5.2.1 The electrical double layer
Our discussion of electrostatic forces would not be complete without a
brief overview of the electrostatic double layer and its role in the interactions between surfaces in liquids at the nanoscale. The electrostatic
double layer is the term given to the diffuse layer of counterions in a
solution that are associated with a charged surface. As shown below, the
electrostatic double layer plays an important role in determining the
forces that operate between charged surfaces in liquids.
When a surface comes into contact with a liquid, it may become charged by
adsorbing ions from solution or releasing ions into solution. Many surfaces,
for instance, are pH labile, and at either high or low pH might become
positively or negatively charged. For example, a surface containing primary
amine groups becomes positively charged at pH < 10 as the amine groups
acquire an extra proton. Another common example of a surface-charging
mechanism is the binding of Ca
2+ ions by the zwitterionic headgroups of
many phospholipid bilayers, resulting in a positively charged surface.
In solution, we would expect the charges on a surface to be balanced by
the appropriate counterions that have been released from the surface
itself or drawn in from the surrounding solution. Indeed, this is the case
and the result is the formation of two regions of counterion charges to
neutralize the surface charge. The first region is a compact layer of
counterions that is closely bound to the charged surface. This compact
region of bound counterions is called the Stern or Helmholtz layer. It
should be noted that the counterions in the Stern layer are not necessarily
irreversibly bound to the surface and can often be exchanged with those
in the surrounding solution. The second region is a more diffuse and
extensive layer of counterions that is in rapid equilibrium with the surrounding solution. This region is referred to as the electrical double layer
or the diffuse electrical double layer and is the focus of our discussion.
These regions are shown schematically in Figure 5.12.
A common example of the electrical double layer can be found in milk,
which is a solid–liquid colloid. As a mixture of primarily nonpolar butterfat droplets in water, milk particles would seem to be expected to
aggregate and coagulate into butter due to hydrophobic interactions.
However, trace amounts of the highly polar phosphoprotein casein at
the water–milk interface result in an electrical double layer forming
around each milk particle. This double layer creates enough repulsion to
overcome the hydrophobic particle’s tendency to aggregate. Inks, paints,
ELECTROSTATIC FORCES BETWEEN SURFACES: THE ELECTRICAL DOUBLE LAYER 157
Our discussion of electrostatic forces would not be complete without a
brief overview of the electrostatic double layer and its role in the interactions between surfaces in liquids at the nanoscale. The electrostatic
double layer is the term given to the diffuse layer of counterions in a
solution that are associated with a charged surface. As shown below, the
electrostatic double layer plays an important role in determining the
forces that operate between charged surfaces in liquids.
When a surface comes into contact with a liquid, it may become charged by
adsorbing ions from solution or releasing ions into solution. Many surfaces,
for instance, are pH labile, and at either high or low pH might become
positively or negatively charged. For example, a surface containing primary
amine groups becomes positively charged at pH < 10 as the amine groups
acquire an extra proton. Another common example of a surface-charging
mechanism is the binding of Ca
2+ ions by the zwitterionic headgroups of
many phospholipid bilayers, resulting in a positively charged surface.
In solution, we would expect the charges on a surface to be balanced by
the appropriate counterions that have been released from the surface
itself or drawn in from the surrounding solution. Indeed, this is the case
and the result is the formation of two regions of counterion charges to
neutralize the surface charge. The first region is a compact layer of
counterions that is closely bound to the charged surface. This compact
region of bound counterions is called the Stern or Helmholtz layer. It
should be noted that the counterions in the Stern layer are not necessarily
irreversibly bound to the surface and can often be exchanged with those
in the surrounding solution. The second region is a more diffuse and
extensive layer of counterions that is in rapid equilibrium with the surrounding solution. This region is referred to as the electrical double layer
or the diffuse electrical double layer and is the focus of our discussion.
These regions are shown schematically in Figure 5.12.
A common example of the electrical double layer can be found in milk,
which is a solid–liquid colloid. As a mixture of primarily nonpolar butterfat droplets in water, milk particles would seem to be expected to
aggregate and coagulate into butter due to hydrophobic interactions.
However, trace amounts of the highly polar phosphoprotein casein at
the water–milk interface result in an electrical double layer forming
around each milk particle. This double layer creates enough repulsion to
overcome the hydrophobic particle’s tendency to aggregate. Inks, paints,
ELECTROSTATIC FORCES BETWEEN SURFACES: THE ELECTRICAL DOUBLE LAYER 157