What is the difference between physisorption and chemisorption?

Physisorption involves weak van der Waals attraction, is easily reversible, and can build up multiple layers, whereas chemisorption forms actual chemical bonds with the surface, is much stronger, usually limited to a monolayer, and is central to catalysis.

Why does surface tension make small droplets spherical?

Surface tension is the energy cost of interface area, so a system minimizes its free energy by minimizing surface area; for a given volume the sphere has the least surface area, which is why free droplets adopt that shape.

Adsorption and Surface Thermodynamics

Surfaces store excess free energy as surface tension, and molecules accumulate at interfaces through adsorption, described by isotherms that relate surface coverage to pressure or concentration.

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Definition

Adsorption is the accumulation of molecules at an interface, and surface thermodynamics is the thermodynamic description of interfaces through surface tension, surface free energy, and the relations governing curved and adsorbing surfaces.

Scope

This topic covers the thermodynamics of interfaces and the phenomenon of adsorption: surface tension and surface free energy, the Young-Laplace and Kelvin equations for curved interfaces, and the Gibbs adsorption isotherm relating surface excess to changes in surface tension. It develops adsorption isotherms, including the Langmuir model for monolayer adsorption and the BET extension to multilayers, the distinction between physisorption and chemisorption, and the measurement of surface area. The catalytic role of adsorption is linked to kinetics, and colloidal and self-assembly phenomena are treated in sibling topics.

Core questions

How does surface tension arise as the free energy cost of creating interface?
How does the Gibbs adsorption isotherm relate surface excess to surface tension?
How do the Langmuir and BET isotherms describe monolayer and multilayer adsorption?
What distinguishes physisorption from chemisorption?

Key concepts

Surface tension and surface free energy
Young-Laplace and Kelvin equations
Gibbs adsorption isotherm
Langmuir and BET isotherms
Physisorption versus chemisorption

Key theories

Langmuir adsorption isotherm: Modelling a surface as a fixed array of equivalent, non-interacting sites gives a coverage that rises with pressure or concentration and saturates at a complete monolayer, the reference model for adsorption.
Gibbs adsorption isotherm: The amount of a solute that accumulates at an interface is related thermodynamically to how the solute lowers the surface tension, explaining why surface-active agents concentrate strongly at interfaces.

Clinical relevance

Adsorption and surface thermodynamics govern heterogeneous catalysis, gas storage and separation by porous materials, the action of detergents and surfactants, chromatographic separation, surface area measurement of catalysts and pharmaceuticals, and wetting and capillarity in soils and biological tissues.

History

Gibbs founded the thermodynamics of surfaces in the 1870s; Langmuir's study of monolayers and adsorption in the 1910s, recognized with the 1932 Nobel Prize, and the Brunauer-Emmett-Teller multilayer theory of 1938 established the quantitative analysis of adsorption and surface area.

Key figures

Irving Langmuir
J. Willard Gibbs
Stephen Brunauer

Seminal works

langmuir1916
adamson1997

Frequently asked questions

What is the difference between physisorption and chemisorption?: Physisorption involves weak van der Waals attraction, is easily reversible, and can build up multiple layers, whereas chemisorption forms actual chemical bonds with the surface, is much stronger, usually limited to a monolayer, and is central to catalysis.
Why does surface tension make small droplets spherical?: Surface tension is the energy cost of interface area, so a system minimizes its free energy by minimizing surface area; for a given volume the sphere has the least surface area, which is why free droplets adopt that shape.