Chemistry Reference
In-Depth Information
4.1.1 S
u r f a c e
f
I l m
b
a l a n c e
Since Langmuir reported monolayer studies, a great many instruments have been
designed about this method. The clean surface of water shows no change in surface
tension if a barrier is moved across it. However, if a surface-active agent is present,
then the latter molecules will be compressed, and this will give rise to a decrease in
surface tension.
The aim in these systems is to study the properties of a monomolecular thin-lipid
film spread on the surface of water. Lately, it has been shown that such lipid films
are useful membrane models for biological membrane structure and function studies.
It is thus obvious that much careful arrangement is required in order to achieve this
goal, with a high degree of cleanliness required so that contaminants do not interfere
with the data. Modern methods allow measurement of monolayer properties. It is
important to note that these monomolecular films can only be studied by using the
surface balance. There exists no other method through which any direct information
can be obtained about molecular packing or interactions.
Monolayer films were studied by using a Teflon trough with a barrier (also of
Teflon) that could move across the surface (Figure 4.1). The change in γ was moni-
tored using a Wilhelmy plate attached to a sensor. The accuracy could be as high as
mmN/m (m dyne/cm).
The film balance (also called the Langmuir trough) consists of a Teflon (or Teflon-
lined) rectangular trough (typically 20 cm × 10 cm × 1 cm). Teflon (PTFE;
poly(tetraflouroethylene)) permits keeping the apparatus clean. Clean water (distilled
water and purified with active charcoal to remove any minute organic contaminants)
is used to fill the trough just over the edge. A barrier of Teflon is placed on one end
of the trough, which is used to compress the lipid molecules. Recently, many com-
mercially available film balances are found (monomolecular films).
Lipid (or protein or other film-forming substance) is applied from its solution to
the surface. A solution with a concentration of 1 mg/mL is generally used. Lipids are
dissolved in CHCl
3
or ethanol or hexane (as found suitable). If the surface area of the
trough is 100 cm
2
, 1-100 μL of this solution may be used. After the solvent has evap-
orated (about 15 min), the barrier is made to compress the lipid film at a rate of 1 cm/s
or as suitable. The amount of lipid or protein applied is generally calculated so as to
give a compressed film. Most substances cover 1 mg/m
2
to give a solid film. If there
is 100 cm
2
solution in the trough, then 10 μg or less of the film-forming substance
is enough for such an experiment. It is thus obvious that such studies can provide
much useful surface chemical information with very minute amounts of material.
Most proteins can also be studied as monomolecular films (1 mg of protein spreads
to cover about 1 m
2
surface area; Birdi, 1999). This method is useful when studying
systems where the magnitude of surface pressure (the change in the surface tension)
is less than 1 mN/m, and using very high-sensitivity apparatus (± 0.001 mN/m).
Constant area method:
One can also study Π versus surface concentration (Cs=
area/molecule) isotherms by keeping the area constant. The surface concentration,
Cs, is changed by adding small amounts of a substance to the surface (by using a
microliter syringe—1 or 5 μL). In general, one obtains a good correlation with the Π
versus area isotherms (Gaines, 1966; Adamson and Gast, 1997; Birdi, 1989, 1999).
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