  We have carried out experiments to probe the static and dynamic interfacial
properties of $\beta$--casein monolayers spread at the air-buffer interface,
and analysed these results in the context of models of weak polyampholytes.
Measurements have been made systematically over a wide range of ionic strength
and pH. In the semi-dilute regime of surface concentration a scaling exponent,
which can be linked to the degree of chain swelling, is found. This shows that
at pH close to the isoelectric point, the protein is compact. At pH away from
the isoelectric pH the protein is extended. The transition between compact and
extended states is continuous. As a function of increasing ionic strength, we
observe swelling of the protein at the isoelectric pH but contraction of the
protein at pH values away from it. These behaviours are typical of a those
predicted theoretically for a weak polyampholyte. Dilational moduli
measurements, made as a function of surface concentration exhibit maxima that
are linked to the collapse of hydrophilic regions of the protein into the
subphase. Based on this data we present a configuration map of the protein
configuration in the monolayer. These findings are supported by strain (surface
pressure) relaxation measurements and surface quasi-elastic light scattering
(SQELS) measurements which suggest the existence of loops and tails in the
subphase at higher surface concentrations.

Soft Condensed Matter;Statistical Mechanics;Biomolecules