Ionic liquid confined in silica nanopores: molecular dynamics in the isobaric–isothermal ensemble

Molecular dynamics simulations in the isobaric–isothermal ensemble are used to investigate the structure and dynamics of an ionic liquid confined at ambient temperature and pressure in hydroxylated amorphous silica nanopores. The use of the isobaric–isothermal ensemble allows estimating the effect of confinement and surface chemistry on the density of the confined ionic liquid. The structure of the confined ionic liquid is investigated using density profiles and structural order parameters while its dynamics is assessed by determining the mobility and ionic conductivity of the confined phase. Despite the important screening of the electrostatic interactions (owing to the small Debye length in ionic liquids), the local structure of the confined ionic liquid is found to be mostly driven by electrostatic interactions. We show that both the structure and dynamics of the confined ionic liquid can be described as the sum of a surface contribution arising from the ions in contact with the surface and a bulk-like contribution arising from the ions located in the pore centre; as a result, most properties of the confined ionic liquid are a simple function of the surface-to-volume ratio of the host porous material. In contrast, the ionic conductivity of the confined ionic liquid, which is a collective dynamical property, is found to be similar to the bulk. This study sheds light on the complex behaviour of hybrid materials made up of ionic liquid confined in inorganic porous materials.     

Dynamics of polydisperse hard-spheres under strong confinement

We use molecular simulation to probe the connection between local structure and the unusual re-entrant dynamics observed for polydisperse hard-sphere liquids confined in thin slit pores. The local structure is characterised by calculating 2-D bond-orientational order parameters associated with square and hexatic order for particles in the layer adjacent to the confining walls. When the wall separation is commensurate with the average particle size, the particles primarily exhibit local hexatic order, whereas local square order increases in prevalence for incommensurate geometries. The relaxation time extracted from the ensemble-averaged mean-square displacement increases exponentially with the static correlation length associated with hexatic local order in strongly confined commensurate geometries, in agreement with theoretical predictions for dynamical slowing. Square order, by contrast, is not associated with a growing length scale for either commensurate or incommensurate geometries, indicating that it is strongly geometrically frustrated. Our results suggest that the influence of bond-orientational order on dynamical slowing may be altered by changing the extent of confinement.     

Connection between Adam-Gibbs theory and spatially heterogeneous dynamics

We investigate the spatially heterogeneous dynamics in the extended simple point charge model of water using molecular dynamics simulations. We relate the average mass n* of mobile particle clusters to the diffusion constant and the configurational entropy. Hence, n* can be interpreted as the mass of the "cooperatively rearranging regions" that form the basis of the Adam-Gibbs theory of the dynamics of supercooled liquids. We also examine the time and temperature dependence of these transient clusters.     

Molecular dynamics studies of slow dynamics in random media: Type A-B and reentrant transitions

Molecular dynamics simulations of mobile particles confined in disordered immobile particles are carried out. Slow dynamics in random media are characterized by two types of dynamics: Type B dynamics for large mobile particle density and Type A dynamics for small mobile particle density. The crossover from Type A to B dynamics is studied by the mean square displacement and the density correlation function. Our results are qualitatively consistent with the results of recent numerical and theoretical studies on relevant spatially heterogeneous systems. We also investigate the effect of random matrix generation on the dynamics of mobile particles in order to examine the reentrant transition predicted by the recent mode-coupling theory. Our simulations demonstrate that the diffusion of the mobile particles largely depends on the protocol of the random matrix generation and that the reentrant transition is observed for a particular protocol.     

Enthalpy recovery of a glass-forming liquid constrained in a nanoporous matrix: Negative pressure effects

The T _g of organic liquids confined to nanoporous matrices and that of thin polymer films can decrease dramatically from the bulk value. One possible explanation for this phenomenon is the development of hydrostatic tension during vitrification under confinement that results in a concomitant increase in the free volume. Here we present experimental evidence and modeling results for ortho-terphenyl (o-TP) confined in pores as small as 11.6 nm that indicate that, although there is an important hydrostatic tension in the liquid in the pores, it does not develop until near the reduced T _g of the constrained material --well below the bulk T _g. Enthalpy recovery for the o-TP in the nanopores exhibits accelerated physical aging relative to the bulk, as well as a leveling off of the fictive temperature at equilibrium values greater than the aging temperature. An adaptation of the structural recovery model that incorporates vitrification under isochoric conditions is able to provide a quantitative explanation for the apparently anomalous aging observed in nanopore confined liquids and in thin polymeric films. The results strongly support the existence of an intrinsic size effect as the cause of the reduced T _g. Received 3 September 2001     

Molecular-Dynamics Simulations of Droplets on a Solid Surface

By using a semi-empirical Lennard-Jones embedded-atom-method potential, we study the influence of many-body forces and atomic size mismatch on the wetting behavior of nano droplets on a solid surface. With molecular dynamics simulations, we find that the contact angle decreases with increasing many-body forces. The increase of atomic size mismatch between solid and liquid results in the decrease of contact angles. Our calculation also shows that the interface structure is strongly affected by the interaction between liquid and solid as well as the atomic size mismatch. For weak solid-liquid interaction, the interface layer of the droplet nearest to the solid exhibits a typical simple liquid structure regardless of the size mismatch. For strong solid-liquid interaction, evident ordering in the interface layer is observed for well matched cases.     

A molecular dynamics investigation of dynamical heterogeneity in supercooled water

We investigate the presence of dynamical heterogeneity in supercooled water with molecular dynamics simulations using the new water model proposed by Mahoney and Jorgensen [M.W. Mahoney, W.L. Jorgensen J. Chem. Phys. 112, 8910 (2000)]. Prompted by recent theoretical results [J.P. Garrahan, D. Chandler, Phys. Rev. Lett. 89, 35704 (2002)] we study the dynamical aggregation of the least and the most mobile molecules. We find dynamical heterogeneity in supercooled water and string-like dynamics for the most mobile molecules. We also find the dynamical aggregation of the least mobile molecules. The two kinds of dynamical aggregation appear however to be very different. Characteristic times are different and evolve differently. String-like motions appear only for the most mobile molecules, a result predicted by the facilitation theory. The aggregation of the least mobile molecules is more organized than the bulk while the opposite is observed for the most mobile molecules.     

Phase coexistence and dynamic properties of water in nanopores

The dynamical properties of a confined fluid depend strongly on the (spatially varying) density. Its knowledge is therefore an important prerequisite for molecular-dynamics (MD) simulations and the analysis of experimental data. In a mixed Gibbs ensemble Monte Carlo (GEMC)/MD simulation approach we first apply the GEMC method to find possible phase states of water in hydrophilic and hydrophobic nanopores. The obtained phase diagrams evidence that a two-phase state is the most probable state of a fluid in incompletely filled pores in a wide range of temperature and level of pore filling. Pronounced variations of the average and local densities are observed. Subsequently, we apply constant-volume MD simulations to obtain water diffusion coefficients and to study their spatial variation along the pore radius. In general, water diffusivity slightly decreases in a hydrophilic pore and noticeably increases in a hydrophobic pore (up to about 40% with respect to the bulk value). In the range of gradual density variations the local diffusivity essentially follows the inverse density and the water binding energy. The diffusivity in the quasi-two-dimensional water layers near the hydrophilic wall decreases by 10 to 20% with respect to the bulk value. The average diffusivity of water in incompletely filled pore is discussed on the basis of the water diffusivities in the coexisting phases.Received: 1 January 2003, Published online: 14 October 2003PACS: 61.20.Ja Computer simulation of liquid structure - 64.70.Fx Liquid-vapor transitions     

Hydrophobic and Ionic-Interactions in Bulk and Confined Water with Implications for Collapse and Folding of Proteins

Water and water-mediated interactions determine the thermodynamics and kinetics of protein folding, protein aggregation and self-assembly in confined spaces. To obtain insights into the role of water in the context of folding problems, we describe computer simulations of a few related model systems. The dynamics of collapse of eicosane shows that upon expulsion of water the linear hydrocarbon chain adopts an ordered helical hairpin structure with 1.5 turns. The structure of dimer of eicosane molecules has two well ordered helical hairpins that are stacked perpendicular to each other. As a prelude to studying folding in confined spaces we used simulations to understand changes in hydrophobic and ionic interactions in nano-sized water droplets. Solvation of hydrophobic and charged species change drastically in nano-scale water droplets. Hydrophobic species are localized at the boundary. The tendency of ions to be at the boundary where water density is low increases as the charge density decreases. The interactions between hydrophobic, polar, and charged residue are also profoundly altered in confined spaces. Using the results of computer simulations and accounting for loss of chain entropy upon confinement we argue and then demonstrate, using simulations in explicit water, that ordered states of generic amphiphilic peptide sequences should be stabilized in cylindrical nanopores.     

过冷液体钾形核初期微观动力学机理的模拟研究

采用分子动力学方法对液态金属钾凝固过程进行了模拟,根据凝固过程体系平均原子能量、原子成键类型和成团类型,以及均方位移和非Gauss参数等动力学参数的演化特征,对过冷熔体形核初期微观动力学机理进行了研究.结果表明:根据过冷液体钾结晶形核过程热力学、动力学和结构特性的演化规律, 其过冷温度区间可以分为两个明显不同的阶段,潜在结晶核心出现在过冷液体较低温区.过冷熔体钾在形核初期,二十面体团簇结构在α-弛豫阶段逐渐解体,同时具有体心立方(bcc)结构的潜在结晶核心逐步形成,其临界晶核包含约300个原子.     

尺寸效应对微通道内固液界面温度边界的影响

采用非平衡分子动力学方法模拟不同浸润性微通道内液体的传热过程,分析了尺寸效应对固液界面热阻及温度阶跃的影响.研究结果表明,界面热阻随微通道尺寸的变化可分为两个阶段,即小尺寸微通道的单调递增阶段和大尺寸微通道的恒定值阶段.随着微通道尺寸的增加,近壁区液体原子受对侧固体原子的约束程度降低,微通道中央的液体原子自由移动,固液原子振动态密度近似不变,使得尺寸效应的影响忽略不计.上述两种阶段的微通道尺寸过渡阈值受固液作用强度与壁面温度的共同作用:减弱壁面浸润性,过渡阈值向大尺寸区域迁移;相较于低温壁面,高温壁面处的过渡阈值更大.增加微通道尺寸,固液界面温度阶跃呈单调递减趋势,致使壁面温度边界和宏观尺度下逐渐符合.探讨尺寸效应有助于深刻理解固液界面能量输运及传递机制.     

Relaxation, crystallization, and glass transition in supercooled liquid Ni

In this Letter, molecular dynamics (MD) simulations based on EAM many-body potential have been performed to investigate the differences of dynamical heterogeneity in the course of crystallization and glass transition, respectively. The crystallization of liquid, detected at a cooling rate of , is characterized by the appearances of the second plateau in mean square displacement (MSD) and the nonzero plateau in non-Gaussian parameter (NGP). It implies that the non-diffusive rearrangement of atoms occurring at a certain temperature and relaxation time leads to nucleus forming. The glass phase forms as the cooling rate increases to . There is no second plateau in MSD appearing in the formation of metallic glass, indicating the diffusive motion of atoms. The non-Gaussian characteristic in NGP is more obvious at low temperatures.     

Molecular Dynamics Simulations of Helium Behaviour in Titanium Crystals

Molecular dynamics simulations are performed to investigate the behaviour of helium atoms in titanium at a temperature of 30OK. The nucleation and growth of helium bubble has been simulated up to 50 helium atoms. The approach to simulate the bubble growth is to add helium atoms one by one to the bubble and let the system evolve. The titanium cohesion is based on the tight binding scheme derived from the embedded atom method, and the helium-titanium interaction is characterized by fitted potential in the form of a Lennard-Jones function. The pressure in small helium bubbles is approximately calculated. The simulation results show that the pressure will decrease with the increasing bubble size, while increase with the increasing helium atoms. An analytic function about the quantitative relationship of the pressure with the bubble size and number of helium atoms is also fitted.     

Effects of confinement on static and dynamical properties of water

Molecular-dynamics results on water confined in a silica pore are reviewed and discussed in connection with experiments performed on water in Vycor and with studies of water in contact with proteins. The properties of confined water are studied as a function of both temperature and hydration level. The interaction of water in the film close to the substrate with the silica atoms induces a strong distortion of the hydrogen bond network. At high hydration levels a double dynamical regime is observed. At low hydration an anomalous diffusion is found upon supercooling with a transition from a Brownian to a non-Brownian regime on approaching the substrate in agreement with results found in studies of water in contact with globular proteins.Received: 1 January 2003, Published online: 14 October 2003PACS: 61.20.-p Structure of liquids - 61.20.Ja Computer simulation of liquid structure     

Water and Ion Permeation through Electrically Charged Nanopore

The behaviour of water and small solutes in confined geometries is important to a variety of chemical and nanofluidic applications. Here we investigate the permeation and distribution of water and ions in electrically charged carbon cylindrical nanopore during the osmotic process using molecular dynamics simulations. In the simulations, charges are distributed uniformly on the pores with diameter of 0.9 nm. For nanopores with no charge or a low charge, ions are difficult to enter. With the increasing of charge densities on the pores, ions will appear inside the nanopores because of the large electronic forces between the ions and the charged pores. Different ion entries induce varying effects on osmotic water flow. Our simulations reveal that the osmotic water can flow through the negatively charged pore occupied by K^＋ ions, while water flux through the positively charged pores will be disrupted by Cl^- ions inside the pores. This may be explained by the different radial distributions of K^＋ ions and Cl^- ions inside the charged nanopores.     

Molecular dynamics study of confined ionic liquids in Au nanopore

Molecular dynamics simulations were carried to investigate the structure and dynamics of [BMIM][PF₆] ionic liquid (IL) confined inside a slit-like Au metal nanopore with a pore size of 5.0 nm. The calculations show that the mass and number densities of the confined ILs are oscillatory; the solid-like high density layers are formed in the vicinity of the metal surface. The orientational investigation shows that the imidazolium ring of [BMIM] cations prefers to form a small tilt angle with the pore walls. Furthermore, the mean squared displacement (MSD) calculation indicates that the dynamics of confined ILs are remarkably slower than those observed in bulk systems. Our results suggest that the confinement of the Au nanopore can strongly affect the structural and dynamical properties of the confined ILs.     

Enhanced diffusion of a needle in a planar array of point obstacles

The transport of an infinitely thin, hard rod in a random, dense array of point obstacles is investigated by molecular dynamics simulations. Our model mimics the sterically hindered dynamics in dense needle liquids. Transport becomes increasingly fast at higher densities, and we observe a power-law divergence of the diffusion coefficient with exponent 0.8. This phenomenon is connected with a new divergent time scale, reflected in a zigzag motion of the needle, a two-step decay of the velocity-autocorrelation function, and a negative plateau in the non-Gaussian parameter. Finally, we provide a heuristic scaling argument for the new exponent.     

4He Luttinger liquid in nanopores

We study the low-temperature properties of a 4He fluid confined in nanopores, using large-scale quantum Monte Carlo simulations with realistic He-He and He-pore interactions. In the narrow-pore limit, the system can be described by the quantum hydrodynamic theory known as Luttinger liquid theory with a large Luttinger parameter, corresponding to the dominance of solid tendencies and strong susceptibility to pinning by a periodic or random potential from the pore walls. On the other hand, for wider pores, the central region appears to behave like a Luttinger liquid with a smaller Luttinger parameter, and may be protected from pinning by the wall potential, offering the possibility of experimental detection of a Luttinger liquid.     

Stratification kinetics of polyelectrolyte solutions confined in thin films

We have studied free horizontal liquid films made with semidilute polyelectrolyte solutions. A stratification phenomenon is observed during film thinning, with a step size close to the mesh size of the polymer network: dark domains nucleate and expand, the outer polymer layer dewetting a thinner film. The kinetics of dark spot expansion is not simply related to bulk viscosity and becomes faster when the film thickness decreases, suggesting an increase of the chain mobility of the confined polymer chains. These findings are similar to recent ones for other confined liquids and are the first reported so far for freely standing films.     

Mechanical properties of thin confined polymer films close to the glass transition in the linear regime of deformation: Theory and simulations

Over the past twenty years experiments performed on thin polymer films deposited on substrates have shown that the glass transition temperature T(g) can either decrease or increase depending on the strength of the interactions. Over the same period, experiments have also demonstrated that the dynamics in liquids close to the glass transition temperature is strongly heterogeneous, on the scale of a few nanometers. A model for the dynamics of non-polar polymers, based on percolation of slow subunits, has been proposed and developed over the past ten years. It proposes a unified mechanism regarding these two features. By extending this model, we have developed a 3D model, solved by numerical simulations, in order to describe and calculate the mechanical properties of polymers close to the glass transition in the linear regime of deformation, with a spatial resolution corresponding to the subunit size. We focus on the case of polymers confined between two substrates with non-negligible interactions between the polymer and the substrates, a situation which may be compared to filled elastomers. We calculate the evolution of the elastic modulus as a function of temperature, for different film thicknesses and polymer-substrate interactions. In particular, this allows to calculate the corresponding increase of glass transition temperature, up to 20 K in the considered situations. Moreover, between the bulk T(g) and T(g) + 50 K the modulus of the confined layers is found to decrease very slowly in some cases, with moduli more than ten times larger than that of the pure matrix at temperatures up to T(g) + 50 K. This is consistent with what is observed in reinforced elastomers. This slow decrease of the modulus is accompanied by huge fluctuations of the stress at the scale of a few tens of nanometers that may even be negative as compared to the solicitation, in a way that may be analogous to mechanical heterogeneities observed recently in molecular dynamics simulations. As a consequence, confinement may result not only in an increase of the glass transition temperature, but in a huge broadening of the glass transition.