Freezing of hard spheres confined in narrow cylindrical pores

Monte Carlo simulations for the equation of state and phase behavior of hard spheres confined inside very narrow hard tubes are presented. For pores whose radii are greater than 1.1 hard sphere diameters, a sudden change in the density and the microscopic structure of the fluid is neatly observed, indicating the onset of freezing. In the high-density structure the particles rearrange in such a way that groups of three particles fit in sections across the pore.     

Critical endpoint and analytical phase diagram of attractive hard-core Yukawa spheres

We analytically calculate the gas-liquid critical endpoint (cep) for hard spheres with a Yukawa attraction. This cep is a boundary condition for the existence of a liquid. We use an analytical Helmholtz energy expression for the attractive Yukawa (hard) spheres based on the first-order mean spherical approximation to the attractive Yukawa potential by Tang and Lu (J. Chem. Phys. 1993, 99, 9828). This theory and our analytical simplification of it predict the gas-liquid and fluid-solid phase behavior, as found from computer simulations, very accurately as long as the range 1/kappa of attraction is not too short. We find that the cep is situated at kappasigma approximately 6 and at a contact potential around 2 kT. It follows that a liquid state is only possible when the attraction range is longer than (1/6) of the particle diameter sigma, and the attraction strength is smaller than 2 kT. The liquid region does not span more than 0.6 kT in strength, and there is also a relatively narrow window for the attraction range.     

Close-packed colloidal crystalline arrays composed of silica spheres coated with titania

Titania coated monodisperse silica spheres have been synthesized and fabricated as a close-packed colloidal crystalline array. We have demonstrated that the coated colloidal sphere can be used to control the peak position of the optical stop band through variation of the coating thickness. The titania coated silica spheres were prepared by the layer-by-layer assembly coating process, which reciprocally laminates the cationic polyelectrolyte and the anionic titania nanosheets on a monodisperse silica spheres, and were sintered to change the titania nanosheets to anatase. The Bragg diffraction peak of the colloidal crystalline array shifted to the long wavelength region with an increase of thickness of the titania layer. Angle-resolved reflection spectra measurements clarified that the red shift was caused by increasing of the refractive index with increase of the thickness of the layer. The current work suggests new possibilities for the creation of advanced colloidal crystalline arrays with tunable optical properties from tailored colloidal spheres.     

Influence of surface chemical heterogeneities on adsorption/desorption hysteresis and coexistence diagram of metastable states within cylindrical pores

Grand canonical Monte Carlo simulations are performed to determine the adsorption/desorption isotherms at different temperatures of a Lennard-Jones fluid confined within a simple model of cylindrical pores presenting chemical heterogeneities. A complex hysteresis loop is observed, showing hysteresis subloops (scanning curves). This is shown to be consistent with the existence of several metastable states (local minima in the system free energy). A recent extension to the Gibbs ensemble technique is then used to calculate the complete coexistence diagram of these local minima.     

Isotactic polystyrene/cis-decaline mixtures: phase diagram and molecular structures

Organized structures grown from iPS/cis-decalin solutions (ranging from 5% to 50% w/w) have been studied by several techniques. From DSC experiments the thermal behavior has been investigated as a function of the annealing temperature. The temperature-concentration phase diagram has been established. From neutron diffraction experiments the short range molecular order has been determined. We show that between the gel state formed at high undercooling and the crystalline state obtained at low undercooling there exist two intermediate phases: thes-phase, already described by Klein et al., and thep-phase. Except for the crystalline state all the other phases contain intercalated solvent molecules. Thegel state displays nematic-like order, whereas thes-phase is reminiscent of smectic arrangements. Thep-phase is less solvated and can be described as a peritectic system. Preliminary neutron scattering experiments show that chain-folding reappears in thes-phase, whereas it was shown to be absent in thegel phase.Formerly Laboratoire de Spectrométrie et d'imagerie Ultrasonores     

Nature of adsorption and desorption branches in cylindrical pores

To examine the nature of the adsorption and desorption branches in hysteretic adsorption isotherms of gases on mesoporous materials, we measured the temperature dependence of the adsorption and desorption isotherms of argon, oxygen, and carbon dioxide onto MCM-41 with a pore diameter of 4.4 nm. The results clearly show that in the open-ended cylindrical pores of MCM-41, capillary condensation rather than evaporation takes place near a thermodynamical equilibrium transition, as opposed to the general statement that capillary evaporation can occur via a meniscus formed at the pore mouth, and, thus, takes place at equilibrium.     

Entropic selectivity of binary mixtures in cylindrical pores

We show that a simple model consisting of a binary hard-sphere mixture in a narrow cylindrical pore can lead to strong size selectivity by considering a situation where each species of the mixture sees a different radius of the cylinder. Two mechanisms are proposed to explain the observed results depending on the radius of the cylinder: for large radii the selectivity is driven by an enhancement of the depletion forces at the cylinder walls whereas for the narrowest cylinders excluded-volume effects lead to a shift of the effective chemical potential of the particles in the pore.     

Structure of liquid and glassy methanol confined in cylindrical pores

We present a neutron scattering analysis of the density and the static structure factor of confined methanol at various temperatures. Confinement is performed in the cylindrical pores of MCM-41 silicates with pore diameters D=24 and 35 A. A change of the thermal expansivity of confined methanol at low temperature is the signature of a glass transition, which occurs at higher temperature for the smallest pore. This is evidence of a surface induced slowing down of the dynamics of the fluid. The structure factor presents a systematic evolution with the pore diameter, which has been analyzed in terms of excluded volume effects and fluid-matrix cross correlation. Conversely to the case of Van der Waals fluids, it shows that stronger fluid-matrix correlations must be invoked most probably in relation with the H-bonding character of both methanol and silicate surface.     

Critical temperature of capillary condensation in narrow cylindrical pores

The dependence of the critical temperature of capillary condensation of an adsorbate in cylindrical pores on the diameter of pores is investigated. The calculation is carried out in the quasi-chemical approximation with a calibration function providing the agreement of results for small pores with exact values obtained by the fragment method. The contribution of the size effect to the calibration function as a function of the diameter of pores is considered in terms of the concept of quasi-one-dimensional behavior of the adsorbate in narrow pores. Various adsorbate-adsorbate potentials are considered.     

Nucleation in hydrophobic cylindrical pores: a lattice model

We consider the nucleation process associated with capillary condensation of a vapor in a hydrophobic cylindrical pore (capillary evaporation). The liquid-vapor transition is described within the framework of a simple lattice model. The phase properties are characterized both at the mean-field level and with Monte Carlo simulations. The nucleation process for the liquid to vapor transition is then specifically considered. Using umbrella sampling techniques, we show that nucleation occurs through the condensation of an asymmetric vapor bubble at the pore surface. Even for highly confined systems, good agreement is found with macroscopic considerations based on classical nucleation theory. The results are discussed in the context of recent experimental work on the extrusion of water in hydrophobic pores.     

Calculations of the distribution of trace impurities in cylindrical pores

The equilibrium distribution of a trace impurity and the self-diffusion coefficients of molecules of the base component and the trace impurity in narrow cylindrical pores were calculated using the lattice-gas model. Two types of lattice structures with six and eight closest neighbors were considered. The sizes of the base component and impurity molecules were taken to be identical. Lateral interactions were taken into account in the quasi-chemical approximation. The equilibrium distributions of the trace impurity across a pore section in the gas and liquid phases of the base component and at the interface for the case of capillary condensation were considered. The probability of existence of isolated dimeric clusters was estimated and the self-diffusion coefficients of the base component and trace impurity for a single-phase distribution of the base component were calculated. The effects of the energy of interaction of impurities with the pore walls and the concentration of the base component on the diffusion mobility of the impurities were analyzed. The concentration dependences of the partition coefficient for the trace impurity between the pore center and the pore wall and the concentration dependences of the self-diffusion coefficients for the trace impurity molecules become nonmonotonic with an increase in the base component concentration. These effects are due to the displacement of the impurity from the near-surface area to the bulk of a pore following an increase in the pore coverage by the base component and to higher mobility of the impurity in the free bulk of the pore. Further filling of the pore bulk reduces the mobility of all molecules. The energetics of intermolecular interactions also plays a certain role. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 605–615, April, 2000.     

Hydraulic and surface characteristics of membranes with parallel cylindrical pores

Pore size distributions and pore densities of track-etched polycarbonate ultrafiltration (UF) membranes with pore sizes ranging from 10 to 100 nm (0.01–0.10 μm) were characterized by image analysis of field emission scanning electron micrographs (FESEM) of membranes. Porosity data obtained from image analysis compared well with those derived from manufacturer's specifications, but this may have been coincidental, as pore size and pore density results differed by 20–40% and 25–70%, respectively. The experimentally determined flux through each membrane type varied by up to 30–45% within a batch, and were about 8–46 times higher than the theoretical over the range of membranes. The disparity between theoretical and experimental flux was beyond the bounds of physical variability of the membranes. The membranes with smaller pore size tended to show a greater disparity. Water flux of all membranes increased with increasing temperature, generally in accord with the decreasing viscosity of water. However, unlike the linear increase for the membranes with larger pores (> 50 nm), the membranes with smaller pores (10 and 30 nm) showed exponential increase with temperature. Water flux also increased with a pressure increase from 50 to 300 kPa. Raised pressure appear to enlarge pores resulting in exponential flux enhancement at higher pressure, particularly for membranes with smaller pores (PC10). The pores may have stretched open under pressure to deliver the higher than expected fluxes due to flexibility of polycarbonate films, although FESEM showed no visible evidence of fracturing or tearing of the membranes. The flux results from filtration of aqueous protein solution were a little lower and correlated well with water permeability of the membranes, but remained in discord with the pore size distribution results.     

Polymer brushes in cylindrical pores: simulation versus scaling theory

The structure of flexible polymers endgrafted in cylindrical pores of diameter D is studied as a function of chain length N and grafting density sigma, assuming good solvent conditions. A phenomenological scaling theory, describing the variation of the linear dimensions of the chains with sigma, is developed and tested by molecular dynamics simulations of a bead-spring model. Different regimes are identified, depending on the ratio of D to the size of a free polymer N(3/5). For D>N(3/5) a crossover occurs for sigma=sigma*=N(-6/5) from the "mushroom" behavior (R(gx)=R(gy)=R(gz)=N(35)) to the behavior of a flat brush (R(gz)=sigma(1/3)N,R(gx)=R(gy)=sigma(-1/12)N(1/2)), until at sigma**=(D/N)3 a crossover to a compressed state of the brush, [R(gz)=D,R(gx)=R(gy)=(N(3)D/4sigma)(1/8)相似文献   

Nematic structures in cylindrical cavities

We present the results of Monte Carlo simulations of a nematic liquid crystal confined to a cylinder with homeotropic surface anchoring. The nematic is modelled using the LebwohlLasher model. For low values of the cylinder radius or anchoring strength, a stable planar polar configuration with two line defects is readily formed, consistent with the predictions of elastic theory. At larger values of the radius or anchoring strength we observe a metastable escaped radial configuration. However, this structure eventually collapses to a planar configuration, contrary to the predictions of elastic theory.     

New terms in binaries phase diagram

Some cases and reactions in binary isobaric phase diagram presenting variation of temperature and composition have no names; we propose to attribute names for some cases and reaction that we identified. The names of the invariant reactions are issued from Greek language as eutectic but concerning the curves of separation between domains, they have a Latin origin. We have respected these choices.     

Melting behavior of water in cylindrical pores: carbon nanotubes and silica glasses

We report a study of the effects of confinement in multi-walled carbon nanotubes and mesoporous silica glasses (SBA-15) on the solid structure and melting of both H(2)O and D(2)O ice, using differential scanning calorimetry, dielectric relaxation spectroscopy, and neutron diffraction. Multi-walled nanotubes of 2.4, 3.9 and 10 nm are studied, and the SBA-15 studied has pores of mean diameter 3.9 nm; temperatures ranging from approximately 110 to 290 K were studied. We find that the melting point is depressed relative to the bulk water for all systems studied, with the depression being greater in the case of the silica mesopores. These results are shown to be consistent with molecular simulation studies of freezing in silica and carbon materials. The neutron diffraction data show that the cubic phase of ice is stabilized by the confinement in carbon nanotubes, as well as in silica mesopores, and persists up to temperatures of about 240 K, above which there is a transition to the hexagonal ice structure.     

Synthesis of micrometer-sized hard silica spheres with uniform mesopore size and textural pores

Micrometer-sized silica spheres were prepared using a new pH-induced rapid colloid aggregation method in water-in-oil (W/O) emulsion separately with F127 and the mixture of Pluronic triblock copolymer (F127, P123, or P105) and PEG20000 as templates. All the mesoporous silica spheres exhibited high surface areas (657-1145 m2/g) and large pore volumes (0.46-2.16 ml/g). Through optimizing the synthetic conditions, hard silica spheres with narrow particle size distribution, uniform pore size, and textural pores were obtained. Finally, the mechanism of this synthetic route is discussed.     

Electrophoretic mobility of cylindrical soft particles

 A general theory for the electrophoresis of a cylindrical soft particle (i.e., a cylindrical hard colloidal particle coated with a layer of ion-penetrable polyelectrolytes) in an electrolyte solution in an applied transverse or tangential electric field is proposed. This theory unites two different electrophoresis theories for cylindrical hard particles and for cylindrical polyelectrolytes. That is, the general mobility expression obtained in this paper tends to the mobility expression for a cylindrical hard particle for the case where the polyelectrolyte layer is absent or the frictional coefficient in the poly-electrolyte layer becomes infinity, whereas it tends to that for a cylin-drical polyelectrolyte in the absence of the particle core. Simple approximate analytic mobility expressions are also presented. Received: 29 August 1996 Accepted: 7 November 1996