Chemistry Reference
In-Depth Information
Plate
FIGure 4.11
Transfer of monolayers of lipids to LB films.
LB films with multilayers (varying from a few layers to thousands) could be made,
and the adsorption monitored by measuring the decrease in Π on each stroke. If no
adsorption takes place, then no change is observed. There are some lipids, such as
cholesterol, which do not form LB films. There are also other methods, such as
Light reflection
IR spectroscopy
STM (scanning tunneling microscope) and AFM (atomic force microscope)
Change in contact angle
that can provide a detailed information on these LB films.
The LB deposition is traditionally carried out in the “solid” phase. The surface
pressure is then high enough to ensure sufficient cohesion in the monolayer; for
example, the attraction between molecules in the monolayer is high enough so that
the monolayer does not fall apart during transfer to the solid substrate. This also
ensures the buildup of homogeneous multilayers. The surface pressure value that
gives the best results depends on the nature of the monolayer and is usually estab-
lished empirically. However, amphiphiles can seldom be successfully deposited at
surface pressures lower than 10 mN/m, and at surface pressures above 40 mN/m,
collapse and film rigidity often pose problems. When the solid substrate is hydro-
philic (glass, SiO
2
, etc.), the first layer is deposited by raising the solid substrate from
the subphase through the monolayer, whereas if the solid substrate is hydrophobic
(HOPG, silanized SiO
2
, etc.), the first layer is deposited by lowering the substrate
into the subphase through the monolayer. It is found that, in some LB systems, the
magnitude of Π drops about 1-2 mN/m as each time a plate (with surface area of 5
cm
2
) is moved down or up through the monolayer. It is thus a very sensitive method
to study the LB deposition phenomenon directly.
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