Type I isotherm is reversible and related to microporousmaterials (I(a) is for micropores of width < 1 nm, I(b) applies to a widerrange of widths).

The limiting value approached relies on the accessiblemicropore volume. Type II is still reversibile, but is given for nonporous ormacroporous solids. The sharpness of the knee of the isotherm (Point B) is anindication of the completion of a monolayer.

Type III still applies tononporous or macroporous materials but Point B is absent, thus monolayerformation is not identifiable. Furthermore, the adsorbed amount stays finite atp/p0=1, as opposed to Type II isotherm. Type IV and V isotherm are relatedto mesoporous adsorbents, where pore condensation follows monolayer ormultilayer adsorption. The plateu is a typical characteristic of such isotherm.

The difference between Types IV(a) and IV (b) lies in the hysteresis, which ispresent when pore width overcomes a critical value. Type VI, lastly, relates toadsorption on highly uniform nonporous surfaces. 

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