230 Surface Chemistry
A study of physical and chemical processes at the boundary between phases that is used to understand catalysis, lubrication, corrosion, and membrane function.
The high-energy surface layer. Where phases meet, a thin transition layer only a few molecules thick separates them. Molecules inside a bulk phase are pulled evenly by neighbors; molecules at the boundary have missing neighbors and higher energy. This principle is used to explain why surfaces have properties distinct from the bulk.
Surface tension. A liquid surface shrinks to cut that high-energy area. Surface tension is the extra Gibbs energy per unit area of the surface. This principle is used to compute the work of stretching a liquid surface.
The surface-tension definition is
\[ \gamma = \Bigl(\dfrac{\partial G}{\partial A}\Bigr)_{T,p} \]
where
- \(\gamma\) is the surface tension.
- \(G\) is the Gibbs energy.
- \(A\) is the surface area.
- \(T\) is the absolute temperature.
- \(p\) is the pressure.
Adsorption. Gas or liquid molecules can cling to a solid boundary instead of soaking into its interior. Adsorption is that accumulation at the surface. This principle is used to describe coverage of a catalyst or adsorbent.
Physisorption and chemisorption. Sticking can be weak through long-range attractions or strong through chemical bonds to surface atoms. This principle is used to distinguish the two standard classes of adsorption.
The Langmuir isotherm. At equilibrium the fraction of occupied surface sites rises with pressure and saturates when the sites are full. A site is a place on the surface that can hold one adsorbed molecule. This principle is used to fit adsorption data and to estimate the number of surface sites.
The Langmuir isotherm is
\[ \theta = \dfrac{Kp}{1+Kp} \]
where
- \(\theta\) is the fraction of occupied sites.
- \(K\) is the adsorption equilibrium constant.
- \(p\) is the pressure of the adsorbing gas.
Heterogeneous catalysis. Rough spots and steps on real solids trap molecules and break bonds. This principle is used to explain why surfaces often catalyze reactions.
Note: Weak sticking is called physisorption. Strong chemical sticking is called chemisorption.