nitrogen adsorption, gas desorption, isotherm, surface area and

Technical Files

Adsorptive Gas Choosing for Cryogenic Temperature Adsorption

Gas sorption principle is based on the adsorption characteristic that gas molecules will adsorb on solid surface under certain pressure. The analyzed particles’ (adsorbent) surface will have reversible physisorption to gas molecules (adsorbate) under super cryogenic temperature and exists equilibrium adsorption amount in certain pressure. Figure out this equilibrium adsorption amount, then employs theories to calculate analyzed samples’ surface area, pore volume and pore size distribution.

High purity nitrogen and liquid nitrogen (coolant) are the most typical used adsorbate due to their ready availability and reversibility. However, the nitrogen has certain limitation in playing the role as adsorbate for micropores (such as molecular sieve, active carbon etc.) and small surface area samples (such as natural minerals, organic materials etc.), in these cases, Ar, CO2 and Kr can be work as adsorbate.

The Ar molecules, applies more common in molecular sieve analysis, can be stabile adsorbed on materials’ surface in 87K liquid Ar or 77K liquid N2 temperature. Following are three reasons for this:

Nitrogen molecule belongs to polarmolecule, also has quadrupole which can strength the acting force between adsorbate molecules and molecular sieve’s wall of hole, easy to occur specific adsorption gives difficult for identifying different sieves’ pore sizes. But Ar molecules are sphere-shaped and non-polar molecule which helps to achieve more accurate micropore distribution data.
N2 needs lower P/Po points than Ar for a certain pore width, thus, takes Ar as adsorbate for higher P/PO points micropore adsorption.
Ar can be adsorbed in 87K liquid Ar temperature, so, will shorten equilibrium time and improve analysis efficiency if operator raises coolant bath temperature.

Argon limits in the capillary condensation will disappear if pore size is bigger than 12nm, therefore, only be used for micropore analysis.

For micropores dominated active carbon samples, CO2 is a good adsorbate. CO2 freezing point adsorption is mainly employed for measuring active carbon saturation adsorption capacity because its high efficiency, quick spread speed and easy to achieved saturation adsorption amount features. CO2 freezing point (273K) adsorption temperature is much higher than Ar (77K) and N2 (87K) adsorption temperature. But, CO2 freezing point’s saturation vapor pressure (3485.3 KPa) is too high and only suits micropores adsorption, can not reach higher P/Po points unless adopts high pressure adsorption analyzer.

For small surface area’s metal powder, organic materials and some natural minerals, krypton is a better choice to be the adsorbate.

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In the measurement procedures for gas sorption, a few factors will influence the processes and results heavily, so they need to take into consideration for analysis the efficiency. These factors are sample pretreatment conditions, adsorbate characteristics and differences between analysis methods.

Sample Pretreatment Conditions:

Specific surface area and porosity measurement is closely connected with particle external surface area. Besides, the key of gas sorption is the adsorbate can be efficiently attached onto particle surface or be filled into pores, thus, no more important than particle surface purity. The purpose for sample pretreatment is to remove atmospheric contaminants on samples’ surface and make room for adsorbate. Most samples need pretreatment and ways are changed with samples’ characteristics. Normally, water molecule is the item need to be removed, thus, to dry samples in atmospheric pressure and temperature over 100℃ (usually 105℃-120℃) is enough which can simplifies operation procedures. It is easy to adsorbed contaminants in atmospheric pressure and temperature for microporous and strong adsorptive samples, but sometimes need to be degassed under vacuum condition, even to inlet rare gases for better desorption. All in all, pretreatment is to clean and purify sample external surface to ensure more precision results.

Adsorbate Characteristics:

Stable chemical properties, reversible and no any influences on sample performance and adsorptive characteristic are three basic requirements in gas sorption measurement. Practices show that nitrogen is the most commonly used adsorbate to analysis majority samples and it can produce an ideally high precision and reproducible data. For microporous samples, it can not adsorb fully if micropore sizes are very small and nearly equal to nitrogen molecule diameter, one reason is nitrogen molecules are hard or absolutely can not enter into micropores, another is adsorbate molecules can be easily affected by unexpected factors and the adsorption amount can not reflect sample surface area real size. Under this circumstance, argon or krypton, have smaller molecule diameter, will be employed as the adsorbate to guarantee adsorption and efficient measurement data.

Analysis Methods’ Differences:

Dynamic contrast method can effectively reduce sample pretreatment influences on analysis data because it uses “contrast” way. In a way, reference materials (RM) and samples’ errors, caused by improper pretreatment, maybe offset, but prerequisites are RM and sample share alike surface structures and adsorption characteristics, also be handled under same pretreatment conditions. This method has great value in field control of product quality, also can reduce sample treatment time and improve measurement efficiency.

In contrast, static volumetric method demands a very strict sample pretreatment owing to this method adopts absolutely adsorption amount to measure, unclean surface or any factors trouble the adsorption processes can result in immediate effect for measurement data.

Using a Gas Pycnometer to Measure Density and Porosity of Grain Kernels

Porosity and density of grain kernels are important parameters that affect the kernel hardness, breakage susceptibility, milling, drying rate, and resistance to fungal development.

Information on the porosity, apparent density, and true density of grain kernels is very limited. Some reports values of true kernel density were determined by a gas pycnometer. Because of the isolation of void spaces or intercellular spaces in grain kernels from the kernel surface, kernel volume determined by a pycnometer could include some of these spaces. Therefore, the density determined might not be the true density.

Zink determined the volume of grain and the void spaces between grain kernels by the displacement of mercury. Lorenzen measured the volumes of wheat, corn, barley, rice and milo by the displacement of toluene. Ross determined the volumes of corn, oats, and soybeans by water displacement. He noted that oats presented a particular problem, because their rough and hairy hulls prevented water from entering the spaces between the kernels. Mercury might present even more serious problems in filling the void spaces between grain kernels because of its high surface tension. Due to this surface tension and possible absorbance, liquid probably are not adequately suited for determining void spaces for most grain and seed kernels. Thompson and Isaacs determined the bulk porosity of various grains and seeds using as air-comparison pycnometer. The porosity values they obtained were 5 to 10% higher than those determined by the mercury displacement method.

But, by many time analyses, we found that the porosity, apparent density, and true density of grain kernels can be determined by a gas pycnometer which operates on Archimedes principle of gas displacement to determine the volume. Below is the schematic of one gas pycnometer from Gold APP Instruments G-DenPyc 2900.