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Physical chemistry of solid surfaces
Lecture 4
郭修伯
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
Table 2.1
Surface energy
• Origin– 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
Surface energy
• Definition– the energy required to create a unit area of
“new” surface
2
1
,,ab
PTn
NA
G
i
surface area half bond length
surface atomic density
number of broken bonds
when brake into two pieces
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
– surface restructuring• through combining surface dangling bonds into
strained new chemical bonds
Fig 2.5
– 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
– composition segregation or impurity enrichment on the surface
• enrichment of surfactants on the surface of a liquid
• through solid-state diffusion
Fig 2.7
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
Sintering & Ostwald ripening
Fig 2.9
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
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
Fig 2.14
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
Electrostatic repulsion stabilization
Fig 2.18
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
Polymeric stabilization
Fig 2.21