文档介绍：8 Dispersion Forces
The Work of Adhesion and Cohesion
and Dispersion Forces
The Work of Adhesion and Cohesion
The term, soap, is applied to the sodium or potassium salts of long-chain fatty
acid. In general, these are examples of amphiphilic or amphipathic molecules
which consist of two well-defined parts: one which is oil-soluble (lyophilic, oleo-
philic, or hydrophobic) and another which is water-soluble (hydrophilic). The hy-
drophobic part is non-polar and usually consists of aliphatic or aromatic hydrocar-
bons. The hydrophilic part consisting of halogen groups is polar and interact
strongly with water.
The hydrophilic or hydrophobic properties are related with the work of adhe-
sion and cohesion. The work of adhesion per unit area is defined by the work
done on the system when two condensed phases (1 and 2), forming an interface
of unit area, are separated in vacuum reversibly to form unit areas of each of the
1 and 2 surfaces.
adh
W12 c1 c2
c12 8:1
where c's are surface tensions. The work of adhesion is the Helmholtz free energy
of separation. The work of cohesion, on the other hand, is the work of adhesion
when a substance, 1, is separated into two by creating two free surfaces of the
same substance, 1, in vacuum.
coh
W11 2c1 8:2
Note that since all substances are attracted each other in vacuum Wadh and W coh
are positive.
The Work of Adhesion and Cohesion and Dispersion Forces 143
van der Waals' Forces
The intermolecular forces are responsible for the surface tension. The important
interaction energies among nonpolar electrically neutral particles are van der
Waals' or dispersion energies, which are attractive and pairwise additive. Quan-
6
tum-mechanics gives an expression, ±b12/r12, for the attractive energy between
two molecules, 1 and 2, where b12 is a constant and r12 is the distance between
the two molecules. (The center of the molecules is suitably defined in determining
r12.)