Fluids In Cylindrical Pores Research Articles

Water Vapor Adsorption on Desiccant Materials for Rotary Desiccant Air Conditioning Systems

In order to determine the water vapor adsorption performance of a rotary desiccant-based air conditioning system, the behavior of water adsorption on cylindrical pores of different sizes was studied by using classical density functional theory (CDFT) based on perturbated chain statistical associating fluid theory (PC-SAFT). Firstly, the structural parameters of the desiccant material were characterized by scanning electron microscopy (SEM), X-ray Energy Dispersive Spectrum (EDS), and N2 adsorption/desorption isotherms, as well as adsorption equilibrium measurements of water vapor at temperature range 293–308 K. Secondly, the potential energy equation of water molecules in cylindrical pores was determined, and contribution of various terms of PC-SAFT for simulating fluid in cylindrical pores were established. Finally, the pore size distribution (PSD) of the desiccant materials is determined by the PC-SAFT kernel. Moreover, water vapor condensation was investigated with the PC-SAFT model in micropores. The results indicate that the rotary desiccant materials have a large number of micropores with a volume of 0.3669 cm3/g and the amount of water adsorption is about 0.285 g/g. The condensation pressure and the pore width corresponding to the saturated pressure P0 grow with an increase in the temperature, signifying that adjusting the PSD of the material has a significant effect on improving the dehumidification performance. The research concludes that the PSD range of the oxide cylindrical pore between 1.09 and 1.53 nm is particularly beneficial for dehumidification. This study provides valuable theoretical guidance for optimizing dehumidification materials.

Effect of pore geometry on the compressibility of a confined simple fluid.

Fluids confined in nanopores exhibit properties different from the properties of the same fluids in bulk; among these properties is the isothermal compressibility or elastic modulus. The modulus of a fluid in nanopores can be extracted from ultrasonic experiments or calculated from molecular simulations. Using Monte Carlo simulations in the grand canonical ensemble, we calculated the modulus for liquid argon at its normal boiling point (87.3 K) adsorbed in model silica pores of two different morphologies and various sizes. For spherical pores, for all the pore sizes (diameters) exceeding 2 nm, we obtained a logarithmic dependence of fluid modulus on the vapor pressure. Calculation of the modulus at saturation showed that the modulus of the fluid in spherical pores is a linear function of the reciprocal pore size. The calculation of the modulus of the fluid in cylindrical pores appeared too scattered to make quantitative conclusions. We performed additional simulations at higher temperature (119.6 K), at which Monte Carlo insertions and removals become more efficient. The results of the simulations at higher temperature confirmed both regularities for cylindrical pores and showed quantitative difference between the fluid moduli in pores of different geometries. Both of the observed regularities for the modulus stem from the Tait-Murnaghan equation applied to the confined fluid. Our results, along with the development of the effective medium theories for nanoporous media, set the groundwork for analysis of the experimentally measured elastic properties of fluid-saturated nanoporous materials.

Melting transition of Lennard-Jones fluid in cylindrical pores.

Three-stage pseudo-supercritical transformation path and multiple-histogram reweighting technique are employed for the determination of solid-liquid coexistence of the Lennard-Jones (12-6) fluid, in a structureless cylindrical pore of radius, R, ranging from 4 to 20 molecular diameters. The Gibbs free energy difference is evaluated using thermodynamic integration method by connecting solid and liquid phases under confinement via one or more intermediate states without any first order phase transition among them. The thermodynamic melting temperature, Tm, is found to oscillate for pore size, R < 8, which is in agreement with the behavior observed for the melting temperature in slit pores. However, Tm for almost all pore sizes is less than the bulk case, which is contrary to the behavior seen for the slit pore. The oscillation in Tm decays at around pore radius R = 8, and beyond that shift in the melting temperature with respect to the bulk case is in line with the prediction of the Gibbs-Thomson equation.

Modeling Self-Diffusion of Simple Fluids in Nanopores

The recent frictional model of the transport of fluid mixtures in nanopores developed in this laboratory is extended here to formulate a new theory of the self-diffusion of Lennard-Jones fluids in cylindrical pores by considering the problem of diffusion of identical molecules that differ only in color. The new theory is found to predict the self-diffusivity accurately over a wide range of densities and pore sizes, extending from molecularly narrow pores to large mesopores. However, deviations from the theory appear near to the critical temperature where the correlation length of the fluid diverges and when intermolecular interactions are important in molecularly narrow pores. Under such circumstances, local averaging of the fluid-fluid density to obtain a local viscosity does not adequately capture the effects of viscous friction. A new criterion is developed for determining the significance of fluid-fluid intermolecular interactions in a nanopore by considering the ratio of oscillation times of a fluid molecule in the force field of the surrounding fluid molecules and that in the force field of the pore wall. The ratio is shown to give good predictions of the region where intermolecular interactions are important and explains the region of deviation between theory and simulation in molecularly narrow pores.

A Density Functional Theory for Lennard-Jones Fluids in Cylindrical Pores and Its Applications to Adsorption of Nitrogen on MCM-41 Materials

A density functional theory (DFT) constructed from the modified fundamental-measure theory and the modified Benedict-Webb-Rubin equation of state is presented. The Helmholtz free energy functional due to attractive interaction is expressed as a functional of attractive weighted-density in which the weight function is a mean-field-like type. An obvious advantage of the present theory is that it reproduces accurate bulk properties such as chemical potential, bulk pressure, vapor-liquid interfacial tension, and so forth when compared with molecular simulations and experiments with the same set of molecular parameters. Capabilities of the present DFT are demonstrated by its applicability to adsorption of argon and nitrogen on, respectively, a model cylindrical pore and mesoporous MCM-41 materials. Comparison of the theoretical results of argon in the model cylindrical pore with those from the newly published molecular simulations indicates that the present DFT predicts accurate average densities in the pore, slightly overestimates the pore pressure, and correctly describes the effect of the fluid-pore wall interaction on average densities and pressures in the pore. Application to adsorption of nitrogen on MCM-41 at 77.4 K shows that the present DFT predicts density profiles and adsorption isotherms in good agreement with those from molecular simulations and experiments. In contrast, the hysteresis loop of adsorption calculated from the mean-field theory shifts toward the low pressure region because a low bulk saturated pressure is produced from the mean-field equation of state. The present DFT offers a good way to describe the adsorption isotherms of porous materials as a function of temperature and pressure.

The effect of discrete attractive fluid–wall interaction potentials on adsorption isotherms of Lennard-Jones fluid in cylindrical pores

The dependence of the adsorption isotherm and phase behavior of Lennard-Jones (LJ) fluid in cylindrical pores on the fluid–wall interaction was investigated using a discrete attractive potential model and the density functional theory. The discrete attractive potential contains two parts, a deep well (dw) and a tail. The results show that the chemical potential corresponding to capillary condensation has a weak dependence on the deep well potential near the pore wall, but exhibits a strong dependence on the tail potential near the pore center. On the contrary, the chemical potential of the formation of the first monolayer is strongly dependent on the deep well potential but almost independent of the tail potential. The shape of the adsorption isotherm plotted as density versus chemical potential is determined by the shape of the fluid–wall interaction, namely εtail–εdw, rather than either the deep well potential or the weak tail potential separately. The critical temperature exhibits similar behavior in its dependence on εtail–εdw. There exists a region where layer transition and capillary condensation intersect. Adsorption isotherms, coexistence curves, and critical temperatures in this region exhibit distinct phase behaviors from other regions.

Nucleation of liquid bridges and bubbles in nanoscale capillaries

Nucleation of liquid bridges and bubbles during condensation and evaporation of Lennard-Jones fluid in cylindrical pores is explored by Monte Carlo simulation. The isotherm of constrained critical nuclei is constructed using the gauge cell method. We confirm the Everett–Haynes scenario of bridging through the formation of a bump/undulation on the adsorption film. The molecular structure of growing bridges and cavitating bubbles is revealed. A new simulation approach is introduced to calculate the nucleation energy barriers. The method is based on the introduction and subsequent removal of a virtual “ghost” potential field with a tunable magnitude. Two computation schemes for determining the free energy of nuclei are elaborated based on the thermodynamic integration along a trajectory of states generated in the tunable ghost field and on the umbrella sampling. The methods developed are applicable to study various nucleation phenomena.

Properties of Confined Square-well Fluids using the Gibbs Ensemble Simulation Technique

In this work we have used the extension of the Gibbs ensemble simulation technique to inhomogeneous fluids [Panagiotopoulos, A.Z. (1987) "Adsorption and capillary condensation of fluid in cylindrical pores by Monte Carlo simulation in the Gibbs ensemble", Mol. Phys. , 62 (3), 701-719], which has been applied to adsorption phenomena of confined fluids. Fluid molecules are described by spherical particles interacting via a square-well potential. The fluid is confined in two types of walls: symmetrical (two hard walls) and non-symmetrical (one square-well wall and one hard wall). In order to analyze the behavior of the confined fluid by varying the potential parameters, we evaluated the bulk and confined densities, the internal energies and the density profiles for different supercritical temperatures. A variety of adsorption profiles can be obtained by using this model. The simulation data reported here complements the available simulation data for this system and can be useful in the development of inhomogeneous fluid theories. Since the square-well parameters can be related to real molecules this system can also be used to understand real adsorption systems.

Inside the hysteresis loop: multiplicity of internal states in confined fluids.

We study the equilibrium and stability of metastable states and capillary condensation hysteresis of a Lennard-Jones fluid in cylindrical pores by means of the canonical ensemble density functional theory and gauge cell Monte Carlo simulations. We demonstrate a possibility for the existence of multiple laterally uniform internal states of equal density inside the hysteresis loop. The region of multiple states is bounded by the states of zero compressibility. The internal states can be stabilized in Monte Carlo simulations constraining the density fluctuations.

Self-diffusion in single-file pores of finite length

Molecular dynamics simulation results are reported for single-file self-diffusion in pores of infinite and finite length. The model system involves moderately dense, nonovertaking, hard-sphere fluids in cylindrical pores, and the particle mobility is investigated as a function of time and pore length. It is shown that, while stationary Fickian diffusion in infinitely long pores with diffusely reflecting pore walls is nonexistent, a diffusivity may be defined for single-file pores of finite length. For pores of moderate length, it is also demonstrated that the self-diffusion coefficient scales as 1/√L where L is the pore length.

Molecular dynamics studies of adsorption and flow of fluids through geometrically non-uniform cylindrical pores

Adsorption and flow of Lennard-Jones fluids in cylindrical pores composed of sections having different diameters are studied by using molecular dynamics. Equilibrium simulations are performed at two temperatures, one is higher and one lower than the bulk critical temperature. The studies also include investigations of the structure of a confined fluid. Moreover, in the case of geometrically uniform pores, the equilibrium results are compared with the predictions of the density functional theory. Studies of the Hagen-Poiseuille flow are performed only at higher temperature by assuming a gravitational type driving force. The aim of these studies is to compare the flow patterns that develop for different shapes of pore walls.

Lennard-Jones fluids in cylindrical pores: Nonlocal theory and computer simulation

We present adsorption isotherms, phase diagrams, and density profiles for a Lennard-Jones fluid confined to a cylindrical pore. In particular, we concentrate on the gas–liquid transition in the pore (capillary condensation). We compare simulations for a series of radii and different temperatures with mean field density functional theory (MFT). Two forms of MFT are considered, the simple local density approximation (LDA) and Tarazona’s nonlocal or smoothed density approximation (SDA). We find that the SDA provides a quite accurate description of fluid structure in the pore and that it produces phase diagrams in good agreement with the simulation data. For larger radii and temperatures T/Tc≳0.6 the SDA shows steep rises in adsorption close to the transition. This strongly affects the shape of the coexistence curve in the T, ρ̄ plane. Here ρ̄ is defined as the average density inside the pore. This behavior is confirmed by the simulation. In contrast, LDA gives a poor representation of the fluid structure and this underlies the failure to reproduce the phase diagrams and adsorption isotherms found with SDA or simulation. For extremely small radii (R*≈1) the simulation adsorption isotherms are smooth, and for not too low a temperature they are accurately described by an approach which starts from the potential distribution theorem and uses perturbation theory for the true one-dimensional fluid.

Adsorption and capillary condensation of fluids in cylindrical pores by Monte Carlo simulation in the Gibbs ensemble

A recently proposed technique [1] for the direct determination of phase coexistence properties of fluids by simulation was extended for the prediction of adsorption and capillary condensation of simple fluids in narrow cylindrical pores. Two variations of the technique were developed, the first for equilibrium between a bulk fluid phase and the interior of a pore, and the second for the determination of capillary condensation points. The metholology was applied for the calculation of equilibria for the pure Lennard-Jones (6, 12) fluid in a structureless cylindrical pore that interacts with the fluid via an integrated Lennard-Jones potential. The potential parameters were chosen to approximate fluid Ar in a pore of solid CO2. The phase diagram of the fluid was determined as a function of temperature and pore radius. The gas/liquid critical temperature decreases as the pore radius is reduced. The results from the proposed technique were found to be in good agreement with recent results from independent mole...

Molecular dynamics simulation of hindered diffusion in microcapillaries

A general model for diffusion in membranes is employed to illustrate that the individual contributions of diffusion relative to the membrane and self-diffusion in the pore fluid are simply related to the autocorrelation function for the instantaneous centre of mass velocity of the pore fluid and the usual autocorrelation function for the particle velocities. These results are used to analyse molecular dynamics simulation data for hindered diffusion of rigid sphere fluids in cylindrical pores over a wide range of pore fluid densities and pore sizes. Two reflection conditions, specular and diffuse scattering from the pore wall, are also investigated. The results obtained for diffuse scattering are in qualitative agreement with experimental data reported in the literature. These results also suggest that dense fluids and liquids may undergo a small, nonzero degree of slip during flow past a rigid boundary.