92168.pdf

102 Liquid-gas and liquid-liquid interfaces

designed^158 for studying film creep under a constant shearing stress.

A platinum ring suspended from a torsion wire is kept under constant

torsional stress and its rotation in the plane of the interface is

measured as a function of time.

The study of surface rheology is useful in connection with the

stability of emulsions and foams (Chapter 10) and the effectiveness of

lubricants, adhesives, etc.

Electron micrographs of monolayers

Insoluble surface films can be studied by electron microscopy. The

films are transferred from the substrate on to a collodion support and

shadow-cast by a beam of metal atoms directed at an angle a (about

15°) to the surface (Figure 4.21). If the width x of the uncoated region

is measured, the thickness of the film, x tan a, can be calculated; for

example, a «-C 36 H73COOH film has been shown to be about 5 nm

thick - i.e. consistent with a vertically orientated monomolecular

layer. The technique has also been used for following the state of the

surface as a film is compressed.

Mefol beam

Support

Figure 4,21

The physical states of monomolecular films

Two-dimensional monolayers can exist in different physical states
which bear some resemblance to the solid, liquid and gaseous states
in three-dimensional matter. Surface films are best classified according
to the lateral adhesion between the film molecules, including end-
groups. Factors such as ionisation (and, hence, the pH of the