1. Physical chemistry of solid surfaces
Lecture 4
郭修伯

2. Surface A large fraction of surface atoms per unit volume
1 cm3 cube of iron -> surface atom 10-5%
1000 nm3 cube of iron -> surface atom 10%
Fig 2.1

3. Table 2.1

4. Surface energy Origin Huge surface energy for nanomaterials
Atoms or molecules on a solid surface posses fewer nearest neighbors or coordination numbers, thus have unsatisfied bonds exposed to the surface
Huge surface energy for nanomaterials
Thermodynamically unstable/metastable
tend to growth to reduce the surface energy

5. Surface energy Definition
the energy required to create a unit area of “new” surface
number of broken bonds
surface atomic density
when brake into two pieces
surface area
half bond length

6. Surface energy
For a given surface with a fixed surface area, the surface energy can be reduced through
surface relaxation
the surface atoms or ions shift inwardly
Fig 2.4

7. surface restructuring
through combining surface dangling bonds into strained new chemical bonds
Fig 2.5

8. surface adsorption
through chemical or physical adsorption of terminal chemical species onto the surface by forming chemical bonds or weak attraction forces such as electrostatic or van der Waals forces
Fig 2.6
chemical adsorption

9. composition segregation or impurity enrichment on the surface
enrichment of surfactants on the surface of a liquid
through solid-state diffusion
Fig 2.7

10. Reduction of overall surface energy at the overall system level
Combining individual nanostructure together to form large structures so as to reduce the overall surface area
sintering: high temp (~70% melting pt.)
Ostwald ripening: wide range temp + solvent (large grow and small eliminate)
agglomeration of individual nanostructures without altering the individual nanostructures

11. Sintering & Ostwald ripening
Fig 2.9

12. Electrostatic stabilization
a solid emerges in a polar solvent or an electrolyte solution
surface charge develops by
preferential adsorption of ions
dissociation of surface charged species
isomorphic substitution of ions
accumulation or depletion of electrons at the surface
physical adsorption of charged species onto the surface

13. Surface charge distribution
The distributions of ions and counter ions are controlled by
Coulomic force or electrostatic force
Entropic force or dispersion
Brownian motion
Inhomogenous distribution
double layer structure
separated by the Helmholtz plane

14. Fig 2.14

15. Van der Waals attraction potential
The sum of the molecular interaction for all pairs of molecules
weak force and becomes significant only at a very short distance
agglomeration of nanoparticles: the combination of van der Waals force and Brownian motion
Prevent agglomeration: electrostatic repulsion and steric exclusion

16. Electrostatic repulsion stabilization
Fig 2.18

17. Steric exclusion stabilization
Also “polymeric stabilization”
Widely used in stabilization of colloidal dispersions
thermodynamic stabilization: particles are always redispersible
high concentration can be accommodated
not electrolyte sensitive
suitable to multiple phase systems

18. Polymeric stabilization
Fig 2.21