porosimetry technique by Gold APP Instruments China

Porosimetry is an analytical technique used to determine various quantifiable aspects of a material's porous nature, such as pore diameter, total pore volume, surface area, and bulk and absolute densities.

The technique involves the intrusion of a non-wetting liquid (often mercury) at high pressure into a material through the use of a porosimeter. The pore size can be determined based on the external pressure needed to force the liquid into a pore against the opposing force of the liquid's surface tension.

A force balance equation known as Washburn's equation for the above material having cylindrical pores is given as:

PL= pressure of liquid
PG= pressure of gas
O= surface tension of liquid
日= contact angle of intrusion liquid
DP= pore diameter

Since the technique is usually done under vacuum, the gas pressure begins at zero. The contact angle of mercury with most solids is between 135° and 142°, so an average of 140° can be taken without much error. The surface tension of mercury at 20 °C under vacuum is 480 mN/m. With the various substitutions, the equation becomes:

As pressure increases, so does the cumulative pore volume. From the cumulative pore volume, one can find the pressure and pore diameter where 50% of the total volume has been added to give the median pore diameter.

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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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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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Activated Carbon Can be Used for CH4 Storage and Transportation

 

Activated Carbon Can be Used for CH4 Storage and Transportation

 

Stored natural gas plays a vital role in ensuring that any excess supply delivered during the summer months is available to meet the increased demand of the winter months. However, with the recent trend towards natural gas fired electric generation, demand for natural gas during the summer months is now increasing (due to the demand for electricity to power air conditioners and the like). At the time, the natural gas industry noted that seasonal demand increases could not feasibly be met by pipeline delivery alone. Natural gas in storage also serves as insurance against any unforeseen accidents, political conflicts, natural disasters, or other occurrences that may affect the production or delivery of natural gas. In order to be able to meet seasonal demand increases, underground storage fields were the only option. Before the storage process, transportation takes places an important role. Scientists developed an option for gas storage and economical method for transportation of natural gas by the help of active carbon’s adsorption property. Via the high adsorption rate and capacity of activated carbon, the higher mole of methane, can be possible to store at low pressures by comparison with conventional methods.

 

Adsorption is the process by which molecules of a substance, such as a gas or a liquid, collect on the surface of another substance, such as a solid. The molecules are attracted to the surface but do not enter the solid's minute spaces as in absorption.

 

Activated carbon is a form of carbon processed to be riddled with small, low-volume pores that increase the surface area available for adsorption or chemical reactions. Activated carbon is a carbonaceous adsorbent with a high internal porosity, and hence a large internal surface area. Commercial activated carbon grades have an internal surface area of 500 up to 1500 m2/g. People studied the feasibility of storing CH4 in an abandoned coal mine and transportation of CH4 by tankers in adsorbed state using activated carbon as medium. It can be concluded that active carbon can be used for safe storage and transportation of methane as an alternative to conventional methods.

 

Now, we need to use good performance active carbons that help people to transport as much CH4 as possible in one time. Problem is how to know that very active carbon is suitable for us. Under this situation, we need one instrument to help us, to analysis that active carbon adsorption performance, and that instrument is HIGH PRESSURE GAS SORPTION ANALYZER.

 

We, Gold APP Instruments, had researched and produced high pressure gas sorption analyzer H-Sorb 2600 series by static volumetric principle. Capable of testing nanomaterials adsorption and desorption performance, PCT (pressure composition temperature) or PCI (pressure composition isotherm) curves. H-Sorb 2600 series with 2 analysis ports and 2 degassing ports, fully automated operation, supports night operation without operators watch, longest supporting test time can be last as long as half month (can be upgrade), all-featured data reports can help researchers to get all analysis details they want.

 

Gold APP Instruments China is a professional manufacturer for analytical and research instruments, such as BET surface area analyzers, surface area and porosity analyzers, helium true density analyzers, high pressure gas sorption analyzers and so on, to determine nanomaterials specific surface area, pore size, pore volume, pore size distribution, true density, open and closed spaces, adsorption/desorption isotherms, pressure-composition-isotherm, pressure-composition-temperature curve and so on.

 

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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

micropore size distribution analysis, laboratory equipment, gas sorption analyser

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.