Capillary sublimation of Ar in mesoporous glass

PHYSICAL REVIEW B, 2003

Ar condensates in a porous Vycor glass with an average pore diameter of 10 nm have been studied by optical transmission. Cooling-heating cycles have been performed through the freezing and melting transition for samples with different fractional fillings. Freezing leads to a decrease of the transmission up to four orders of magnitude due to material transport in the pore network that leads to a coarsening of the pore filling. The results are compared with transmission data on isothermal filling and draining and with heat-capacity data on freezing and melting.

Ar condensed into a porous glass matrix has been investigated by simultaneous measurements of adsorptiondesorption isotherms and x-ray diffraction patterns as function of the pore filling above and below the melting point. The chemical-potential-temperature phase diagram has been established. It is consistent with a firstorder phase transition between the adsorbate state and the capillary condensed state, above and below the melting temperature. The adsorbate and the capillary condensed state can also be distinguished in the diffraction patterns. A consistent picture of the structure and the thermodynamics is obtained.

Applied Surface Science, 2002

The pore condensation and hysteresis behavior of nitrogen and argon was studied on well-defined, ordered porous materials like MCM-48, MCM-41 silica (mode pore diameters, 2-5 nm) and SBA-15 (6.7 nm) at 87 and 77 K. A comparison with the results of similar sorption experiments carried out using more disordered adsorbents like controlled-pore glasses (CPG) (mode pore diameters, 11 and 16 nm) is made. The results show clearly that the shape of sorption isotherms (in particular the shape and the width of sorption hysteresis loops) depend both on temperature and pore diameter, i.e. the thermodynamic states of pore fluid and bulk fluid, but-in particular at temperatures below the bulk triple point-also strongly on the texture (and degree of disorder) of the porous material. Analyses of nitrogen (at 77 K) and argon (at 87 K) adsorption-desorption isotherms in MCM-48 silica lead to the conclusion that in this well-defined, interconnected pore network the desorption branch of the hysteresis loop represents the equilibrium transition. In addition, pore condensation of argon can still be observed at 77 K, i.e. ca. 6.5 K below the bulk triple point in MCM-48/41 and SBA-15 silica materials with pore diameters < 8 nm. However, pore condensation of argon at 77 K vanishes in case the pore diameter exceeds ca. 12 nm (based on BJH method), which limits the range for mesopore-size analysis of silica materials using argon sorption at 77 K. #

Journal of Colloid and Interface Science, 1998

The adsorption of argon, nitrogen, and carbon monoxide in porous Vycor glass has been studied by volumetric and microcalorimetric methods and by thermoporometry. Samples with particle sizes ranging from <50 to >200 m have been selected and treated by sample controlled thermal analysis (SCTA). Subsequent characterization indicates that the particle size has no influence on the pore texture and nature. Thermal treatment, however, modifies the chemical nature of the surface. It would seem that nitrogen and carbon monoxide assume a distinct mean orientation leading to smaller effective cross-sectional areas than those usually accepted. Carbon monoxide clearly distinguishes two different types of adsorption site for samples treated at low temperature.

Physical Review Letters, 2008

Freezing and melting of Ar condensed in a granular packing of template-grown arrays of linear mesopores (SBA-15, mean pore diameter 8 nm) has been studied by specific heat measurements C as a function of fractional filling of the pores. While interfacial melting leads to a single melting peak in C, homogeneous and heterogeneous freezing along with a delayering transition for partial fillings of the pores result in a complex freezing mechanism explainable only by a consideration of regular adsorption sites (in the cylindrical mesopores) and irregular adsorption sites (in niches of the rough external surfaces of the grains, and at points of mutual contact of the powder grains). The tensile pressure release upon reaching bulk liquid/vapor coexistence quantitatively accounts for an upward shift of the melting/freeezing temperature observed while overfilling the mesopores. PACS numbers: 64.70.Nd, 65.80.+n, 65.40.Ba

Frontiers in Chemistry

Nitrogen sorption and melting and freezing of water in a small pore size mesoporous glass with irregular pore structure is studied. The analysis of the experimentally obtained data is performed using the recently developed serially connected pore model (SCPM). The model intrinsically incorporates structural disorder by introducing coupling between nucleation and phase growth mechanisms in geometrically disordered mesopore spaces. It is shown that, in contrast to the independent pore models prevailing in the literature, SCPM self-consistently describes not only boundary transitions, but also the entire family of the scanning transitions. The scanning behavior is shown to be very sensitive to microscopic details of the fluid phase distribution within the porous materials, hence can be used to check the validity of the thermodynamic models and to improve the structural analysis. We show excellent quantitative agreement between the structural information evaluated from the cryoporometry and gas sorption data using SCPM.

Advances in Colloid and Interface Science, 2011

Hysteresis in capillary condensation is important for the fundamental study and application of porous materials, and yet experiments on porous materials are sometimes difficult to interpret because of the many interactions and complex solid structures involved in the condensation and evaporation processes. Here we make an overview of the significant progress in understanding capillary condensation and hysteresis phenomena in mesopores that have followed from experiment and simulation applied to highly ordered mesoporous materials such as MCM-41 and SBA-15 over the last few decades.

We have generated molecular models for adsorption in porous glasses that reproduce the complex structures of these materials, using simulations that mimic the quench processes by which Vycor and Controlled Pore Glasses are produced. We have simulated the adsorption and desorption isotherms for two of these model materials. In this work we compare pore size distributions calculated with the Barrett-Joyner-Halenda (BJH) method of isotherm analysis with geometrically-defined pore size distributions.

Langmuir, 2009

A simple phenomenological model that describes capillary condensation and evaporation of pure fluids confined in cylindrical mesopores is presented. Following the work of Celestini (Phys. Lett. A, 1997, 228, 84), the free energy density of the system is derived using interfacial tensions and a corrective term that accounts for the interaction coupling between the vapor/adsorbed liquid and the adsorbed liquid/adsorbent interfaces. This corrective term is shown to be consistent with the Gibbs adsorption isotherm and assessed by standard adsorption tests. This model reveals that capillary condensation and evaporation are metastable and equilibrium processes respectively, hence exhibiting the existence of a hysteresis loop in adsorption/desorption isotherms that is well known in experiment. We extend the phenomenological model of Celestini to give a quantitative description of adsorption on the pore wall and hysteresis width evolution with temperature and confinement. Direct quantitative comparison is made with experimental data for confined argon. Used as a characterizing tool, this integrated model allows in a single fit of an experimental adsorption/desorption isotherm assessing essential characterization data such as the specific surface area, pore volume, and mean pore size.

The optical transmission of O 2 and CO condensates embedded in porous Vycor glass has been studied as function of the filling fraction and of the thermal history of the samples. The freezing transition as well as the solidsolid transitions (b-a of CO and c-b of O 2 ) induce a coarsening of the separation into empty and filled regions which results from the hysteretic behavior of the transitions in the presence of a pore size distribution. In the birefringent b phase of O 2 domains with a fixed crystallographic orientation extend over distances much larger than the pore diameter (10 nm).