Surface area analysis

Physisorption system for surface area and pore size analysis
Example adsorption isotherm for a soil sample

Surface area and porosity are two critical physical properties when considering the characteristics of rocks and soils, particularly with regard to their engineering properties, industrial mineral use, water/hydrocarbon potential, gas storage and possible effects on soil quality and human health.

BGS offers two different approaches to determine the surface area of Earth Science materials.

Gas adsorption

This method derives the amount of adsorbed nitrogen (or other inert gas) on a solid surface at monolayer coverage from either a single-point analysis or a multipoint plot of adsorption isotherm data using the nitrogen/BET method, named after its inventors Brunauer, Emmett and Teller (Brunauer et al., 1938). From a knowledge of the projected cross-sectional area per molecule in the monolayer and the quantity of gas adsorbed, the surface area of the material may be calculated. By extending the process of gas adsorption so that gas is allowed to condense in material pore spaces, the fine pore structure of a material may be evaluated. As pressure is increased, the gas condenses in pores of increasing dimensions until saturation is reached when all pores are filled with liquid. Incremental reduction in the pressure of the adsorbed gas then evaporates the condensed gas. Comparison of the adsorption and desorption isotherms and the hysteresis between them using a range of different models (e.g. BJH method; Barrett, Joyner and Halenda, 1951) reveals information about the material's pore size distribution, pore volume, pore area and pore shape.

Polar liquid adsorption

Since inert gases do not penetrate between layers of expanding clay minerals in general, only the external surface area is determined using the BET method. To measure the total (internal and external) surface area of clay-bearing samples, a different technique is employed requiring the adsorption of polar molecules such as, for example, 2-ethoxyethanol (ethylene glycol monoethyl ether, EGME, Carter et al., 1965). As smectite clay minerals have a total surface area of about 800 m2/g (an order of magnitude greater than other clay minerals and typically two orders of magnitude greater than non-clay minerals), total surface area may be used as proxy for the smectite content of rocks and soils. Smectite-group clay minerals produce the greatest degree of shrink-swell behaviour in rocks and soils and therefore identifying and quantifying their presence is critical to engineering projects.

References

Barrett, E P, Joyner, L G and Halenda. 1951. The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms, Journal of the American Chemical Society, 73, 373—380.

Brunauer, S, Emmet, P H and Teller, E. 1938. Adsorption of gases in multimolecular Layers. Journal of the American Chemical Society, 60, 309—319.

Carter, D L, Heilmen, M D and Gonzalez, F L. 1965. Ethylene glycol monoethyl ether for determining surface area of silicate minerals. Soil Science, 100, 356—360.

Contact

Please contact Simon Kemp for further information