Molecular-dynamics simulations of ion size effects on the fluid structure of aqueous electrolyte systems between charged model electrodes

Water-in-Salt: Fast Dynamics, Structure, Thermodynamics, and Bulk Properties

We present concentration-dependent dynamics of highly concentrated LiBr solutions and LiCl temperature-dependent dynamics for two high concentrations and compare the results to those of prior LiCl concentration-dependent data. The dynamical data are obtained using ultrafast optical heterodyne-detected optical Kerr effect (OHD-OKE). The OHD-OKE decays are composed of two pairs of biexponentials, i.e., tetra-exponentials. The fastest decay (t1) is the same as pure water's at all concentrations within error, while the second component (t2) slows slightly with concentration. The slower components (t3 and t4), not present in pure water, slow substantially, and their contributions to the decays increase significantly with increasing concentration, similar to LiCl solutions. Simulations of LiCl solutions from the literature show that the slow components arise from large ion/water clusters, while the fast components are from ion/water structures that are not part of large clusters. Temperature-dependent studies (15-95 °C) of two high LiCl concentrations show that decreasing the temperature is equivalent to increasing the room temperature concentration. The LiBr and LiCl concentration dependences and the two LiCl concentrations' temperature dependences all have bulk viscosities that are linearly dependent on τcslow, the correlation time of the slow dynamics (weighted averages of t3 and t4). Remarkably, all four viscosity vs 1/τCslow plots fall on the same line. Application of transition state theory to the temperature-dependent data yields the activation enthalpies and entropies for the dynamics of the large ion/water clusters, which underpin the bulk viscosity.

Molecular Simulation Study on the Wettability of a Surface Texturized with Hierarchical Pillars

By using molecular dynamics simulation, we investigate the wettability of a surface texturized with a periodic array of hierarchical pillars. By varying the height and spacing of the minor pillars on top of major pillars, we investigate the wetting transition from the Cassie-Baxter (CB) to Wenzel (WZ) states. We uncover the molecular structures and free energies of the transition and meta-stable states existing between the CB and WZ states. The relatively tall and dense minor pillars greatly enhance the hydrophobicity of a pillared surface, in that, the CB-to-WZ transition requires an increased activation energy and the contact angle of a water droplet on such a surface is significantly larger.

Molecular Dynamics Simulation on the Wettability of Nanoscale Wrinkles: High Water Adhesion of Rose Petals

Understanding the high water adhesion of rose petals is of great significance in artificial surface design. With all-atom molecular dynamics simulation, the wettability of nanoscale wrinkles was explored and compared to that of nanoscale strips with favorable hydrophobicity. The dewetting and wetting of gaps between nanoscale structures represent the Cassie-Baxter (CB) and Wenzel (WZ) states of the macroscopic droplet deposited on the textured surface, respectively. We uncovered the intermediate state, which is different from the CB and WZ states for wrinkles. Structures and free-energy profiles of metastable and transition states under various pressures were also investigated. Moreover, free-energy barriers for the (de)wetting transitions were quantified. On this basis, the roles of pressure and the unique structures of nanoscale wrinkles in the high water adhesion of rose petals were identified.

Molecular Insights on the Wetting Behavior of a Surface Corrugated with Nanoscale Domed Pillars

Using all-atom molecular dynamics simulation, we investigated the wettability of a surface texturized with nanoscale pillars of domed, rectangular, or cylindrical shapes. The dewetted and wetted states of the gaps between the pillars were related to the Cassie-Baxter (CB) and Wenzel (WZ) states of a macroscopic water droplet resting on top of the pillars. We uncovered the structures and free energies of the intermediate states existing between the CB and WZ states. The contact line of the liquid-vapor-solid interface could not be depinned for the domed pillars due to their smooth curvatures unlike for the rectangular or cylindrical pillars. The liquid symmetrically penetrated down into the gap between the domed pillars by a liquid-vapor interface shape like a paraboloid, while the penetration for the rectangular or cylindrical pillars was often asymmetrical, giving a half-tubular liquid-vapor interface.

Capacitance and Structure of Electric Double Layers: Comparing Brownian Dynamics and Classical Density Functional Theory

We present a study of the structure and differential capacitance of electric double layers of aqueous electrolytes. We consider electric double layer capacitors (EDLC) composed of spherical cations and anions in a dielectric continuum confined between a planar cathode and anode. The model system includes steric as well as Coulombic ion-ion and ion-electrode interactions. We compare results of computationally expensive, but “exact” , Brownian Dynamics (BD) simulations with approximate, but cheap, calculations based on classical Density Functional Theory (DFT). Excellent overall agreement is found for a large set of system parameters, including variations in concentration, ionic size- and valency-asymmetries, applied voltages and electrode separation, provided the differences between the canonical ensemble of the BD simulations and the grand-canonical ensemble of DFT are properly taken into account. In particular, a careful distinction is made between the differential capacitance $$C_N$$ C N at fixed number of ions and $$C_\mu $$ C μ at fixed ionic chemical potential. Furthermore, we derive and exploit their thermodynamic relations. In the future these relations will also be useful for comparing and contrasting experimental data with theories for supercapactitors and other systems. The quantitative agreement between simulation and theory indicates that the presented DFT is capable of accounting accurately for coupled Coulombic and packing effects. Hence it is a promising candidate to cheaply study room temperature ionic liquids at much lower dielectric constants than that of water.

Molecular Simulation of Electrode-Solution Interfaces

Many key industrial processes, from electricity production, conversion, and storage to electrocatalysis or electrochemistry in general, rely on physical mechanisms occurring at the interface between a metallic electrode and an electrolyte solution, summarized by the concept of an electric double layer, with the accumulation/depletion of electrons on the metal side and of ions on the liquid side. While electrostatic interactions play an essential role in the structure, thermodynamics, dynamics, and reactivity of electrode-electrolyte interfaces, these properties also crucially depend on the nature of the ions and solvent, as well as that of the metal itself. Such interfaces pose many challenges for modeling because they are a place where quantum chemistry meets statistical physics. In the present review, we explore the recent advances in the description and understanding of electrode-electrolyte interfaces with classical molecular simulations, with a focus on planar interfaces and solvent-based liquids, from pure solvent to water-in-salt electrolytes.

Cation folding and the thermal stability limit of the ionic liquid [BMIM+][BF4 -] under total vacuum

Molecular dynamics simulations reveal the behavior of the bimodal distribution of cation conformations (folded/unfolded) in ionic liquids based on alkylated imidazoles, such as [BMIM+][BF4 -]. The alkyl chains of the cations can fold and block interactions between the cations and anions, thereby reducing the cohesivity of the liquid. At room temperature, the folded conformations represent less than one-third of the total conformations. In contrast to the behavior observed during the thermal denaturation of proteins, in ionic liquids, the concentration of folded cations grows when the temperature increases. At the equimolar concentration, the system reaches the reported experimental temperature of thermal stability (similar to the thermal denaturation behavior). There is an outermost layer of cations at the interface that can tilt toward the interface and cover a layer of anions adsorbed at the interface. This interfacial conformation makes the system stable in transverse directions and unstable in the normal direction at temperatures in the region of thermal instability, limiting the rate of vaporization of neutral ion pairs, which are observed as rare events at temperatures as low as 773.15 K.

Ionic Layering and Overcharging in Electrical Double Layers in a Poisson-Boltzmann Model

Electrical double layers (EDLs) play a significant role in a broad range of physical phenomena related to colloidal stability, diffuse-charge dynamics, electrokinetics, and energy storage applications. Recently, it has been suggested that for large ion sizes or multivalent electrolytes, ions can arrange in a layered structure inside the EDLs. However, the widely used Poisson-Boltzmann models for EDLs are unable to capture the details of ion concentration oscillations and the effect of electrolyte valence on such oscillations. Here, by treating a pair of ions as hard spheres below the distance of closest approach and as point charges otherwise, we are able to predict ionic layering without any additional parameters or boundary conditions while still being compatible with the Poisson-Boltzmann framework. Depending on the combination of ion valence, size, and concentration, our model reveals a structured EDL with spatially oscillating ion concentrations. We report the dependence of critical ion concentration, i.e., the ion concentration above which the oscillations are observed, on the counter-ion valence and the ion size. More importantly, our model displays quantitative agreement with the results of computationally intensive models of the EDL. Finally, we analyze the nonequilibrium problem of EDL charging and demonstrate that ionic layering increases the total charge storage capacity and the charging timescale.

Correlation between mobility and the hydrogen bonding network of water at an electrified-graphite electrode using molecular dynamics simulation

Mobility and hydrogen bonding network of water at a graphite electrode: effects of dissolved ions and applied potential.

Application of a Zwitterionic Hydrophobic Associating Polymer with High Salt and Heat Tolerance in Brine-Based Fracturing Fluid

ZID₁₆PM, a zwitterionic hydrophobic associating polymer, has equivalent positive and negative charges and some hydrophobic monomers with twin-tailed long hydrophobic chains. It exhibits a great heat resistance and salt tolerance to the common salt in formation brine (MgCl₂, CaCl₂, NaCl, and KCl), which is attributed to its anti-polyelectrolyte effect and strong association force. High-salinity water (seawater or formation water) can be prepared as a fracturing fluid directly. In this paper, the formation water of the West Sichuan Gas Field is directly prepared into fracturing fluid with a concentration of 0.3% ZID₁₆PM (Fluid-1), and the seawater of the Gulf of Mexico is directly prepared into fracturing fluid with a concentration of 0.3% ZID₁₆PM (Fluid-2). Finally, rheological measurements, proppant suspension tests, and core matrix permeability damage rate tests for the Fluid-1 and Fluid-2 are conducted. Results show that after 120 min of shearing at 140 and 160 °C, respectively, the viscosity of Fluid-1 remains in the range of 50–85 mPa·s, and the viscosity of Fluid-2 remains in the range of 60–95 mPa·s. And the wastewater produced by an oilfield in Shaanxi, Xinjiang, and Jiangsu are also prepared into fracturing fluids with a concentration of 0.3% ZID₁₆PM, the viscosity of these fracturing fluids can remain 32, 42, and 45 mPa·s, respectively, after 120 min of shearing at 160 °C. All results demonstrate that the polymer ZID₁₆PM displays prominent performance in fracturing fluids.

Hydration of monovalent and divalent cations near a cathode surface

Hydration of monovalent (Li⁺, Na⁺, K⁺, and Cs⁺) and divalent (Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺) cations on a cathode surface was studied by a classical molecular dynamics simulation. The potential of mean force (PMF) for each cation species was calculated as a function of the distance from the cathode surface, and the potential barriers for dehydrating the first and second hydration shells near the cathode surface were estimated. The positions of the minimum of the PMF closest to the cathode surface were found to be in the order Li⁺ < Na⁺ < Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺ < K⁺ < Cs⁺. It was found that Li⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ ions are most likely doubly hydrated when they are adsorbed on the cathode surface without an applied voltage, whereas Na⁺, K⁺, and Cs⁺ ions are most likely singly hydrated at room temperature. On the other hand, when a voltage of 1 V was applied to the electrodes, all the cation species that we studied appeared most likely to be singly hydrated on the cathode surface. The depths of the potential well closest to the cathode surface under an applied voltage of 1 V were found to be in the order Ba²⁺ < Sr²⁺ < Ca²⁺ < Mg²⁺ for the divalent cations and Li⁺ < Na⁺ < K⁺ < Cs⁺ for the monovalent cations in the set of models that we used. These orders coincide with the Hofmeister series from the kosmotropic to the chaotropic.

Molecular features of hydration layers probed by atomic force microscopy

Structurally-ordered layers of water are universally formed on a solid surface in aqueous solution or under ambient conditions. Although such hydration layers are commonly probed via atomic force microscopy (AFM), the current understanding on how the hydration layers manifest themselves in an AFM experiment is far from complete. By using molecular dynamics simulation, we investigate the hydration layers on a hydrophilic or hydrophobic surface probed by a nanoscale tip. We study the density and molecular orientation of water, the free energy, and the force on the tip by varying the tip-surface distance. The force-distance curve oscillates due to the transition between the mono-, bi-, and tri-layers of water confined between the tip and the surface. If both the tip and the surface are hydrophobic, water confined between the tip and the surface evaporates due to the dewetting transition, giving a hydrophobic force without oscillation. The periodicity of oscillation in the force differs from the structural periodicity of water. With a close proximity of the tip, the molecular dipoles align parallel to the surface, regardless of whether the tip and the surface are hydrophilic or hydrophobic.

Water desorption from rapidly-heated metal oxide surfaces-first principles, molecular dynamics, and the Temkin isotherm

Quantitative understanding and control of water and impurity desorption from steel surfaces are crucial for high-voltage, pulsed power, vacuum technology, catalysis, and environmental applications. We apply a suite of modeling techniques, ranging from electronic density functional theory, to classical molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) methods to study the thermodynamics and kinetics of fast water desorption from different surfaces of hematite Fe₂O₃ and Cr₂O₃. Water binding energies on chromium oxide are found to be higher than iron oxide at zero temperature. MD simulations are conducted on Fe₂O₃ surfaces using thermodynamically consistent initial water inventory deduced with GCMC. The resulting time- and temperature-dependent desorption profiles on the Fe₂O₃ [Formula: see text] surfaces show multi-water cooperative behavior which cannot be deduced from zero temperature predictions, but which are in reasonable agreement with simple Temkin isotherm model estimates if finite temperature effects are incorporated into the Temkin binding energy parameter. Qualitatively different desorption behaviors associated with the [Formula: see text] and [Formula: see text] facets are discussed.

Novel Hydrophobic Associating Polymer with Good Salt Tolerance

A hydrophobic associating polymer named DiPHAM (acrylamide/sodium acrylamide-2-methylpropanesulfonic/sodium acrylate/N,N-di-n-dodecylacrylamide) with good salt tolerance was synthesized via photo-initiation polymerization. The critical association concentration (CAC) of DiPHAM was determined by viscosity changes to be 490 mg/L with different DiPHAM concentrations and particle sizes varied under such dynamic conditions. The influences of aqueous metal ions with different charges on its aqueous solution were investigated by measuring apparent viscosity, viscoelasticity, thixotropy, rheology, and particle size, and by SEM observation. The apparent viscosity of the DiPHAM solution was affected by metal ions to some extent, but the viscosity of the polymer can be still maintained at 55 mPa·s under 20 × 10⁴ mg/L NaCl. Divalent metal ions show greater impact on DiPHAM aqueous solutions, but the polymer solutions showed resistance to the changes caused in viscosity, structure, and viscoelasticity by Ca2+ and Mg2+ ions. The salt tolerance of DiPHAM is due to the combination of hydrophobic association, the electrostatic shield, and double layer compression of the hydration shell. Increasing the ion concentration enhances the dehydration and further compresses the hydration shell, making the non-structural viscosity decrease, even "salting out". Measurements of rheological properties showed that DiPHAM solutions could maintain a relatively high viscosity (0.6%-71 mPa·s/0.3%-50 mPa·s) after 120 min of continuous shearing (170 s-1) at 140 °C. Under high-salinity (5000 mg/L Ca2+/3000 mg/L Mg2+) conditions, the solution with 0.6 wt% DiPHAM still maintained a high viscosity (50 mPa·s/70 mPa·s) after continuously shearing for 120 min at 120 °C and 170 s-1. The good salt tolerance of DiPHAM can lead to a variety of applications, including in fracturing fluids for enhanced oil recovery (EOR) and in sewage treatment.

Hydration and dehydration of monovalent cations near an electrode surface

The mechanism of hydration and dehydration of monovalent ions, Li⁺, Na⁺, K⁺, and Cs⁺, in a dilute solution near an electrode surface was studied by molecular dynamics simulations. The potentials of mean force for these ions were calculated as a function of the distance from the electrode surface and the potential barriers for dehydrating the first and the second hydration shell near the electrode surface and were estimated for each ion species. It was found that the mechanism of hydration for Li⁺ is distinct from those for Na⁺, K⁺, and Cs⁺. Penetration of ions into the first layer of water molecules on the electrode surface is unlikely to occur for the case of Li⁺, while that would occur with certain probabilities for the case of Na⁺, K⁺, or Cs⁺, whether or not voltage is applied to the electrode. Li⁺ ions would be adsorbed on the electrode surface in a doubly hydrated form with a significant probability, while Na⁺, K⁺, and Cs⁺ ions would be adsorbed most likely in a singly hydrated form. Furthermore, the theory of ionic radii, which has been successfully used in the analysis of bulk solutions, was applied to the electrode/electrolyte interface. It was found that the theory of ionic radii is also useful in explaining the structural behaviors of ions near an electrode surface. The distance between an ion and the layers of water molecules on the electrode surface showed almost linear dependence on the radius of the ion, as predicted by the theory of ionic radii. Analysis of the deviation from the linearity showed that Li⁺ ions are most likely adsorbed in the first layer of water molecules on the electrode surface, while Na⁺, K⁺, and Cs⁺ ions are adsorbed on the second layer of water molecules. These analyses indicate that Li⁺ is a structure maker, while Na⁺, K⁺, and Cs⁺ are structure breakers, which is consistent with the widely accepted idea in explaining the behaviors of the bulk solutions.

Contrasting water adhesion strengths of hydrophobic surfaces engraved with hierarchical grooves: lotus leaf and rose petal effects

The (de)wetting transitions of hierarchical grooves periodically engraved on a hydrophobic surface were investigated using a fully atomistic molecular dynamics simulation. The (meta) stable and transition states with sagging or depinning liquid surfaces were identified by calculating the free energy profiles of the (de)wetting transitions. The dewetting transitions for wide and narrow minor grooves have large and small activation free energies, respectively, exhibiting contrasting water adhesion forces as found for rose petals and lotus leaves.

Using molecular dynamics to quantify the electrical double layer and examine the potential for its direct observation in the in-situ TEM

Understanding the fundamental processes taking place at the electrode-electrolyte interface in batteries will play a key role in the development of next generation energy storage technologies. One of the most fundamental aspects of the electrode-electrolyte interface is the electrical double layer (EDL). Given the recent development of high spatial resolution in-situ electrochemical fluid cells for scanning transmission electron microscopy (STEM), there now exists the possibility that we can directly observe the formation and dynamics of the EDL. In this paper we predict electrolyte structure within the EDL using classical models and atomistic Molecular Dynamics (MD) simulations. Classical models are found to greatly differ from MD in predicted concentration profiles. It is thus suggested that MD must be used in order to accurately predict STEM images of the electrode-electrolyte interface. Using MD and image simulation together for a high contrast electrolyte (the high atomic number CsCl electrolyte), it is determined that, for a smooth interface, concentration profiles within the EDL should be visible experimentally. When normal experimental parameters such as rough interfaces and low-Z electrolytes (like those used in Li-ion batteries) are considered, observation of the EDL appears to be more difficult.

Thermal conductivity of ordered-disordered material: a case study of superionic Ag2Te

Thermoelectric devices, which can generate electricity from waste heat, offer an attractive pathway for addressing an important niche in the globally growing landscape of energy demand. In the past few decades, the search for high-efficiency thermoelectrics has been guided by the concept of 'phonon-glass electron-crystal' (PGEC), i.e. an ideal thermoelectric material should have high carrier mobility and low thermal conductivity. Although remarkable progress has already been made along this line, the efficiency of thermoelectrics is still too poor to compete with other electricity producing methods. Ordered-disordered material, an emerging trend of high performance thermoelectrics under the concept of PGEC, is a new hot topic in the current thermoelectric research community. Taking superionic phase silver telluride (α-Ag2Te) as an example, we performed a comprehensive study of the thermal transport properties and of its physical mechanism by means of equilibrium molecular dynamic simulations. The results show that the thermal conductivity of α-Ag2Te is intrinsically very low. By analyzing the different contributions to the overall thermal conductivity, we revealed for the first time from atomistic simulations that the vibration of the Te(2-) sublattice dominates the thermal transport of α-Ag2Te, while the collision between the randomly diffusing Ag(+) ions and the Te(2-) sublattice yields a significant negative contribution to the thermal transport. We also studied the effect of isotropic compressive stain and carrier concentration on the thermal conductivity of α-Ag2Te. It has been found that the thermal conductivity can be largely reduced by applying compressive strain or with stoichiometric quantity modulation. Our studies shed light on the governing mechanism of thermal transport in ordered-disordered materials and could offer useful guidance for engineering the thermal transport properties of superionic conductors in terms of enhancing their thermoelectric performance.

Effect of water on structural and frictional properties of self assembled monolayers

Self-assembled monolayers (SAMs) of n-alkanethiols [(CH3(CH2)(n-1), n = 14, 15] on Au(111) in the presence of water have been simulated by molecular dynamics simulation. The behavior and effects of compression on structural characteristics and water penetration into monolayers under different ranges of normal pressures have been investigated. Frictional properties of hydrated SAM systems under various sliding velocities, and loading conditions are examined to explore correlation between the amount of water penetration and friction. Simulations for one odd and one even SAM (C14 and C15) systems have revealed interesting odd-even effects in water penetration and frictional properties. We have also compared the frictional and structural properties of hydrated systems to that of dry SAM-Au (one surface of gold is covered by SAM) and SAM-SAM (both gold substrates are covered by SAM) contacts. The results reveal that the even hydrated SAM (C14) shows lower friction coefficient compared with the odd hydrated SAM (C15). We found the presence of water reduces the friction only at lower pressures; and at higher pressures, dry SAM-Au contacts offer lower friction. It was interesting to see that the lubricity effect of water was much stronger for the odd system and persisted to slightly higher pressures (300 MPa for the even SAM and 700 MPa for the odd SAM). At higher pressures, for both odd and even systems, the presence of water increased the friction. We also found that at low sliding velocities and higher pressures apparent water viscosity was enhanced by up to 3 orders of magnitude, indicating possible solidification.

Examining the frictional forces between mixed hydrophobic-hydrophilic alkylsilane monolayers

Monolayers presenting methyl-terminated (hydrophobic) and hydroxyl-terminated (hydrophilic) surfaces on silica have been studied by molecular dynamics simulation and the effects of hydrogen bonding, chain length, and chain mixing on the frictional properties determined. The hydroxyl-terminated monolayers were found to show large adhesion zones as a result of strong interfacial interlayer hydrogen bonds; the interfacial sliding forces observed in the hydroxyl-terminated monolayers being one order of magnitude higher than the interfacial forces for the hydrophobic surfaces at the characteristic point of zero-load. Mixed hydroxyl- and methyl-terminated monolayers of equal length were found to exhibit intermediate shear stress values between those observed for pure monolayers, with the magnitude of the shear stress depending on the surface content of the hydroxyl-terminated chains. For mixed monolayers of unequal chain lengths, at high loads a maximum in the magnitude of the shear stress as a function of the length of the methyl-terminated chain was observed due to the creation of a buffer zone between the hydroxyl-terminated chains that produces strong hydrogen-bonding interactions. The effect of a constant normal load or constant separation simulation ensemble on the results has also been studied and in general found to have minimal influence on the observed behavior, although some differences are observed for the shear stress at intermediate normal loads due to the formation of stronger hydrogen bond networks at constant load compared to constant separation.

Layer-by-layer assembly of polyelectrolyte chains and nanoparticles on nanoporous substrates: molecular dynamics simulations

We performed molecular dynamics simulations of a multilayered assembly of oppositely charged polyelectrolyte chains and nanoparticles on porous substrates with cylindrical pores. The film was constructed by the sequential adsorption of oppositely charged species in a layer-by-layer fashion from dilute solutions. The multilayer assembly proceeds through surface overcharging after the completion of each deposition step. The substrate overcharging fraction fluctuates around 0.5 for nanoparticle-polyelectrolyte systems and around 0.4 for polyelectrolyte-polyelectrolyte systems. The surface coverage increases linearly with the number of deposition steps. The rate of surface coverage increases as a function of the number of deposition step changes when the pore is blocked. The closing of the pore occurs from the pore entrance for nanoparticle-polyelectrolyte systems. In the case of polyelectrolyte-polyelectrolyte systems, the pore plug is formed inside the pore and then spreads toward the pore ends.

Friction between brush layers of charged and neutral bottle-brush macromolecules. molecular dynamics simulations

Using molecular dynamics simulations, we study the lubricating properties of neutral and charged bottle-brush coatings as a function of the compression and shear stresses and brush grafting density. Our simulations have shown that in charged bottle-brush systems under shear there is a layer with excess counterions located in the middle between brush-bearing surfaces. The main deformation mode of the charged bottle-brush layers is associated with the backbone deformation, resulting in the backbone deformation ratio, α, and shear viscosity, η, being universal functions of the Weissenberg number. In the case of neutral bottle-brush systems, in addition to the backbone deformation there is also side chain deformation. The coupling between backbone and side chain deformation violates universality in the deformation ratio, α, dependence on the Weissenberg number and results in scaling exponents varying with the compression stress and brush grafting density. The existence of different length scales controlling deformation of neutral bottle brushes manifests itself in the shear viscosity, η, dependence on the shear rate, ̇γ. Shear viscosity, η, as a function of the shear rate, ̇γ, has two plateaus and two shear thinning regimes. The low shear rate plateau and shear thinning regime correspond to the backbone deformation, while the second plateau and shear thinning regime at moderate shear rates are due to side chain deformation. For both systems the value of the friction coefficient increases with increasing shear rate. The values of the friction coefficient for charged bottle-brush systems are about ten times smaller than corresponding values for neutral systems at the same shear rate.

Tribological durability of silane monolayers on silicon

We report the frictional performance and long-term tribological stability of various alkyl silane monolayer films on silicon by using pin-on-disk tribometry at ambient conditions. We show that the durability of monolayers derived from n-alkyltrichlorosilanes on silicon increases exponentially with the chain length of the silane precursor, which we relate to the cohesive energy of these monolayers through molecular dynamics simulations. X-ray photoelectron spectroscopy (XPS) was used to show that tribological damage consisted of the loss of molecular components that could be partially replaced upon exposure to a solution containing perfluorinated silane precursors. For monolayers derived from n-octadecyltrichlorosilane, a critical load was identified to be approximately 250 mN (200 MPa), above which failure of films occurred within 100 cycles of testing. Monolayers with hydroxyl surfaces exhibited reduced stabilities due to stronger tip-surface interactions. Monolayers with the capability for cross-linking exhibited much greater stabilities than monolayers where cross-linking was limited or prevented. Collectively, these results demonstrate that the mechanical durability of monolayers when subjected to a tribological load is greatly enhanced by maximizing dispersional interactions and cross-linking and minimizing tip-surface interactions.

Spontaneous asymmetry of coated spherical nanoparticles in solution and at liquid-vapor interfaces

Spherical nanoparticles (NPs), uniformly coated with a simple polymer used to passivate NPs in solution, are shown to produce highly asymmetric coating arrangements. We show that a class of spherical particles is not symmetrically coated even when extremely uniform grafting arrangements and full coverages are employed. We demonstrate, using explicit-atom molecular dynamics simulations of model NPs, that geometric properties dictate when a coating's spherical symmetry will be unstable and show that the chain end group and the solvent play a secondary role in determining the properties of surface patterns. At the water-vapor interface the coatings are significantly distorted and oriented by the surface. These asymmetric and oriented coatings are expected to have a dramatic effect on the interactions between NPs and will likely influence the structures of aggregated NPs which self-assemble in the bulk and at surfaces.

Temperature control in molecular dynamic simulations of non-equilibrium processes

Thermostats are often used in various condensed matter problems, e.g. when a biological molecule undergoes a transformation in a solution, a crystal surface is irradiated with energetic particles, a crack propagates in a solid upon applied stress, two surfaces slide with respect to each other, an excited local phonon dissipates its energy into a crystal bulk, and so on. In all of these problems, as well as in many others, there is an energy transfer between different local parts of the entire system kept at a constant temperature. Very often, when modelling such processes using molecular dynamics simulations, thermostatting is done using strictly equilibrium approaches serving to describe the NVT ensemble. In this paper we critically discuss the applicability of such approaches to non-equilibrium problems, including those mentioned above, and stress that the correct temperature control can only be achieved if the method is based on the generalized Langevin equation (GLE). Specifically, we emphasize that a meaningful compromise between computational efficiency and a physically appropriate implementation of the NVT thermostat can be achieved, at least for solid state and surface problems, if the so-called stochastic boundary conditions (SBC), recently derived from the GLE (Kantorovich and Rompotis 2008 Phys. Rev. B 78 094305), are used. For SBC, the Langevin thermostat is only applied to the outer part of the simulated fragment of the entire system which borders the surrounding environment (not considered explicitly) serving as a heat bath. This point is illustrated by comparing the performance of the SBC and some of the equilibrium thermostats in two problems: (i) irradiation of the Si(001) surface with an energetic CaF(2) molecule using an ab initio density functional theory based method, and (ii) the tribology of two amorphous SiO(2) surfaces coated with self-assembled monolayers of methyl-terminated hydrocarbon alkoxylsilane molecules using a classical atomistic force field. We discuss the differences in behaviour of these systems due to applied thermostatting, and show that in some cases a qualitatively different physical behaviour of the simulated system can be obtained if an equilibrium thermostat is used.

Water nanoconfinement induced thermal enhancement at hydrophilic quartz interfaces

We report the effect of water nanoconfinement on the thermal transport properties of two neighbor hydrophilic quartz interfaces. A significant increase and a nonintuitive, nonmonotonic dependence of the overall interfacial thermal conductance between the quartz surfaces on the water layer thickness were found. By probing the interfacial structure and vibrational properties of the connected components, we demonstrated that the mechanism of the peak occurring at submonolayer water originates from the freezing of water molecules at extremely confined conditions and the excellent match of vibrational states between trapped water and hydrophilic headgroups on the two contact surfaces. Our results show that incorporation of polar molecules into hydrophilic interfaces is very promising to enhance the thermal transport through thermally smooth connection of these interfaces.

Molecular dynamics simulations of water confined between matched pairs of hydrophobic and hydrophilic self-assembled monolayers

We have conducted a molecular dynamics (MD) simulation study of water confined between methyl-terminated and carboxyl-terminated alkylsilane self-assembled monolayers (SAMs) on amorphous silica substrates. In doing so, we have investigated the dynamic and structural behavior of the water molecules when compressed to loads ranging from 20 to 950 MPa for two different amounts of water (27 and 58 water molecules/nm2). Within the studied range of loads, we observe that no water molecules penetrate the hydrophobic region of the carboxyl-terminated SAMs. However, we observe that at loads larger than 150 MPa water molecules penetrate the methyl-terminated SAMs and form hydrogen-bonded chains that connect to the bulk water. The diffusion coefficient of the water molecules decreases as the water film becomes thinner and pressure increases. When compared to bulk diffusion coefficients of water molecules at the various loads, we found that the diffusion coefficients for the systems with 27 water molecules/nm2 are reduced by a factor of 20 at low loads and by a factor of 40 at high loads, while the diffusion coefficients for the systems with 58 water molecules/nm2 are reduced by a factor of 25 at all loads.

Water penetration of damaged self-assembled monolayers

The interaction of water with self-assembled monolayers (SAMs) on amorphous silica is investigated using classical molecular dynamics simulation. Damage is induced through shear simulations with model atomic force microscopy (AFM) tips and separately with controlled extraction. We find that SAM coatings that have been slightly damaged (by normal loads close to 10 nN from a 10-nm-diameter AFM tip) are susceptible to water penetration and migration to the underlying hydrophilic substrate. The controlled damage studies indicate that the presence of water tends to heal damage below a threshold radius and exploits and magnifies damage above this threshold. For the systems studied here, Si(OH)3(CH2)10CH3 alkylsilane chains on amorphous silica, this threshold radius is between 0.5 and 1.0 nm.

Water in nanoconfinement between hydrophilic self-assembled monolayers

Molecular dynamics (MD) simulations of water confined to subnanometer thicknesses between carboxyl-terminated alkanethiol self-assembled monolayers (SAMs) on gold were performed to address conflicts in the literature on the structure and response of water in confinement. The amount of water was varied to yield submonolayer to bilayer structures. The orientation of the water is affected by the confinement, especially in the submonolayer case. We find that the diffusion coefficient decreases as the film becomes thinner and at higher pressures. However, in all cases studied, liquid diffusion is always found. At maximal suppression, the diffusion constant is 2 orders of magnitude smaller than the bulk value.

Simulations of electrophoretic RNA transport through transmembrane carbon nanotubes

The study of interactions between carbon nanotubes and cellular components, such as membranes and biomolecules, is fundamental for the rational design of nanodevices interfacing with biological systems. In this work, we use molecular dynamics simulations to study the electrophoretic transport of RNA through carbon nanotubes embedded in membranes. Decorated and naked carbon nanotubes are inserted into a dodecane membrane and a dimyristoylphosphatidylcholine lipid bilayer, and the system is subjected to electrostatic potential differences. The transport properties of this artificial pore are determined by the structural modifications of the membrane in the vicinity of the nanotube openings and they are quantified by the nonuniform electrostatic potential maps at the entrance and inside the nanotube. The pore is used to transport electrophoretically a short RNA segment and we find that the speed of translocation exhibits an exponential dependence on the applied potential differences. The RNA is transported while undergoing a repeated stacking and unstacking process, affected by steric interactions with the membrane headgroups and by hydrophobic interaction with the walls of the nanotube. The RNA is structurally reorganized inside the nanotube, with its backbone solvated by water molecules near the axis of the tube and its bases aligned with the nanotube walls. Upon exiting the pore, the RNA interacts with the membrane headgroups and remains attached to the dodecane membrane while it is expelled into the solvent in the case of the lipid bilayer. The results of the simulations detail processes of molecular transport into cellular compartments through manufactured nanopores and they are discussed in the context of applications in biotechnology and nanomedicine.

Hydrogen bonding and binding of polybasic residues with negatively charged mixed lipid monolayers

Phosphoinositides, phosphorylated products of phosphatidylinositol, are a family of phospholipids present in tiny amounts (1% or less) in the cytosolic surface of cell membranes, yet they play an astonishingly rich regulatory role, particularly in signaling processes. In this letter, we use molecular dynamics simulations on a model system of mixed lipid monolayers to investigate the interaction of phosphatidylinositol 4,5-bisphosphate (PIP2), the most common of the phosphoinositides, with a polybasic peptide consisting of 13 lysines. Our results show that the polybasic peptide sequesters three PIP2 molecules, forming a complex stabilized by the formation of multiple hydrogen bonds between PIP2 and the Lys residues. We also show that the polybasic peptide does not sequester other charged phospholipids such as phosphatidylserine because of the inability to form long-lived stable hydrogen bonds.

Simulations of nanotribology with realistic probe tip models

We present the results of massively parallel molecular dynamics simulations aimed at understanding the nanotribological properties of alkylsilane self-assembled monolayers (SAMs) on amorphous silica. In contrast to studies with opposing flat plates, as found in the bulk of the simulation literature, we use a model system with a realistic AFM tip (radius of curvature ranging from 3 to 30 nm) in contact with a SAM-coated silica substrate. We compare the differences in response between systems in which chains are fully physisorbed, fully chemisorbed, and systems with a mixture of the two. Our results demonstrate that the ubiquitous JKR and DMT models do not accurately describe the contact mechanics of these systems. In shear simulations, we find that the chain length has minimal effects on both the friction force and coefficient. The tip radius affects the friction force only (i.e., the coefficient is unchanged) by a constant shift in magnitude due to the increase in pull-off force with increasing radius. We also find that at extremely low loads, on the order of 10 nN, shearing from the tip causes damage to the physisorbed monolayers by removal of molecules.

Monte Carlo simulation of electrolytes in the constant voltage ensemble

The authors studied the structural, electrostatic, and electromechanical properties of the terlamellar structure composed of the anode, the cathode, and the electrolyte layer separating them. They used the Monte Carlo simulation technique in the constant voltage ensemble, where the electrical potential difference between the anode and the cathode is introduced as an external field. For ions, they used the primitive models of different sizes and valences in order to investigate how they affect the physical properties when an electrical field is applied between the electrodes. For electrodes, they used impermeable and permeable models, which mimic planar and porous electrodes, respectively. The asymmetry between the anions and the cations in size or valence was found to be responsible for the asymmetry in the concentration profile, the potential drop, and the stress distribution, in comparing the anode and the cathode sides. The charging/discharging process in the planar and porous electrodes is discussed at molecular level.

Charge inversion of divalent ionic solutions in silica channels

Recent experiments [F. H. J. van der Heyden, Phys. Rev. Lett. 96, 224502 (2006)] of streaming currents in silica nanochannels with divalent ions report charge inversion, i.e., interfacial charges attracting counterions in excess of their own nominal charge, in conflict with existing theoretical and simulation results. We reveal the mechanism of charge inversion by using all-atomic molecular dynamics simulations. Our results show excellent agreement with experiments, both qualitatively and quantitatively. We further discuss the implications of our study for the general problem of ionic correlations in solutions as well as in regards to the properties of silica-water interfaces.

A Monte Carlo Study of Spherical Electrical Double Layer of Macroions−Polyelectrolytes Systems in Salt Free Solutions

A canonical Monte Carlo simulation is performed to investigate the microstructure and the electrical double layer (EDL) of polyelectrolytes around macroions in the bulk systems based on the primitive model. We explore the influences of particles size, chain length, and charge density of polyelectrolytes on the microscopic behavior of the macroions-polyelectrolytes systems. The simulation results show that the surface charge density and the chain length of the polyelectrolytes are two key factors that affect the microstructure of polyelectrolytes around the macroions and potential of mean force between the macroions as well as the zeta potential of the spherical EDL constructed by polyelectrolytes. The high surface charge density of a polyelectrolyte leads to the polyelectrolyte acting as a bridge for the aggregation of macroions, causing the presence of the attraction between macroions. The polyelectrolytes with a long chain length present a cooperativity effect for the adsorption of the polyelectrolytes on the surface of the macroions. Furthermore, the two key factors both induce the overcharge of the macroions. The longer the chain length and the higher surface charge density of the polyelectrolytes, the stronger is the overcharge.

Atomistic simulations of langmuir monolayer collapse

Monolayers at the vapor/water interface collapse by exploring the third dimension at sufficient lateral compression, either by forming three-dimensional structures or by solubilization into the aqueous solution. In this paper, we provide an atomistic description of collapse from molecular dynamics (MD) simulations. More specifically, we investigate monolayers of arachidic acids spread on pure water and in an aqueous solution with Ca2+ ions in the subphase. In both cases, it is found that the collapsed systems generally lead to the formation of multilayer structures, which in the system with Ca2+ ions, proceeds by an intermediate regime where the monolayer exhibits significant roughness (of the order of 4 A). If no roughness is present, the system forms collapsed structures into the aqueous solution. The computational cost of atomic MD limits our simulations to relatively small system sizes, fast compression rates, and temporal scales on the order of a nanosecond. We discuss the issues caused by these limitations and present a detailed discussion of how the collapse regime proceeds at long time scales. We conclude with a summary of the implications of our results for further theoretical and experimental studies.

Tribological properties of alkylsilane self-assembled monolayers

In this study, we perform molecular dynamics simulations of adhesive contact and friction between alkylsilane Si(OH)(3)(CX(2))(10)CX(3) and alkoxylsilane Si(OH)(2)(CX(2))(10)CX(3) (where X = H or F) self-assembled monolayers (SAMs) on an amorphous silica substrate. The alkylsilane SAMs are primarily hydrogen-bonded or physisorbed to the surface. The alkoxylsilane SAMs are covalently bonded or chemisorbed to the surface. Previously, we studied the chemisorbed systems. In this work, we study the physisorbed systems and compare the tribological properties with the chemisorbed systems. Furthermore, we examine how water at the interface of the SAMs and substrate affects the tribological properties of the physisorbed systems. When less than a third of a monolayer is present, very little difference in the microscopic friction coefficient mu or shear stresses is observed. For increasing amounts of water, the values of mu and the shear stresses decrease; this effect is somewhat more pronounced for fluorocarbon alkylsilane SAMs than for the hydrocarbon SAMs. The observed decrease in friction is a consequence of a slip plane that occurs in the water as the amount of water is increased. We studied the frictional behavior using relative shear velocities ranging from v = 2 cm/s to 2 m/s. Similar to previously reported results for alkoxylsilane SAMs, the values of the measured stress and mu for the alkylsilane SAM systems decrease monotonically with v.

Mixed bilayer containing dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylserine: lipid complexation, ion binding, and electrostatics

Two mixed bilayers containing dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylserine at a ratio of 5:1 are simulated in NaCl electrolyte solutions of different concentration using the molecular dynamics technique. Direct NH.O and CH.O hydrogen bonding between lipids was observed to serve as the basis of interlipid complexation. It is deduced from our results and previous studies that dipalmitoylphosphatidylcholine alone is less likely to form interlipid complexes than in the presence of bound ions or other bilayer "impurities" such as dipalmitoylphosphatidylserine. The binding of counterions is observed and quantitated. Based upon the calculated ion binding constants, the Gouy-Chapman surface potential (theta) is calculated. In addition we calculated the electrostatic potential profile (Phi) by twice integrating the system charge distribution. A large discrepancy between and the value of Phi at the membrane surface is observed. However, at "larger" distance from the bilayer surface, a qualitative similarity in the z-profiles of Phi and psi(GC) is seen. The discrepancy between the two potential profiles near the bilayer surface is attributed to the discrete and nonbulk-like nature of water in the interfacial region and to the complex geometry of this region.

Density-functional theory of spherical electric double layers and ζ potentials of colloidal particles in restricted-primitive-model electrolyte solutions

A density-functional theory is proposed to describe the density profiles of small ions around an isolated colloidal particle in the framework of the restricted primitive model where the small ions have uniform size and the solvent is represented by a dielectric continuum. The excess Helmholtz energy functional is derived from a modified fundamental measure theory for the hard-sphere repulsion and a quadratic functional Taylor expansion for the electrostatic interactions. The theoretical predictions are in good agreement with the results from Monte Carlo simulations and from previous investigations using integral-equation theory for the ionic density profiles and the zeta potentials of spherical particles at a variety of solution conditions. Like the integral-equation approaches, the density-functional theory is able to capture the oscillatory density profiles of small ions and the charge inversion (overcharging) phenomena for particles with elevated charge density. In particular, our density-functional theory predicts the formation of a second counterion layer near the surface of highly charged spherical particle. Conversely, the nonlinear Poisson-Boltzmann theory and its variations are unable to represent the oscillatory behavior of small ion distributions and charge inversion. Finally, our density-functional theory predicts charge inversion even in a 1:1 electrolyte solution as long as the salt concentration is sufficiently high.