Effect of added electrolytes on ζ
The addition of electrolytes to a
colloidal system reduces zeta potential or even reverses it. The electrolytes
supply additional anions and cations so that the density of ions of opposite
charge to the charge on the wall is increased. Thus the ionic atmosphere is
compressed near the surface and the potential gradient in the fixed layer is
greatly increased and the potential in the diffuse layer (as well as the zeta
potential) is decreased. As the concentration of the counterions in the liquid
increases, the ζ is continuously reduced tending to zero, and may even
be reduced. The higher the valence of the electrolyte, the higher the reduction
of potential. Thus trivalent ions are more effective than divalent ions, which
are in turn more effective than monovalent ions.
The presence and magnitude or
absence of charge on colloidal particles is an important factor in stability of
colloidal systems. However, lyophilic and lyophobic colloids differ in their
stability requirements and hence each is approached separately.
Stability of lyophobic
colloids
A lyophobic sol is
thermodynamically unstable. It is produced from insoluble substances which have
been subdivided as polymolecular aggregates throughout the liquid phase. The
enormous increase in surface area results in an increase in surface energy.
Particles therefore have a tendency to aggregate and precipitate in order to
reduce this energy. The tendency to aggregate is offset by the repulsive forces
between particles due to the charge at the surface of the particles. Not
however that there exist Van der Waal forces of attraction between particles.
Thus there are two opposing forces between any two colloidal particles,
attraction and repulsion.
Forces acting between two
particles in colloidal sol
Consider two particles in close
proximity. They attract each other by Van der Waals forces of attraction
which vary inversely as x7 ( where x = distance between them). The
particles also repel each other due to electrostatic forces which vary
inversely as x2. The resultant force between the particles as a function of
their distance apart was explained independently by Derjaguin and Landau in the
USSR and by Verwey and
Overbeek in the Netherlands
in the early 1940’s. This is now called DLVO theory, and is summarized in the
following figure:
Consider two particles
approaching each other. Curve A represents the Van der Waals attractive energy (which decrease with
second power of the inter-particle distance). Curve R represents the
electrostatic repulsive forces (which decrease exponentially with the seventh
power of the inter-particle distance). The combination of these two opposing forces,
attraction dominates at large distance and shallow trough is exhibited at x3.
This so-called secondary minimum is significant in controlled flocculation
in preparation of suspensions.. As the particles still approach each other,
repulsion begin to dominate at intermediate distance, to reach maximum at x2,
so called primary maximum. Repulsion occurs because when particles
approach each other the two atmospheres of counterions surrounding them begin
to interpenetrate or overlap at point o. Repulsion occurs because of
the work involved in distorting the diffuse double layer and in pushing water
and counterions aside, which increase as the particles approach further to
reach maximum at point x2. At this point the potential energy
barrier exceeds the kinetic energy of the approaching particles, and the
particles will not come close to each other, and the dispersion shall be
stable. But if the kinetic energy exceeds the potential energy, at x2
or beyond, the particles continue to approach each other past this point, where
van der Waals forces of attraction become more important than the repulsive
forces. The net potential energy decrease to zero and then becomes
negative as the particles continue to approach each other, until when they
touch at point a (x1) otherwise known as primary minimum.
This corresponds to a very stable situation in which particles adhere, and are
attached permanently (irreversible caking in suspensions). In other words, the
particles agglomerate or the sol is said to coagulate. This situation is
irreversible because no kinetic energy can be applied to overcome the potential
energy. In practice the particles cannot approach further than this point, but
theoretically, if the particles were to approach further than the touching
distance, they will meet with very rapid rise in potential energy, because they
will be interpenetrating each other, causing the atomic orbitals to overlap and
face repulsive Bonn
repulsive forces.
Addition of Electrolytes decrease
the height of the primary maxima and the range over which the
electrostatic repulsion is effective, and hence lead to coagulation. The lowest
concentration at which an electrolyte coagulates the sol is called coagulation
value, Cv. It does not depend on valence of the anion, but
depends on the valence of the counterions, decreasing by one or two orders of
magnitude for each increase in heir valencies. This is the Schulze-Hardy
rule.
Effects of added solvents:
The addition of water
miscible solvents e.g. alcohol, glycerin, propylene glycol or polyethylene
glycol (to aqueous dispersions) lowers the dielectric constant ε of the
medium. This reduces the thickness of the double layer and therefore the range
over which electrostatic repulsion is effective, and lowers the size of
potential energy barrier. Thus addition solvents to aqueous dispersion lead to
coagulation. At concentration to low to cause coagulation, solvents make
dispersions more sensitive to coagulation by added electrolytes, i.e. they
lower the Cv
Stabilization by adsorbed
surfactants:
Recall that surfactant molecules
are amphiphiles having a polar and
non-polar parts. When placed in a medium of dispersed system, they accumulate
at interfaces and orient themselves in such a way that the hydrocarbon
(non-polar) part is in contact with non-polar particles or oil droplets while
the polar group is oriented towards the polar (usually water) phase. This is
known as oriented physical absorption. The adsorption of ionic
surfactants increases the charge density and the zeta potential ζ of disperse
particles. This in turn increases electrostatic repulsion among non-polar
organic particles and hence stabilization of the system. This is known as charge
stabilization.
Water-soluble non-ionic
surfactants are usually hydrated making the particle surface where they are
adsorbed to be surrounded by a thin layer of water. This hydrophilic shell
forms a steric barrier which prevents close contact between particles and hence
coagulation. This known as steric stabilization. Moreover, non-ionic surfactants
reduce the sensitivity of hydrophobic sols towards coagulation by salts, i.e.
they increase the coagulation value Cv.
Surfactants added to a
flocculated sol tend to pry apart the flocs by wedging themselves between the
particles and their area of contact. This allows in more surfactant molecules
and hence breaks-up the flocs. The system becomes deflocculated or undergoes peptization.
Peptization is defined as
formation of colloidal dispersion by action of solvent i.e. breaking up of
large particles into colloidal range. Peptization can be spontaneous e.g.
albumin and water form a “solution” due to extensive solvation. But some action
may be needed to bring about peptization. E.g. precipitated substances may be
brought into colloidal dispersions either by providing a peptizing agent usually
a surfactant, or by removing a flocculating agent similar to
deflocculation). When activated charcoal is added to water with stirring,
grains are broken up incompletely. The suspension is no good. Addition of 0.1%
sodium laurylsulphate or octoxynol disintegrates the particles to fine ones and
the dispersion changes color from gray to deep black. Alumium hydroxide Al(OH)3
in the presence of aluminium chloride (AlCl3) is a poor colloidal
dispersion. Washing with water removes AlCl3 and a good colloidal
dispersion forms.
Ophthalmic and parenteral
suspensions should be deflocculated to prevent irritation and capillary
blockage respectively. But because deflocculated systems tend to cake, a
process of controlled flocculation is used in preparing such suspensions.
Stabilization by adsorbed
polymers;
Water-soluble polymers act like
surfactants and are adsorbed at interface. The polar part attracts and encloses
water of hydration forming a sheath that surrounds the particles. This layer is
part of the particles and prevents the particles from touching each other to
coagulate. This kind of steric stabilization is said have protective
action. The sol thus protected has resistance to coagulation by salts. The
water-soluble polymers whose adsorption stabilizes hydrophilic sols
against coagulation are called protective colloids, e.g. gelatin, serum
albumin, povidone and dextran. The stabilizing efficiency of a protective
colloid is determined by its gold number. (define the gold number
of a polymer)
Some protective colloids at
concentrations below those at which they exert a protective action may cause sensitization,
i.e. a decrease in the stability of hydrophobic sols.
