Page 120 - Introduction to Colloid and Surface Chemistry

110 Liquid-gas and liquid-liquid interfaces

thousand-fold. Tarry material, dust, etc., in town fogs is responsible
for delay in dispersal because of monolayer formation.

Surface films of proteins

Surface films of high-polymer material, particularly proteins, offer
another wide field of study.
Proteins consist of a primary structure of amino acid residues
connected in a definite sequence by peptide linkages to form
polypeptide chains:
T r r

-CH-NH-CO-CH-NH~CO-CH-etc.

which may embrace hundreds of such residues. These polypeptide
chains normally take up a helical configuration, which is stabilised
mainly by hydrogen bonding between spatially adjacent -NH- and
™CO~ groups. The helical polypeptide chains of the globular
proteins are in turn folded up to give compact and often nearly
spherical molecules. This configuration is maintained by hydrogen
bonding, van der Waals forces between the non-polar parts,
disulphide cross-linking, etc.
Any significant alteration in this arrangement of the polypeptide
chains without damage to the primary structure is termed denatura-
tion. The common agents of denaturation are those which would be
expected to modify hydrogen bonds and other weak stabilising
linkages - e.g. acids, alkalis, alcohol, urea, heat, ultraviolet light and
surface forces. Protein denaturation is accompanied by a marked loss
of solubility and is usually, but not necessarily, an irreversible
process. Proteins adsorb and denature at high-energy air-water and
oil-water interfaces because unfolding allows the polypeptide chains
to be orientated with most of their hydrophilic groups in the water
phase and most of the hydrophobic groups away from the water
phase.
If a small amount of protein solution is suitably spread at the
surface of an aqueous substrate, most of the protein will be surface-
denatured, giving an insoluble monomolecular film before it has a
chance to dissolve. The techniques already described for studying
spread monolayers of insoluble material can, therefore, be used in