Hysteresis
Definition and meaning of Hysteresis in chemistry.
Hysteresis is the lag between a change in a driving variable, such as pressure or temperature, and a system's response, so that the path traced going forward differs from the path traced in reverse. As a result, the system's state at a given value of the variable depends on its prior history, not on that value alone.
In more detail
Hysteresis arises when the forward and reverse processes proceed through different metastable states or mechanisms rather than retracing the same equilibrium pathway. A classic case is capillary condensation in mesoporous solids: pores fill with liquid at a higher relative pressure than the pressure at which they empty, because filling and emptying involve different meniscus curvatures. The area enclosed by the resulting loop corresponds to energy dissipated (irreversibility) rather than stored reversibly. IUPAC classifies these adsorption hysteresis loops into types (H1 through H5) based on pore shape and connectivity.
Key facts
| Field | Physical Chemistry |
|---|---|
| Common context | Gas adsorption-desorption isotherms in porous materials |
| IUPAC classification | Hysteresis loop types H1-H5 (physisorption isotherms) |
| Underlying cause | Forward and reverse processes follow different metastable pathways |
Nitrogen gas adsorption on mesoporous silica (pore diameters 2 to 50 nm) at 77 K shows a hysteresis loop between roughly p/p0 = 0.4 and 0.9: the adsorption branch (pore filling) lies below the desorption branch (pore emptying) on a plot of adsorbed volume versus relative pressure.
Frequently asked questions
Why does adsorption hysteresis occur in porous materials?
Capillary condensation (pore filling) and capillary evaporation (pore emptying) occur via different meniscus geometries and mechanisms, so the two isotherm branches do not coincide. This is typical for mesopores (2-50 nm) at temperatures below the adsorbate's critical point.
Is hysteresis unique to adsorption?
No. It also appears in other chemistry-adjacent phenomena, such as thermal hysteresis in phase transitions (for example, supercooling versus melting point in differential scanning calorimetry) and in magnetic materials studied alongside chemical systems.