# Inner surface

The inner surface of porous or granular solids is a dimensional parameter that is used in various scientific and technical disciplines. It encompasses all of the surfaces it contains , including those that are between grains and within pores. The actual measured variable of the inner surface is the specific surface .

In contrast to this, the outer surface is the surface that can be seen directly from the outside, i.e. that which would be obtained when the material system was packaged.

## Basics

All porous ( sponge or foam-like ) materials as well as heaps (such as powder and debris ) are pervaded by numerous cavities. Since all chemical reactions essentially depend on how large their area of ​​attack is compared to the volume, the inner surface is of great importance. For example, fine iron filings are highly flammable ( pyrophoric ), while larger pieces of iron are not. The weathering stability of a rock is also largely determined by its size and the number of cracks. There are similar effects with numerous purely physical phenomena, such as capillary action and moisture storage , thermal insulation , light reflection and others.

Technical adsorbents, for example, are often highly porous solids with a heavily fissured inner surface. They have internal surfaces that can be more than a thousand m 2 per gram.

## specific surface

The specific surface area (of English. Surface ) is a surface measurement determined. ${\ displaystyle S}$

### Mass related

The mass -related specific surface

${\ displaystyle S_ {m} = {\ frac {A} {m}}}$

indicates which surface (in m 2 ) a kilogram of a material has ( unit ). ${\ displaystyle \ mathrm {\ frac {m ^ {2}} {kg}}}$

### Volume related

The volume -related specific surface

${\ displaystyle S_ {V} = {\ frac {A} {V}}}$

indicates the surface area A (in m 2 ) of a cubic meter of a material (unit ). ${\ displaystyle \ mathrm {\ frac {m ^ {2}} {m ^ {3}}} = \ mathrm {\ frac {1} {m}}}$

The smallest specific surface (for a given volume) has the sphere:

${\ displaystyle S_ {V_ {K}} = 6 / d.}$