FROM AUTHOR]

Point defects, particularly vacancies, exist abundantly in the structure of a variety of clay minerals (e.g., kaolinite). Such defects significantly alter the physical/chemical characteristics of the minerals. Dissolution of kaolinite in an alkali medium is a critical step in the geopolymerization process and it determines the properties of the final product. In this work, a series of molecular dynamics (MD) simulations were carried out in the isothermal-isobaric (NPT) ensemble at 298 K and 1 atm to study the influence of structural vacancies (Al and Si vacancies in particular) on the interaction/dissolution of the two basal surfaces of kaolinite (partially deprotonated octahedral and tetrahedral surfaces) exposed to alkali media. Two different alkali media were used. One contained sodium cation (Na + ) only while the other potassium cation (K + ) and their concentrations were 3 M and 5 M. The MD results showed that regardless of the type of cation and cation concentration, Al vacancies on the octahedral surface promoted the dissolution of Al into the solution compared to the surface without Al vacancies. However, there existed a vacancy concentration (2 Al vacancies per 576 Al atoms) at which the dissolution amount was the maximum and above which the dissolution decreased with increasing Al vacancy concentration. The presence of Si vacancies in the tetrahedral surface did not show significant effect on the dissociation of Si. Radial distribution function (RDF) analyses indicated that the degree of crystallinity of the octahedral surface decreased as the number of Al vacancies increased but this was not the case for the tetrahedral surface. [ABSTRACT FROM AUTHOR]