Malgorzata Sliwinska-Bartkowiak

Phase separation in confined systems

We review the current state of knowledge of phase separation and phase equilibria in porous materials. Our emphasis is on fundamental studies of simple fluids (composed of small, neutral molecules) and well-characterized materials. While theoretical and molecular simulation studies are stressed, we also survey experimental investigations that are fundamental in nature. Following a brief survey of the most useful theoretical and simulation methods, we describe the nature of gas‐liquid (capillary condensation), layering, liquid‐liquid and freezing/melting transitions. In each case studies for simple pore geometries, and also more complex ones where available, are discussed. While a reasonably good understanding is available for phase equilibria of pure adsorbates in simple pore geometries, there is a need to extend the models to more complex pore geometries that include effects of chemical and geometrical heterogeneity and connectivity. In addition, with the exception of liquid‐liquid equilibria, little work has been done so far on phase separation for mixtures in porous media.

Effects of confinement on freezing and melting

We present a review of experimental, theoretical, and molecular simulation studies of confinement effects on freezing and melting. We consider both simple and more complex adsorbates that are confined in various environments (slit or cylindrical pores and also disordered porous materials). The most commonly used molecular simulation, theoretical and experimental methods are first presented. We also provide a brief description of the most widely used porous materials. The current state of knowledge on the effects of confinement on structure and freezing temperature, and the appearance of new surface-driven and confinement-driven phases are then discussed. We also address how confinement affects the glass transition.

Effect of the fluid-wall interaction on freezing of confined fluids: Toward the development of a global phase diagram

We report molecular simulation studies of the freezing behavior of fluids in nano-porous media. The effect of confinement is to induce spatial constraints as well as energetic heterogeneity on the confined fluid, thereby altering the bulk phase behavior drastically. We consider the effect of the fluid-wall interaction energy on the shift of the freezing temperature and on the fluid structure, using a novel approach to calculate the free energy surface based on Landau theory and order parameter formulation. Corresponding states theory is then used to map out the global freezing behavior of a Lennard-Jones (LJ) fluid in model slit-shaped pores of varying fluid-wall interaction strengths. Using LJ parameters fitted to thermophysical property behavior, we predict the qualitative freezing behavior for a variety of fluids and nano-porous materials, based on a global freezing diagram. We have attempted to verify these predictions by comparing with experimental data for several systems, and show that in these cas...

Global phase diagrams for freezing in porous media

Using molecular simulations and free energy calculations based on Landau theory, we show that freezing/melting behavior of fluids of small molecules in pores of simple geometry can be understood in terms of two main parameters: the pore width H* ~expressed as a multiple of the diameter of the fluid molecule ! and a parameter a that measures the ratio of the fluid-wall to the fluid‐fluid attractive interaction. The value of the a parameter determines the qualitative nature of the freezing behavior, for example, the direction of change in the freezing temperature and the presence or absence of new phases. For slit-shaped pores, larger a values lead to an increase in the freezing temperature of the confined fluid, and to the presence of a hexatic phase. For pores that accommodate three or more layers of adsorbate molecules several kinds of contact layer phase ~inhomogeneous phases in which the contact layer has a different structure than the inner layers! are observed. Smaller a values lead to a decrease in the freezing temperature. The parameter H* determines the magnitude of shift in the freezing temperature, and can also affect the presence of some of the new phases. Results are presented as plots of transition temperature vs a for a particular pore width. Experimental results are also presented for a variety of adsorbates in activated carbon fibers~ACF! covering a wide range of a values; the ACF have slit-shaped pores with average pore width 1.2 nm. The experimental and simulation results show qualitative agreement.

Melting/freezing behavior of a fluid confined in porous glasses and MCM-41: Dielectric spectroscopy and molecular simulation

We report both experimental measurements and molecular simulations of the melting and freezing behavior of fluids in nanoporous media. The experimental studies are for nitrobenzene in the silica-based pores of controlled pore glass, Vycor, and MCM-41. Dielectric relaxation spectroscopy is used to determine melting points and the orientational relaxation times of the nitrobenzene molecules in the bulk and the confined phase. Monte Carlo simulations, together with a bond orientational order parameter method, are used to determine the melting point and fluid structure inside cylindrical pores modeled on silica. Qualitative comparison between experiment and simulation are made for the shift in the freezing temperatures and the structure of confined phases. From both the experiments and the simulations, it is found that the confined fluid freezes into a single crystalline structure for average pore diameters greater than 20σ, where σ is the diameter of the fluid molecule. For average pore sizes between 20σ and...

Freezing behavior in porous glasses and MCM-41

We report experimental measurements of the melting and freezing behavior of fluids in nano-porous media. The experimental studies are for nitrobenzene in the silica based pores of controlled pore glass (CPG), Vycor and MCM-41. Dielectric relaxation spectroscopy was used to determine melting points and the orientational relaxation times of the nitrobenzene molecules in the bulk and the confined phase. It was found that the confined fluid freezes into a single crystalline structure for average pore diameters greater than 20 , where is the diameter of the fluid molecule. For average pore sizes between 20 and 15 , part of the confined fluid freezes into a frustrated crystal structure with the rest forming an amorphous region. For pore sizes smaller than 15 , even the partial crystallization did not occur.

Molecular modeling of freezing of simple fluids confined within carbon nanotubes

We report Monte Carlo simulation results for freezing of Lennard-Jones carbon tetrachloride confined within model multiwalled carbon nanotubes of different diameters. The structure and thermodynamic stability of the confined phases, as well as the transition temperatures, were determined from parallel tempering grand canonical Monte Carlo simulations and free-energy calculations. The simulations show that the adsorbate forms concentric molecular layers that solidify into defective quasi-two-dimensional hexagonal crystals. Freezing in such concentric layers occurs via intermediate phases that show remnants of hexatic behavior, similar to the freezing mechanism observed for slit pores in previous works. The adsorbate molecules in the inner regions of the pore also exhibit changes in their properties upon reduction of temperature. The structural changes in the different regions of adsorbate occur at temperatures above or below the bulk freezing point, depending on pore diameter and distance of the adsorbate molecules from the pore wall. The simulations show evidence of a rich phase behavior in confinement; a number of phases, some of them inhomogeneous, were observed for the pore sizes considered. The multiple transition temperatures obtained from the simulations were found to be in good agreement with recent dielectric relaxation spectroscopy experiments for CCl(4) confined within multiwalled carbon nanotubes.

Dielectric studies of freezing behavior in porous materials: Water and methanol in activated carbon fibres

We report both experimental measurements and molecular simulations of the melting and freezing behavior of two dipolar fluids, water and methanol, in activated carbon fibres. Differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DS) were used to determine the melting point in these porous materials. The melting point was found to be very sensitive to the relative strength of the fluid–wall interaction compared to the fluid–fluid interaction. Monte Carlo simulations and the Landau free energy formalism were used to determine the shift in the melting point, Tm, for simple fluids in pores having weakly attractive and strongly attractive walls. The strength of the interaction of the fluid with the pore wall is shown to have a large effect on the shift in Tm, with Tm being reduced for weakly attracting walls and elevated for strongly attracting walls.

Effect of Pressure on the Freezing of Pure Fluids and Mixtures Confined in Nanopores

Monte Carlo simulations combined with the parallel tempering technique are used to study the freezing of Ar, CH(4), and their mixtures in a slit graphite nanopore. For all systems, the solid/liquid coexistence line is located at higher temperature than that for the bulk phase, as expected for fluids for which the wall/fluid interaction is stronger than the fluid/fluid interaction. In the case of the mixtures, the phase diagram for the confined system is of the same type as that for the bulk (azeotropic). It is also found that the freezing temperatures for the confined fluids and mixture are much more affected by pressure than those for the bulk phase. By calculating the isothermal compressibility of the confined fluids and determining the slope of the solid/liquid coexistence line (T,P) from the Clapeyron equation, we show that such a strong effect of pressure is not related to reduced compressibility within the pores. On the other hand, the pressure dependence of the in-pore freezing temperature is correctly described in the frame of the model proposed by Miyahara et al. [ Miyahara , M. ; Kanda , H. ; Shibao , M. ; Higashitani , K. J. Chem. Phys. 2000 , 112 , 9909. ], which is based on the pressure difference between the bulk and confined phases (capillary effect). In this model, a change in the in-pore freezing temperature with pressure is explained by a drastic change in the in-pore pressure, which varies very sharply with the bulk external pressure. We present an extended version of this model to confined systems for which an increase in the freezing temperature is observed.

Freezing/melting behaviour within carbon nanotubes

We report a study of the freezing and melting of fluids confined within multi-walled carbon nanotubes with an internal diameter of 5 nm, using experimental measurements and molecular simulations. Dielectric relaxation spectroscopy was used to determine the experimental melting points and relaxation times of nitrobenzene and carbon tetrachloride within carbon nanotubes, and parallel tempering Monte Carlo simulations in the grand canonical ensemble were performed for confined carbon tetrachloride. The simulations show that the adsorbate forms concentric layers that solidify into quasi-two-dimensional hexagonal crystals with defects; highly defective microcrystalline regions are formed in the inner layers, owing to the strong geometrical constraints. Our simulations show no formation of common three-dimensional crystalline structures (fcc, hcp, bcc, sc or icosahedral) in confinement. The results suggest the presence of inhomogeneous phases (i.e., combinations of crystalline and liquid regions) within the pore over extended temperature ranges. Our results indicate that the outer layers of adsorbate solidify at temperatures slightly higher than the bulk freezing point, whereas the inner layers freeze at lower temperatures. The simulation results are in good agreement with the experimental measurements.