The article by Mackie (pp. 357–361) describes experimental findings from imaging studies of protein + surfactant layers using Langmuir-Blodgett techniques combined
with high-resolution atomic force microscopy (AFM). An
enhanced appreciation of the general applicability of the
orogenic displacement concept has come from recent
measurements involving mixtures of proteins with a range
of anionic, cationic and zwitterionic surfactants. This has
been usefully complemented by a comparison of the
qualitative features of the AFM images with those
generated by Brownian dynamics simulations of
viscoelastic protein layers using a bonded particle network
model. The structural changes observed during globular
protein displacement by charged surfactants show similar
general trends to those previously reported by the Norwich
group for non-ionic surfactants. The main difference,
however, lies in the much more limited surfactant domain
growth, before the gel-like protein network starts to fail,
and the surfactant forms the continuous phase at the
surface. This can be explained—at least qualitatively—in
terms of the role of surfactant/protein electrostatic interactions. Screening of charges caused by addition of
electrolyte converts this evolving structural change during
competitive displacement into behaviour more resembling
that found for protein +non-ionic surfactant systems, as
also predicted by the computer simulations.