Surface Area Measurement

Determination of Surface Areas of Solids by a Gravimetric BET method

A CI MK2 Vacuum Microbalance has been incorporated into a purpose-designed apparatus for the determination of surface areas of solids by a gravimetric BET method, (Glasson 1956), using nitrogen gas as the absorbate at a temperature of 77K. The sample, in an aluminium open top container, is suspended form the balance by a fine Pyrex suspension rod 25 cm long. A Pyrex limb around the sample allows the sample to be cooled in a Dewar of liquid nitrogen, and the complete balance to be evacuated. Surface areas in the range <1 m² to >100 m² per g can be determined with a sample of 250 mg.

The apparatus can also be used to obtain the full absorption and desorption isotherms in the pressure range 0 to 1 atmosphere, and thus give an indication of the sample pore shapes and pore distribution. A second CI Microbalance has been set up in a similar apparatus but with the added facility to use carbon monoxide as the absorbate at 196K as well as nitrogen at 77K.

The samples studied included commercial catalysts, silts and muds obtained from estuaries and rivers, cokes and coals at various stages of oxidation and metal oxides and oxide layers.

Measuring Surface Tension using a Langmuir Trough

The Langmuir Blodgett technique of depositing thin films, one molecule thick, onto a solid is, in principle, very simple. The material to be deposited is spread onto the surface of a pool of water, and the solid is coated by the film as it passes through the water surface.

The material used to form the film should be amphipathic. When such molecules are floated on water, they 'stand' on the water surface. By applying sufficient material to the surface of the pool of water, a compact layer exactly one molecule thick may be formed. However, when adopting this procedure, films are often produced with small holes. By weighing the sample, a feedback mechanism may be employed to control the surface area of the water, and so maintain the correct concentration of molecules on the surface.