Role of Electrostatic Interactions on Molecular Self-Assembly of Protein + Phospholipid Films at the Air–Water Interface - Food Colloids: Self-Assembly and Material Science - page 233

5.0x10 -6

pH 5

pH 7

4.0x10 -6

pH 9

COLLAPSE 3D
STRUCTURES

3.0x10 -6

2.0x10 -6

LE

1.0x10 -6

S

LC

0.0

0 0 0 0 0 0 0

(mN / m)
Figure 3 The effect of pH on the reflectivity of BAM images (shutter time ¼ 1/250 s) as a
function of the surface pressure for DPPC monolayers spread at the air-water
interface at 201C and ionic strength 0.05M; O, pH 5; D ,pH7; r ,pH9

π

from the LE to the LC region, the thickness increases with increasing p because
the hydrocarbon chains acquire a greater inclination with respect to the
interfacial plane. At higher p the thickness becomes constant, coinciding with
the formation of the solid structure within the monolayer. The increase in the
monolayer reflectivity with pH in Figure 3 could be due to the increased affinity
between water molecules and the DPPC polar head-group ionized in the more
basic pH aqueous phase. That is, the higher reflectivity of DPPC monolayers at
pH 9 may be due to compression against the greater electrostatic repulsion.
This will force some head-groups to be displaced further into the aqueous phase
to maintain head-group spacing as required by the electrostatic repulsion
forces. This will give the impression that A lim remains independent of pH,
but only in the two-dimensional plane parallel to the interface.
The BAM and AFM images for DPPC monolayers as a function of pH show
similar structures when viewed at equivalent magnifications. However, the
AFM data reveal a previously unexpected heterogeneity within the domains
and at the surfaces of the domain structures. Figure 4 shows that the pH of the
aqueous phase has a significant effect on the topography of the DPPC mono-
layers. The topographic images suggest the presence of numerous small holes in
the interior of the domains (possibly LE regions) and ragged surface protru-
sions at the domain boundaries. The main geometrical characteristics and
topographical parameters deduced from the BAM images for the LE and LC
domains are included in Table 2. Electrostatic interactions clearly play a role in
the self-assembly of domains within the DPPC layers as observed in the BAM
images. At pH 5, the DPPC domains are small and compact in shape. As the
pH increases, so the domains increase in size. In addition, the shape of the
domains changes: they become clearly dendrite like in appearance. This is
presumably because increasing the boundary length minimizes the effects of
repulsive interactions between DPPC molecules within the domain. The great-
est thickness of DPPC domains is observed at pH 9 because these domains are