Atomic-scale Molecular Dynamics Simulations Research Articles

Interaction of chondroitin sulfate with zwitterionic lipid membranes

Chondroitin sulfates (CSs) are important components of the extracellular matrix and side chains of membrane proteoglycans. These polysaccharides are, therefore, likely to interact with plasma membranes and play a significant role in modulating cellular functions. So far, the details of the processes occurring at the interface between the extracellular matrix and cellular membranes are not fully understood. In this study, we used experimental methods and atomic-scale molecular dynamics (MD) simulations to reveal the molecular picture of the interactions between CS and phosphocholine (PC) membranes, used as a simplified model of cell membranes. MD simulations reveal that the polysaccharide associates to the PC bilayer as a result of electrostatic interactions between the positively charged quaternary ammonium groups of choline and the negatively charged sulfate groups of CS. Compared to an aqueous medium, the adsorbed polysaccharide chains adopt more elongated conformations, which facilitates the electrostatic interactions with the membrane, and have a high degree of freedom to change their conformations and to adhere to and detach from the membrane surface. Penetrating slightly between the polar groups of the bilayer, they form a loosely anchored layer, but do not intrude into the hydrophobic region of the PC bilayer. The CS adsorption spread the PC headgroups apart, which is manifested by an increase in the value of the area pre lipid. The expansion of the lipid polar groups weakens the dispersion interactions between the lipid acyl chains. As a result, the lipid membrane in the membrane-polysaccharide contact areas becomes more fluid. Our outcomes may help to understand in detail the interaction of chondroitin sulfate with zwitterionic membranes at the molecular level, which is of biological interest since many biological processes depend on lipid-CS interactions.

Direct pen writing and atomic-scale molecular dynamics simulation study of a novel silver nano-ink

Direct pen writing and atomic-scale molecular dynamics simulation study of a novel silver nano-ink

A multiscale predictive digital twin for neurocardiac modulation.

Cardiac function is tightly regulated by the autonomic nervous system (ANS). Activation of the sympathetic nervous system increases cardiac output by increasing heart rate and stroke volume, while parasympathetic nerve stimulation instantly slows heart rate. Importantly, imbalance in autonomic control of the heart has been implicated in the development of arrhythmias and heart failure. Understanding of the mechanisms and effects of autonomic stimulation is a major challenge because synapses in different regions of the heart result in multiple changes to heart function. For example, nerve synapses on the sinoatrial node (SAN) impact pacemaking, while synapses on contractile cells alter contraction and arrhythmia vulnerability. Here, we present a multiscale neurocardiac modelling and simulator tool that predicts the effect of efferent stimulation of the sympathetic and parasympathetic branches of the ANS on the cardiac SAN and ventricular myocardium. The model includes a layered representation of the ANS and reproduces firing properties measured experimentally. Model parameters are derived from experiments and atomistic simulations. The model is a first prototype of a digital twin that is applied to make predictions across all system scales, from subcellular signalling to pacemaker frequency to tissue level responses. We predict conditions under which autonomic imbalance induces proarrhythmia and can be modified to prevent or inhibit arrhythmia. In summary, the multiscale model constitutes a predictive digital twin framework to test and guide high-throughput prediction of novel neuromodulatory therapy. KEY POINTS: A multi-layered model representation of the autonomic nervous system that includes sympathetic and parasympathetic branches, each with sparse random intralayer connectivity, synaptic dynamics and conductance based integrate-and-fire neurons generates firing patterns in close agreement with experiment. A key feature of the neurocardiac computational model is the connection between the autonomic nervous system and both pacemaker and contractile cells, where modification to pacemaker frequency drives initiation of electrical signals in the contractile cells. We utilized atomic-scale molecular dynamics simulations to predict the association and dissociation rates of noradrenaline with the β-adrenergic receptor. Multiscale predictions demonstrate how autonomic imbalance may increase proclivity to arrhythmias or be used to terminate arrhythmias. The model serves as a first step towards a digital twin for predicting neuromodulation to prevent or reduce disease.

Property investigation for high-Performance Polyimides fabricated via compression molding in solid-like state

A compacted body was fabricated by pulverulent polyimide (PI) block copolymers using solid-like state compression molding (SCM) technique. Polymer heated to solid-like state, i.e. the high-elastic non-melting state above the glass transition temperature ( Tg) and well below melting temperature, could achieve plasticity due to dramatic decreases in elastic modulus. Tensile properties were taken as response values, and the results of single-factor experiments indicated that molding temperature was the dominant parameter on mechanical performances, followed by molding pressure and holding time. Within this context, the SCM process possesses a longer processing time window whereas the processing temperature is narrow. The manufacturing defects induced by inappropriate processing conditions also hurt the tribological performance of PIs. Particles in a solid-like state could coalesce tightly only by exerting both high temperature and pressure in the SCM process. Thermoforming mechanism examined by atomic-scale molecular dynamics simulation indicated that non-bonding interaction forces, especially van der Waals forces play a key role in fusing among polymeric particles. This study is devoted to establishing the interdependence of structure-formability-property for high-temperature polymers that are not melt processible.

Cooling-Rate Computer Simulations for the Description of Crystallization of Organic Phase-Change Materials

A molecular-level insight into phase transformations is in great demand for many molecular systems. It can be gained through computer simulations in which cooling is applied to a system at a constant rate. However, the impact of the cooling rate on the crystallization process is largely unknown. To this end, here we performed atomic-scale molecular dynamics simulations of organic phase-change materials (paraffins), in which the cooling rate was varied over four orders of magnitude. Our computational results clearly show that a certain threshold (1.2 × 1011 K/min) in the values of cooling rates exists. When cooling is slower than the threshold, the simulations qualitatively reproduce an experimentally observed abrupt change in the temperature dependence of the density, enthalpy, and thermal conductivity of paraffins upon crystallization. Beyond this threshold, when cooling is too fast, the paraffin's properties in simulations start to deviate considerably from experimental data: the faster the cooling, the larger part of the system is trapped in the supercooled liquid state. Thus, a proper choice of a cooling rate is of tremendous importance in computer simulations of organic phase-change materials, which are of great promise for use in domestic heat storage devices.

Plasma-assisted self-assembled monolayers for reducing thermal resistance across graphite films/polymer interfaces

Graphite-related materials are commonly used as fillers in polymers to prepare thermal interface materials (TIMs) with high thermal conductivity. The thermal contact resistance (TCR) between graphite and polymer is a crucial factor affecting the thermal conductivity of TIMs. However, reducing the TCR is challenging due to the highly stable surface of graphite. Here, we propose a plasma-assisted strategy to form a self-assembled monolayer (SAM) on the surface of graphite films to reduce the TCR of the graphite film/silicone systems. A series of measurements, including the contact angle and Fourier infrared spectrum, confirm the formation of SAM. Using the non-contact photothermal technique, we investigate the effects of plasma bombardment times and powers on the TCR. Under 30 min bombardment, the TCR between modified graphite and silicone is reduced by almost one order of magnitude than the original one. Atomic-scale molecular dynamics simulations further confirm the effectiveness of SAM in reducing the TCR. This plasma-assisted SAM strategy can help graphite-based TIMs to provide better thermal performance to meet the requirements of future high-power devices.

Theoretical and experimental study on plasma-induced atom-migration manufacturing (PAMM) of glass

Optical manufacturing plays an important role in various fields, such as optical lenses, telescopes, and laser optics. Generally, traditional optical manufacturing techniques are all subtractive processes based on the micro-shearing effect between the tool and surface protrusions. This paper reports a plasma-induced atom migration manufacturing (PAMM) process, which is a nonsubtractive finishing approach by which angstrom level surface roughness of fused silica has been successfully achieved. Inductively coupled plasma (ICP), which is characterized by high temperature and high radical density, is used as the tool of PAMM. After obtaining instantaneous plasma energy input on the fused silica surface, the peak site atoms migrate to the valley sites, thereby reducing the roughness. According to the energy minimization principle, this migration process continues until an ultra-smooth surface with reduced surface energy is formed. Atomic-scale molecular dynamics simulations were performed to clarify the microscopic mechanisms of PAMM, and experiments were conducted to verify its smoothing capability. The roughness of a ground silica surface was drastically reduced from Sa 391 nm to Sa 0.16 nm. This study demonstrates the feasibility of the PAMM as an alternative approach for atomic-level surface manufacturing.

Rejuvenation of plasticity via deformation graining in magnesium

Magnesium, the lightest structural metal, usually exhibits limited ambient plasticity when compressed along its crystallographic c-axis (the “hard” orientation of magnesium). Here we report large plasticity in c-axis compression of submicron magnesium single crystal achieved by a dual-stage deformation. We show that when the plastic flow gradually strain-hardens the magnesium crystal to gigapascal level, at which point dislocation mediated plasticity is nearly exhausted, the sample instantly pancakes without fracture, accompanying a conversion of the initial single crystal into multiple grains that roughly share a common rotation axis. Atomic-scale characterization, crystallographic analyses and molecular dynamics simulations indicate that the new grains can form via transformation of pyramidal to basal planes. We categorize this grain formation as “deformation graining”. The formation of new grains rejuvenates massive dislocation slip and deformation twinning to enable large plastic strains.

Effect of incident angle on the microstructure proprieties of Cu thin film deposited on Si (001) substrate

The deposition processes of Cu atoms onto Si substrate are studied in atomic-scale molecular dynamics (MD) simulation. Our aim here is to check how the microstructure properties of Cu thin film are affected by the incident angles. To this end, we have adopted various analytical techniques such as surface roughness, layer coverage, film density, and internal stress. Our results show that at low incident angles (0∘−45∘), the deposition of Cu atoms on the substrate generates a compact and uniform thin film with approximately a smooth surface. However, at higher incident angles (60∘−85∘) the effects become more pronounced, where the thin film morphology is changed from a dense structure to a columnar structure with more voids and vacancies. The surface roughness was also significantly increased as the incident angle increases. On the other hand, the analysis of interface mixing revealed that there are little effects of the deposition angles on the film-substrate mixing, and the penetration of Cu atoms onto Si substrate is limited to the two first upper layers. In the same context, our calculations show that the film density decreases significantly along the growth direction when the incident angles are above 60∘. Finally, the result of residual stress after the deposition process demonstrates that the film has compressive stress at low incident angles, but it changed to tensile stress when deposition angles exceed 60∘.

How to control interactions of cellulose-based biomaterials with skin: the role of acidity in the contact area.

Being able to control the interactions of biomaterials with living tissues and skin is highly desirable for many biomedical applications. This is particularly the case for cellulose-based materials which provide one of the most versatile platforms for tissue engineering due to their strength, biocompatibility and abundance. Achieving such control, however, requires detailed molecular-level knowledge of the dominant interaction mechanisms. Here, we employed both biased and unbiased atomic-scale molecular dynamics simulations to explore how cellulose crystals interact with model stratum corneum bilayers, ternary mixtures of ceramides, cholesterol, and free fatty acids. Our findings show that acidity in the contact area directly affects binding between cellulose and the stratum corneum bilayer: Protonation of free fatty acids in the bilayer promotes attractive cellulose-bilayer interactions. We identified two major factors that control the cellulose-skin interactions: (i) the electrostatic repulsion between a cellulose crystal and the charged (anionic due to deprotonated fatty acids) surface of a stratum corneum bilayer and (ii) the cellulose-stratum corneum hydrogen bonding. When less than half of the fatty acids in the bilayer are protonated, the first factor dominates and there is no binding to skin. At a larger degree of fatty acid protonation the cellulose-stratum corneum hydrogen bonding prevails yielding a tight binding. Remarkably, we found that ceramide molecules are the key component in hydrogen bonding with cellulose. Overall, our findings highlight the critical role of fatty acid protonation in biomaterial-stratum corneum interactions and can be used for optimizing the surface properties of cellulose-based materials aimed at biomedical applications such as wound dressings.

Polycation-Anionic Lipid Membrane Interactions.

Natural or synthetic polycations are used as biocides or as drug/gene carriers. Understanding the interactions between these macromolecules and cell membranes at the molecular level is therefore of great importance for the design of effective polymer biocides or biocompatible polycation-based delivery systems. Until now, details of the processes at the interface between polycations and biological systems have not been fully recognized. In this study, we consider the effect of strong polycations with quaternary ammonium groups on the properties of anionic lipid membranes that we use as a model system for protein-free cell membranes. For this purpose, we employed experimental measurements and atomic-scale molecular dynamics (MD) simulations. MD simulations reveal that the polycations are strongly hydrated in the aqueous phase and do not lose the water shell after adsorption at the bilayer surface. As a result of strong hydration, the polymer chains reside at the phospholipid headgroup and do not penetrate to the acyl chain region. The polycation adsorption involves the formation of anionic lipid-rich domains, and the density of anionic lipids in these domains depends on the length of the polycation chain. We observed the accumulation of anionic lipids only in the leaflet interacting with the polymer, which leads to the formation of compositionally asymmetric domains. Asymmetric adsorption of the polycation on only one leaflet of the anionic membrane strongly affects the membrane properties in the polycation–membrane contact areas: (i) anionic lipid accumulates in the region near the adsorbed polymer, (ii) acyl chain ordering and lipid packing are reduced, which results in a decrease in the thickness of the bilayer, and (iii) polycation–anionic membrane interactions are strongly influenced by the presence and concentration of salt. Our results provide an atomic-scale description of the interactions of polycations with anionic lipid bilayers and are fully supported by the experimental data. The outcomes are important for understanding the correlation of the structure of polycations with their activity on biomembranes.

Carrot-based covalently bonded saccharides as a new 2D material for healing defective calcium-silicate-hydrate in cement: Integrating atomistic computational simulation with experimental studies

Concrete is currently produced at a rate of 20 billion tonnes per year and contributes 5–10% of mankind's CO2 production. If the strength of the calcium-silicate-hydrate (C–S–H), the main binding material of concrete, could be improved, the volume of cementitious material needed for a given structure would be reduced and its environmental impact would be decreased. Here, we show that the constitutive behavior of C–S–H can be improved significantly by complexation with carrot-based cellulose nanosheets (CNSs). This environmentally friendly, reinforcing material heals the defective microstructure of C–S–H, which is responsible for structural deformation and failure at larger length scales. CNSs are built from repeating saccharide units that are covalently linked by a β-1-4 glycosidic (C–O–C) bond. The CNSs show remarkable affinity to C–S–H due to the interfacial Ca–O coordination and H-bond interaction. The functional groups on the surface of the CNS sheet act as a root network, cross-linking the neighboring silicate calcium layers and inhibiting the water dynamics at the silicate nanochannel, thereby significantly improving the interfacial properties of the C–S–H/CNS hybrid structure. The macro experimental results show that the mechanical properties of the composites increase with increasing the concentration of CNSs up to 0.4-wt%. At 28 days and CNS concentration of 0.20-wt%, the flexural strength increases by about 23.2% and the compressive strength increases by about 17.5%. The developed atomic-scale molecular dynamics simulations, combined with top-down experimental measurements of their mechanical properties reveal that the proposed C–S–H/CNS composites show significant enhancement in strength, stiffness and ductility, and provide a foundation for the development of new high-performance construction materials with lower carbon footprint.

Coupling between clay swelling/collapse and cationic partition

The semipermeability of clay aggregates in mudstones, shales, and marine sediments gives rise to geological ultrafiltration of ions and water. This behavior has been numerically analyzed based on the theory of the electric double layer (EDL). The model of EDL is suited for describing ionic partitions in clay pore with a width of a few nanometers. However, the interlayer space of smectite might be hardly described by the EDL model. An explicit atomic-scale description is more appropriate for the interlayer space. Besides, clay swelling or collapse corresponding to a variation of environmental conditions, and its impact on partitions of water and ions between interlayers and environment, have not been taken into account numerically. With atomic-scale molecular dynamics simulations and thermodynamic integrations based on them, the coupling between montmorillonite swelling/collapse and partitions of water and cations under different aqueous activity conditions is systematically disclosed here. With the combination of the Horinek’s force field to describe cations and ClayFF force field for atoms in solid layers, consistent swelling/collapse of Na- and K-montmorillonite with experimental observations as a function of water activity is shown. The swelling/collapse is connected to the transition between monolayer and bilayer hydration states. Decomposition of the swelling/collapse free energy shows both energy and entropy contributions are important in determining the thermodynamically stable hydration state. Taking environmental water activity and Na+/K+ ionic activity ratio as variables, a picture of cationic partition in response to those variables is established. It shows, under all circumstances, K+ ions are enriched in montmorillonite, which might explain K+ depletion in pore fluids of sediments. A low water activity improves K+ enrichment in montmorillonite. In addition, cationic partition accompanies variations of hydration states and alters the ionic concentration of the aqueous environment. These results can be applied to explain water and ionic partitions during subsurface water flow and the diagenesis processes of sediments.

Atomic-Scale Friction on Monovacancy-Defective Graphene and Single-Layer Molybdenum-Disulfide by Numerical Analysis.

Using numerical simulations, we study the atomic-scale frictional behaviors of monovacancy-defective graphene and single-layer molybdenum-disulfide (SLMoS2) based on the classical Prandtl–Tomlinson (PT) model with a modified interaction potential considering the Schwoebel–Ehrlich barrier. Due to the presence of a monovacancy defect on the surface, the frictional forces were significantly enhanced. The effects of the PT model parameters on the frictional properties of monovacancy-defective graphene and SLMoS2 were analyzed, and it showed that the spring constant of the pulling spring cx is the most influential parameter on the stick–slip motion in the vicinity of the vacancy defect. Besides, monovacancy-defective SLMoS2 is found to be more sensitive to the stick–slip motion at the vacancy defect site than monovacancy-defective graphene, which can be attributed to the complicated three-layer-sandwiched atomic structure of SLMoS2. The result suggests that the soft tip with a small spring constant can be an ideal candidate for the observation of stick–slip behaviors of the monovacancy-defective surface. This study can fill the gap in atomic-scale friction experiments and molecular dynamics simulations of 2D materials with vacancy-related defects.

The Interaction of Flavonols with Membrane Components: Potential Effect on Antioxidant Activity.

Flavonols are the most widely distributed class of dietary flavonoids with a wide range of pharmacological properties due to their potent lipid peroxidation inhibition activity. The permeability and orientation of these compounds in lipid bilayers can provide an understanding of their antioxidant and lipid-peroxidation inhibition activity based on their structures at the molecular level. For this purpose, we studied antioxidant activity and atomic-scale molecular dynamics simulations of 3-hydroxyflavone (fisetin), 5-hydroxyflavone (apigenin) and 3,5-hydroxyflavone (morin) in palmitoyloleylphosphatidylcholine (POPC) membrane models with 0mol% and 40mol% cholesterol concentration. In pure POPC bilayer with 0mol% cholesterol concentration, the flavonols penetrated into bilayer with lowest free energy profiles, however, incorporation of 40% cholesterol concentration reduced the permeability of the flavonols. Higher cholesterol concentrations in the POPC lipid bilayer resulted in an increase of the bilayer thickness and corresponding decrease in the area per lipid which rationalizes the reduced partitioning of flavonols due to cholesterol. In the presence of cholesterol, the flavonols reside at the polar interfacial region of the lipid bilayer to form higher H-bonding interactions with cholesterol molecules in addition to water and lipid oxygens. Among all the selected flavonols, morin showed the highest affinity which was driven by the hydrophobic effect as also depicted by ITC (Isothermal titration calorimetry) experiments and thus, more efficient antioxidant in scavenging superoxide, nitric oxide radicals as well as lipid peroxyl radicals. Furthermore, our simulations also confirmed that the permeability of compounds is sensitive towards the cholesterol content in the membrane.

Phospholipid-Cellulose Interactions: Insight from Atomistic Computer Simulations for Understanding the Impact of Cellulose-Based Materials on Plasma Membranes.

Cellulose is an important biocompatible and nontoxic polymer widely used in numerous biomedical applications. The impact of cellulose-based materials on cells and, more specifically, on plasma membranes that surround cells, however, remains poorly understood. To this end, here, we performed atomic-scale molecular dynamics simulations of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) bilayers interacting with the surface of a cellulose crystal. Both biased umbrella sampling and unbiased simulations clearly show the existence of strong attractive interactions between phospholipids and cellulose: the free energy of the cellulose-bilayer binding was found to be -1.89 and -1.96 kJ/mol per cellulose dimer for PC and PE bilayers, respectively. Although the values are similar, there is a pronounced difference between PC and PE bilayers. The driving force in both cases is the formation of hydrogen bonds. There are two distinct types of hydrogen bonds: (1) between the lipid head groups and the hydroxyl (hydroxymethyl) groups of cellulose, and (2) lipid-water and cellulose-water bonds. The former is the dominant component for PE systems whereas the latter dominates in PC systems. This suggests that achieving controlled binding via new cellulose modifications must pay close attention to the lipid head groups involved. The observed attractive phospholipid-cellulose interactions have a significant effect on bilayer properties: a cellulose crystal induces noticeable structural perturbations on the bilayer leaflet next to the crystal. Given that such perturbations can be undesirable when it comes to the interactions of cellulose-based materials with cell membranes, our computational studies suggest that the impact of cellulose could be reduced through chemical modification of the cellulose surface which prevents cellulose-phospholipid hydrogen bonding.

Molecular-Level Insight into the Interaction of Phospholipid Bilayers with Cellulose.

Molecular-level insight into the interactions of phospholipid molecules with cellulose is crucial for the development of novel cellulose-based materials for wound dressing. Here we employ the state-of-the-art computer simulations to unlock for the first time the molecular mechanisms behind such interactions. To this end, we performed a series of atomic-scale molecular dynamics simulations of phospholipid bilayers on a crystalline cellulose support at various hydration levels of the bilayer leaflets next to the cellulose surface. Our findings clearly demonstrate the existence of strong interactions between polar lipid head groups and the hydrophilic surface of a cellulose crystal. We identified two major types of interactions between phospholipid molecules and cellulose chains: (i) direct attractive interactions between lipid choline groups and oxygens of hydroxyl (hydroxymethyl) groups of cellulose and (ii) hydrogen bonding between phosphate groups of lipids and cellulose's hydroxymethyl/hydroxyl groups. When the hydration level of the interfacial bilayer/support region is low, these interactions lead to a pronounced asymmetry in the properties of the opposite bilayer leaflets. In particular, the mass density profiles of the proximal leaflets are split into two peaks and lipid head groups become more horizontally oriented with respect to the bilayer surface. Furthermore, the lateral mobility of lipids in the leaflets next to the cellulose surface is found to slow down considerably. Most of these cellulose-induced effects are likely due to hydrogen bonding between lipid phosphate groups and hydroxymethyl/hydroxyl groups of cellulose: the lipid phosphate groups are pulled toward the water/lipid interface due to the formation of hydrogen bonds. Overall, our findings shed light on the molecular details of the interactions between phospholipid bilayers and cellulose nanocrystals and can be used for identifying possible strategies for improving the properties of cellulose-based dressing materials via, e.g., chemical modification of their surface.

Adsorption of Synthetic Cationic Polymers on Model Phospholipid Membranes: Insight from Atomic-Scale Molecular Dynamics Simulations

Although synthetic cationic polymers represent a promising class of effective antibacterial agents, the molecular mechanisms behind their antimicrobial activity remain poorly understood. To this end, we employ atomic-scale molecular dynamics simulations to explore adsorption of several linear cationic polymers of different chemical structure and protonation (polyallylamine (PAA), polyethylenimine (PEI), polyvinylamine (PVA), and poly-l-lysine (PLL)) on model bacterial membranes (4:1 mixture of zwitterionic phosphatidylethanolamine (PE) and anionic phosphatidylglycerol (PG) lipids). Overall, our findings show that binding of polycations to the anionic membrane surface effectively neutralizes its charge, leading to the reorientation of water molecules close to the lipid/water interface and to the partial release of counterions to the water phase. In certain cases, one has even an overcharging of the membrane, which was shown to be a cooperative effect of polymer charges and lipid counterions. Protonated amine groups of polycations are found to interact preferably with head groups of anionic lipids, giving rise to formation of hydrogen bonds and to a noticeable lateral immobilization of the lipids. While all the above findings are mostly defined by the overall charge of a polymer, we found that the polymer architecture also matters. In particular, PVA and PEI are able to accumulate anionic PG lipids on the membrane surface, leading to lipid segregation. In turn, PLL whose charge twice exceeds charges of PVA/PEI does not induce such lipid segregation due to its considerably less compact architecture and relatively long side chains. We also show that partitioning of a polycation into the lipid/water interface is an interplay between its protonation level (the overall charge) and hydrophobicity of the backbone. Therefore, a possible strategy in creating highly efficient antimicrobial polymeric agents could be in tuning these polycation's properties through proper combination of protonated and hydrophobic blocks.

Simulations of threshold displacement in beryllium

Atomic scale molecular dynamics simulations of radiation damage have been performed on beryllium. Direct threshold displacement simulations along a geodesic projection of directions were used to investigate the directional dependence with a high spatial resolution. It was found that the directionally averaged probability of displacement increases from 0 at 35 eV, with the energy at which there is a 50% chance of a displacement occurring is 70 eV and asymptotically approaching 1 for higher energies. This is, however, strongly directionally dependent with a 50% probability of displacement varying from 35 to 120 eV, with low energy directions corresponding to the nearest neighbour directions. A new kinetic energy dependent expression for the average maximum displacement of an atom as a function of energy is derived which closely matches the simulated data.

Polyethyleneimine loaded inverse SDS micelle in pentanol/toluene media

An atomic scale molecular dynamics simulation (100ns) was carried out to reveal the conformational features of a cationic polyelectrolyte, i.e., hyperbranched polyethyleneimine (PEI), inside of water-in-oil microemulsion droplets stabilized by the anionic sodium dodecyl sulfate surfactant (SDS) layer. Simulations show that the polymer reorients very quickly and is localized at the headgroup region, i.e., the polymer nitrogens are close to SDS sulfur atoms. In spite of the availability of surface roughness caused by the polymer, we track a stable inverse micelle during the production run. In overall, the obtained parameters are well compared with experimental findings.