Isothermal adsorption equilibrium adsorption isotherm - Database & Sql Blog Articles

**Isothermal Adsorption Equilibrium – Adsorption Isotherm** At a constant temperature, a specific amount of gas can be adsorbed on a solid surface depending on the pressure of the adsorbate. By measuring the adsorption amount at various relative pressures, an adsorption isotherm can be obtained. This isothermal data is fundamental for understanding adsorption phenomena and characterizing the surface and pore structure of solids. It helps determine key properties such as specific surface area and pore size distribution. There are six main types of adsorption isotherms (see Figure 1). The first five have been traditionally classified, while the sixth was introduced more recently. The shape of the isotherm directly reflects the pore size and structure of the material. **Type I Isotherm:** This type corresponds to the Langmuir isotherm, representing a monolayer adsorption process. It typically occurs in materials with micropores where the adsorption is governed by the pore volume rather than the external surface area. The isotherm shows a sharp rise followed by a plateau when pores are completely filled with liquid. Examples include microporous silica gel, zeolites, and carbon molecular sieves. Near saturation vapor pressure, some particles may exhibit behavior similar to larger pores, causing a rapid increase in adsorption. **Type II Isotherm:** Also known as the S-shaped isotherm, this represents multilayer adsorption on non-porous or macroporous solids. At low P/P ratios, the curve rises steeply, indicating monolayer coverage. As pressure increases, additional layers form, and at high pressures, the number of layers becomes infinite. This type is common in materials with pores larger than 20 nm. The curve's shape reflects the strength of interaction between the adsorbate and the solid. **Type III Isotherm:** This is characterized by a convex downward curve without an inflection point B. It typically occurs when the adsorbate–adsorbent interaction is weaker than the adsorbate–adsorbate interaction. For example, water vapor adsorbed on graphite or hydrophobic metal oxides. Adsorption increases gradually with pressure, showing pore filling behavior. Some systems, like nitrogen adsorption on polymers, display smooth curves without a clear inflection point. **Type IV Isotherm:** This is the most common type for mesoporous solids. In the low P/P region, it resembles Type II, with a convex upward curve. As pressure increases, capillary condensation leads to a sharp rise in adsorption. When all pores are filled, the curve flattens, indicating saturation. Hysteresis loops may appear due to differences between adsorption and desorption processes, reflecting pore structure and shape. This is widely used in industrial catalysts and other mesoporous materials. **Physical Adsorption Mechanism of Type IV Isotherm:** The first stage involves monolayer adsorption, with the inflection point B marking the completion of the first layer. The second stage includes multilayer adsorption and capillary condensation. The hysteresis loop indicates the range of pore sizes, with the start and end points corresponding to the smallest and largest pores, respectively. Once fully filled, the curve plateaus, showing no further increase in adsorption. **Type V Isotherm:** Less common, this type exhibits behavior between Type III and Type IV. It often shows weak interactions and may include capillary condensation at higher pressures. Hysteresis loops can also occur in certain cases. **Type VI Isotherm:** Known as the stepped isotherm, it represents harmonic multilayer adsorption on a uniform surface, such as ruthenium on clean metal surfaces. However, real-world surfaces are usually heterogeneous, making this type rare in practice. Adsorption isotherms provide critical insights into the nature of both the adsorbent and adsorbate. They help calculate surface area, pore size, and distribution. For instance, Type II and IV isotherms are commonly used for determining surface area and mesopore distribution, respectively. Understanding these patterns allows researchers to design better materials for applications ranging from catalysis to environmental remediation.

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