Nature of capillary condensation and evaporation processes in ordered porous materials

We report on a detailed experimental study of capillary condensation-evaporation processes of N(2) in ordered mesoporous SBA-15 silica. We have carried out measurements of boundary hysteresis loops, reversal curves, and subloops in order to test whether this material behaves as an assembly of independent cylindrical pores open at both ends. With these data, we come to the conclusion that, whereas the boundary hysteresis loop has the classical shape of type H1 associated with condensation-evaporation in cylindrical pores open at both ends, the capillary evaporation does not take place at equilibrium as it is generally assumed. Moreover, the pores do not desorb independently of one another.     

Grand canonical Monte Carlo simulation study of capillary condensation between nanoparticles

Capillary condensation at the nanoscale differs from condensation in the bulk phase, because it is a strong function of surface geometry and gas-surface interactions. Here, the effects of geometry on the thermodynamics of capillary condensation at the neck region between nanoparticles are investigated via a grand canonical Monte Carlo simulation using a two-dimensional lattice gas model. The microscopic details of the meniscus formation on various surface geometries are examined and compared with results of classical macromolecular theory, the Kelvin equation. We assume that the system is composed of a lattice gas and the surfaces of two particles are approximated by various shapes. The system is modeled on the basis of the molecular properties of the particle surface and lattice gas in our system corresponding to titania nanoparticles and tetraethoxy orthosilicate molecules, respectively. This system was chosen in order to reasonably emulate our previous experimental results for capillary condensation on nanoparticle surfaces. Qualitatively, our simulation results show that the specific geometry in the capillary zone, the surface-surface distance, and the saturation ratio are important for determining the onset and broadening of the liquid meniscus. The meniscus height increases continuously as the saturation ratio increases and the meniscus broadens faster above the saturation ratio of 0.90. The change of the radius of curvature of the particle surface affects the dimensions of the capillary zone, which drives more condensation in narrow zones and less condensation in wide zones. The increase of surface-surface distance results in the decrease of the meniscus height or even the disappearance of the meniscus entirely at lower saturation ratios. These effects are significant at the nanoscale and must be carefully considered in order to develop predictive relationships for meniscus height as a function of saturation conditions.     

Static and dynamic critical behavior of a symmetrical binary fluid: a computer simulation

A symmetrical binary, A+B Lennard-Jones mixture is studied by a combination of semi-grand-canonical Monte Carlo (SGMC) and molecular dynamics (MD) methods near a liquid-liquid critical temperature T(c). Choosing equal chemical potentials for the two species, the SGMC switches identities (A-->B-->A) to generate well-equilibrated configurations of the system on the coexistence curve for TT(c). A finite-size scaling analysis of the concentration susceptibility above T(c) and of the order parameter below T(c) is performed, varying the number of particles from N=400 to 12 800. The data are fully compatible with the expected critical exponents of the three-dimensional Ising universality class. The equilibrium configurations from the SGMC runs are used as initial states for microcanonical MD runs, from which transport coefficients are extracted. Self-diffusion coefficients are obtained from the Einstein relation, while the interdiffusion coefficient and the shear viscosity are estimated from Green-Kubo expressions. As expected, the self-diffusion constant does not display a detectable critical anomaly. With appropriate finite-size scaling analysis, we show that the simulation data for the shear viscosity and the mutual diffusion constant are quite consistent both with the theoretically predicted behavior, including the critical exponents and amplitudes, and with the most accurate experimental evidence.     

Stability of Ca-montmorillonite hydrates: a computer simulation study

Classic simulations are used to study interlayer structure, swelling curves, and stability of Ca-montmorillonite hydrates. For this purpose, NP(zz)T and muP(zz)T ensembles are sampled for ground level and given burial conditions. For ground level conditions, a double layer hydrate having 15.0 A of basal spacing is the predominant state for relative vapor pressures (p/p0) ranging 0.6-1.0. A triple hydrate counting on 17.9 A of interlaminar distance was also found stable for p/p0 = 1.0. For low vapor pressures, the system may produce a less hydrated but still double layer state with 13.5 A or even a single layer hydrate with 12.2 A of interlaminar distance. This depends on the established initial conditions. On the other hand, the effect of burial conditions is two sided. It was found that it enhances dehydration for all vapor pressures except for saturation, where swelling is promoted.     

Capillary condensation in a geometrically and a chemically heterogeneous pore: a molecular simulation study

A computer simulation study has been carried out, using an extended Gibbs ensemble Monte Carlo technique, to examine the influence of so-called geometric and chemical disorder on the thermodynamic behavior of simple fluids confined in porous media. The technique allows the equilibrium coexistence of gas and liquid phases to be calculated in a single run. The phase diagram of Lennard-Jones fluid has been calculated in a perfectly cylindrical pore as a reference. Some disorder is then introduced in the porous material, first by spatially modifying the external potential of the initially cylindrical pore, to imitate the geometric disorder of a more realistic pore (undulation, constrictions, etc.) and second by modulating the amplitude of the same initially cylindrical potential to reproduce the energetic disorder of realistic pores due to chemical variations along it. It is shown that the chemical disorder has a much stronger effect on the phase diagram of the confined fluid. The complete adsorption/desorption isotherms are also calculated to help in understanding the large effects of chemical disorder.     

Water as a lubricant for graphite: a computer simulation study

The phase state and shear behavior of water confined between parallel graphite sheets are studied using the grand canonical Monte Carlo technique and TIP4P model for water. In describing the water-graphite interaction, two orientation-dependent potentials are tried. Both potentials are fitted to many-body polarizable model predictions for the binding energy and the equilibrium conformation of the water-graphite complex [K. Karapetian and K. D. Jordan in Water in Confining Geometries, edited by V. Buch and J. P. Devlin (Springer, Berlin, 2003), pp. 139-150]. Based on the simulation results, the property of water to serve as a lubricant between the rubbing surfaces of graphitic particles is associated, first, with the capillary condensation of water occurring in graphitic pores of monolayer width and, second, with the fact that the water monolayer compressed between graphite particles retains a liquidlike structure and offers only slight resistance to shear.     

Hydrogen bonding in liquid alcohols: a computer simulation study

A series of molecular dynamics simulations has been performed to investigate hydrogen bonding in liquid alcohols. The systems considered have been methanol, ethanol, ethylene glycol and glycerol at 298 K. The hydrogen bonding statistics as well as the mean lifetime of the hydrogen bonds are analyzed. The results are compared with those corresponding to liquid water.     

Water condensation in hydrophobic silicalite-1 zeolite: a molecular simulation study

We report grand canonical Monte Carlo simulations of the gas and liquid phase adsorption of water in silicalite-1 zeolite. Simple but effective models and simulation methods, found useful for studying gas adsorption in nanoporous materials, have been extended to describe the intrusion/extrusion cycle of water in this hydrophobic solid. The picture of water confined to hydrophobic spaces of nanoscopic dimensions that emerges from this study is one of a strongly depleted and highly inhomogeneous fluid.     

On the thermodynamic theory of the strength of solids: 4. Capillary condensation and capillary evaporation in wedge-shaped crack

The processes of capillary condensation and capillary evaporation in a wedge-shaped crack are considered. Capillary evaporation is a comparatively new phenomenon that is opposite to capillary condensation and occurs upon the cleavage of a solid in a nonwetting liquid. For both cases, the positions of a meniscus inside a wedge-shaped crack have been calculated as functions of the meniscus curvature radius, liquid-contact angle, and crack-opening angle. The effect of temperature on the meniscus position has been analyzed; it has been established that the meniscus shifts from the gaseous toward the liquid phase as temperature rises. The regularities of meniscus displacements in the course of crack growth have been established: under the conformal mechanism of crack growth, the absolute position of the meniscus remains unchanged (i.e., the meniscus and the crack frontal line move at the same velocity), while, under the depth mechanism of growth, the relative position of the meniscus is retained.     

Arrest of fluid demixing by nanoparticles: a computer simulation study

We use lattice Boltzmann simulations to investigate the formation of arrested structures upon demixing of a binary solvent containing neutrally wetting colloidal particles. Previous simulations for symmetric fluid quenches pointed to the formation of "bijels": bicontinuous interfacially jammed emulsion gels. These should be created when a glassy monolayer of particles forms at the fluid-fluid interface, arresting further demixing and rigidifying the structure. Experimental work has broadly confirmed this scenario, but it shows that bijels can also be formed in volumetrically asymmetric quenches. Here, we present new simulation results for such quenches, compare these to the symmetric case, and find a crossover to an arrested droplet phase at strong asymmetry. We then make extensive new analyses of the postarrest dynamics in our simulated bijel and droplet structures, on time scales comparable to the Brownian time for colloid motion. Our results suggest that, on these intermediate time scales, the effective activation barrier to ejection of particles from the fluid-fluid interface is smaller by at least 2 orders of magnitude than the corresponding barrier for an isolated particle on a flat interface.     

Anesthetic molecules embedded in a lipid membrane: a computer simulation study

The effect of four general anesthetic molecules, i.e., chloroform, halothane, diethyl ether and enflurane, on the properties of a fully hydrated dipalmitoylphosphatidylcholine (DPPC) membrane is studied in detail by long molecular dynamics simulations. Furthermore, to address the problem of pressure reversal, the effect of pressure on the anesthetic containing membranes is also investigated. In order to ensure sufficient equilibration and adequate sampling, the simulations performed have been at least an order of magnitude longer than the studies reported previously in the literature on general anesthetics. The results obtained can help in resolving several long-standing contradictions concerning the effect of anesthetics, some of which were the consequence of too short simulation time used in several previous studies. More importantly, a number of seeming contradictions are found to originate from the fact that different anesthetic molecules affect the membrane structure differently in several respects. In particular, halothane, being able to weakly hydrogen bound to the ester group of the lipid tails, is found to behave in a markedly different way than the other three molecules considered. Besides, we also found that two changes, namely lateral expansion of the membrane and increasing local disorder in the lipid tails next to the anesthetic molecules, are clearly induced by all four anesthetic molecules tested here in the same way, and both of these effects are reverted by the increase in pressure.     

Adhesive transitions in Newton black films: a computer simulation study

We report molecular dynamics simulations of Newton black films (NBFs), ultra thin films of aqueous solutions stabilized with two monolayers of ionic surfactants, sodium dodecyl sulfate. We show that at low water content conditions and areas per surfactant corresponding to experimental estimates in NBFs, homogeneous films undergo an adhesion "transition," which results in a very thin adhesive film coexisting with a thicker film. We identify the adhesive film with the equilibrium structure of the Newton black film. We provide here a direct microscopic view of the formation of these important structures, which have been observed in experimental studies of emulsions and foams. We also report a detailed investigation of the structural properties and interfacial fluctuation spectrum of the adhesive film. Our analysis relies on the definition of an "intrinsic surface," which is used to remove the averaging effect that the capillary waves have on the film properties.     

Capillary condensation hysteresis in overlapping spherical pores: a monte carlo simulation study

The mechanisms of hysteretic phase transformations in fluids confined to porous bodies depend on the size and shape of pores, as well as their connectivity. We present a Monte Carlo simulation study of capillary condensation and evaporation cycles in the course of Lennard-Jones fluid adsorption in the system of overlapping spherical pores. This model system mimics pore shape and connectivity in some mesoporous materials obtained by templating cubic surfactant mesophases or colloidal crystals. We show different mechanisms of capillary hysteresis depending on the size of the window between the pores. For the system with a small window, the hysteresis cycle is similar to that in a single spherical pore: capillary condensation takes place upon achieving the limit of stability of adsorption film and evaporation is triggered by cavitation. When the window is large enough, the capillary condensation shifts to a pressure higher than that of the isolated pore, and the possibility for the equilibrium mechanism of desorption is revealed. These finding may have important implications for practical problems of assessment of the pore size distributions in mesoporous materials with cagelike pore networks.     

Free volume properties of a linear soft polymer: a computer simulation study

Molecular dynamics simulation of a linear soft polymer has been performed and the free volume properties of the system have been analyzed in detail in terms of the Voronoi polyhedra of the monomers. It is found that there are only small density fluctuations present in the system. The local environment of the monomers is found to be rather spherical, even in comparison with liquids of atoms or small molecules. The monomers are found to be, on average, eight coordinated by their nearest geometric neighbors, including intra-chain and inter-chain ones. The packing of the monomers is found to be rather compact, in a configuration of 1900 monomers there are, on average, only three voids large enough to incorporate a spherical particle as large as a monomer, indicating that the density of the large vacancies in the system is considerably, i.e., by a few orders of magnitude lower than in molecular liquids corresponding to roughly the same reduced densities.     

Dipolar and dispersion interactions in nematogens A computer simulation study

A classical system, consisting of identical cylindrically symmetric particles, with centres fixed on a simple-cubic lattice, may or may not support nematic—like orientational order depending on the interaction potential. For example, a Lebwohl-Lasher potential model produces orientational order, as does an anisotropic London—de Boer dispersion potential restricted to nearest neighbours, although increasing its range destroys the orientational order and brings about a staggered configuration of the system. In consequence, a Maier-Saupe molecular field treatment is appropriate in the first two cases, but not in the last. On the other hand, according to computer simulation results, a purely dipolar interaction produces a low temperature antiferroelectric phase which can be regarded as an extreme case of a nematogen. We carried out Monte Carlo simulations on a potential model defined by a linear combination of dipolar and full ranged London-de Boer dispersion terms, in order to study their competition. To save computer time and yet monitor the physically relevant changes, we have varied their relative weights, while keeping fixed both the temperature and the anisotropy parameter in the dispersion potential. Simulation results show that in energetic terms the two interactions are cooperative, whereas in structural terms their interplay brings about first a weak but recognizable increase of nematic ordering, and then its collapse with the onset of the staggered configuration.     

Molecular dynamics study on evaporation and condensation of n-dodecane at liquid-vapor phase equilibria

Molecular dynamics simulations are performed to study the evaporation and condensation of n-dodecane (C(12)H(26)) at temperatures in the range 400-600 K. A modified optimized potential for liquid simulation model is applied to take into account the Lennard-Jones, bond bending and torsion potentials with the bond length constrained. The equilibrium liquid-vapor n-dodecane interface thickness is predicted to be ~1.2-2.0 nm. It is shown that the molecular chains lie preferentially parallel to the interface in the liquid-vapor transition region. The predicted evaporation/condensation coefficient decreased from 0.9 to 0.3 when temperature increased from 400 to 600 K. These values can be used for the formulation of boundary conditions in the kinetic modeling of droplet heating and evaporation processes; they are noticeably different from those predicted by the transition state theory. We also present the typical molecular behaviors in the evaporation and condensation processes. The molecular exchange in condensation, typical for simple molecules, has never been observed for n-dodecane molecular chains.     

On the Debye-Waller factor of hexagonal ice: a computer simulation study

We investigate by molecular dynamics (MD) simulations the temperature dependence of the Debye-Waller (DW) factor of hexagonal ice with 25 different proton-disordered configurations. Each initial configuration is composed of 288 water molecules with no net dipole moment. The intermolecular interaction of water is described by TIP4P potential. Each production run of the simulation is 15 ns or longer. We observe a change in slope of the DW factor around 200 K, which cannot be explained within the framework of either classical or quantum harmonic approximation. Configurations generated by MD simulations are subjected to the steepest descent energy minimization. Analysis of the local energy minimum structures reveals that water molecules above 200 K jump to other lattice sites via some local energy minimum structures which contain some water molecules sitting on the locations other than the lattice sites. As time evolves, these defect molecules move back and forth to the lattice sites yielding defect-free structures. Those motions are responsible for the unusual increase in the DW factor at high temperatures. In making a transition from an energy-minimum structure to another one, a small number of water molecules are involved in a highly cooperative fashion. The larger DW factor at higher temperature arises from jump-like motions of water molecules among these locally stable configurations which may or may not be a family of the proton-disordered ice forms satisfying the "ice rule".     

Accurate relations between pore size and the pressure of capillary condensation and the evaporation of nitrogen in cylindrical pores

To examine the theoretical and semiempirical relations between pore size and the pressure of capillary condensation or evaporation proposed so far, we constructed an accurate relation between the pore radius and the capillary condensation and evaporation pressure of nitrogen at 77 K for the cylindrical pores of the ordered mesoporous MCM-41 and SBA-15 silicas. Here, the pore size was determined from a comparison between the experimental and calculated X-ray diffraction patterns due to X-ray structural modeling recently developed. Among the many theoretical relations that differ from each other in the degree of theoretical improvements, a macroscopic thermodynamic approach based on Broekhoff-de Boer equations was found to be in fair agreement with the experimental relation obtained in the present study.     

Liquid crystal phases of achiral banana-shaped molecules: a computer simulation study

The phase behaviour of achiral banana-shaped molecules was studied by computer simulation. The banana-shaped molecules were described by model intermolecular interactions based on the Gay-Berne potential. The characteristic molecular structure was considered by joining two calamitic Gay-Berne particles through a bond to form a biaxial molecule of point symmetry group C 2v with a suitable bending angle. The dependence on temperature of systems of N =1024 rigid banana-shaped molecules with bending angle ϕ=140° has been studied by means of Monte Carlo simulations in the isobaric-isothermal ensemble ( NpT ). On cooling an isotropic system, two phase transitions characterized by phase transition enthalpy, entropy and relative volume change have been observed. For the first time by computer simulation of a many-particle system of banana-shaped molecules, at low temperature an untilted smectic phase showing a global phase biaxiality and a spontaneous local polarization in the layers, i.e. a local polar arrangement of the steric dipoles, with an antiferroelectric-like superstructure could be proven, a phase structure which recently has been discovered experimentally. Additionally, at intermediate temperature a nematic-like phase has been proved, whereas close to the transition to the smectic phase hints of a spontaneous achiral symmetry breaking have been determined. Here, in the absence of a layered structure a helical superstructure has been formed. All phases have been characterized by visual representations of selected configurations, scalar and pseudoscalar correlation functions, and order parameters.