Liquid-gas and liquid-liquid interfaces 111Figure 4.27 Tt-A and r)s-A curves for a spread monolayer of /3-globulin at the petrol
ether-water interface^159 (By courtesy of The Faraday Society)the study of protein films. One frequently used spreading solution
contains about 0.1 per cent protein in a mixture of alcohol and
aqueous sodium acetate.
Compression of protein films below about 1 m^2 mg"^1 results in
close packing of the polypeptide chains and the gradual development
of a gel-like structure. At a surface pressure of c. 15-20 mN m"^1 a
time-dependent collapse of the film into bundles of insoluble fibres
takes place. The area occupied by the compressed protein film is
often characterised by the limiting area obtained by extrapolating the
approximately linear part of the IT-A curve to zero pressure. A more
satisfactory characterisation of the close-packed film, especially in
relation to the areas indicated by X-ray diffraction measurements on
protein fibres, is given by the area at minimum compressibility - i.e.
where the TT~A curve is at its steepest. This area corresponds
approximately to the onset of film collapse. Not surprisingly, these
areas show little variation from protein to protein, since the average
size of the constituent amino acid residues does not alter greatly.