Thin Liquid Films - Surface and Colloid Chemistry: Principles and Applications - page 161

8 Thin Liquid Films
8.1 IntroductIon
The formation and structure of thin liquid ilms (TLF; such as in foams or bubbles) is
a fascinating phenomena that humans have studied over many decades. This struc-
ture is the closest one comes to observing molecules with the eye. TLF is thus the
thinnest object one can see without the aid of any kind of microscope. One of the
most commonly known thin liquid film structures is the soap bubble or bubbles
formed on detergent solutions (as in dishwashing solutions). Everyone has enjoyed
this formation and its display of rainbow colors. It may look as if a bubble formation
and its characteristic stability is of little consequence, but, in fact, in everyday life,
bubbles play an important role (e.g., from the functioning of the lungs to the enjoy-
ment of beer and champagne!). It is a common observation that, ordinary water,
when shaken, does not form any bubbles at the surface. On the other hand, all soap
and detergent solutions (shampoo, washing powder, beer, champagne, and seawater),
on shaking, may form very extensive bubbles. In this chapter, the formation and sta-
bility of bubbles will be described. Further, even though one cannot see or observe
the surface layer of a liquid directly, TLFs allow one to make some observations that
can provide much useful information (Ivanov, 1988; Birdi, 2002).
8.2 bubbleS and FoamS
If pure water is shaken, no bubbles are observed at the surface. All pure organic
fluids exhibit no bubble formation on shaking. It means that, as an air bubble rises to
the surface of the liquid, it merely exits into the air. However, if an aqueous detergent
(surface-active substance) solution is shaken or an air bubble is created under the
surface, a bubble is formed (Figure 8.1).
The formation of a bubble can be described as follows:
The process begins with an air bubble inside the liquid phase. At the surface, the
bubble detaches and moves up under gravity. The detergent molecule forms a bilayer
in the bubble film. The water in between is the same as the bulk solution. This may
be depicted as follows: a surface layer of detergent is applied, a bubble forms with air
and a layer of detergent, and the bubble at the surface forms a double layer of deter-
gent with some water in between TLF; varying from 10 μm to 100 μm).
The bubble test of shaking a water solution, although simple, is very sensitive and
may be used to determine the presence of very minute (around parts per million)
contents of surface-active substances.
A bubble is composed of a TLF with two surfaces, each with a polar end pointed
inward and the hydrocarbon chains pointing outward (Figure 8.2). The water inside
the films will move away (due to gravity), effecting the thinning of the film. Since the
161