Characterisation of monolayers classical methods

The expression classical methods is used here to describe methods which were pioneered before the Second World War and which determine the 

Surface viscosity has an important influence on the deformation of films and can also provide information about structure. Gaines [14] describes various methods of measuring this quantity. The damped torsion pendulum as developed by Langmuir and Schaefer [65] is probably the best device for making such measurements. Recent measurements of this type have been made by Buhaenko et al. [66]. Malcolm [67, 68] and Daniel and Hart [69] have carried out experiments which illustrate the important influence which viscosity has on the study of isotherms. 

The influence of the presence of an organic monolayer on the surface potential measured at the water surface can also provide interesting information. However, this information can be difficult to interpret as it depends on the dipole moments associated with both the upper and the lower ends of the molecules and also the surface double layer in the water immediately below the film. There are two different ways of measuring this quantity. Both, of course, involve a counter electrode below the water surface. 

In the Kelvin method, a planar electrode is vibrated up and down just above the surface. This electrode and the surface form the two plates 

A more satisfactory procedure involves a radioactive probe held a little above the water surface. The radioactive radiations ionise the air between the probe and the surface and ensure that they are at the same potential. The probe is connected to an electrometer having a very high input impedance which reads the surface potential. This procedure was pioneered by Guyot (70J and Frumkin [71J in the 1920s but has become much more convenient to use with the introduction of artificial radioactive isotopes and modern electronics. Americium 241 is particularly useful as the low energies of the a and y radiations produced only ionise the air in the immediate vicinity of the probe. 

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