Biomedical Engineering Reference
In-Depth Information
20.3.1 Air/Water and Air/Solid Properties of Thin Solid Films
of Lipids
Amphiphiles can self-assemble at an air/water or air/ solid interface. Based on the
nature of thin fi lm formed from different amphiphiles, they can be deposited onto
a polymer substrate by various techniques, such as thermal evaporation, sputter-
ing, electro deposition, molecular beam epitaxy, and adsorption from solution,
Langmuir - Blodgett Technique, and Self - assembly
7
. Langmuir - Blodgett trough
facility is versatile for studies at air/solid and air/water interface. Irwing Langmuir
was the fi rst to perform systematic studies on fl oating surfactant monolayers
8
.
However, the fi rst detailed description of sequential monolayer transfer was
given several years later by Katherine Blodgett
9
. These built up thin solid fi lms
are therefore known as Langmuir-Blodgett fi lms.
The most important indicator of the monolayer properties of an amphiphilic
molecule (depending upon the properties) is given by measuring the surface
pressure as a function of the area of water surface available for each molecule.
This is carried out at constant temperature and is known as surface pressure-
area isotherm. Figure 20.1 shows the typical pattern of surface pressure-area
isotherm.
A number of distinct regions are immediately apparent on examining the
isotherm. These regions are called phases. A simple terminology used to classify
different monolayer phases of fatty acids has been proposed by W. D. Harkins
10
.
This is applicable to different kinds of molecules at the air/water interface. At
large, the monolayers exist in the gaseous state (G); on compression, they undergo
a phase transition to the liquid expanded state (L
1
). Upon further compression
Figure 20.1.
π
-A isotherm. Gaseous state (G), liquid expanded (L
1
), Liquid Condensed (L
2
),
Solid (S) state.
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