reversal in surface charge compared to the film prior to deposition. As
happened for the previous layer, the charged surface is balanced by
positive counterions from the polyelectrolyte solution. An assembly with
one polycationic and one polyanionic layer comprises a single bilayer.
The presence of counterions at the charged surface can be better
explained through the use of image forces. Image forces result from the
differences in the dielectric constants of water and the film interface.
Stronger dielectric environments polarize to a greater extent in response to
an applied electric field, providing more effective screening of that electric
field than a weaker dielectric environment. Water has a very high dielectric
constant (~80), while organic polymers typically have relatively low
dielectric constants (2–10). The difference in the dielectric environment
causes a repulsion of water charges from the surface, allowing a polyelectrolyte counterion to bind at the surface instead. These counterions
will subsequently be displaced by a polyelectrolyte with the same charge
opposite to the surface—a process driven by gains in entropy because
more salt ions are displaced from the surface than the number of polyelectrolyte molecules that adsorb.
To maintain the overall neutral charge of the film, charge compensation—
either intrinsic or extrinsic—must be present. Extrinsic compensation
refers to polymer charges that are balanced by extrinsic materials, such as
the salt counterions. It has been found that there is actually very little salt
concentration observed within the film. Instead, there is a 1:1 stoichiometric ratio of polycation to polyanion charge. As a result, the overall
charge balance within the multilayers must be attributed primarily to
intrinsic compensation. However, some extrinsic compensation has been
found to occur at the substrate surface, where charge overcompensation is
neutralized by salt counterions.
Within the bulk of the film, we might expect clearly defined layers of
alternating charge. This is not what occurs. Upon adsorption the polyelectrolyte does not simply “lie” on the surface. It has been commonly
observed that after adsorption, the outer polyelectrolyte then diffuses
slightly into the inner polyelectrolyte layer. This effect results in blurred or
“fuzzy” distinctions from layer to layer (Figure 10.7). The effects of
interpenetration underscore the lack of organized structure observed
within the bulk film.
The degree to which interpenetration occurs, while not completely
understood, is linked to ionic concentration and charge density of the
polyelectrolyte. In general, ionic concentration is correlated with interfacial
CHAPTER 10: Fabrication, Properties, and Applications of Thin Films
394
happened for the previous layer, the charged surface is balanced by
positive counterions from the polyelectrolyte solution. An assembly with
one polycationic and one polyanionic layer comprises a single bilayer.
The presence of counterions at the charged surface can be better
explained through the use of image forces. Image forces result from the
differences in the dielectric constants of water and the film interface.
Stronger dielectric environments polarize to a greater extent in response to
an applied electric field, providing more effective screening of that electric
field than a weaker dielectric environment. Water has a very high dielectric
constant (~80), while organic polymers typically have relatively low
dielectric constants (2–10). The difference in the dielectric environment
causes a repulsion of water charges from the surface, allowing a polyelectrolyte counterion to bind at the surface instead. These counterions
will subsequently be displaced by a polyelectrolyte with the same charge
opposite to the surface—a process driven by gains in entropy because
more salt ions are displaced from the surface than the number of polyelectrolyte molecules that adsorb.
To maintain the overall neutral charge of the film, charge compensation—
either intrinsic or extrinsic—must be present. Extrinsic compensation
refers to polymer charges that are balanced by extrinsic materials, such as
the salt counterions. It has been found that there is actually very little salt
concentration observed within the film. Instead, there is a 1:1 stoichiometric ratio of polycation to polyanion charge. As a result, the overall
charge balance within the multilayers must be attributed primarily to
intrinsic compensation. However, some extrinsic compensation has been
found to occur at the substrate surface, where charge overcompensation is
neutralized by salt counterions.
Within the bulk of the film, we might expect clearly defined layers of
alternating charge. This is not what occurs. Upon adsorption the polyelectrolyte does not simply “lie” on the surface. It has been commonly
observed that after adsorption, the outer polyelectrolyte then diffuses
slightly into the inner polyelectrolyte layer. This effect results in blurred or
“fuzzy” distinctions from layer to layer (Figure 10.7). The effects of
interpenetration underscore the lack of organized structure observed
within the bulk film.
The degree to which interpenetration occurs, while not completely
understood, is linked to ionic concentration and charge density of the
polyelectrolyte. In general, ionic concentration is correlated with interfacial
CHAPTER 10: Fabrication, Properties, and Applications of Thin Films
394