Showing papers by "Lev D. Gelb published in 1999"

Phase separation in confined systems

Abstract: 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.

Pore size distributions in porous glasses : A computer simulation study

Abstract: We have prepared a series of molecular models of porous glass using a recently developed procedure (Gelb, L. D.; Gubbins, K. E. Langmuir 1998, 14, 2097) that mimics the experimental processes that produce Vycor and controlled-pore glasses. We calculate nitrogen adsorption isotherms in these precisely characterized model glasses using Monte Carlo simulations. These isotherms are analyzed using the Barrett−Joyner−Halenda (BJH) method to yield pore size distributions, which are tested against exact pore size distributions directly measured from the pore structures. The BJH method yields overly sharp distributions that are systematically shifted (by about 1 nm) to lower pore sizes than those from our geometric method.

Phase Transitions in Pores: Experimental and Simulation Studies of Melting and Freezing†

Abstract: We report both experimental measurements and molecular simulations of the melting and freezing behavior of simple fluids in porous media. The experimental studies are for carbon tetrachloride and nitrobenzene in controlled pore glass (CPG) and Vycor. Differential scanning calorimetry (DSC) was used to determine the melting point in the porous materials for each of the glass samples. In the case of nitrobenzene (which has a nonzero dipole moment), dielectric spectroscopy was also used to determine melting points. Measurements by the two methods were in excellent agreement. The melting point was found to be depressed relative to the bulk value for both fluids. With the exception of smallest pores, the melting point depression was proportional to the reciprocal of the pore diameter, in agreement with the Gibbs-Thomson equation. Structural information about the different confined phases was obtained by measuring the dielectric relaxation times using dielectric spectroscopy. Monte Carlo simulations were used to determine the shift in the melting point, T m , for a simple fluid in pores having both repulsive and strongly attractive walls. The strength of attraction to the wall was shown to have a large effect on the shift in T m , with T m being reduced for weakly attracting walls. For strongly attracting walls, such as graphitic carbon, the melting point increases for slit-shaped pores. For such materials, the adsorbed contact layer is shown to melt at a higher temperature than the inner adsorbed layers. A method for calculating the free energies of solids in pores is presented, and it is shown that the solid-liquid transition is first order in these systems.

Correlation functions of adsorbed fluids in porous glass: a computer simulation study

Abstract: We have prepared a series of model porous glasses using a recently developed molecular dynamics procedure (Gelb, L. D. and Gubbins, K. E., 1998, Langmuir, 14, 2097) that mimics the experimental processes that produce Vycor and controlled pore glasses. Using Grand Canonical Monte Carlo simulations we have measured nitrogen adsorption isotherms in these models and determined the radial distribution functions and structure factors for the adsorbate at several different pressures. These correlation functions show long-wavelength fluctuations which at low density are controlled by the glasses surface—surface correlation functions and at high density are controlled by the glasses void—void correlation functions. The structure factor data at low density can be used to extract a characteristic pore diameter which agrees well with that obtained by a geometrical analysis of the pore structure. Significant qualitative differences in S(k) are found between 30% porous and 50% porous materials.