Chemistry Reference
In-Depth Information
The foam consists of
Air on one side
Outer monolayer of detergent molecule
Some amount of water
Inner monolayer of detergent molecule
Air on outer layer
This can be depicted as follows (schematic):
DETERGENT
WATE
R
DETERGENT
DETERGENT
WATER
DETERGENT DETERGENT
WATER
DETERGENT
DETERGENT
WAT E R
DETERGENT
The orientation of the detergent molecule in TLF is such as that the polar group (OO)
is pointing toward the water phase, and the apolar alkyl part (CCCCCCCCCC) is
pointing toward the air, as
AirCCCCCCCCCCOOWATEROOCCCCCCCCCCAir
AirCCCCCCCCCCOOWATEROOCCCCCCCCCCAir
AirCCCCCCCCCCOOWATEROOCCCCCCCCCCAir
AirCCCCCCCCCCOOWATEROOCCCCCCCCCCAir
AirCCCCCCCCCCOOWATEROOCCCCCCCCCCAir
AirCCCCCCCCCCOOWATEROOCCCCCCCCCCAir
The thickness of the water phase can vary from over 100 μm to less than 100 nm.
Foams are thermodynamically unstable since there is a decrease in total free energy
when they collapse. As the thickness increases around the wavelength of light (nm),
one starts to observe rainbow colors (arising from the interference). The TLF forms
at an even smaller thickness (50 Å or 5 nm).
However, certain kinds of foams are known to persist for very long periods of
time, and many attempts have been made to explain their metastability.
The TLF may be regarded as a kind of condenser. The repulsion between the two
surfactant layers (Figure 8.3) will be determined by the EDL. The effect of added
ions to the solution is to make the EDL contract, and this leads to thin films.
The film looks black-gray, and its thickness is around 50 Å (5 nm), which is almost
the size of the bilayer structure of the detergent (i.e., twice the length [ca. 25 Å] of
a typical detergent molecule). Actually, it is a remarkable fact that one can
see
two
molecules' thin structure. The rainbow colors are observed since the light is reflected
by the varying thickness of the TLF of the bubble.
In the beer industry, foaming behavior is vital to the product. The beer bottle is
produced under CO
2
gas at high pressure. As soon as a beer bottle is opened, the pres-
sure drops and the gas (CO
2
) is released, which gives rise to foaming. Commonly,
the foam stays inside the bottle. Foaming is caused by the presence of different
amphiphilic molecules (fatty acids, lipids, and proteins). The foam is very rich as the
liquid film is very thick and contains a substantial aqueous phase (such foams are
Search WWH ::
Custom Search