Texture of divided materials Specific surface area of powders or nanoporous materials

The aim of this article is to use gas adsorption to determine one of the main characteristics of porous or powdery materials: the extent of their surface area. The scale considered ranges from 1 nm to 1 mm for particles and from 0.1 to 50 nm for pores.

Divided solids" (either powdery or porous) are ubiquitous in nature and in industrial processes.

In nature, the surface area and porosity of a soil largely determine its capacity to retain not only water, but also fertilizing, weed-killing, phytosanitary or even toxic substances (heavy metals, possibly radioactive). Sand, in its most divided form, is capable of remaining suspended in the air, where it forms an aerosol of dust that can be transported over thousands of kilometers, before being precipitated to the ground by rainfall: this is how pink or yellow dust from the Sahara ends up on cars in the south of France the day after it rains.

In industrial processes, the same phenomenon, once mastered, controlled and, above all, channeled, is today the basis for pneumatic conveying of powders: food flours, cements, sulfur, talcum powder, etc.... Another special feature of very fine powders is their ability to "sinter", i.e. to weld particles together at a temperature well below the material's melting point. This property is the basis of the pottery and ceramics industries, but it also applies to metal powders: this is how the Etruscans made gold statuettes in furnaces that were incapable of reaching the melting temperature of gold (1,063°C), thanks to the mass setting of compacted gold powder.

However, it is the adsorbent properties of porous materials that have given rise to the most diverse applications. Man has long exploited the adsorbent properties of coal or volcanic porous stones for medical purposes (to draw venom from a wound), or the porosity of pottery to enable cooling by evaporation of the water passing through it. Today, we're inventing new adsorbents that can be fine-tuned (in terms of granulometry, pore size and surface chemical functions) to suit the intended applications.

It is easy to understand that the efficiency of these adsorbents is not unrelated to the extent of their surface area, hence the interest in reliable measurement of this characteristic. To this end, this article successively examines the textural complexity of divided materials and the way in which the adsorption of a gas by a solid can provide access to its surface area using the most common methods. The quality and exact significance of the results obtained are also discussed.