Spread Lipid Films on Liquid Surfaces (Langmuir–Blodgett Films) - Surface and Colloid Chemistry: Principles and Applications - page 82

Δγ = k s (1/ A ) (d A /dt) (4.8)

where k s is the fractional change in area per unit time per unit surface tension differ-
ence. From this relation, C s and E s can be derived.
Monolayers are thus very useful in understanding various aspects of molecular
packing (such as liquid crystals, etc.). With the information from area/molecule, the
packing and other interaction parameters can be estimated. These monolayer studies
have been found to be important in understanding the thin-liquid film (TLF) struc-
tures (bubbles, foams).

4.3.2.2 Surface Potential ( Δ v) of lipid monolayers
Any liquid surface, especially aqueous solutions, will exhibit asymmetric dipole or
ions distribution at the surface as compared to the bulk phase. If SDS is present in
the bulk solution, then we will expect that the surface will be covered with SD ions.
This would impart a negative surface charge (as is also found from experiments).
It is thus seen that the addition of SDS to water not only changes (reduces) surface
tension but also imparts negative surface potential. Of course, the surface molecules
of methane (in liquid state) will obviously exhibit symmetry in comparison to water
molecules. This characteristic can also be associated to the force field resulting from
induced dipoles of the adsorbed molecules or spread lipid films (Adamson and Gast,
1997; Birdi, 1989).
The surface potential arises because the lipid molecule orients with polar part
toward the aqueous phase. This effects a change in dipole at the surface. There would
thus be a change in surface potential when a monolayer is present, as compared to a
clean surface. The surface potential, ΔV, is

Δ V = potential monolayer − potential clean = V monolayer − V clean (4.9)

The magnitude of ΔV is measured most conveniently by placing an air electrode (a
radiation emitter Po 210 [alpha-emitter]), near the surface (ca. millimeter in air), con-
nected to an impedance electrometer. This is required since resistance in air is very
high, but it is appreciably reduced by the radiation electrode. However, proper isola-
tion is essential for the data to be reliable.
Since these monolayers are found to be very useful biological cell-membrane
structures, it is seen that such studies can provide information on many systems
where ions are carried actively through cell membranes (Chattoraj and Birdi, 1984;
Birdi, 1999).
The transport of K + ions through cell membranes by antibiotics (valinomycin) has
been a very important example. Addition of K + ions to the subphase of a valinomycin
monolayer showed that the surface potential became positive. This clearly indicates
the ion-specific binding of K + to valinomycin (Birdi, 1989).

4.3.3 c h a r g e d l I p I d m of n of l a y e r S
The spread monolayers have provided much useful information about the role of
charges at interfaces. In the case of an aqueous solution consisting of fatty acid or