Capilliary condensation introduction and relation with pore size distribution

Capillary condensation is the "process by which multilayer adsorption from the vapor [phase] into a porous medium proceeds to the point at which pore spaces become filled with condensed liquid from the vapor [phase]." The unique aspect of capillary condensation is that vapor condensation occurs below the saturation vapor pressure, P_sat, of the pure liquid. This result is due to an increased number of van der Waals interactions between vapor phase molecules inside the confined space of a capillary. Once condensation has occurred, a meniscus immediately forms at the liquid-vapor interface which allows for equilibrium below the saturation vapor pressure. Meniscus formation is dependent on the surface tension of the liquid and the shape of the capillary, as shown by the Young-Laplace equation. As with any liquid-vapor interface involving a menisci, theKelvin equation provides a relation for the difference between the equilibrium vapor pressure and the saturation vapor pressure. A capillary does not necessarily have to be a tubular, closed shape, but can be any confined space with respect to its surroundings.

Capillary condensation isan important factor in both naturally occurring and synthetic porousstructures. In these structures, scientists use the concept of capillarycondensation to determine pore size distribution and surface area thoughadsorption isotherms. Syntheticapplications such as sintering ofmaterials are also highly dependent on bridging effects resulting fromcapillary condensation. In contrast to the advantages of capillarycondensation, it can also cause many problems in materials science applicationssuch as Atomic Force Microscopyand Microelectromechanical Systems See belowfigure 1.

 

Figure 1: An example of a porous structure exhibiting capillary condensation.

 

Pores that are not of the same size will fill atdifferent values of pressure, with the smaller ones filling first. This difference in filling rate can bea beneficial application of capillary condensation. Many materials havedifferent pore sizes with ceramics being one of the most commonly encountered.In materials with different pore sizes, curves can be constructed similar toFigure 2. A detailed analysis of the shape of these isotherms is done using the Kelvin equation. This enables the pore size distribution to be determined. While this is a relatively simplemethod of analyzing the isotherms, a more in depth analysis of the isotherms isdone using the BET method. Anothermethod of determining the pore size distribution is by using a procedure knownas Mercury Injection Porosimetry. This uses the volume of mercury taken up bythe solid as the pressure increases to create the same isotherms mentionedabove. An application where pore size is beneficial is in regards to oilrecovery. When recovering oil from tiny pores, it is useful to inject gas andwater into the pore. The gas will then occupy the space where the oil once was,mobilizing the oil, and then the water will displace some of the oil forcing itto leave the pore.

Figure 2: Capillary condensation profile showing a sudden increase in adsorbed volume due to a uniform capillary radius (dashed path) among a distribution of pores and that of a normal distribution of capillary radii (solid path)

 

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Adsorption isotherm--GOLD APP INSTRUMENTS

Adsorption isotherm

Adsorption isotherm is the relationship between the pressure and adsorption amount at a constant temperature. The horizontal axis is the relative pressure (P/P₀) which is the equilibrium pressure divided by the saturation pressure. The relative pressure can be 0 to 1 and P/P₀ =1.0 means that the condensation of adsorptive occurs in the sample cell. So an adsorption isotherm is the measurement of adsorptive density which becomes higher than the than the bulk (gas) phase density due to the interaction between the adsorptive and solid surface atoms below its condensation pressure. Adsorption amount in the vertical axis is commonly expressed as V/ml(STP)g^-1 which is expressed by the standard gas volume (at 0^oC and 1 atm).

The figure indicates the classification of adsorption isotherms defined by IUPAC. The type of adsorption isotherm is determined by the pore size and surface character of the material.

I : Microporous materials (e.g. Zeolite and Activated carbon)

II : Non porous materials (e.g. Nonporous Alumina and Silica)

III : Non porous materials and materials which have the weak interaction between the adsorbate and adsorbent (e.g. Graphite/water)

IV : Mesoporous materials (e.g. Mesoporous Alumina and Silica)

V : Porous materials and materials that have the weak interaction between the adsorbate and adsorbent (e.g. Activated carbon/water)

VI : Homogeneous surface materials (e.g. Graphite/Kr and NaCl/Kr)

 

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A novel method for deriving true density of pharmaceutical solids including hydrates and water-containing powders

True density is commonly measured using helium pycnometry. However, most water-containing powders, for example, hydrates, amorphous drugs and excipients, and most tablet formulations, release water when exposed to a dry helium atmosphere. Because released water brings significant errors to the measured true density and drying alters the nature of water-containing solids, the helium pycnometry is not suitable for those substances. To overcome this problem, a novel method has been developed to accurately calculate powder true density from compaction data. No drying treatment of powder samples is required. Consequently, the true density thus obtained is relevant to tableting characterization studies because no alteration to the solid is induced by drying. This method involves nonlinear regression of compaction pressure-tablet density data based on a modified Heckel equation. When true density values of water-free powders derived by this novel method were plotted against values measured using pycnometry, a regression line with slope close to unity and intercept close to zero was obtained. Thus, the validity of this method was supported. Using this new method, it was further demonstrated that helium pycnometry always overestimates true densities of water containing powders, for example, hydrates, microcrystalline cellulose (MCC), and tablet formulations. The calculated truedensities of powders were the same for different particle shapes and sizes of each material. This further suggests that true density values calculated using this novel method are characteristic of given materials and independent of particulate properties.

 

Copyright 2004 Wiley-Liss, Inc. and the American Pharmacists Association

 

 

 

 

published papers

Surface Area|Pore Size|Pore Volume|Pore Size Distribution|Gas Pycnometer|Helium Density Analyzer|High Pressure Volumetric Analyzer|Powder and Porous Analysing|Laboratory Equipment|Research Instruments-Gold APP Instruments

Gas Pycnometer HeliumTrue Density Equipment|High Pressure Volumetric Analyzer| Langmuir/BET/Gibbs/BJH/MP/SF/DR/DA/HK/T-Plot Analyzer

BET Surface Area|Porosity Analyzer|Gas Pycnometer Helium Abosulte Density Equipment|High Pressure Volumetric Analyzer| Langmuir/BET/Gibbs/BJH/MP/SF/DR/DA/HK/T-Plot Analyzer

specific surface area introduction--GOLD APP INSTRUMENTS

Specific surface area "SSA" is a property of solids which is the total surface area of a material per unit of mass, solid or bulk volume, or cross-sectional area.

It is a derived scientific value that can be used to determine the type and properties of a material (e.g. soil). It is defined either by surface area divided by mass (with units of m²/kg), or surface area divided by the volume (units of m²/m³ or m⁻¹)

It has a particular importance for adsorption, heterogeneous catalysis, and reactions on surfaces.

Measurement

The value obtained for specific surface area depends upon the method of measurement. Several techniques have been developed to measure the specific surface area of clays, including methylene blue (MB) stain test, ethylene glycol monoethyl ether (EGME) method, Brenauer-Emmett-Teller (BET)adsorption method and Protein Retention (PR) method.

Calculation 

The SSA can be simply calculated from a particle size distribution, making some assumption about the particle shape. This method, however, fails to account for surface associated with the surface texture of the particles.

Adsorption

The SSA can be measured by adsorption using the BET isotherm. This has the advantage of measuring the surface of fine structures and deep texture on the particles. However, the results can differ markedly depending on the substance adsorbed.

Gas permeability

This depends upon a relationship between the specific surface area and the resistance to gas-flow of a porous bed of powder. The method is simple and quick, and yields a result that often correlates well with the chemical reactivity of a powder. However, it fails to measure much of the deep surface texture.