Control of multilayer thickness is determined by a variety of factors, such
as polymer chain length, polymer charge, and strength of ionic solution.
The most obvious way to control film thickness is to control the number
of multilayers formed on the substrate surface. A greater number of
multilayers will result in a thicker overall film, whereas fewer multilayers
will decrease the film thickness. In addition to the number of multilayers,
layer thickness is highly dependent on ionic concentration with an almost
linear relationship. Nearly linear multilayer growth has been observed, up
to 100 bilayers (Reveda and Petkanchin, 1997). Low salt concentrations
will allow the polymer to assume a flat, extended conformation. Higher
salt concentrations result in greater shielding of the charges on the polyelectrolyte, allowing the charges to be in closer proximity and resulting in
a more coiled structure. Low ionic concentration screens charges on the
polyelectrolyte and the surface charges, allowing for a flat conformation.
The looped and coiled conformation that results from increased ionic
concentration creates thicker layers and thus an overall thicker film,
although it will not necessarily change the amount of polymer adsorbed
to the surface. Charge density on the polymer can also affect film thickness. A lower charge density correlates with thicker layer formation.
Similar to the results at high ionic concentration, a polymer with lower
charge density will assume a more coiled conformation due to fewer
electrostatic interactions between the substrate surface and the weakly
charged polyelectrolyte. Perhaps contrary to intuition, it is important to
note that polymer molecular weight and polymer branching do not cause
significant changes in multilayer thickness.
10.3 MODEL PHOSPHOLIPID BILAYER
FORMATION AND CHARACTERIZATION
Biological membranes are complex structures that contain a large amount
of proteins and are difficult to study in vivo. By using model phospholipid
bilayers, we can study how specific membrane components interact by
incorporating only these specific components into the model membrane
(Castellana et al., 2006). Furthermore, phospholipid bilayers closely
resemble cell membranes in many ways. They retain two-dimensional
fluidity and are capable of hosting membrane proteins. Therefore, model
membranes have historically been used to study membrane properties in
vitro. They have also been used for the investigation of biological processes involving membranes that occur at the cellular level, such as ligandreceptor interactions, viral attack, and cellular signaling events.
CHAPTER 10: Fabrication, Properties, and Applications of Thin Films
396
as polymer chain length, polymer charge, and strength of ionic solution.
The most obvious way to control film thickness is to control the number
of multilayers formed on the substrate surface. A greater number of
multilayers will result in a thicker overall film, whereas fewer multilayers
will decrease the film thickness. In addition to the number of multilayers,
layer thickness is highly dependent on ionic concentration with an almost
linear relationship. Nearly linear multilayer growth has been observed, up
to 100 bilayers (Reveda and Petkanchin, 1997). Low salt concentrations
will allow the polymer to assume a flat, extended conformation. Higher
salt concentrations result in greater shielding of the charges on the polyelectrolyte, allowing the charges to be in closer proximity and resulting in
a more coiled structure. Low ionic concentration screens charges on the
polyelectrolyte and the surface charges, allowing for a flat conformation.
The looped and coiled conformation that results from increased ionic
concentration creates thicker layers and thus an overall thicker film,
although it will not necessarily change the amount of polymer adsorbed
to the surface. Charge density on the polymer can also affect film thickness. A lower charge density correlates with thicker layer formation.
Similar to the results at high ionic concentration, a polymer with lower
charge density will assume a more coiled conformation due to fewer
electrostatic interactions between the substrate surface and the weakly
charged polyelectrolyte. Perhaps contrary to intuition, it is important to
note that polymer molecular weight and polymer branching do not cause
significant changes in multilayer thickness.
10.3 MODEL PHOSPHOLIPID BILAYER
FORMATION AND CHARACTERIZATION
Biological membranes are complex structures that contain a large amount
of proteins and are difficult to study in vivo. By using model phospholipid
bilayers, we can study how specific membrane components interact by
incorporating only these specific components into the model membrane
(Castellana et al., 2006). Furthermore, phospholipid bilayers closely
resemble cell membranes in many ways. They retain two-dimensional
fluidity and are capable of hosting membrane proteins. Therefore, model
membranes have historically been used to study membrane properties in
vitro. They have also been used for the investigation of biological processes involving membranes that occur at the cellular level, such as ligandreceptor interactions, viral attack, and cellular signaling events.
CHAPTER 10: Fabrication, Properties, and Applications of Thin Films
396