文档介绍：10 Electrostatic and ic Phenomena
Our discussion so far has avoided the cases involving electrolytic solutions. Non-
electrolytes have no effect on sols and suspensions. In reality, however, many dis-
perse systems which we encounter contain electrolytes and the stability of the dis-
persion is due to electrostatic effects. An important cause of dispersion instability
is aggregation and ulation. Clay particles (usually negatively charged) and
silt carried in suspensions by rivers are electrically charged to repel each other,
but coagulate upon encountering salt sea and form huge deltas. Electrostatic stabi-
lization is also responsible for the long shelf-life of certain latex paints. Electro-
static forces governs the behavior of biological systems.
When charged particles are under an electric field, they move in a dispersion
medium, which is usually an electrolyte. When there is a relative motion between
the particles and the medium, the phenomena observed include electrophoresis,
electroosmosis, streaming potential, sedimentation potential, etc. The technique of
electrophoresis is widely used in measuring the charge on a particle. The electric
conductivity of a suspension reflects ic processes.
The Electric Stress Tensor
in pressible Fluid
In order to understand the interaction forces among particles in an electrolyte so-
lution, we must discuss the electric stress tensor. Let us consider an electrostatic
field, E, arising from charges located on the surfaces of conductors embedded in
an isotropic dielectric or an electrolyte solution. We assume that the dielectric is
continuous outside the conductors and the dielectric constant, e, depends on posi-
tion. The total electrostatic energy of the system is then given by a well-known
expression:
Z
1
U e eE2dV ;
E 0 ; inside conductors
10:1
2 0
Now suppose that the dielectric is, at every point, subject to an arbitrary, infini-
tesimal displacement s, which gives rise to