Self-Assembly of Synthetic and Biological Polymeric Systems of ...

where w is the meniscus’ weight detected by the balance, is the contact angle defined by the
meniscus shape on the wet platinum surface plate, and l is the width and d the plate thickness,
respectively (Figure 2.1). Considering very small and the plate thickness negligible compared
with its width, the former expression is simplified to:
Figure 2.1. Illustration of a fully wet Wilhelmy plate in contact with an aqueous solution.
2.2.2.- Monolayers and Langmuir-Blodgett films
Many insoluble substances such as long chain fatty acids, alcohols, surfactants and polymers
can be spread from a solvent onto water to form a one-molecule thick film, called monolayer.
Molecules in a monolayer can be arranged in different ways, especially when they are closely
packed, depending on the lateral forces between them (17). The obtention of surface
pressure-area isotherms is one of the main methods used to study monolayer formation.
Surface pressure, , is the reduction of the surface tension due to the presence of the
monolayer: , where 0 is the surface tension of the solvent (usually water), and
the surface tension of the system. Surface pressure-area isotherms are often measured for
films/monolayers compressed using a Langmuir trough (Figure 2.2).
In general, an insoluble substance is spread from a volatile solvent to ensure the formation of a
uniform monolayer, and its area is controlled through a moveable barrier. The horizontal force
necessary to maintain the float at a fixed position is measured using a torsion balance, which
provides the surface tension values. Since only micrograms of materials are present in
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