Steric stabilization vs electrostatic stabilization
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Steric stabilization by polymers adsorbed on inorganic particle surfaces is gaining more
and more attention in both industry and academia because it plays an important role in
stabilizing colloidal dispersions. In this review, basic concepts and some related topics
about steric stabilization are introduced. In addition the application of steric stabilization
in dispersing ceramic particles in non-aqueous media is summarized. It is found that
functional groups, such as carboxyl, hydroxyl, amine, and ester groups in the molecular
polymer structure generally play an important role in steric stabilization. Polymers
containing carboxyl groups turn out to be the most effective steric stabilizers because
carboxyl groups are supposed to interact strongly with basic sites, often present on the
particle surface. On the other hand, the long chain hydrocarbons in the molecular
structures extend from the surface into nonaqueous solvent and act as good moieties in
nonaqueous media. The longer the hydrocarbon chains, the better the stabilization effect.
It is also shown in the literature that copolymers are usually more effective in steric
stabilization than homopolymers because copolymers consist of more than one type of
repeated unit. One type of repeated unit can act as anchor; the other type can act in the
moieties extending into a nonaqueous solution.
Steric stabilization in the modified emulsion precipitation method to prepare ceramic
nanoparticles is discussed at the end of this review. Poly(octadecyl methacrylate)
(PODMA) is used as the steric stabilizer in that method. It is suggested that the ceramic
nanoparticles are stabilized by a bilayer of polymer (PODMA) and surfactant (DiDAB).
The exact stabilization mechanism still needs further study.
Electrostatic stabilization:-
======================
An effective way to counterbalance the VDWL attraction between colloidal particles in
polar liquids is to provide the particles with Coulombic repulsion. In liquid dispersion
media, ionic groups can adsorb to the surface of a colloidal particle through different
mechanisms to form a charged layer. To maintain electroneutrality, an equal number of
counterions with the opposite charge will surround the colloidal particles and give rise to
overall charge-neutral double layers. In charge stabilization, it is the mutual repulsion of
these double layers surrounding particles that provides stability.
The thickness of the double layer depends, amongst others, on the ionic strength of the
dispersion medium. The ionic strength can be expressed as l =1/2 ∑ i z ^2i c^2i
I
2
2
1
, where z is the
charge number of ions, i, and c is the molar concentration of the ions. For 1:1
electrolytes, the ionic strength is proportional to the concentration. Here we will use the
concentration c to represent ionic strength. At low ionic strengths (electrolyte c=10-3 M),
the thickness of the double layer is about 5-10 nm, which is of the same order as the
VDWL attraction. This explains the observation of charge stabilization in dispersion
media of low ionic strength. The thickness of the double layer is reduced significantly
with increasing the ionic strength. At ionic strengths for electrolyte c>10-1M, the
thickness of the double layer is less than 1 nm. In that case, the range of double layer
electrostatic repulsion is usually insufficient to counterbalance the VDWL attraction.
This accounts for the fact that most charge-stabilized dispersions coagulate when
increasing the ionic strength of the dispersion medium [3]. Hence, one great disadvantage
of charge stabilization of particles is its great sensitivity to the ionic strength of the
dispersion medium. In addition it only works in polar liquid which can dissolve
electrolytes. However, due to the advantages in simplicity and cost price, charge
stabilization is still widely used in stabilizing dispersions in aqueous media.
Steric stabilization;-
=================
Steric stabilization of colloidal particles is achieved by attaching (grafting or
chemisorption) macromolecules to the surfaces of the particles (figure 1.1). The
stabilization due to the adsorbed layers on the dispersed particle is generally stabilization...
Steric stabilization by polymers adsorbed on inorganic particle surfaces is gaining more
and more attention in both industry and academia because it plays an important role in
stabilizing colloidal dispersions. In this review, basic concepts and some related topics
about steric stabilization are introduced. In addition the application of steric stabilization
in dispersing ceramic particles in non-aqueous media is summarized. It is found that
functional groups, such as carboxyl, hydroxyl, amine, and ester groups in the molecular
polymer structure generally play an important role in steric stabilization. Polymers
containing carboxyl groups turn out to be the most effective steric stabilizers because
carboxyl groups are supposed to interact strongly with basic sites, often present on the
particle surface. On the other hand, the long chain hydrocarbons in the molecular
structures extend from the surface into nonaqueous solvent and act as good moieties in
nonaqueous media. The longer the hydrocarbon chains, the better the stabilization effect.
It is also shown in the literature that copolymers are usually more effective in steric
stabilization than homopolymers because copolymers consist of more than one type of
repeated unit. One type of repeated unit can act as anchor; the other type can act in the
moieties extending into a nonaqueous solution.
Steric stabilization in the modified emulsion precipitation method to prepare ceramic
nanoparticles is discussed at the end of this review. Poly(octadecyl methacrylate)
(PODMA) is used as the steric stabilizer in that method. It is suggested that the ceramic
nanoparticles are stabilized by a bilayer of polymer (PODMA) and surfactant (DiDAB).
The exact stabilization mechanism still needs further study.
Electrostatic stabilization:-
======================
An effective way to counterbalance the VDWL attraction between colloidal particles in
polar liquids is to provide the particles with Coulombic repulsion. In liquid dispersion
media, ionic groups can adsorb to the surface of a colloidal particle through different
mechanisms to form a charged layer. To maintain electroneutrality, an equal number of
counterions with the opposite charge will surround the colloidal particles and give rise to
overall charge-neutral double layers. In charge stabilization, it is the mutual repulsion of
these double layers surrounding particles that provides stability.
The thickness of the double layer depends, amongst others, on the ionic strength of the
dispersion medium. The ionic strength can be expressed as l =1/2 ∑ i z ^2i c^2i
I
2
2
1
, where z is the
charge number of ions, i, and c is the molar concentration of the ions. For 1:1
electrolytes, the ionic strength is proportional to the concentration. Here we will use the
concentration c to represent ionic strength. At low ionic strengths (electrolyte c=10-3 M),
the thickness of the double layer is about 5-10 nm, which is of the same order as the
VDWL attraction. This explains the observation of charge stabilization in dispersion
media of low ionic strength. The thickness of the double layer is reduced significantly
with increasing the ionic strength. At ionic strengths for electrolyte c>10-1M, the
thickness of the double layer is less than 1 nm. In that case, the range of double layer
electrostatic repulsion is usually insufficient to counterbalance the VDWL attraction.
This accounts for the fact that most charge-stabilized dispersions coagulate when
increasing the ionic strength of the dispersion medium [3]. Hence, one great disadvantage
of charge stabilization of particles is its great sensitivity to the ionic strength of the
dispersion medium. In addition it only works in polar liquid which can dissolve
electrolytes. However, due to the advantages in simplicity and cost price, charge
stabilization is still widely used in stabilizing dispersions in aqueous media.
Steric stabilization;-
=================
Steric stabilization of colloidal particles is achieved by attaching (grafting or
chemisorption) macromolecules to the surfaces of the particles (figure 1.1). The
stabilization due to the adsorbed layers on the dispersed particle is generally stabilization...
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