Freezing and melting of binary mixtures confined in a nanopore

This paper reports on a Grand Canonical Monte Carlo study of the freezing and melting of Lennard–Jones Ar/Kr mixtures confined in a slit pore composed of two strongly attractive structureless walls. For all molar compositions and temperatures, the pore, which has a width of 1.44?nm, accommodates two contact layers and one inner layer. Different wall/fluid interactions are considered, corresponding to pore walls that have a larger affinity for either Ar or Kr. The solid/liquid phase diagram of the confined mixture is determined and results compared with data for the bulk mixture. The structure of the confined mixture is studied using 2D order parameters and both positional g(r) and bond orientational G₆(r) pair correlation functions. It is found that in the confined solid phase, both the contact and inner layers have a hexagonal crystal structure. It is shown that the freezing temperature of the Ar/Kr confined mixture is higher than the bulk freezing point for all molar compositions. Also, it is found that the freezing temperature becomes larger as the ratio α of the wall/fluid to the fluid/fluid interactions increases, in agreement with previous simulation studies on pure substances confined in nanopores. In the case of pore walls having a stronger affinity for Kr atoms (ε _Ar/W<ε _Kr/W), it is observed that both the contact and inner layers of the confined mixture undergo, at the same temperature, a transition from the liquid phase to the crystal phase. The freezing of Ar/Kr mixtures confined between the walls having a stronger affinity for Ar (ε _Ar/W?>?ε _Kr/W) is more complex: for Kr molar concentration lower than 0.35, we observe the presence of an intermediate state between all layers being 2D hexagonal crystals and all the layers being liquid. This intermediate state consists of a crystalline contact layer and a liquid-like inner layer. It is also shown that the qualitative variations of the increase of freezing temperature with the molar composition depend on the affinity of the pore wall for the different components. These results confirm that, in addition to the parameter α the ratio of the wall/fluid interactions for the two species, η=?_Ar/W/?_Kr/W, is a key variable in determining the freezing and melting behaviour of the confined mixture.     

Demixing transitions in a binary Gaussian-core fluid confined in narrow slit-like pores

Using a mean-field density functional approach we investigate phase separation transition in a binary mixture of Gaussian-core molecules confined in narrow slit-like pores. We consider pores with repulsive and attractive walls. In the case of fluid confinement in pores with repulsive and non-selective attracting walls, no phase separation in the confined fluid, prior to the bulk separation transition, was observed. However, in the case of pores with the walls selectively attracting fluid particles, we reveal that the separation transition may take place as a two-step process. During the first step the composition change occurs within a few layers adjacent to the pore walls, whereas in the second step, it takes place in the pore interior.     

Influence of capillary condensation on the near-critical solvation force

We argue that in a fluid, or magnet, confined by adsorbing walls which favor liquid, or the (+) phase, the solvation (Casimir) force in the vicinity of the critical point is strongly influenced by capillary condensation which occurs below the bulk critical temperature T(c). At T slightly below and above T(c), a small bulk field h<0, which favors gas, or the (-) phase, leads to residual condensation and a solvation force which is much more attractive (at the same large wall separation) than that found exactly at the critical point. Our predictions are supported by results obtained from density-matrix renormalization-group calculations in a two-dimensional Ising strip subject to identical surface fields.     

Surface-induced ordering of a molecular fluid of flat hexagonal structure in a narrow graphite slit

We systematically investigate by Monte Carlo simulations the role of the wall structure on a fluid of flat hexagonal molecules confined between two graphite walls. Our simulations show that the centers of mass of the molecules in different layers undergo an order-disorder transition as the wall separation increases, irrespective of the details of the wall structure. The wall structure thus becomes insignificant for the intervening fluid even down to a surprisingly low wall separation.     

Dewetting in associating lattice gas model confined by hydrophobic walls

The phase behavior of a two dimensional fluid confined within hydrophobic walls is obtained by Monte Carlo simulations. The fluid is described by the associating lattice gas model which reproduces the density and diffusion anomalous behavior of water.The confined fluid exhibits a liquid-liquid critical temperature which decreases with the decrease of the distance between the confining walls. In contact with the wall a dewetting is observed. The thickness of this interfacial layer is independent of the distance between the two walls. Even for very small distances between the two walls no total depletion is observed and consequently no drying transition is present.     

具有不同作用势的两平行墙间流体的浸润相变

用解析方法讨论了当两平行墙对流体施加不同的作用势时流体的浸润相变。系统的状态很敏感地依赖于两墙的作用势和两墙间距离的大小。即使两墙差别很小,在一定条件下仍可发生浸润一墙而不浸润另一墙的现象。系统中发生的浸润相变可能是一级的,也可能是二级的。 关键词：     

Second-order Percus-Yevick theory for a confined hard-sphere fluid

A fluid of hard spheres confined between two hard walls and in equilibrium with a bulk hard-sphere fluid is studied using a second-order Percus-Yevick approximation. We refer to this approximation as second-order because the correlations that are calculated depend upon the position of two hard spheres in the confined fluid. However, because the correlation functions depend upon the positions of four particles (two hard spheres and two walls treated as giant hard spheres), this is the most demanding application of the second-order theory that has been attempted. When the two walls are far apart, this calculation reduces to our earlier second-order approximation calculations of the properties of hard spheres near a single hard wall. Our earlier calculations showed this approach to be accurate for the single-wall case. In this work we calculate the density profiles and the pressure of the hard-sphere fluid on the walls. We find, by comparison with grand canonical Monte Carlo results, that the second-order approximation is very accurate, even when the two walls have a small separation. We compare with a singlet approximation (in the sense that correlation functions that depend on the position of only one hard sphere are considered). The singlet approach is fairly satisfactory when the two walls are far apart but becomes unsatisfactory when the two walls have a small separation. We also examine a simple theory of the pressure of the confined hard spheres, based on the usual Percus-Yevick theory of hard-sphere mixtures. Given the simplicity of the latter approach the results of this simple (and explicit) theory are surprisingly good.     

Capillary smectization and layering in a confined liquid crystal

Using density-functional theory, we have analyzed the phase behavior of a model liquid crystal confined between two parallel, planar surfaces (i.e., the so-called slit pore). As a result of confinement, a rich phase behavior arises. The complete liquid-crystal phase diagram of the confined fluid is mapped out as a function of wall separation and chemical potential. Strong commensuration effects in the film with respect to wall separation lead to enhanced smectic ordering, which gives capillary smectization (i.e., formation of a smectic phase in the pore), or frustrated smectic ordering, which suppresses capillary smectization. These effects also produce layering transitions. Our nonlocal density-functional-based analysis provides a unified picture of all the above phenomena.     

Phase behaviour of a Lennard-Jones fluid in a pore with permeable walls of a finite thickness: a density functional approach

A confinement of a Lennard-Jones fluid in a system of slitlike pores separated by semipermeable walls of a finite width is studied. The walls are modelled by square-well repulsive potential wells. The structure of the confined fluid is investigated by means of a density functional method. For high potential barriers separating the pores, the phase behaviour of the system is similar to that for a single slitlike pore with impenetrable walls. For intermediate and low potential barriers the system shows different phase behaviour. Within some temperature range the isotherms exhibit two hysteresis loops, which characterize the condensation of the fluid in different parts of the system, namely in the pore and inside the semipermeable walls. The systems characterized by low and intermediate potential barriers exhibit the triple point, such that at temperatures below that triple point the condensation instantaneously takes place in both the pore and inside the permeable wall.     

Monte Carlo simulation of a confined hard ellipse fluid

The structural and thermodynamic properties of a confined hard ellipse fluid are studied using Monte Carlo simulation. The angular, average number densities and order parameters of hard ellipses confined between hard parallel walls are obtained for various bulk densities, aspect ratios and wall separations. The results show that the effect of the existence of the wall on the molecular fluid structure, either on their directions or their locations, with respect to the bulk, especially close to the walls, is significant. For this system the pressure is also obtained and it is shown that the average density at the wall is proportional to the pressure, βP=〈ρ_w〉. Our simulation results show that the order parameters depend on the number of the particles in the box unless it exceeds thousand.     

约束条件下的硬球流体

利用密度泛函理论和分子动力学方法 ,对处于两平行硬墙之间的硬球流体的密度分布进行了计算 .通过比较两种方法的结果 ,发现在墙之间距离较大时 ,Rosenfeld密度泛函理论的结果与分子动力学模拟的结果符合很好 ;当两堵墙间的距离很小时 ,这两个结果之间存在明显的不一致 .另外 ,还研究了约束条件下密度分布的结构     

Phase behaviors in a binary mixture of diblock copolymers confined between two parallel walls

The phase behaviors in a binary mixture of diblock copolymers confined between two parallel walls are investigated by using a cell dynamics simulation of the time-dependent Ginzburg-Landau theory.The morphological dependence of the wall-block interaction and the distance between walls(confinement degree) has been systematically studied,and the effect of repulsive interactions between different monomers is also discussed.It is interesting that multiple novel morphological transitions are observed by changing these factors,and various multilayered sandwich structures are formed in the mixture.Furthermore,the parametric dependence and physical reasons for the microdomain growth and orientational order transitions are discussed.From the simulation,we find that much richer morphologies can form in a binary mixture of diblock copolymers than those in a pure diblock copolymer.Our results provide an insight into the phase behaviors under parallel wall confinement and may provide guidance for experimentalists.This model system can also give a simple way to realize orientational order transition in soft materials through confinement.     

限制在疏水板中的新的六边形-四边形三层冰结构 (cited: 1)

对限制在两个光滑的疏水板间的水进行了分子动力学模拟,观察到了两种晶体结构,都满足冰规则. 在1 GPa的压强和1.0 nm的板间距下获得的新的冰相是平坦的六边形-四边形三层冰. 在此结构中,靠近板的两层(外层)中的水分子形成六边形环,中间层的水分子形成四边形环. 对于外层的水分子,其四个氢键中的三个在同一层中,另一个氢键与中间层连接. 对于中间层的水分子,四个氢键中的两个在同一层中,而另外两个氢键与两个不同的外层相连. 虽然三层的形状不同,但其面密度却接近相等. 另一种结构是在0.8 nm的板间距和100     

The phase behaviour of a fluid in a slitlike pore with permeable walls

The confinement of a lattice fluid in a set of slitlike pores separated by semipermeable walls with a finite width has been studied. The walls are modelled by a square-well repulsive potential with a finite height. The thermodynamic properties and the phase behaviour of the system are evaluated by means of Monte Carlo simulations. For some states theoretical calculations have been made using a mean-field-type theory. These investigations confirm previous findings for confined Lennard-Jones fluids, obtained from a density functional approach. For intermediate and low potential barriers that separate the pores, the isotherms exhibit two hysteresis loops and the liquid-vapour coexistence curve divides into two branches describing condensation inside the pore and inside the permeable wall. These two branches are separated by a triple point. At temperatures lower than the triple point temperature, the condensation takes place instantaneously in both the pore and inside the permeable wall. It was found that when the temperature is scaled by the bulk critical temperature, the phase diagram emerging from this simple mean-field treatment is close to the phase diagram obtained from simulation.     

Thin films of a binary polymer mixture: calculations of the concentration profile for asymmetric walls

A numerical method for studying equilibrium behavior of thin films between symmetric and asymmetric walls using mean field theory is described. Calculated critical lengths for phase separation between symmetric walls increase as the effective wall composition deviates from the average concentration. Concentration profiles between asymmetric walls are presented.     

Free energy studies of freezing in slit pores: an order-parameter approach using Monte Carlo simulation

We report a molecular simulation study of freezing transitions for simple fluids in narrow slit pores. A major stumbling block in previous studies of freezing in pores has been the lack of any method for calculating the free energy difference between the confined solid and liquid phases. Conventional thermodynamic integration methods often fail for confined systems, due to the difficulty in choosing a suitable path of integration. We use a different approach that involves calculating the Landau free energy as a function of a suitable order parameter, using the grand canonical Monte Carlo simulation method. The grand free energy for each phase can then be obtained by one-dimensional integration of the Landau free energy over the order parameter. These calculations are carried out for two types of wall—fluid interaction, a hard wall and a strongly attractive wall modelled on carbon. The grand free energy results for both cases clearly indicate a first order fluid to solid transition. In the case of the attractive carbon wall, there are three phases. Phase A corresponds to all layers having a liquid-like structure; phase B corresponds to the contact layers (the layers adjacent to the two pore walls) being frozen and the rest of the layers being fluid-like; phase C corresponds to all the layers being frozen. Our results for the angular structure function in the individual molecular layers show strong evidence of a transition from a two-dimensional liquid phase to a hexatic phase. This is followed by a transition from the hexatic to a crystal phase.     

Depolarised Rayleigh light scattering in argon layer confined between graphite plains: MD simulation

The interaction-induced light scattering many-body correlation functions and their spectra in a thin argon layer located between two parallel graphite walls have been investigated by molecular dynamics simulation method. The calculations have been performed for three different distances between graphite plates. Our simulations show the increased intensity of the interaction-induced light scattering spectra at low frequencies for argon atoms in confined space, in comparison to the bulk (unconfined) sample. Moreover, we show a substantial dependence of the interaction-induced light scattering correlation functions of argon on the distances between graphite walls, that is, on the density of argon layer. The mean square displacement and related diffusion coefficient of argon atom in the confined space has been also investigated. Moreover, the structural feature of the thin layer has been studied by calculating the argon density profile, perpendicular to the graphite walls. An interesting observation is the development of a fluid phase in the innermost region of the confined argon layer.     

Sound transmission at low frequencies through the walls of distorted circular ducts

A theoretical treatment of sound transmission through the walls of distorted circular ducts is given, for plane mode transmission within the duct. The transmission mechanism is essentially that of “mode coupling”, whereby higher structural modes in the duct walls are excited, because of the wall distortion, by the internal sound field. The theory is in two parts: an approximate analytical model for the structural response of the walls to the internal sound field, and a structural radiation model. Computed results, based on the theory, are compared to measurements on “long-seam” air conditioning ducts. Where the duct geometry can be reliably specified, reasonably good agreement is obtained between theoretical and experimental data. It is concluded that mode coupling effects serve to account for the discrepancies between ideal and observed behaviour in sound transmission through duct walls.     

Studies of diffusion coefficients in disordered porous matrices confined in a slit-pore

We present a series of molecular dynamics simulations to study the structure of porous matrices confined in a slit-pore. The matrices were prepared by two different methods. In the first method we used direct simulations of a fluid at a fixed density and the matrix was taken from the last configuration of its particles. In the second method we simulated a binary mixture where one of the components served as a template material and the final porous matrix configuration was obtained by removing template particles from the mixture. In both methods the matrices were confined by two parallel walls (slit-pore) modeled by continuous solid surfaces. The results show that the matrix structure and porosity were affected by the method of preparation of the porous matrices. Moreover, we found smaller void cavities in these matrices than in matrices prepared without walls. Finally, diffusion of a fluid inside the matrices was investigated and it was found that the diffusion coefficient did not decrease with the fluid density, and presented a maximum at certain values of the fluid density.