Biology Reference
In-Depth Information
Figure 8.8.
Schematic of the layer-by-layer deposition process on a
substrate bearing an initial negative charge excess. Taken with permission
from [134], M. F. Castelnono and J.-F. Joanny,
Langmuir
16
, 7524-7532
(2000). c
American Chemical Society.
technique has been applied to the deposition of many different
charged species, including conducting polymers, DNA, and proteins.
Some recent reviews of the applications are givenin Refs. 64 to 66.
The main driving force for the adsorption of, for example, a
positive PE onto a negative surface is electrostatic attraction. Zeta
potential measurements of such adsorption [67] have shown that
charge overcompensation occurs, that is, the PE/solid does not
become neutral, but is positive overall and so can then adsorb a
negative PE. As the layers are built up, the zeta potential oscillates
symmetrically around the zero value [67]. Neutron reflectommetry
experiments indicate the polymer layers are not flat, but penetrate
into each other [68]. Apart from Coulombic attraction, secondary
forces such as van der Waals, hydrogen bonding and hydrophobic
interactionsalsocontribute,andtheseattractionsgivetheprocessa
negative enthalpy. Also, small counterions and solvent shell water
molecules are liberated when the PEs come together and hence
entropy is increased. These two factors are responsible for the
negative free energy of l-b-l deposition according to
G
=
H
-
T
S
.
