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

that they exert little force one on another. As the surface area of the monolayer is reduced,
the hydrophobic chains of the molecules will begin to interact. The liquid state formed is
generally called the expanded monolayer phase (E). At this stage, molecules at the water
surface are in a random rather than in a regular orientation, with their polar groups in contact
with the subphase. As the molecular area is progressively reduced, a condensed (C) phase may
appear. In the condensed monolayer state the molecules are closely packed and are oriented
with the hydrophobic groups pointing away from the water surface.
Figure 2.3. Surface pressure-area isotherm for a chain organic monolayer.
2.2.3.- Axisymetric drop shape analysis (ADSA)
Axisymetric drop shape analysis (ADSA) is a drop shape technique for measuring the interfacial
surface tension, γ. This method can be performed on the measured profiles of captive bubbles,
sessile drops, pendant drops or bubbles. The principle of the technique is based on the
determination of the drop shape by the balance between: i) the force of gravity which deforms
the drop and ii) the surface tension which resists the deformation by minimizing the drop
surface area (17). The relationship between these two forces is described by the Laplace
equation (19):
The Laplace equation is written in terms of the surface tension, , the principal radii of the
drop, and , and the pressure difference across the curved interface, . The pressure
drop has two contributions, , where is the pressure difference at a
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