Transcript Colloids PHT 312 lec..


When colloidal solutions have been
observed through ultra microscope, the
colloidal particles are seen in constant
and rapid zigzag motion called Brownian
movement. Sir Robert Brown first
observed the phenomenon in 1827.
Suspensions and true solutions do not
exhibit Brownian movement.
(area/unit time)
 In osmosis, the solvent water moves
through a semipermeable membrane

Water flows from the side with the lower
solute concentration into the side with
the higher solute concentration
 Eventually, the concentrations of the two
solutions become equal.

Equal to the pressure that would prevent
the flow of additional water into the more
concentrated solution

Increases as the number of dissolved
particles increase
When a strong beam of light is passed
through a colloidal solution, the path of
the light becomes visible when viewed
from a direction at right angle to that of
the incident light. This occurs because the
colloidal particles absorb light energy
and then scatter it in all directions. The
phenomenon of scattering of light by sol
particles to form illuminated beam or
cone is called Tyndall effect or Tyndall
beam or Tyndall cone.
< 1 nm
solutions
> 100 nm
colloids
suspensions
Absorption of light
Passage of light
Scattering in beam
Scattering in
all directions
C) Electrical properties
Colloidal particles of a sol either carry positive or
negative charge. Sols in, which the colloidal particles
carry positive charge are called positive sols. When
colloidal particles carry negative charge, the sols are
called negative sols. The existence of charge on the
colloidal particles can be demonstrated by a
phenomenon called electrophoresis where the
colloidal particles, when placed in an electric field,
move towards either cathode or anode depending
upon the charge on them. Sols of basic dyestuffs, ferric
hydroxide, aluminium hydroxide etc., are some
common examples of positive sols. Colloidal solutions
of gums, starch, soap solution, metals (Ag, Cu, Au, Pt
etc.), metal sulphides, and some acid dyestuffs are the
examples of negative sols
- If all the particles have a large negative or positive zeta potential they will
Repel each other and there is dispersion stability.
-If the particles have low zeta potential values then there is no force to prevent the
Particles coming together and there is dispersion instability.
Zeta potential is not measurable directly but it can be calculated using theoritical
models and an experimently-determined electrophoretic mobility or dynamic
electrophoretic mobility.
Involves the movement of a charged particle through a liquid under the
influence of an applied potential difference.
An electrophoresis cell, fitted with two electrodes, contains the dispersion.
When a potential is applied across the electrodes, the particles migrate to the
oppositely charged electrode.
Electrophoresis: The movement of a charged particle relative to the liquid it
suspended in under the influence of an applied electric field
This technique finds application in
Measurements of zeta potentials of model systems (like polystyrene latex
dispersion),
To test colloidal stability theory,
To asses the stability of coarse dispersion,
In identification of charge groups
The particles move with a characteristic velocity which is dependent on the
strength of the electric field (measured by the instrument), the dielectric
constant and the viscosity of the medium.
The velocity of a particle in a unit electric field is referred to as its
electrophoretic mobility