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Gas Adsorption


The gas adsorption technique may used to measure the specific surface area and pore size distribution of powdered or solid materials. The dry sample is usually evacuated of all gas and cooled to a temperature of 77K, the temperature of liquid nitrogen. At this temperature inert gases such as nitrogen, argon and krypton will physically adsorb on the surface of the sample. This adsorption process can be considered to be a reversible condensation or layering of molecules on the sample surface during which heat is evolved. Nitrogen gas is ideal for measuring surface area and pore size distribution.

Adsorption Isotherm

An adsorption isotherm (one temperature) is usually recorded as volume of gas adsorbed (cc/g @ STP) versus relative pressure (i.e., sample pressure / saturation vapor pressure). Using relative pressure to construct the isotherm eliminates changes in pressure from small changes in temperature. A small change in temperature changes the saturation vapor pressure considerably. For example, 0.1K increase in temperature changes the saturation pressure of nitrogen from approx. 760 mm Hg to 800 mm Hg. The use of relative pressure is convenient and is scaled from 0 to 1. A relative pressure of 1 represents a completely saturated sample, i.e., all of the available surface structure is filled with liquid-like gas.

Different Methods of Measurement

There are three instrument methods in common use today for measuring adsorption isotherm data. These volumetric methods use the "GAS LAWS" to calculate the volume of gas adsorbed at measured relative pressures and are known as 1) Static, (classic) fully equilibrated, 2) Continuous Flow, or quasi-equilibrated, and Dynamic or Chromatographic. Each have their own advantages and disadvantages. Coulter manufactures instruments which use the static and continuous flow methods.

BET Surface Area Determination

One or more data points of the adsorption isotherm must be measured and the BET (after Brunauer, Emmett and Teller) equation is used to give specific surface area from this data. The BET equation is used to give the volume of gas needed to form a monolayer on the surface of the sample. The actual surface area can be calculated from knowledge of the size and number of the adsorbed gas molecules.

Nitrogen is used most often to measure BET surface, but if the surface area is very low, argon or krypton may be used as both give a more sensitive measurement, because of their lower saturation vapor pressures at liquid nitrogen temperature.

Coulter Corporation manufactures both the OMNISORP and SA 3100 ranges of gas adsorption analyzers.

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