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 and Absorption--Gold APP Instruments

Adsorption and Absorption

 

Absorption is the process in which a fluid is dissolved by a liquid or a solid (absorbent).

Adsorption is the process in which atoms, ions or molecules from a substance (it could be gas, liquid or dissolved solid) adhere to a surface of the adsorbent. Adsorption is a surface-based process where a film of adsorbate is created on the surface while absorption involves the entire volume of the absorbing substance.

 

Comparison Chart

Items	Absorption	Adsorption
Definition	Assimilation of molecular species throughout the bulk of the solid or liquid is termed as absorption.	Accumulation of the molecular species at the surface rather than in the bulk of the solid or liquid is termed as adsorption.
Phenomenon	It is a bulk phenomena	It is a surface phenomena.
Heat Exchange	Endothermic process	Exothermic process
Temperature	It is not affected by temperature	It is favored by low temperature
Rate of Reaction	It occurs at a uniform rate.	It steadily increases and reach to equilibrium
Concentration	It is same throughout the material.	Concentration on the surface of adsorbent is different from that in the bulk

 

Process

Gas-liquid absorption (a) and liquid-solid adsorption (b) mechanis.

Blue spheres are solute molecules

 

Adsorption and absorption are both sorption processes.

Absorption occurs when atoms pass through or enter a bulky material. During absorption, the molecules are entirely dissolved or diffused in the absorbent to form a solution. Once dissolved, the molecules cannot be separated easily from the absorbent.

Adsorption is generally classified into physisorption (weak van der Waal’s forces) and chemisorption. It may also occur due to electrostatic attraction. The molecules are held loosely on the surface of the adsorbent and can be easily removed.

 

 

Uses

 

Adsorption: Some of the industrial applications for adsorption are air-conditioning, adsorption chillers, synthetic resin and water purification. An adsorption chiller does not require moving parts and hence is quiet. In pharmaceutical industry applications, adsorption is used as a means to prolong neurological exposure to specific drugs or parts thereof. Adsorption of molecules onto polymer surfaces is used in various applications such as in the development of non-stick coatings and in various biomedical devices.

 

Absorption: The common commercial uses of absorption cycle are absorption chillers for space cooling applications, ice production, cold storage, turbine inlet cooling. High efficiency operation, environmentally friendly refrigerants, clean-burning fuels and few moving parts that require maintenance make absorption a very good choice for consumers. 

The process of gas absorption by a liquid is used in hydrogenation of oils and carbonation of beverages.

 

Video Link for comparison of adsorption and absorption http://v.youku.com/v_show/id_XNjIxNzAxOTc2.html

 

 

 

 

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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

 

 

 

 

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.