Effect of the fluid-wall interaction on freezing of confined fluids: Toward the development of a global phase diagram
14 Jun 2000-Journal of Chemical Physics (American Institute of PhysicsAIP)-Vol. 112, Iss: 24, pp 11048-11057
TL;DR: In this article, the effect of the fluid-wall interaction energy on the shift of the freezing temperature and on the fluid structure is considered, using a novel approach to calculate the free energy surface based on Landau theory and order parameter formulation.
Abstract: 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 cases, the experimental observations and the predictions are in agreement.
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TL;DR: High levels of selective sorption of acetylene molecules as compared to a very similar molecule, carbon dioxide, onto the functionalized surface of a MOM are reported.
Abstract: Metal-organic microporous materials (MOMs) have attracted wide scientific attention owing to their unusual structure and properties, as well as commercial interest due to their potential applications in storage, separation and heterogeneous catalysis. One of the advantages of MOMs compared to other microporous materials, such as activated carbons, is their ability to exhibit a variety of pore surface properties such as hydrophilicity and chirality, as a result of the controlled incorporation of organic functional groups into the pore walls. This capability means that the pore surfaces of MOMs could be designed to adsorb specific molecules; but few design strategies for the adsorption of small molecules have been established so far. Here we report high levels of selective sorption of acetylene molecules as compared to a very similar molecule, carbon dioxide, onto the functionalized surface of a MOM. The acetylene molecules are held at a periodic distance from one another by hydrogen bonding between two non-coordinated oxygen atoms in the nanoscale pore wall of the MOM and the two hydrogen atoms of the acetylene molecule. This permits the stable storage of acetylene at a density 200 times the safe compression limit of free acetylene at room temperature.
1,301 citations
TL;DR: In this article, the effects of size and confinement at the nanometre size scale on both the melting temperature and the glass transition temperature, Tm, are reviewed, and it seems that the existing theories of Tg are unable to explain the range of behaviours seen at the nano-scale.
Abstract: In this article, the effects of size and confinement at the nanometre size scale on both the melting temperature, Tm, and the glass transition temperature, Tg, are reviewed. Although there is an accepted thermodynamic model (the Gibbs–Thomson equation) for explaining the shift in the first-order transition, Tm, for confined materials, the depression of the melting point is still not fully understood and clearly requires further investigation. However, the main thrust of the work is a review of the field of confinement and size effects on the glass transition temperature. We present in detail the dynamic, thermodynamic and pseudo-thermodynamic measurements reported for the glass transition in confined geometries for both small molecules confined in nanopores and for ultrathin polymer films. We survey the observations that show that the glass transition temperature decreases, increases, remains the same or even disappears depending upon details of the experimental (or molecular simulation) conditions. Indeed, different behaviours have been observed for the same material depending on the experimental methods used. It seems that the existing theories of Tg are unable to explain the range of behaviours seen at the nanometre size scale, in part because the glass transition phenomenon itself is not fully understood. Importantly, here we conclude that the vast majority of the experiments have been carried out carefully and the results are reproducible. What is currently lacking appears to be an overall view, which accounts for the range of observations. The field seems to be experimentally and empirically driven rather than responding to major theoretical developments.
900 citations
TL;DR: In this article, the coordination space is defined as the space where the coordination bond plays an important role in the formation of the spatial structures and where various physical properties are exhibited, and the coordination spaces provided by porous coordination polymers, and their uniqueness is illustrated with current representative results.
830 citations
TL;DR: Both simple and more complex adsorbates that are confined in various environments (slit or cylindrical pores and also disordered porous materials) are considered and how confinement affects the glass transition is addressed.
Abstract: 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
640 citations
TL;DR: A review of experimental work on freezing and melting in confinement is presented in this paper, where a range of systems, from metal oxide gels to porous glasses to novel nanoporous materials, are discussed.
Abstract: A review of experimental work on freezing and melting in confinement is presented. A range of systems, from metal oxide gels to porous glasses to novel nanoporous materials, is discussed. Features such as melting-point depression, hysteresis between freezing and melting, modifications to bulk solid structure and solid-solid transitions are reviewed for substances such as helium, organic fluids, water and metals. Recent work with well characterized assemblies of cylindrical pores like MCM-41 and graphitic microfibres with slit pores has suggested that the macroscopic picture of melting and freezing breaks down in pores of molecular dimensions. Applications of the surface force apparatus to the study of freezing and melting phenomena in confinement are discussed in some detail. This instrument is unique in allowing the study of conditions in a single pore, without the complications of pore blockage and connectivity effects. The results have confirmed the classical picture of melting-point depression in larger pores, and allowed the direct observation of capillary condensation of solid from vapour. Other results include the measurement of solvation forces across apparently fluid films below the bulk melting point and a solid-like response to shear of films above the bulk melting point. These somewhat contradictory findings highlight the difficulty of using bulk concepts to define the phase state of a substance confined to nanoscale pores.
515 citations
References
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TL;DR: In this article, a molecular simulation study of freezing transitions for simple fluids in narrow slit pores was performed using the grand canonical Monte Carlo simulation method, which indicated a first order fluid to solid transition.
Abstract: We report a molecular simulation study of freezing transitions for simple fluids in narrow slit pores. A major stumbling block in previous studies of freezing in pores has been the lack of any method for calculating the free energy difference between the confined solid and liquid phases. Conventional thermodynamic integration methods often fail for confined systems, due to the difficulty in choosing a suitable path of integration. We use a different approach that involves calculating the Landau free energy as a function of a suitable order parameter, using the grand canonical Monte Carlo simulation method. The grand free energy for each phase can then be obtained by one-dimensional integration of the Landau free energy over the order parameter. These calculations are carried out for two types of wall—fluid interaction, a hard wall and a strongly attractive wall modelled on carbon. The grand free energy results for both cases clearly indicate a first order fluid to solid transition. In the case of the attr...
131 citations
TL;DR: In this paper, a Monte Carlo study of the split-pore with structured walls reveals that at critical inter-wall separations the fluid layers take on solid-like order, that is, the fluid 'freezes' to a solid reflecting the structure of the wall.
Abstract: The behaviour of fluids near solid-fluid interfaces is important in a number of fields1–4 and is receiving renewed attention5. Previous computational studies6–11 of the prototype split-pore—an atomic fluid contained between two infinite, plane-parallel solid surfaces (walls)—demonstrate that the fluid piles up in layers parallel to the walls. Yet none of these investigations finds long-range order within the fluid layers in directions parallel to the walls. In contrast, our Monte Carlo study of the prototype with structured walls reveals that at critical inter-wall separations the fluid layers take on solid-like order, that is, the fluid 'freezes' to a solid reflecting the structure of the wall. This epitaxy decreases with increasing inter-wall separation, but persists indefinitely in the contact layer. Such epitaxy may occur at aqueous-solid interfaces in geological1,2,4 and biological2,3 systems.
114 citations
TL;DR: Theoretical methods have been developed that view crystallization from the liquid side, in contrast to the traditional lattice-instability theories of melting as discussed by the authors, revealing the ordering of colloidal particles suspended in solution and the disordering of surface layers of crystals, giving direct information about the mechanisms of freezing and melting.
Abstract: Over the past decade, new theoretical and experimental techniques have greatly advanced our understanding of the transition between liquid and crystalline phases. Theoretical methods have been developed that view crystallization from the liquid side, in contrast to the traditional lattice-instability theories of melting. Experiments have revealed the ordering of colloidal particles suspended in solution and the disordering of surface layers of crystals, giving direct information about the mechanisms of freezing and melting.
100 citations
TL;DR: In this paper, the melting and freezing behaviors of benzene confined in graphitic micropores of different pore widths were examined by the use of differential scanning calorimetry (DSC) over the temperature range of 180-320 K.
95 citations
TL;DR: In this paper, the authors determine les temperatures de solidification and de fusion for ces trois corps and estime une limite inferieure aux valeurs de l'energie de surface liquide-solide.
Abstract: On determine les temperatures de solidification et de fusion pour ces trois corps et on estime une limite inferieure aux valeurs de l'energie de surface liquide-solide. Ces temperatures sont composees entre elles et avec la temperature unique de fusion du corps libre correspondant
94 citations