Chemistry Reference
In-Depth Information
tend to exist as coils. This can lead to a more complex behaviour of the
damping curves.
4.4 Film elasticities
In order to retrieve film elasticities from the damping coefficient values we
assumed that the films can be described as purely elastic and the relation
between the damping coefficient and the film elasticity parameter is given
by expression (3). The problem of retrival of the film elasticity from for-
mula (3) is that there are two elasticity values (two roots) corresponding, in
general, to a given damping coefficient value measured at a given wave
frequency. The problem of the proper choice of retrieved elasticity values
seems to be solved, if the damping coefficients will be measured at several
wave frequencies. Then for purely elastic insoluble films the real elasticity
value should be equal at different wave frequencies. Usually, however,
both the roots for the retrieved elasticity values are different at different
frequencies because of experimental errors and because a purely elastic
film is only an idealization of real films. Therefore, some additional argu-
ments have to be included to choose a physically significant elasticity
value retrieved from damping measurements at a given frequency.
The film elasticity roots retrieved from the data presented in Figure 5
are shown in Figure 6. One should note that the two roots tend to each
other at concentrations corresponding to the maxima of the damping coef-
ficients. We assume that a transition of the film elasticity values from one
root to another should occur. It will give us a smooth dependence of the
elasticity on concentration with a continuous derivative of the elasticity on
concentration, which is reasonable from a physical point of view. If to as-
sume the elasticity as one of the roots in the whole concentration range,
then the derivative of the retrieved elasticity will have a step-like behav-
iour at concentrations of the damping maxima.
The resulting elasticity values, using the above criterion are given in
Figure 7. Obviously, the elasticity for the ordinary surfactants increases
monotonously with concentration tending to some large limiting values at
concentrations of the order and larger than monomolecular coverage. The
physical arguments in favour of this behaviour are as those presented
above. Namely, at concentrations reflecting monomolecular coverage the
surface-active molecules are directed practically normal to the water sur-
face and strongly interact with each other, so that the monolayer exhibits
high elasticity. At
mean
concentrations exceeding monomolecular cover-
age the excess of surfactant materials will form microscopic drops. If the
periods of surface waves are small compared with the characteristic times
of surfactant exchange between the drops and the monolayer, then the
