Adsorption isotherm shapes.

The model and its limits.

An isotherm records uptake as pressure changes at fixed temperature. Here p₀ is saturation vapor pressure at that temperature. Relative pressure is appropriate for a subcritical adsorptive, not every gas-storage experiment.

These curves are illustrative drawings generated from shape functions, not measured data or adsorption-model fits. Uptake is in arbitrary units. Heights do not compare capacities; the cursor is a reading aid. The endpoint at relative pressure 1 is illustrative, not a prediction at bulk saturation.

Reading the families

The descriptions above summarize six broad IUPAC families. Types I(a) and I(b) distinguish microporous shapes. IV(a) has hysteresis; IV(b) can be reversible. The plotted IV illustrates IV(a). Loop branches represent different adsorption and desorption paths.

Classification describes morphology. Choosing a fitting equation such as Langmuir or BET requires appropriate assumptions and a valid pressure range. Shape alone does not determine pore size or identify a material.

Why this matters for MOFs

Flexible MOFs can change structure as gas enters or leaves. Gate opening and structural transitions can create steps and hysteresis. A loop is not sufficient evidence for capillary condensation: structural measurements and experimental conditions matter. This explorer does not model framework flexibility or infer a mechanism from an unknown curve.

For measured high-pressure methane curves, explore methane storage.

Literature

• Thommes et al. (2015), Physisorption of gases — IUPAC Technical Report. Classification, subtypes, and interpretation limits.
• High-throughput gas separation by flexible metal–organic frameworks with fast gating and thermal management capabilities, Nature Communications (2020). Experimental gate-opening behavior.
• Temperature dependence of adsorption hysteresis in flexible metal organic frameworks, Communications Chemistry (2020). Framework transitions, temperature, and hysteresis.