Chemistry Reference
In-Depth Information
Figure 3.10. Idealized pressure vs. area isotherm indicating phase transitions
of the ML.
···
O-H
O hydrogen bonds at the water/air interface. In general, the state of the ML
on the water surface is monitored by measuring the surface pressure
, defined as
the difference between
γ
0
and that of the film-covered surface
γ
.
is of the order
of a few mN m
−
1
.
As the ML is compressed on the water surface by the moveable barrier or piston,
it will undergo several phase transitions, which can be readily identified by moni-
toring
as a function of the area occupied by the film. An idealized pressure vs.
area isotherm is shown in Fig. 3.10. This is the 2D equivalent of the pressure vs.
volume isotherm for a gas-liquid-solid system. If the area per molecule is suffi-
ciently high, the floating film will behave as a 2D gas phase (disordered) where the
molecules are far enough apart, resulting in negligible intermolecular interactions.
As the ML is compressed the pressure rises, signalling a change in phase to a 2D
liquid state, called the expanded ML phase. Upon further compression, the pressure
begins to rise more steeply as the liquid expanded phase transforms to a condensed
phase, in fact to a series of condensed phases. The emergence of each condensed
phase can be accompanied by constant pressure regions of the isotherm (plateaus),
associated with enthalphy changes in the ML. MLs can be compressed to pressures
considerably higher than the equilibrium spreading pressure. The surface pressure
continues to increase with decreasing surface area until a point is reached where it is
not possible to increase the pressure any further. This is referred to as collapse. The
forces acting on a ML at this point are quite high. When collapse occurs, molecules
are forced out of the ML.
