Page 122 - Introduction to Colloid and Surface Chemistry

1 1 2 Liquid-gas and liquid-liquid interfaces

Many of the proteins from which these films can be formed are
approximately spherical in the native state, with diameters of c.
2 !
5-10 nm. Since a limiting area of 1 m mg~ corresponds to about
2
0.15 nm per peptide residue, or a film only 0.8-1.0 nm thick, then
clearly some unfolding of the polypeptide chains takes place at the
surface. Proteins unfold even further at oil- water interfaces.
1
At low compressions, up to c. I mN m" , protein films tend to be
gaseous, thus permitting relative molecular mass determination.
For an ideal gaseous film,

= RT
irA m
is the molar area of the film material. Therefore,
where A m
= RT

where A represents the area per unit mass and M is the molar mass of
the film material. To realise ideal gaseous behaviour experimental
data must be extrapolated to zero surface pressure -i.e.

RT
limir/t= — (4.37)
»-*o M
The relative molecular masses of a number of spread proteins have
54
been determined from surface-pressure measurements . In many
cases they compare with the relative molecular masses in bulk
solution. Relative molecular masses significantly lower than the bulk-
solution values have also been reported, which suggests surface
dissociation of the protein molecules into sub-units.

Interactions in mixed films

Mixed surface films, especially those likely to be of biological
importance, have been subject to a great deal of investigation.
Often there is evidence of interaction between stoichiometric
proportions of the components of a mixed monolayer. Evidence of
interaction can be sought by measuring partial molecular areas or by
studying the collapse of the mixed film. The partial molecular areas of
the components of a mixed film are usually different from the
molecular areas of the pure components when interaction occurs. A
mixed monolayer may collapse in one of two ways: (a) with no