Anodic aluminium oxide: Concurrent SEM and SANS characterisation. Influence of AAO confinement on PEO mean-square displacement

Anodic aluminium oxide (AAO) membranes are macroscopically highly ordered systems made of oriented nano-scale parallel cylindrical channels. The membrane topology is defined by the pore radius, the inter-pores distance and the pore length. We show by a combined SANS and SEM study that the membrane morphology can be tailored by the main anodization parameters: voltage, temperature and anodization duration. We show that the confinement of an hydrophilic polymer (poly(ethylene oxide)) inside the pores is possible. We observe a striking effect of the confinement on the polymer mean-square displacement as measure by high resolution neutron quasi-elastic scattering.     

Effect of confinement in nano-porous materials on the solubility of a supercritical gas

By combining Gibbs Ensemble Monte Carlo simulations and density functional theory, we investigate the influence of confinement in a slit-shaped carbon pore on the solubility of a supercritical solute gas in a liquid solvent. In the cases studied here, competing adsorption of the solvent and solute determines whether the solubility is enhanced or suppressed for larger pores. We find that the solubility in the confined system is strongly dependent on pore width, and that molecular packing effects are important for small pore widths. In addition, the solubility decreases on increase in the temperature, as for the bulk mixture, but the rate of decrease is greater in the pore due to a decrease in the partial molar enthalpy of the solute in the pore; this effect becomes greater as pore width is decreased. The solubility is increased on increasing the bulk pressure of the gas in equilibrium with the pore, and obeys Henry's law at lower pressures. However, the Henry constant differs significantly from that for the bulk mixture, and the range of pressure over which Henry's law applies is reduced relative to that for the bulk mixture. The latter observation indicates that solute–solute interactions become more important in the pore than for the bulk at a given bulk pressure. Finally, we note that different authors use different definitions of the solubility in pores, leading to some confusion over the reported phenomenon of ‘oversolubility’. We recommend that solubility be defined as the overall mole fraction of solute in the pores, since it takes into account the increase in density of the solvent in the pores, and avoids ambiguity in the definition of the pore volume.     

Fluids absorbed in structured pores

A model of fluids absorbed in a pore with walls patterned with parallel channels is used to demonstrate some of the unity that can be proved to hold between the statistical mechanics of fluids absorbed in structured pores and of fluids adsorbed at unstructured walls and at edges/wedges where walls meet. In particular, the work done to reversibly shear a corrugated pore immersed in liquid is related to the difference in the density profile structure of liquid adsorbed near the edges of the channels. When the channel dimensions are mesoscopic or macroscopic but the minimum pore width is microscopic, statistical mechanics generates remarkable links between the surface tension of planar wall-fluid interfaces or, more generally, the solvation free energy of a planar pore, and the density profile at the sides of a channel wall in the vicinity of edges and wedges.     

Oscillatory structure of confined fluids

The confinement of fluids in pores and wedges is associated with exponentially damped oscillatory packing structure, as observed with the surface forces apparatus. This paper reviews the statistical mechanics of confined fluids and then illustrates the results with density functional data for hard-sphere solvent. The free energy of the pore fluid and its functional derivatives with respect to thermodynamic fields all oscillate, as a function of pore width, with a wavelength close to the solvent diameter. In contrast, the density at the centre of pores oscillates with twice this wavelength, as a function of pore width. The development of a unified physics of confined fluids is considered. Approximations based on one-dimensional physics do extraordinarily well in planar symmetry at three-dimensions.     

The adsorption of water in finite carbon pores

Grand canonical Monte Carlo simulations were applied to the adsorption of SPCE model water in finite graphitic pores with different configurations of carbonyl functional groups on only one surface and several pore sizes. It was found that almost all finite pores studied exhibit capillary condensation behaviour preceded by adsorption around the functional groups. Desorption showed the reverse transitions from a filled to a near empty pore resulting in a clear hysteresis loop in all pores except for some of the configurations of the 1.0?nm pore. Carbonyl configurations had a strong effect on the filling pressure of all pores except, in some cases, in 1.0?nm pores. A decrease in carbonyl neighbour density would result in a higher filling pressure. The emptying pressure was negligibly affected by the configuration of functional groups. Both the filling and emptying pressures increased with increasing pore size but the effect on the emptying pressure was much less. At pressures lower than the pore filling pressure, the adsorption of water was shown to have an extremely strong dependence on the neighbour density with adsorption changing from Type IV to Type III to linear as the neighbour density decreased. The isosteric heat was also calculated for these configurations to reveal its strong dependence on the neighbour density. These results were compared with literature experimental results for water and carbon black and found to qualitatively agree.     

Structure of liquid-vapor interface of square well fluid confined in a cylindrical pore

By means of the canonical Monte Carlo simulations, the vapor-liquid (VL) equilibrium and structure of square well (SW) fluids confined in a single cylindrical pore with repulsive surface, have been studied. Coexistence curves of the confined VL interface are determined for a wide range of temperatures and pore diameters. It is demonstrated that the confinement not only reduces the VL coexistence region but also induces strong inhomogeneities of the VL interface: coexistence liquid densities are different at the pore center and at the wall surface. It may be considered as a preliminary step for an isolated droplet formation inside the pore, as well as a tentative reason of the two VL phase transitions of simple fluids adsorbed into disordered porous media.     

Probing interfacial dynamics of water in confined nanoporous systems by NMRD

The confined dynamics of water molecules inside a pore involves an intermittence between adsorption steps near the interface and surface diffusion and excursions in the pore network. Depending on the strength of the interaction in the layer(s) close to the surface and the dynamical confinement of the distal bulk liquid, exchange dynamics can vary significantly. The average time spent in the surface proximal region (also called the adsorption layer) between a first entry and a consecutive exit allows estimating the level of ‘nanowettablity’ of water. As shown in several seminal works, NMRD is an efficient experimental method to follow such intermittent dynamics close to an interface. In this paper, the intermittent dynamics of a confined fluid inside nanoporous materials is discussed. Special attention is devoted to the interplay between bulk diffusion, adsorption and surface diffusion on curved pore interfaces. Considering the nano or meso length scale confinement of the pore network, an analytical model for calculating the inter-dipolar spin–lattice relaxation dispersion curves is proposed. In the low-frequency regime (50?KHz–100?MHz), this model is successfully compared with numerical simulations performed using a 3D-off lattice reconstruction of Vycor glass. Comparison with experimental data available in the literature is finally discussed.     

On the critical size of pores

The critical sizes of pores, determined by surface tension forces, have been determined. It is shown that the pore size in a material cannot be lower than some critical value.     

Molecular simulation study of triangle-well fluids confined in slit pores

Grand canonical Monte Carlo simulations are used to study the behaviour of triangle-well (TW) fluids with variable well widths confined inside slit pores. The effect of individual factors influencing the properties of confined fluids such as fluid–fluid interactions, pore size and pore wall–fluid interactions are obtained using simulations as it is difficult to experimentally determine the same. An interesting observation of this study is that inside the narrow pore of slit height h* = 5 at the high-pressure condition of P* = 0.8, for the TW fluid with long-range attraction or for the fluid at a low temperature for even a short-range attraction, the density profiles show layering such that there is a sticking tendency of the particles at centre, while there is a depletion of particles near the wall (as the layers at the centre have higher density peak heights than near the walls).     

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.     

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.     

等温环境中激光惯性约束聚变冷冻靶丸内部液氢层分布（已撤稿）

叙述了低温等温情况下,重力和界面张力平衡的激光惯性约束聚变靶丸内表面液氢层分布的Young-Laplace(YL)方程.为了得到靶丸壳内连续液氢层分布的有效解,考虑了液体与固体(衬底)分子间的London-van der Waals力以及该力的迟滞影响.计算结果表明,只有在靶丸内部引力为零或者固体液体分子间的London常量为无穷大时,才能得到等温环境中有均匀厚度的连续液氢层. 关键词： 温度     

Solid-gas interface thermal conductance for the thermal barrier coating with surface roughness: The confinement effect

The yttria-stabilized zirconia (YSZ) is a famous thermal barrier coating material to protect hot-end components of an engine. As a characteristic feature of the YSZ, the surface roughness shall play an important role in the interface thermal conductance between the YSZ and gas, considering that the gas is typically at an extremely high temperature. We investigate the effect of the surface roughness on the thermal conductance of the YSZ-gas interface with surface roughness described by nanoscale pores on the surface of the YSZ. We reveal two competitive mechanisms related to the microstructure of the pore, i.e., the actual contact area effect and the confinement effect. The increase of the pore depth will enlarge the actual contact area between the YSZ and gas, leading to enhancement of the solid-gas interface thermal conductance. In contrast to the positive actual contact area effect, the geometry-induced confinement effect greatly reduces the interface thermal conductance. These findings shall offer some fundamental understandings for the microscopic mechanisms of the YSZ-gas interface thermal conductance.     

Optical switching in a Ξ system: A comparative study on DROP and EIT

Liquid Lennard-Jones clusters with magic number of atoms N = 55, 147, 309, 561 and 923 were cooled down in Monte Carlo simulations until freezing. Structural properties of the clusters, including the radial dependence of atomic concentration/density and the local regular structure in arrangement of atoms, just before freezing were analysed. Existence of spherical layers in atomic density around the centre of mass of liquid LJ clusters was confirmed. Formation of layers is explained by central net forces acting on every cluster atom and leading to positioning an atom close to the cluster centre of mass. The strong layering in small clusters of N = 55 and 147 affects atomic diffusion in radial and tangential directions inside the cluster, leading to easier movement of atoms on the layer surface. Analysis of radial profiles of four types of structural units detected in liquid clusters reveals that icosahedral units are the most numerous and are located mainly near cluster surface of all clusters and also in the centre of small clusters.     

Liquid-vapor interface in liquid binary alloys: an ab initio molecular dynamics study

We report the results of an ab initio molecular dynamics simulation of the liquid-vapor interface of two binary liquid alloys, Na(0.3)K(0.7) and Li(0.4)Na(0.6), whose bulk behavior exhibits rather differing ordering tendencies. The study has been performed using samples of 2000 and 3000 particles, respectively, in a slab geometry with periodic boundary conditions. In both cases, the total ionic density distributions along the normal to the interface display some layering with a virtually pure monolayer of the lower surface tension component located outermost at the interface. However, the two systems behave very differently below the interface which can be accounted for by their different ordering tendencies in the bulk.     

Capillary pressure and contact line force on a soft solid

The surface free energy, or surface tension, of a liquid interface gives rise to a pressure jump when the interface is curved. Here we show that a similar capillary pressure arises at the interface of soft solids. We present experimental evidence that immersion of a thin elastomeric wire into a liquid induces a substantial elastic compression due to the solid capillary pressure at the bottom. We quantitatively determine the effective surface tension from the elastic displacement field and find a value comparable to the liquid-vapor surface tension. Most importantly, these results also reveal the way the liquid pulls on the solid close to the contact line: the capillary force is not oriented along the liquid-air interface, nor perpendicularly to the solid surface, as previously hypothesized, but towards the interior of the liquid.     

D2/H2 quantum sieving in microporous carbons: a theoretical study on the effects of pore size and pressure

The adsorption of hydrogen and deuterium in slit-shaped carbon pores is studied by grand canonical Monte Carlo simulations. All interactions are assumed to be of Lennard–Jones type, while the Feynman–Hibbs expression is used to account for quantum effects. The interaction energy of both isotopes inside the slit pore space is discussed thoroughly. Furthermore, pure component adsorption isotherms of both isotopes were simulated at 77?K for pressures up to 20?bar in slit pores having widths of up to 2.0?nm. According to our simulations, in equilibrium, slit pores reveal slight deuterium selectivity over hydrogen, and this quantum-based selectivity depends both on pressure and pore size.     

Phonon-roton excitations in liquid 4He at negative pressures

We present neutron scattering measurements of the phonon-roton excitations of superfluid 4He held at negative pressures from zero to -5 bar. The liquid was stretched to negative pressures by immersing it in the porous medium MCM-41. In the wave vector range 0.35< or =Q< or =1.55 A(-1) and temperature T=0.4 K investigated, the phonon and maxon energies decrease systematically below bulk values as the negative pressure is increased. The energies are consistent with extrapolation of positive pressure values from which the negative internal pressure can be estimated. The maximum negative pressure realized is consistent with surface tension arguments and the MCM-41 pore diameter of 47 A.